Ohm's Law: V = I × R

0:00How are voltage, current and resistance linked?

0:04Voltage, in volts, is the push that drives charge around a circuit. This battery gives 12 volts.

0:12Current is the rate at which charge flows, measured in amperes, or amps.

0:18Resistance, in ohms, is how strongly a part opposes the current. This resistor is 4 ohms.

0:27Ohm's law: voltage equals current times resistance.

0:32To rearrange it, use a triangle: V on top, I and R below.

0:38Cover the one you want: covering I leaves V divided by R.

0:43Here, 12 volts divided by 4 ohms gives 3 amps.

0:50A lamp on a 120 volt supply draws 0.5 amps. What is its resistance?

0:57Cover R: 120 volts divided by 0.5 amps is 240 ohms.

1:05Now graph voltage against current for this resistor: 4 volts drives 1 amp, 8 volts 2 amps, and 12 volts 3 amps.

1:16At a steady temperature, a resistor gives a straight line through the origin.

1:21Its slope, rise over run, is 12 volts divided by 3 amps: 4 ohms, the resistance.

1:29A common mistake is to think more resistance gives more current.

1:35Swap in 6 ohms: the line is steeper, and 12 volts drives only 2 amps, not 3.

1:42Voltage equals current times resistance, so at the same voltage, more resistance means less current.