What is alternating current?
Alternating current (AC) keeps changing direction: in European homes it swings back and forth 50 times every second. Batteries give direct current (DC), which flows one way only.
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Physics
Matter, energy, forces and motion — the rules everything follows.
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What it is and why it matters — intuition first, little or no math.
What is an atom?
An atom is a tiny building block that all stuff is made of. Air, water, your toys and even you are built from atoms, far too small to see.
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Why does water boil at different temperatures?
The boiling point is the temperature at which a liquid bubbles into a gas, and it depends on how hard the air presses on the liquid. High on a mountain, water boils below 100 °C; in a pressure cooker, it boils above 100 °C.
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What are conductors and insulators?
Conductors are materials that let electricity flow through them easily, like copper and other metals. Insulators, like plastic, rubber and glass, hardly let it through at all — that is why wires are wrapped in plastic.
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Why does energy never disappear?
Energy can never be made from nothing, and it can never be destroyed. It only moves from place to place or changes form — and when it seems to vanish, it has usually turned into heat.
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What is electric charge?
Electric charge is something tiny particles carry that makes them pull or push on each other. It comes in two kinds, plus and minus: opposites pull together, and two of the same kind push apart.
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What is an electric circuit?
An electric circuit is a closed loop that electricity can flow around: from a battery, through wires and a lamp, and back to the battery. Break the loop anywhere and the flow stops.
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How does electric current work?
Electric current is a flow of tiny particles called electrons through a wire. A battery or a socket pushes them around a closed loop, and that flow makes lamps shine and motors spin.
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What is electric power?
Electric power tells how fast a device uses electrical energy. It is measured in watts (W): a kettle uses thousands of watts, a small LED lamp only a few.
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What is an electron?
An electron is a tiny particle with a minus charge that moves around the middle of every atom. When lots of electrons move along a wire together, electricity flows.
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What is energy?
Energy is what makes things happen. It lets things move, warm up, shine, make sounds and grow — and it never disappears, it only changes from one form into another.
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What are evaporation and condensation?
Evaporation is a liquid slowly turning into a gas, and it soaks up heat — that is why wet skin feels cold. Condensation is the opposite, a gas turning back into a liquid, and it gives that heat back.
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What is heat?
Heat is energy that flows from something warmer to something colder. By itself it always flows from hot to cold, never the other way — that is why a hot drink cools down in a cool room.
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How do air conditioners and heat pumps work?
Air conditioners and heat pumps don’t make heat or cold. They move heat from one place to another, using a special liquid that soaks up heat when it evaporates and gives it back when it condenses.
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What is pressure?
Pressure is how hard something pushes on each bit of a surface. The air around us presses on everything, and squeezing a gas into a smaller space raises its pressure — and warms it up.
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What is a refrigerant?
A refrigerant is the special fluid inside fridges, air conditioners and heat pumps. It boils at very low temperatures, so it can soak up heat even from cold air and carry it somewhere else.
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What is electrical resistance?
Resistance is how much something holds back the flow of electricity. Long, thin wires hold it back more than short, thick ones — and the holding back turns electrical energy into heat.
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What is temperature?
Temperature tells how hot or cold something is. Everything is made of tiny particles that are always jiggling — the faster they jiggle, the higher the temperature.
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What is voltage?
Voltage is the push that makes electricity flow. A battery or a socket pushes tiny electrons through the wires — the bigger the voltage, the harder the push.
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Under the hood
How it works in detail — the mechanism step by step, with precise words.
How does electric current behave?
Electric current follows some simple rules. It needs a closed loop, it splits between paths, it warms up wires, it makes magnets, and in our homes it swings back and forth 50 times a second.
Under the hood
Numbers & formulas
How to calculate it, with formulas and worked examples.
How do we calculate electric current?
We measure current by counting how much electric charge flows past each second, in amperes. With Ohm’s law you can work it out from the voltage and the resistance, current = voltage ÷ resistance.
Numbers & formulasUnder the hood
How efficient is a heat pump?
The coefficient of performance (COP) tells how much heat a heat pump delivers for each unit of electricity it uses. A COP of 3.5 means 3.5 kilowatt-hours of heat from just 1 kilowatt-hour of electricity.
Numbers & formulasUnder the hood
What is Ohm’s law?
Ohm’s law is the rule that connects voltage, current and resistance: current equals voltage divided by resistance. Push harder and more current flows; make the path harder and less flows.
Numbers & formulasUnder the hood
Fun facts
Surprising facts, myths and curious questions.
Why don’t birds on power lines get shocked?
A bird sits with both feet on the same wire, so there is almost no difference in voltage between its feet and no reason for electricity to flow through it. A bird that touches two wires at once is in great danger.
Fun facts
History
Who discovered or invented it, and how it happened.
Who discovered electricity?
Nobody discovered electricity all at once. Over about 2,600 years, curious people — from the ancient Greeks rubbing amber to Volta, Faraday, Edison and Tesla — each added a piece of the puzzle.
History