Current, potential difference and resistance

Key content

An electric current is a flow of electrical charge. A potential difference (p.d.) pushes the charge around a circuit, and resistance opposes the flow.

Standard circuit diagram symbols

Circuit diagrams use standard symbols so that anyone can read them. You need to recognise and draw the symbols for: switch (open and closed), cell, battery, diode, resistor, variable resistor, LED, lamp, fuse, voltmeter, ammeter, thermistor and LDR.

• An ammeter measures current. It is connected in series.
• A voltmeter measures potential difference. It is connected in parallel with (across) the component.

Electrical charge and current

For charge to flow around a circuit, the circuit must be closed and there must be a source of potential difference, such as a cell or battery.

Current is the rate of flow of charge:

Charge $Q$ is measured in coulombs (C), current $I$ in amperes (A) and time $t$ in seconds (s).

In a single closed loop, the current is the same at every point.

Current, resistance and potential difference

The current through a component depends on the potential difference across it and its resistance:

Resistance $R$ is measured in ohms (Ω). For a fixed p.d., a higher resistance means a smaller current.

You investigate how the resistance of a wire depends on its length, and how combinations of resistors behave, in Required practical 3.

Resistors

For some resistors the current is directly proportional to the p.d. at constant temperature. These are called ohmic conductors. Their resistance stays constant.

For other components the resistance changes as the current changes:

• Filament lamp: as the current increases, the filament gets hotter and its resistance increases. Its I–V graph is an S-shaped curve through the origin that gets flatter at higher p.d., in both directions.
• Diode: current flows in one direction only. It has a very high resistance in the reverse direction, so almost no current flows that way. In the forward direction the current stays tiny until about 0.7 V, then rises steeply.
• Thermistor: its resistance decreases as the temperature increases. It is used in thermostats and fire alarms.
• LDR (light-dependent resistor): its resistance decreases as light intensity increases. It is used in automatic night lights.

Only the ohmic resistor gives a straight line through the origin: it is linear. The filament lamp and the diode are non-linear, because their resistance changes.

To measure the resistance of a component, connect it in series with an ammeter and a variable resistor (or a variable power supply), with a voltmeter in parallel across it. Change the p.d. in steps, record the current and p.d. each time, and calculate $R = \dfrac{V}{I}$ for each pair of readings.

You can investigate how a thermistor's resistance changes with temperature by heating it in a water bath, and how an LDR's resistance changes with light intensity by moving a lamp closer to it.

You plot I–V characteristics for a resistor, a filament lamp and a diode in Required practical 4.

Equations

Worked examples

Common misconceptions