AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER
Resistors and I–V characteristics
Electricity · Lesson 3 of 10
Warm-up
Answer each one, then check.
1. What is an ohmic conductor?
A conductor with constant resistance at constant temperature
2. What does I–V stand for?
Current against potential difference
3. What is a straight line graph through the origin?
Directly proportional
4. What happens to a metal when its temperature rises?
Its resistance rises
5. What does LDR stand for?
Light-dependent resistor
Learning Objectives
1. Describe the I–V graphs of a resistor, filament lamp and diode.
2. Explain the changes in resistance of a filament lamp, diode, thermistor and LDR.
3. Describe how to investigate I–V characteristics (Required Practical 4).
4. Use graphs to say whether a component is linear or non-linear.
I–V Characteristics
The current through an ohmic conductor at constant temperature is directly proportional to the potential difference across it, so its resistance is constant.
The resistance of a filament lamp, diode, thermistor and LDR is not constant.
I–V Graphs
|
Straight line: constant resistance. Curve: changing resistance. |
I-V graphs for a fixed resistor, a filament lamp and a diode.
Thermistor and LDR
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Both fall as the quantity increases. |
Graphs showing the resistance of a thermistor falling as temperature rises and the resistance of an LDR falling as light intensity rises.
Required Practical 4: I–V Circuit
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Vary the pd and measure the current each time. |
A circuit with a variable resistor, ammeter in series with the test component and a voltmeter across it.
Component Behaviour
Learn each description.
|
Component |
Resistance |
Graph or behaviour |
|---|---|---|
|
Fixed resistor |
Constant at constant temperature |
Straight line through the origin |
|
Filament lamp |
Increases as the filament gets hotter |
Curve that flattens; gradient decreases |
|
Diode |
Very high in the reverse direction |
Current flows in one direction only |
|
Thermistor |
Decreases as temperature increases |
Used in thermostats and temperature sensors |
|
LDR |
Decreases as light intensity increases |
Used in automatic lights |
Required Practical 4: Method
Find the I–V characteristic of a component.
|
1 Set up Connect the component in series with the ammeter and variable resistor, and the voltmeter across the component. |
2 Vary Change the variable resistor to get different pds. |
3 Record For each setting record V and I. |
4 Reverse Swap the connections and repeat to get negative values (not for a diode with a protective resistor). |
5 Plot Draw current against pd and draw a smooth line. |
6 Repeat For the filament lamp and the diode. |
Resistance of a Filament Lamp
|
The current through a filament lamp is 0.34 A at 2.0 V and 0.62 A at 6.0 V. Calculate the resistance at each pd and explain the difference. |
1. At 2.0 V
R = 2.0 ÷ 0.34 = 5.9 Ω
2. At 6.0 V
R = 6.0 ÷ 0.62 = 9.7 Ω
3. Explain
At the higher pd the current is greater, so the filament is hotter and its resistance is higher
Answer: 5.9 Ω and 9.7 Ω; the resistance increases as the filament gets hotter.
Uses in Circuits
Applications you need to know.
▸ Thermistor. In a thermostat: as the temperature rises the resistance falls, changing the pd across it to switch heating off.
▸ LDR. In automatic lights: as it gets dark the resistance rises, which switches the light on.
▸ Diode. Allows current in one direction only, protecting circuits from being connected the wrong way round.
Key Terms
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Ohmic conductor A conductor whose resistance is constant at constant temperature. |
Filament lamp A lamp with a thin wire that gets hot and glows. |
|
Diode A component that lets current flow in one direction only. |
Thermistor A resistor whose resistance decreases when the temperature increases. |
|
LDR A resistor whose resistance decreases when the light intensity increases. |
Linear Following a straight line graph. |
Your Task: Match the Graph
10 minutes
|
Match each component to its I–V graph description: resistor, filament lamp, diode. (1) Curve that flattens as pd increases. (2) Straight line through the origin. (3) Almost no current one way, then a rapid increase the other way. 1. Look at the shape. 2. Explain the resistance change. |
A good answer shows: Resistor: (2). Filament lamp: (1). Diode: (3).
Can I...?
☐ Draw and describe the I–V graph of a resistor.
☐ Describe the I–V graph of a filament lamp.
☐ Describe the I–V graph of a diode.
☐ Explain why a filament lamp's resistance increases.
☐ Describe how a thermistor behaves.
☐ Describe how an LDR behaves.
☐ Describe Required Practical 4.
☐ Give one use each of a thermistor and an LDR.
Summary
✓ Resistor: constant resistance, straight line through the origin.
✓ Filament lamp: resistance increases with temperature.
✓ Diode: current in one direction only.
✓ Thermistor and LDR resistances decrease as temperature or light increases.
|
EXAM FOCUS Explain why the resistance of a filament lamp increases as the current increases. (3 marks) Say the filament gets hotter and link it to resistance. |