2.1.4a Resistors: ohmic conductors

Resistance can stay constant or change as the current changes

1. The resistance of a component is found from $R=\frac{V}{I}$, where $V$ is the potential difference in volts and $I$ is the current in amperes.
2. For some components the resistance stays constant as the current changes; for others it changes.
3. You can test which is true by checking whether the ratio $\frac{V}{I}$ stays the same at different currents.

Ohmic conductors: current is proportional to potential difference

1. For an ohmic conductor at constant temperature, $I\propto V$.
2. So if the potential difference doubles, the current doubles, and because $\frac{V}{I}$ stays the same, the resistance stays constant.
3. The words $at\; constant\; temperature$ matter: if the temperature changes, the resistance can change and the simple proportional relationship no longer holds.
4. On a graph of current against potential difference, an ohmic conductor gives a straight line through the origin.

Many components do not have constant resistance

1. The resistance of lamps, diodes, thermistors and light-dependent resistors is not constant: it changes as the current through the component changes.
2. So the current is not directly proportional to the potential difference, and $R=\frac{V}{I}$ gives a different value at different currents.
3. You do not call these components ohmic conductors, because their resistance does not stay constant.

2.1.4b Resistors: filament lamp and diode

A filament lamp's resistance rises as it heats up

1. The resistance of a filament lamp is not constant: it increases as the filament gets hotter.
2. When current passes through the filament, energy is transferred to it electrically, so the filament becomes very hot and emits light.
3. As the filament heats up, its metal ions vibrate more, which makes it harder for the electrons to pass through, so the resistance increases.
4. So as the potential difference is increased, the current rises at first, but the increasing resistance means the current does not increase in direct proportion to the potential difference.

A diode lets current flow one way only

1. In its forward direction a diode has a $low\; resistance$, so current flows easily.
2. In the reverse direction it has a $very\; high\; resistance$, so almost no current flows.
3. A diode therefore acts as a one-way device: to decide whether current flows, check which way the diode is connected, and if it is reversed the current is zero or very small.

2.1.4c Resistors: thermistors, LDRs and I–V characteristics (required practical)

A thermistor's resistance falls as it gets hotter

1. As a thermistor gets hotter its resistance falls, so for a fixed potential difference the current through it increases.
2. This makes thermistors useful in temperature-sensing circuits such as thermostats.
3. In a thermostat, a rise in temperature lowers the thermistor’s resistance, and a control circuit uses this change to switch a heater off or switch cooling on.

An LDR's resistance falls as the light gets brighter

1. In bright light an LDR has a low resistance; in dim light or darkness it has a high resistance.
2. This makes LDRs useful in light-sensing circuits such as automatic street lights.
3. When it gets dark, the light on the LDR falls and its resistance rises, and the control circuit uses this change to switch the lamp on.

Measuring the resistance of a component

1. To measure a component’s resistance, measure the current through it with an ammeter in series and the potential difference across it with a voltmeter in parallel, then use $R=\frac{V}{I}$ in ohms, $\Omega$.
2. A suitable circuit has a cell, battery or d.c. supply, a switch, an ammeter in series with the component, the component itself, a voltmeter in parallel across it, and usually a variable resistor in series so the current and potential difference can be changed.
3. Use the correct symbols: a resistor is a rectangle, a thermistor is a resistor with a diagonal line ending in a short bar, and an LDR is a resistor in a circle with arrows pointing towards it.

I–V characteristics identify a component by the shape of its graph

1. An I–V characteristic is a graph of the current through a component against the potential difference across it, and its shape tells you how the component behaves.
2. You obtain the characteristic by changing the potential difference across the component and recording pairs of current and potential difference readings.