In a wire, electrons can move freely from atom to atom. Connect the wire to a battery and they flow. Two numbers describe this flow: how much charge passes each second, and how much energy each bit of charge carries.
Key idea
Current $I$ is the rate of flow of charge: $I = \dfrac{Q}{t}$, in amperes (A), where 1 A is 1 coulomb per second. Voltage $V$ is the energy given to each coulomb of charge, in volts (V), where 1 V is 1 joule per coulomb.
Why it works
Think of a loop of water pipe with a pump. The pump (the battery) gives the water energy; the flow rate (the current) is how much water passes a point each second. In a light bulb the charge gives up its energy as light and heat, but the charge itself keeps going around the loop: what goes into the bulb comes out of it. Current is not used up; energy is transferred.
Worked example: Find the current
30 coulombs of charge flow through a lamp in 10 s. What is the current?
- $I = \dfrac{Q}{t} = \dfrac{30\,\mathrm{C}}{10\,\mathrm{s}} = 3\,\mathrm{A}$.
Answer: 3 A
Worked example: Energy from voltage
A 9 V battery pushes 4 C of charge through a circuit. How much energy does it give the charge?
- Each coulomb gets 9 J.
- $4 \times 9 = 36$ J.
Answer: 36 J
Common mistake
Thinking current gets used up as it passes through a bulb, so less comes out than goes in. An ammeter placed before the bulb and one placed after read the same. What the bulb uses is energy, carried by the charge; the charge itself returns to the battery.
The electrons in a wire drift along slowly, often less than a millimeter per second. A lamp still lights almost at once, because the wire is already full of electrons and they all start moving together, like water in a full hose.
Wire up batteries, bulbs and meters in the Science Lab: Circuit Builder.