Ohm's law
What decides how much electricity flows through a wire?
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Explain
A big battery pushes hard.
Thin wires slow things down.
Three things work together in a circuit: voltage, current and resistance.
Voltage pushes the charge along. Resistance slows it down. Current is how much flows.
More push gives more current, and more resistance gives less current.
Ohm's law: V = I × R, so I = V / R.
Resistance is measured in ohms (Ω). A long, thin wire has more resistance than a short, thick one.
Worked example: a 12 V battery and a 4 Ω resistor give I = 12 / 4 = 3 A.
- Voltage pushes
- Resistance slows
- Current flows
Want the full story?
Ohm's law only holds for some materials at steady temperature. A bulb's filament heats up, so its resistance rises. Superconductors have no resistance at all, but only when they are very cold.
The Formula
voltage = current × resistance
- the voltage, measured in volts (V)
- the current, measured in amps (A)
- the resistance, measured in ohms (Ω)
Try the numbers
Current: 1 A
Try it at home
You need a straw, a drink.
You need: a straw, a drink.
Sip through a thin straw. Then a fat straw.
- Sip water through a wide straw.
- Sip through a thin straw.
- Compare how hard you need to suck.
What's happening: this is only a model. The thin straw has more resistance, so less flows for the same push.
Physics is life
Your body
Your skin has resistance.
Daily life
Toasters use resistance to get hot.
Nature
Electric fish make voltage.
Space
Magnets on big machines use special wires.
Quick check
Question 1 of 3
Words to know
Resistance: How much something slows down current. It is measured in ohms.
- Resistance
- How much something slows down current. It is measured in ohms.
- Voltage
- The push that makes current flow. It is measured in volts.
- Current
- The flow of electric charge. It is measured in amps.