Current, Voltage, Resistance and Ohm's Law

Electric current

Current is the flow of electric charge. In a wire, the moving charges are electrons.

• Symbol: $I$
• Unit: ampere (A), often called "amps". Small currents are in milliamps (mA): 1000 mA = 1 A.
• Measured with an ammeter, connected in series (in line with the component).

A current only flows if there is a complete loop (a closed circuit) and something to push the charge, like a battery.

Voltage (potential difference)

Voltage is the push that drives current around a circuit. More precisely, it is the energy given to or used by each unit of charge.

• Symbol: $V$
• Unit: volt (V)
• Measured with a voltmeter, connected in parallel (across the component, on a side branch).

A battery gives energy to the charges. Components like bulbs use that energy. A 9 V battery gives 9 joules to every coulomb of charge that passes through it.

Resistance

Resistance is how much a component opposes the flow of current.

• Symbol: $R$
• Unit: ohm (Ω)

Resistance is like friction for electricity. When current pushes through a resistance, some electrical energy becomes heat. This is how toasters, kettles and heaters work.

The resistance of a wire is bigger when the wire is:

• longer
• thinner
• hotter (for most metals)
• made of a material that conducts poorly (like nichrome instead of copper)

Ohm's law

For many components, the current is directly proportional to the voltage, as long as the temperature stays the same.

$$V=I\times R$$

$$I=\frac{V}{R}R=\frac{V}{I}$$

I-V graphs

An I-V graph plots current against voltage.

• Ohmic resistor (constant temperature): a straight line through the origin. The resistance is constant.
• Filament bulb: a curve that flattens. As the bulb gets hotter, its resistance increases, so the current grows less and less.
• Diode: current flows only one way, and only once the voltage passes about 0.7 V.

Quick summary

• Current ($I$, amps) is the flow of charge. Measure with an ammeter in series.
• Voltage ($V$, volts) is the push, or energy per charge. Measure with a voltmeter in parallel.
• Resistance ($R$, ohms) opposes current. Longer, thinner, hotter wires have more.
• Ohm's law: $V=IR$.
• Ohmic resistor: straight I-V line. Filament bulb: curve, because resistance rises as it heats.

Quiz