Lesson C: Components | Topic 2 Electricity

What the graph is really telling you

An I–V graph is a picture of $R = V/I$. A straight line through the origin means $R$ is constant (ohmic). If the line gets shallower, $R$ is increasing. A diode’s reverse half is almost on the voltage axis because $R$ is huge and $I \approx 0$.

2.9 · Required practical

Investigate I against V for a wire, a lamp and a diode

Apparatus

Power supply or battery pack, component under test (resistor/wire, filament lamp, diode), ammeter, voltmeter, variable resistor (or variable supply), connecting leads, switch. For a diode, include a protective resistor.

Circuit rule I will not let you get wrong

Ammeter in series with the component. Voltmeter in parallel across the component.

Variables

Independent: voltage (p.d.) across the component.
Dependent: current through the component.
Control: same component, same temperature as far as possible (the lamp will heat — that is the point), same meters.

Method

Build the circuit. Check the ammeter is series and the voltmeter is parallel.
Start at $0\,\text{V}$. Record $V$ and $I$.
Increase the voltage in even steps. Record a pair of readings each time.
Reverse the supply (or the component) and repeat for negative voltages — essential for the diode.
Plot $I$ (vertical) against $V$ (horizontal), or follow the axes the question names. Calculate $R=V/I$ at chosen points.

Safety and errors

Do not leave a lamp on a high voltage — the filament can blow.
A diode without a series resistor can be destroyed by a large forward current.
Zero-error on meters; loose leads; heating of a resistor if the current is large.

Wire / fixed resistor

Straight line through the origin. Ohmic. $R$ constant if temperature is constant.

Filament lamp

Curve that gets shallower. As current rises the filament heats, ions vibrate more, $R$ increases. Non-ohmic.

Diode

Almost no current in reverse. In forward bias, current rises sharply after a threshold (about $0.6\,\text{V}$ for silicon). One-way valve.

Watch

$V=IR$ and I–V graphs for a resistor, lamp and diode

Cognito · Open on YouTube

Watch the method

Setting up the I–V circuit for a diode

MyEdSpace Physics · Open on YouTube

2.11–2.12

LDR, thermistor, lamps and LEDs

LDR: resistance decreases as light intensity increases. In the dark, $R$ is large, so current is small.
Thermistor: resistance decreases as temperature increases (the usual NTC type in this course).
Lamp or LED in a circuit lights when there is a current — they are current indicators (2.12). An LED only lights in the forward direction.

Exam sentence for an LDR in series with a lamp

“As it gets darker, the LDR’s resistance increases, so the current in the circuit decreases and the lamp becomes dimmer.” Do not mention potential dividers — that is Physics-only.