Resistors and IV characteristics.pptx

AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Resistors and I–V characteristics

Electricity

Lesson 3 of 10

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Warm-up

Electricity

Lesson 3 of 10

Answer each one, then check.

1

What is an ohmic conductor?

2

What does I–V stand for?

3

What is a straight line graph through the origin?

4

What happens to a metal when its temperature rises?

5

What does LDR stand for?

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Warm-up - Answers

Electricity

Lesson 3 of 10

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

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Learning Objectives

Electricity

Lesson 3 of 10

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.

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Electricity

Lesson 3 of 10

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.

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I–V Graphs

Electricity

Lesson 3 of 10

Straight line: constant resistance. Curve: changing resistance.

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Thermistor and LDR

Electricity

Lesson 3 of 10

Both fall as the quantity increases.

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Required Practical 4: I–V Circuit

Electricity

Lesson 3 of 10

Vary the pd and measure the current each time.

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Component Behaviour

Electricity

Lesson 3 of 10

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

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Required Practical 4: Method

Electricity

Lesson 3 of 10

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.

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Resistance of a Filament Lamp

Electricity

Lesson 3 of 10

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.

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Uses in Circuits

Electricity

Lesson 3 of 10

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.

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Key Terms

Electricity

Lesson 3 of 10

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.

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Electricity

Lesson 3 of 10

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.

WHAT A GOOD ANSWER SHOWS

Resistor: (2). Filament lamp: (1). Diode: (3).

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Can I...?

Electricity

Lesson 3 of 10

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.

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Summary & Exam Focus

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.

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