Resistors — AQA GCSE Combined Science Trilogy 8464 - Physics Revision Notes & Topical Questions

Section 1 of 8Fixed and variable resistors

Fixed resistor

• Fixed resistor Its resistance stays constant, so current is directly proportional to potential difference.
  • The I–V graph is a straight line through the origin, provided temperature is constant.
    • (ohmic conductor)

Variable resistor

• Variable resistor Its resistance can be changed by the user, e.g. by a sliding contact.
  • It is used to alter the current in a circuit, such as in a dimmer or volume control.

Section 2 of 8Components with changing resistance

Component	Behaviour	I–V graph / use
Filament lamp	Resistance increases as the filament gets hotter	Curve flattening at high p.d.; used for lighting
Diode / LED	Very high resistance in reverse; low resistance in the forward direction	Current flows in one direction only; protects circuits and indicates
Thermistor (NTC)	Resistance falls as temperature rises	Used in thermostats and fire alarms
LDR	Resistance falls as light intensity rises	Used in automatic lights and light meters

Section 3 of 8Why the filament lamp curves

1Current increases

• Current increases More charge flows through the thin filament.

2More collisions

• More collisions Electrons collide with the metal ions, transferring energy.

3Filament heats

• Filament heats Its temperature rises and the ions vibrate harder.

4Resistance rises

• Resistance rises The graph becomes less steep, so current rises more slowly than potential difference.

Section 4 of 8Investigating an I–V characteristic

1

Build a series circuit containing the component, an ammeter and a variable resistor (or variable power supply), and connect a voltmeter in parallel across the component.

2

Set the variable resistor so that the potential difference is small, then record the voltmeter and ammeter readings.

3

Change the setting in equal steps, recording the current and potential difference each time; the independent variable is the potential difference and the dependent variable is the current.

4

Reverse the connections of the cell or supply to take readings for current in the opposite direction.

5

Plot current (y-axis) against potential difference (x-axis) and draw a smooth line or curve. Keep the component cool between readings by switching off, as heating changes the resistance.

Section 5 of 8Reading the graphs

Graph shape	Component	What it tells you
Straight line through the origin	Fixed resistor at constant temperature	Resistance constant; gradient = 1 ÷ R
Curve that flattens as p.d. increases	Filament lamp	Resistance rises as the filament heats up
Almost no current in reverse, then a steep rise forward	Diode	Current flows in one direction only, after a small threshold p.d.

Section 6 of 8Choosing the right component

Matching component to job

Constant current wanted

• Constant current wanted Use a fixed resistor of a suitable value.

Adjustable current

• Adjustable current Use a variable resistor.

Temperature sensing

• Temperature sensing Use a thermistor: as it warms, resistance falls and more current flows.

Light sensing

• Light sensing Use an LDR: brighter light gives lower resistance.

One-way current

• One-way current Use a diode, which blocks current in the reverse direction.

Section 7 of 8Key terms

Term	Meaning
Ohmic conductor	A conductor whose current is directly proportional to potential difference at constant temperature.
Variable resistor	A resistor whose resistance can be adjusted.
Thermistor	A component whose resistance changes with temperature; in the NTC type it falls as temperature rises.
LDR	Light-dependent resistor: resistance decreases as light intensity increases.
Diode	A component that allows current to flow in only one direction.
I–V characteristic	A graph of current against potential difference for a component.

Section 8 of 8Exam technique

1

Saying a filament lamp's resistance falls when it heats. It rises.

2

Claiming a diode has zero resistance in the forward direction; it is low, and only above a small threshold voltage.

3

Mixing up the LDR and thermistor responses.

4

Forgetting that a fixed resistor only follows Ohm's law if its temperature stays constant.

5

Connecting the voltmeter in series instead of in parallel with the component.