100 Best Physics Concepts Explained With Real-World Examples

Physics is easier to learn when every idea is tied to something you have already seen. A seatbelt is Newton's first law. A bike pump that warms up is the first law of thermodynamics. A phone that knows where you are depends on relativity. This guide explains 100 core physics concepts in plain language, and gives each one a real-world example you can picture.

The concepts follow the order most introductory courses use: measurement and motion, forces, energy and momentum, rotation and gravity, fluids, waves and sound, heat, electricity and magnetism, light, and modern physics. Each entry says what the concept means, then shows where you meet it. Equations appear only where they make the idea clearer; for the full set, see our companion list of 100 physics formulas every student should know.

How this was checked. Definitions follow the free OpenStax textbooks University Physics (Volumes 1–3) and College Physics 2e. Constants come from the NIST 2022 CODATA tables, and specific claims link to primary sources such as NASA, the BIPM and the U.S. FDA. Numbers in the examples are rounded, typical values for illustration, not measurements we made. Full references are at the end of the article.

Measurement and motion

Motion is described with a small set of quantities: position, velocity and acceleration, all measured in agreed units (OpenStax University Physics Volume 1, chapters 1–4).

1. SI units and measurement uncertainty

Physics uses the International System of Units (SI): metres, kilograms, seconds, amperes, kelvins, moles and candelas. Since 20 May 2019 all seven base units have been defined by fixing the values of physical constants. The kilogram, for example, is now defined through the Planck constant rather than a metal cylinder kept near Paris (BIPM). Every real measurement also carries uncertainty.

Real-world example: A kitchen scale that reads to the nearest gram cannot tell you whether a letter weighs 20.2 g or 20.4 g. Reporting "20 g" is honest; "20.31 g" would claim precision the scale does not have.

2. Scalars and vectors

A scalar has only a size, such as mass, temperature or speed. A vector has a size and a direction, such as displacement, velocity or force. You add vectors by direction, not just by size.

Real-world example: A weather forecast that says "wind 20 km/h from the northwest" is giving a vector. "20 km/h" alone would be a scalar, and it would not tell a sailor which way to set the sail.

3. Distance vs displacement

Distance is the total length of the path travelled. Displacement is the straight-line change in position from start to finish, with a direction.

Real-world example: One lap of a standard 400 m running track covers a distance of 400 m, but your displacement is zero because you finish where you started.

4. Speed and velocity

Speed is how fast something moves (distance per unit time). Velocity is speed with a direction. Average speed uses the whole trip; instantaneous speed is the value at one moment.

Real-world example: A car's speedometer shows instantaneous speed. A navigation app's estimate of arrival time uses average speed, which includes the time spent stopped at traffic lights.

5. Acceleration

Acceleration is the rate at which velocity changes. Speeding up, slowing down and changing direction are all accelerations.

Real-world example: A car that goes from 0 to 100 km/h (about 27.8 m/s) in 8 seconds has an average acceleration of about 3.5 m/s². Braking is acceleration in the opposite direction to motion.

6. Free fall and the acceleration due to gravity

Without air resistance, every object near Earth's surface falls with the same acceleration, about 9.8 m/s², regardless of its mass.

Real-world example: In 1971, Apollo 15 commander David Scott dropped a hammer and a falcon feather on the Moon, where there is no air. They hit the ground together (NASA). On Earth the feather drifts because air resistance matters for light, broad objects.

7. Projectile motion

A thrown object moves horizontally at a steady speed (ignoring air resistance) while gravity accelerates it downwards. The two motions are independent, and together they make a parabola.

Real-world example: A basketball shot follows a curved arc. If you fire one ball horizontally and drop another from the same height at the same moment, both hit the floor at the same time.

8. Relative velocity

Velocity always depends on what you measure it against. To find the velocity relative to the ground, add the object's velocity relative to its carrier to the carrier's velocity.

Real-world example: Walking at 1.5 m/s on an airport moving walkway that runs at 0.7 m/s carries you past the shops at 2.2 m/s. A plane flying into a headwind has a lower ground speed than air speed, so the flight takes longer.

9. Frames of reference

A frame of reference is the viewpoint, with its own axes and clock, from which motion is measured. The laws of motion look simplest in inertial frames, which are not accelerating.

Real-world example: On a smoothly cruising plane, pouring a drink works exactly as it does on the ground, because the plane is close to an inertial frame. During turbulence the frame accelerates, and the drink seems to jump.

10. Uniform circular motion and centripetal acceleration

An object moving in a circle at constant speed is still accelerating, because its direction keeps changing. The acceleration points to the centre of the circle and equals $v^2/r$.

Real-world example: When a car takes a curve, friction between the tyres and the road supplies the force towards the centre. On ice there is too little friction, and the car slides straight on.

11. Terminal velocity

A falling object speeds up until air resistance grows to match its weight. After that the net force is zero and it falls at a steady terminal velocity.

Real-world example: A skydiver in a belly-down position reaches a terminal speed of roughly 200 km/h. Opening the parachute increases drag a lot, so the terminal speed drops to a safe landing speed.

Forces and Newton's laws

Newton's three laws connect forces to changes in motion (OpenStax University Physics Volume 1, chapters 5–6).

12. Force and net force

A force is a push or pull, measured in newtons (N). What changes an object's motion is the net force: the vector sum of all forces acting on it.

Real-world example: In a tug of war, both teams pull hard, but if their forces are equal the net force is zero and the rope does not move.

13. Newton's first law (inertia)

An object stays at rest, or keeps moving in a straight line at constant speed, unless a net force acts on it. Inertia is this resistance to changes in motion.

Real-world example: When a car brakes suddenly, your body keeps moving forward. The seatbelt supplies the force that slows you down with the car.

14. Newton's second law

The net force on an object equals its mass times its acceleration: $F = ma$. A bigger force gives a bigger acceleration; a bigger mass gives a smaller one.

Real-world example: Pushing a full shopping trolley with the same effort as an empty one produces a much smaller acceleration, because the mass is larger.

15. Newton's third law

Forces come in pairs. If A pushes on B, B pushes back on A with a force of equal size in the opposite direction. The two forces act on different objects, so they do not cancel.

Real-world example: A rocket pushes exhaust gas backwards; the gas pushes the rocket forwards. A swimmer pushes water back, and the water pushes the swimmer ahead.

16. Mass vs weight

Mass is the amount of matter in an object, in kilograms, and it is the same everywhere. Weight is the gravitational force on that mass, $W = mg$, in newtons, and it depends on where you are.

Real-world example: An astronaut with a mass of 80 kg has the same mass on the Moon, but weighs about one sixth as much, because the Moon's surface gravity is about 1.62 m/s² compared with Earth's 9.8 m/s².

17. Normal force and apparent weight

The normal force is the push a surface exerts perpendicular to itself to stop an object passing through it. Bathroom scales measure the normal force, not your mass directly.

Real-world example: Stand on a scale in a lift. As the lift starts upwards, the reading rises above your normal weight; as it slows at the top, the reading drops.

18. Static and kinetic friction

Friction resists sliding between surfaces. Static friction acts before anything slides and can be larger than kinetic friction, which acts once sliding starts.

Real-world example: Anti-lock brakes (ABS) pulse the brakes to stop the wheels from locking. A rolling tyre grips with static friction, which is usually stronger than the kinetic friction of a skidding tyre, and it keeps the car steerable.

19. Drag and air resistance

Drag is the resistance a fluid such as air or water exerts on a moving object. At everyday speeds it grows roughly with the square of speed, so doubling your speed roughly quadruples the drag.

Real-world example: Racing cyclists tuck low and ride in a line, because drag is the main force they work against at high speed. Cars use more fuel per kilometre on a motorway than at moderate speeds for the same reason.

20. Tension

Tension is the pulling force transmitted through a string, rope, cable or chain when it is pulled tight at both ends.

Real-world example: The steel cables of a lift carry tension that supports the cabin. A guitar string under higher tension vibrates faster and plays a higher note.

21. Elasticity and Hooke's law

Many materials stretch or compress in proportion to the force applied, up to their elastic limit. For a spring, $F = kx$, where $k$ is the spring constant.

Real-world example: A luggage scale works because its spring stretches a distance proportional to the weight hanging on it. Car suspension springs compress further when the car is loaded.

22. Inclined planes

On a slope, gravity splits into a part pulling the object down the slope and a part pressing it into the surface. A shallower slope needs less force to push something up, over a longer distance.

Real-world example: Wheelchair ramps are kept shallow; the U.S. ADA standards set a maximum running slope of 1:12 for new ramps (U.S. Access Board), so a person can push up without excessive force.

23. Pressure

Pressure is force divided by the area it acts on: $P = F/A$. The same force spread over a larger area produces less pressure.

Real-world example: Snowshoes spread your weight over a large area so you do not sink. A sharp knife concentrates the force of your hand on a tiny edge, so it cuts easily.

Energy, work and momentum

Energy and momentum are the two big conserved quantities of mechanics (OpenStax University Physics Volume 1, chapters 7–9).

24. Work

In physics, work is done when a force moves an object through a distance in the direction of the force: $W = Fd\cos\theta$. It is measured in joules.

Real-world example: Lifting a 10 kg bag of shopping 1 m onto a table does about 98 J of work against gravity. Carrying it across a level room at constant height does almost no work on the bag in the physics sense, even though your muscles get tired.

25. Kinetic energy

Kinetic energy is the energy of motion: $KE = \tfrac{1}{2}mv^2$. Because speed is squared, it rises fast with speed.

Real-world example: A car at 100 km/h has about four times the kinetic energy of the same car at 50 km/h. The brakes have to remove that energy, which is a major reason braking distances grow so quickly with speed.

26. Gravitational potential energy

An object raised to a height has stored energy, $PE = mgh$, that can be released when it falls.

Real-world example: Pumped-storage hydroelectric plants pump water uphill when electricity is cheap or plentiful and let it run back down through turbines when demand is high.

27. Conservation of energy

Energy cannot be created or destroyed, only transformed from one form to another or transferred. The total stays constant in a closed system.

Real-world example: A roller coaster trades potential energy at the top of a hill for kinetic energy at the bottom. Each later hill is lower than the first, because some energy is converted to heat and sound by friction and air resistance.

28. Power

Power is the rate of doing work or transferring energy, measured in watts (1 W = 1 J/s).

Real-world example: Running up a flight of stairs takes the same energy as walking up, but more power, because you do the work in less time. An appliance's watt rating tells you how fast it uses energy.

29. Efficiency

Efficiency is the fraction of input energy that ends up in the useful form you want. The rest usually becomes heat.

Real-world example: An LED bulb gives about the same light as an old incandescent bulb while using a fraction of the electricity, because the incandescent bulb turns most of its input into heat rather than visible light (U.S. Department of Energy).

30. Momentum

Momentum is mass times velocity, $p = mv$. It measures how hard it is to stop a moving object.

Real-world example: A loaded lorry and a bicycle moving at the same speed have very different momenta. The lorry needs a much larger force, or a much longer time, to stop.

31. Impulse

Impulse is force multiplied by the time it acts, and it equals the change in momentum. Stretching out the stopping time reduces the force needed.

Real-world example: Airbags, crumple zones and bending your knees when you land from a jump all lengthen the stopping time, so the same change in momentum happens with a smaller, less damaging force.

32. Conservation of momentum

In a collision or explosion with no outside force, the total momentum before equals the total momentum after.

Real-world example: Two ice skaters standing still push off each other and glide apart in opposite directions. The lighter skater moves faster, so the momenta stay equal and opposite.

33. Elastic and inelastic collisions

In an elastic collision, kinetic energy is conserved as well as momentum. In an inelastic collision, some kinetic energy turns into heat, sound or deformation. Momentum is conserved in both.

Real-world example: Hard snooker balls collide almost elastically. A car crash is strongly inelastic: the metal crumples and absorbs energy, which is exactly what crumple zones are designed to do.

34. Centre of mass

The centre of mass is the average position of an object's mass. An object balances when supported directly under it, and topples when it moves outside the base of support.

Real-world example: In the Fosbury flop, a high jumper arches over the bar so that their centre of mass can pass just below it while their body goes over.

35. Simple machines and mechanical advantage

Levers, pulleys, wheels and axles, ramps, wedges and screws let a small force move a large load. They do not create energy: you trade a smaller force for a longer distance.

Real-world example: A bottle opener is a lever. The low gears on a bicycle let you climb hills with less force on the pedals, at the cost of turning them more times.

Rotation and gravity

Spinning objects and orbits follow rotational versions of Newton's laws (OpenStax University Physics Volume 1, chapters 10, 11 and 13).

36. Torque

Torque is the turning effect of a force. It equals the force times its perpendicular distance from the pivot.

Real-world example: Door handles are placed far from the hinges, and a longer spanner loosens a stuck bolt more easily, because a larger distance gives more torque for the same push.

37. Moment of inertia

The moment of inertia is the rotational version of mass. It depends on how far the mass sits from the axis: mass far from the axis makes an object harder to spin up or slow down.

Real-world example: A tightrope walker carries a long pole. Its large moment of inertia slows any tipping rotation, which gives the walker time to correct their balance.

38. Conservation of angular momentum

With no outside torque, angular momentum stays constant. If the moment of inertia decreases, the spin rate must increase.

Real-world example: A figure skater who pulls in their arms during a spin rotates much faster. A diver tucks to spin quickly and straightens out to slow the rotation before entering the water.

39. Rolling motion

A rolling object has both translational and rotational kinetic energy. Objects with more mass far from the axis put more energy into spinning and roll down slopes more slowly.

Real-world example: Race a solid can of soup against an empty can down a ramp. The solid can usually wins, because less of its energy goes into rotation.

40. Gyroscopes and precession

A spinning object resists changes to the direction of its spin axis. When a torque acts on it, the axis slowly swings around instead of falling over; this is precession.

Real-world example: A spinning top stays upright while the axis traces a slow circle. Spacecraft use spinning reaction wheels and gyroscopes to point telescopes accurately. (The motion sensors in phones are tiny vibrating MEMS devices, not spinning wheels, but they serve the same purpose of sensing rotation.)

41. Centrifugal effects in rotating frames

Inside a rotating frame, objects seem to be flung outwards. This "centrifugal force" is not a real interaction; it is the object's inertia trying to carry it in a straight line while the frame turns.

Real-world example: A salad spinner: the basket forces the lettuce round in a circle, but water drops pass through the holes and keep going in straight lines, away from the leaves.

42. Newton's law of universal gravitation

Every two masses attract each other with a force proportional to the product of their masses and inversely proportional to the square of the distance between them.

Real-world example: The same law that makes an apple fall keeps the Moon in orbit. Satellite operators use it to calculate orbits for weather, TV and navigation satellites.

43. Tides

Tides are caused mainly by the Moon's gravity being slightly stronger on the side of Earth facing it than on the far side. This difference stretches the oceans into two bulges, and Earth rotates through them.

Real-world example: Most coasts see two high tides and two low tides roughly every day. Extra-large "spring" tides happen near new and full moon, when the Sun's tidal effect lines up with the Moon's (NOAA).

44. Orbits and weightlessness

An orbit is continuous free fall. A satellite moves sideways so fast that as it falls towards Earth, the ground curves away beneath it.

Real-world example: Astronauts on the International Space Station float even though gravity at that height is still about 90% as strong as at the surface. They and the station are falling together, so nothing presses them against the floor.

45. Escape velocity

Escape velocity is the minimum launch speed an object needs to coast away from a planet without falling back, ignoring air resistance. For Earth it is about 11.2 km/s.

Real-world example: Missions to Mars must reach escape velocity. Satellites in low Earth orbit only need about 7.8 km/s, which is why reaching orbit is easier than leaving Earth entirely.

Fluids

Liquids and gases flow, push and float things in ways that follow from pressure and density (OpenStax University Physics Volume 1, chapter 14).

46. Density

Density is mass per unit volume. Things less dense than a fluid float in it; denser things sink.

Real-world example: Ice is about 8% less dense than liquid water, so it floats. That is why lakes freeze from the top down, letting fish survive the winter in the water below.

47. Hydrostatic pressure

Pressure in a fluid increases with depth, because of the weight of the fluid above: $P = \rho g h$.

Real-world example: Your ears feel pressure at the bottom of a swimming pool. Dams are built thicker at the base than at the top, because the water pushes hardest at the bottom.

48. Atmospheric pressure

The weight of the air above us presses on everything with about 101 kPa at sea level. We do not notice because the pressure inside our bodies balances it.

Real-world example: When you drink through a straw, you lower the pressure inside it, and atmospheric pressure pushes the drink up. A suction cup stays on a window because air pressure outside is greater than the pressure under the cup.

49. Pascal's principle

Pressure applied to an enclosed fluid is transmitted equally throughout it. A small force on a small piston can create a large force on a large piston.

Real-world example: A hydraulic car jack lets one person lift a car. Car brakes use the same idea: your foot's push on the pedal is multiplied at the brake pads.

50. Buoyancy and Archimedes' principle

A fluid pushes up on a submerged object with a force equal to the weight of fluid the object displaces.

Real-world example: A steel ship floats because its hull displaces a huge volume of water whose weight equals the ship's. A hot-air balloon rises because the hot air inside is less dense than the cooler air it displaces.

51. Bernoulli's principle

In a steady flow, where a fluid speeds up, its pressure drops. It is a statement of energy conservation for moving fluids.

Real-world example: Perfume atomisers and carburettors draw liquid into a fast airstream using the low pressure. Bernoulli's principle is part of how wings make lift, but the popular "equal transit time" explanation is wrong: air over the top does not have to meet the air from below at the trailing edge (NASA Glenn Research Center). Lift comes from the wing turning the airflow downwards.

52. Viscosity

Viscosity is a fluid's internal friction, its resistance to flowing. It usually falls as a liquid warms up.

Real-world example: Honey pours slowly and water pours quickly. Motor oil grades such as 5W-30 describe how the oil's viscosity behaves when the engine is cold and when it is hot.

53. Surface tension

Molecules at a liquid's surface pull on each other, making the surface act like a stretched, elastic skin.

Real-world example: Water striders walk on ponds without sinking. Soap lowers water's surface tension, which helps it spread into fabric and lift away grease.

Oscillations, waves and sound

Waves carry energy from place to place without carrying matter along with them (OpenStax University Physics Volume 1, chapters 15–17).

54. Simple harmonic motion

When the restoring force is proportional to displacement, an object oscillates back and forth with a fixed period that does not depend on how far it swings (for small swings).

Real-world example: A pendulum clock keeps time because each small swing takes the same time. A swing in a park follows the same physics.

55. Resonance

Every oscillating system has natural frequencies. Push it repeatedly at one of those frequencies and the amplitude grows large.

Real-world example: Pushing a child on a swing at just the right moment each cycle builds up a big swing with small pushes. Engineers design buildings and bridges so their natural frequencies avoid likely sources of vibration. (The 1940 Tacoma Narrows Bridge collapse is often cited as simple resonance, but it is better explained by aeroelastic flutter, where wind and the bridge's motion fed each other.)

56. Damping

Damping is the loss of energy from an oscillation, usually as heat, so the motion dies away.

Real-world example: A car's shock absorbers damp the bouncing of its springs after a bump. Without them, the car would bob up and down for a long time.

57. Transverse and longitudinal waves

In a transverse wave, the medium moves at right angles to the wave's direction. In a longitudinal wave, it moves back and forth along the direction of travel.

Real-world example: Earthquakes send out both. The faster P-waves are longitudinal (compressions), and the slower S-waves are transverse. Seismometers use the gap between their arrival times to estimate the distance to the earthquake.

58. Frequency, wavelength and wave speed

For any wave, speed equals frequency times wavelength: $v = f\lambda$. At a fixed speed, higher frequency means shorter wavelength.

Real-world example: An FM radio station at 100 MHz broadcasts radio waves with a wavelength of about 3 m. That is why FM car aerials are typically tens of centimetres to a metre long, a fraction of the wavelength.

59. The speed of sound

Sound travels through air at about 343 m/s at 20 °C, much faster in water and faster still in steel. Light is nearly a million times faster.

Real-world example: Count the seconds between a lightning flash and the thunder. Every 3 seconds is about 1 km of distance.

60. Loudness and decibels

Sound intensity is measured on the logarithmic decibel scale. An increase of 10 dB means ten times the intensity.

Real-world example: A busy street at about 70 dB carries ten times the sound intensity of a 60 dB conversation. Hearing protection matters because damage depends on both level and exposure time (CDC).

61. Pitch

Pitch is how we perceive a sound's frequency. Higher frequency sounds higher.

Real-world example: Pressing a guitar string against a fret shortens the vibrating length, which raises the frequency and the note. Shorter, thinner and tighter strings all sound higher.

62. The Doppler effect

When a wave source moves towards you, the waves bunch up and you detect a higher frequency. When it moves away, they spread out and the frequency drops.

Real-world example: An ambulance siren sounds higher as it approaches and lower as it passes. Police radar guns and weather radar measure speed from the Doppler shift of reflected waves.

63. Interference

When two waves overlap, they add. Crests meeting crests make a bigger wave (constructive interference); crests meeting troughs cancel (destructive interference).

Real-world example: Noise-cancelling headphones record outside noise and play an inverted copy, so the two cancel. The swirling colours on a soap bubble come from light waves reflecting off its front and back surfaces and interfering.

64. Standing waves

When waves reflect back and forth in a confined space, they can form a pattern that seems to stand still, with fixed nodes that do not move and antinodes that move most.

Real-world example: A guitar string and the air column in a flute vibrate in standing waves, which set the notes they can play. Uneven heating in a microwave oven is partly due to standing-wave hot and cold spots, which is why most ovens have a turntable.

65. Diffraction

Waves spread out when they pass through an opening or around an obstacle, most strongly when the opening is similar in size to the wavelength.

Real-world example: You can hear someone talking around a corner because sound waves, with wavelengths up to a few metres, diffract around the doorway. The rainbow sheen on a CD or DVD comes from light diffracting off its closely spaced tracks.

Heat and thermodynamics

Thermodynamics explains how heat, work and temperature relate, and why some processes only run one way (OpenStax University Physics Volume 2, chapters 1–4).

66. Temperature vs heat

Temperature measures the average kinetic energy of the particles in a substance. Heat is energy transferred because of a temperature difference.

Real-world example: On a cold morning a metal bench feels colder than a wooden one, even though both are at the same temperature. Metal conducts heat away from your hand faster.

67. Thermal expansion

Most materials expand when heated and contract when cooled.

Real-world example: Bridges have expansion joints, the toothed gaps you drive over, so the deck can lengthen in summer without buckling. Running a stuck metal jar lid under hot water loosens it because the lid expands more than the glass.

68. Specific heat capacity

Specific heat capacity is the energy needed to raise the temperature of 1 kg of a substance by 1 °C (or 1 K). Water's is unusually high, about 4,186 J/(kg·K).

Real-world example: Coastal towns have milder climates than inland towns at the same latitude, because the sea warms and cools slowly. Sand on a beach gets much hotter than the water on a sunny afternoon.

69. Latent heat and phase changes

Melting, boiling and condensing absorb or release energy without changing temperature. That energy is called latent heat.

Real-world example: Sweating cools you because evaporating water takes latent heat from your skin. A steam burn is worse than a burn from boiling water at the same temperature, because the steam releases extra latent heat as it condenses on your skin.

70. Conduction

Conduction is heat transfer through direct contact, as faster-vibrating particles pass energy to their neighbours.

Real-world example: Pans are made of metal so heat reaches the food quickly, and their handles are made of plastic or wood, which conduct poorly.

71. Convection

Convection is heat transfer by the movement of a fluid: warm fluid rises, cooler fluid sinks, and a circulating current forms.

Real-world example: On a sunny day, land heats faster than the sea. Warm air over the land rises and cooler air flows in from the sea, creating a sea breeze.

72. Thermal radiation

All objects give off electromagnetic radiation that depends on their temperature. It needs no medium and can cross empty space.

Real-world example: Sunlight warms your face across 150 million km of near-vacuum. Thermal cameras detect the infrared radiation people and buildings give off, which is how energy auditors find heat leaks.

73. The first law of thermodynamics

The change in a system's internal energy equals the heat added to it minus the work it does. It is energy conservation applied to heat.

Real-world example: Pump up a bicycle tyre and the pump barrel gets warm. You do work compressing the air, which raises its internal energy and temperature.

74. The second law of thermodynamics and entropy

Heat flows naturally from hot to cold, never the reverse on its own. Equivalently, the total entropy of an isolated system never decreases.

Real-world example: A cup of coffee cools to room temperature; it never spontaneously heats up by taking energy from the cooler room. A refrigerator can move heat from cold to hot, but only because it uses electrical work to do so.

75. Heat engines and heat pumps

A heat engine turns some of the heat flowing from a hot place to a cold place into work. A heat pump runs the process in reverse, using work to move heat from cold to hot.

Real-world example: A car engine is a heat engine, and much of the fuel energy leaves as waste heat. A home heat pump can deliver up to about three times as much heat as the electricity it uses, because it moves heat from outside rather than generating it (ENERGY STAR).

76. The ideal gas law

For many gases, pressure, volume and temperature are linked by $PV = nRT$. Heat a gas in a fixed volume and its pressure rises.

Real-world example: Tyre pressure drops on cold mornings, which is why a warning light can come on in winter and go off once the tyres warm up.

Electricity and magnetism

Electric and magnetic forces are two sides of one interaction, electromagnetism (OpenStax University Physics Volume 2, chapters 5–16).

77. Electric charge and static electricity

Charge comes in two types, positive and negative. Like charges repel and unlike charges attract. Rubbing two materials together can move electrons from one to the other.

Real-world example: Rub a balloon on your hair and it sticks to the wall. Walking across a carpet in dry weather can charge your body, and you feel a small spark when you touch a metal door handle.

78. Electric fields

An electric field is the region around a charge where other charges feel a force. Field lines point from positive to negative charges.

Real-world example: Laser printers and photocopiers use electric fields: a charged drum attracts toner powder only where the image should be, and then transfers it to the paper.

79. Conductors and insulators

In conductors such as copper, some electrons move freely, so current flows easily. In insulators such as plastic, rubber and glass, electrons are tightly bound.

Real-world example: Electrical cables have a copper core to carry current and a plastic coating to keep it from reaching your hand.

80. Voltage (potential difference)

Voltage is the energy transferred per unit charge between two points, measured in volts. It is what pushes current around a circuit.

Real-world example: An AA battery provides about 1.5 V. Household mains supplies are about 120 V in North America and 230 V in much of Europe and Asia, so appliances must match the local supply.

81. Electric current

Current is the rate at which charge flows past a point, measured in amperes (amps).

Real-world example: A fuse or circuit breaker cuts off the supply when the current exceeds a safe value, preventing wiring from overheating.

82. Resistance and Ohm's law

Resistance opposes current. For many materials at constant temperature, current is proportional to voltage: $V = IR$.

Real-world example: The heating element in a toaster is a wire with relatively high resistance that turns electrical energy into heat. Thin, long wires have more resistance than thick, short ones, which is why heavy appliances need thicker cables.

83. Series and parallel circuits

In a series circuit, components share one path, so one break stops everything. In a parallel circuit, each component has its own path to the supply.

Real-world example: Household sockets are wired in parallel, so each appliance gets the full mains voltage and switching one off does not affect the others. In some older strings of fairy lights wired in series, one failed bulb made the whole string go dark.

84. Electrical power and energy

Electrical power equals voltage times current, $P = VI$. Energy used equals power times time, and utility companies bill it in kilowatt-hours (kWh).

Real-world example: A 2 kW kettle running for 3 minutes uses 0.1 kWh. A 10 W LED bulb would take 10 hours to use the same energy.

85. Capacitance

A capacitor stores energy in the electric field between two conductors separated by an insulator. It can release that energy very quickly.

Real-world example: A camera flash charges a capacitor and then dumps the energy into the flash tube in a fraction of a second. Most phone touchscreens are capacitive: your finger changes the capacitance at the point you touch.

86. Magnetic fields

Magnets and moving charges create magnetic fields. A magnetic field exerts forces on other magnets, on magnetic materials and on moving charges.

Real-world example: A compass needle lines up with Earth's magnetic field, which at the surface is roughly 25 to 65 microteslas. Fridge magnets attract the steel door; they do not stick to aluminium.

87. Electromagnets and motors

A current in a coil of wire creates a magnetic field, which can be switched on and off. In a motor, the force between magnetic fields and currents produces rotation.

Real-world example: Scrapyard cranes lift cars with large electromagnets and drop them by switching the current off. Every electric fan, drill and electric car uses an electric motor.

88. Electromagnetic induction

A changing magnetic field through a coil induces a voltage in it (Faraday's law). This is how most of the world's electricity is generated.

Real-world example: Power station generators spin magnets inside coils. Wireless phone chargers and induction hobs use a rapidly changing magnetic field to induce currents in the phone's coil or the base of the pan.

89. Alternating current and transformers

Mains electricity is alternating current (AC), which reverses direction many times per second. Transformers use induction to step AC voltages up or down.

Real-world example: Power grids transmit electricity at very high voltages to reduce energy lost as heat in the cables, then step it down for homes. A laptop charger converts mains AC to the low-voltage direct current (DC) the laptop uses.

Light and optics

Light is an electromagnetic wave, and its behaviour at surfaces and lenses explains much of what we see (OpenStax University Physics Volume 3, chapters 1–4).

90. The electromagnetic spectrum

Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays are all electromagnetic waves travelling at the speed of light. They differ only in frequency and wavelength.

Real-world example: Wi-Fi uses microwaves, your TV remote uses infrared, and hospital scanners use X-rays. Ultraviolet causes sunburn, which is why sunscreen is rated for UV protection.

91. Reflection

When light bounces off a surface, the angle of reflection equals the angle of incidence. Smooth surfaces give clear images; rough surfaces scatter light in many directions.

Real-world example: Road signs and bicycle reflectors use retroreflectors, tiny corner-shaped or bead surfaces that send light back towards its source, so a driver's headlights make the sign glow.

92. Refraction

Light bends when it passes from one material to another, because its speed changes. Lenses use refraction to focus light.

Real-world example: A straw in a glass of water looks bent at the surface. Glasses and contact lenses refract light so that it focuses correctly on the retina.

93. Total internal reflection

When light inside a dense material hits the boundary at a shallow enough angle (beyond the critical angle), it reflects completely back inside instead of escaping.

Real-world example: Optical fibres carry internet data as pulses of light that bounce along the glass by total internal reflection, even around bends. The sparkle of a well-cut diamond also comes partly from it.

94. Dispersion

The refractive index of a material varies slightly with wavelength, so different colours bend by different amounts and white light spreads into a spectrum.

Real-world example: A rainbow forms when sunlight refracts into raindrops, reflects inside them and refracts out again, with each colour leaving at a slightly different angle.

95. Polarisation

Light waves can vibrate in many directions. A polarising filter only lets through waves vibrating in one direction.

Real-world example: Glare off water and roads is partly polarised, so polarised sunglasses cut it more than ordinary tinted lenses. LCD screens use polarising filters to control which pixels let light through.

Modern physics

Twentieth-century physics showed that light comes in packets, matter has wave properties, and space and time are linked (OpenStax University Physics Volume 3, chapters 5–10).

96. Photons and the photoelectric effect

Light delivers energy in packets called photons, each with energy $E = hf$. Light can knock electrons out of a material only if each photon has enough energy, however bright the light is.

Real-world example: Solar panels and the image sensors in digital cameras work because photons free electrons in a semiconductor, producing a current or a signal.

97. Wave-particle duality

Electrons and other particles also behave like waves, with a wavelength that gets shorter as their momentum increases.

Real-world example: Electron microscopes use the very short wavelength of fast electrons to see details far smaller than an ordinary light microscope can resolve.

98. Atomic energy levels and spectra

Electrons in atoms can only have certain energies. When an electron drops to a lower level it emits a photon of a specific colour, so each element has its own spectral "fingerprint".

Real-world example: Neon signs glow red-orange and old low-pressure sodium street lamps glow a deep orange-yellow because of the specific energy levels of those atoms. Astronomers identify elements in distant stars from the same fingerprints.

99. Special and general relativity

Special relativity says the speed of light is the same for all observers, which makes moving clocks run slow. General relativity says gravity is the curving of spacetime, which makes clocks run slower deeper in a gravitational field.

Real-world example: GPS satellite clocks would drift by about 38 microseconds per day relative to ground clocks without relativity: they lose about 7 µs a day from their speed and gain about 45 µs a day from weaker gravity at their height. The system corrects for both (Ashby, Physics Today).

100. Radioactivity, half-life and nuclear energy

Unstable atomic nuclei decay at random, but a large group of them decays at a predictable rate. The half-life is the time for half of them to decay. Splitting heavy nuclei (fission) or joining light ones (fusion) releases huge amounts of energy, explained by $E = mc^2$.

Real-world example: Radiocarbon dating uses carbon-14, with a half-life of about 5,730 years, to date organic material up to about 50,000 years old. Nuclear power stations use fission, and the Sun shines by fusing hydrogen into helium.

How to learn these concepts so they stick

Reading a definition is not the same as understanding it. A few habits help:

• Predict, then check. Before reading an example, guess what will happen. Being wrong and seeing why is how misconceptions get fixed.
• Test ideas with your hands. Many of these concepts can be seen with household items; try our 101 physics experiments you can do at home.
• Connect concepts to equations. Once you understand the idea, the formula is easier to remember. Our 100 physics formulas every student should know uses the same topic order as this article.
• Look for physics around you. Our guide to 100 applications of physics in everyday life works the other way round, starting from devices and activities.
• Investigate a question properly. When you are ready to design your own investigation, pick one of the 101 physics projects for students.

Common misconceptions

These ideas are widespread, and each is contradicted by one of the concepts above:

• "Heavier objects fall faster." Without air resistance, all objects fall with the same acceleration (concept 6).
• "A moving object needs a force to keep moving." It needs a force only to change its motion (concept 13). Things slow down on Earth because friction and drag are acting.
• "There is no gravity in space." Gravity on the ISS is about 90% of surface gravity; astronauts float because they are in free fall (concept 44).
• "Heat and temperature are the same thing." A bathtub of warm water holds far more heat than a cup of boiling water (concept 66).
• "Cold flows into the house." Heat flows out; "cold" is just less energy (concept 74).
• "Batteries store charge, and circuits use it up." Charge flows around the circuit; what the battery supplies and the circuit uses is energy (concepts 80 and 84).
• "Seasons are caused by Earth being closer to the Sun." They are caused by the tilt of Earth's axis, which changes how directly sunlight strikes each hemisphere. Earth is actually closest to the Sun in early January, during northern winter (NASA Space Place).

For more articles and blogs on the subject, browse the physics category.

References

• University Physics Volume 1, William Moebs, Samuel J. Ling and Jeff Sanny, OpenStax (2016). https://openstax.org/details/books/university-physics-volume-1
• University Physics Volume 2, Samuel J. Ling, Jeff Sanny and William Moebs, OpenStax (2016). https://openstax.org/details/books/university-physics-volume-2
• University Physics Volume 3, Samuel J. Ling, Jeff Sanny and William Moebs, OpenStax (2016). https://openstax.org/details/books/university-physics-volume-3
• College Physics 2e, Paul Peter Urone and Roger Hinrichs, OpenStax (2022). https://openstax.org/details/books/college-physics-2e
• "The International System of Units (SI)", Bureau International des Poids et Mesures (BIPM). https://www.bipm.org/en/measurement-units
• "Fundamental Physical Constants: 2022 CODATA values", NIST Physical Measurement Laboratory. https://physics.nist.gov/cuu/Constants/
• "The Apollo 15 Hammer-Feather Drop", NASA Space Science Data Coordinated Archive. https://nssdc.gsfc.nasa.gov/planetary/lunar/apollo_15_feather_drop.html
• "Incorrect Lift Theory: Equal Transit", NASA Glenn Research Center, Beginner's Guide to Aeronautics. https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/foilw1/
• "What is a spring tide?", NOAA National Ocean Service. https://oceanservice.noaa.gov/facts/springtide.html
• "ADA Standards: 405 Ramps", U.S. Access Board. https://www.access-board.gov/ada/#ada-405
• "Energy-Efficient Lighting" fact sheet, U.S. Department of Energy, Energy Saver (2021). https://www.energy.gov/sites/default/files/2021-08/ES-EE%20Lighting_080921.pdf
• "HVAC Marketing Toolkit", ENERGY STAR (2023). https://www.energystar.gov/sites/default/files/tools/HVAC_Marketing_Toolkit_2023.pdf
• "What Noises Cause Hearing Loss?", U.S. Centers for Disease Control and Prevention. https://www.cdc.gov/hearing-loss/causes/index.html
• Neil Ashby, "Relativity and the Global Positioning System", Physics Today 55(5), 41 (2002). https://physicstoday.aip.org/features/relativity-and-the-global-positioning-system
• Neil Ashby, "Relativity in the Global Positioning System", Living Reviews in Relativity 6, 1 (2003). https://doi.org/10.12942/lrr-2003-1
• "What Causes the Seasons?", NASA Space Place. https://spaceplace.nasa.gov/seasons/en/