Current resistance and potential difference.pptx

AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Current, resistance and potential difference

Electricity

Lesson 2 of 10

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Warm-up

Electricity

Lesson 2 of 10

Answer each one, then check.

1

Write the unit of resistance.

2

What is the unit of potential difference?

3

Where is a voltmeter connected?

4

What does a longer wire do to resistance?

5

What is a current?

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Warm-up - Answers

Electricity

Lesson 2 of 10

1

Write the unit of resistance.

Ohm (Ω)

2

What is the unit of potential difference?

Volt (V)

3

Where is a voltmeter connected?

In parallel across the component

4

What does a longer wire do to resistance?

Increases it

5

What is a current?

A flow of charge

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Learning Objectives

Electricity

Lesson 2 of 10

1

Explain that current depends on resistance and potential difference.

2

Recall and apply V = IR.

3

Describe how to investigate the resistance of a wire and of combinations of resistors (Required Practical 3).

4

Interpret graphs of resistance against length.

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Electricity

Lesson 2 of 10

V = IR

potential difference = current × resistance V = IR

The greater the resistance of a component, the smaller the current for a given potential difference across it. Questions use the term potential difference; voltage also gains credit.

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Required Practical 3: Resistance of a Wire

Electricity

Lesson 2 of 10

Change only the length of wire between the clips.

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Method: Resistance and Length of a Wire

Electricity

Lesson 2 of 10

Required Practical 3, part 1.

1

Set up

Connect the circuit with the ammeter in series and the voltmeter across the wire.

2

Set the length

Clip the crocodile clips at 0.20 m, and record the pd and current.

3

Switch off between readings

This keeps the wire at a constant temperature.

4

Repeat

Take readings at 0.40 m, 0.60 m, 0.80 m and 1.00 m.

5

Calculate

For each length R = V ÷ I.

6

Plot

Draw a graph of resistance against length: a straight line through the origin.

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Method: Resistors in Series and Parallel

Electricity

Lesson 2 of 10

Required Practical 3, part 2.

1

Series

Connect known resistors in series, measure V and I, calculate R = V ÷ I.

2

Parallel

Repeat with the same resistors in parallel.

3

Compare

The series combination has a larger resistance than either resistor; the parallel combination has a smaller one.

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Calculating Current

Electricity

Lesson 2 of 10

A 12 V battery is connected across a 4.0 Ω resistor. Calculate the current.

1

Rearrange

I = V ÷ R

2

Substitute

I = 12 ÷ 4.0

3

Answer

I = 3.0 A

ANSWER

3.0 A

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Calculating Potential Difference

Electricity

Lesson 2 of 10

A current of 0.50 A flows through a 20 Ω resistor. Calculate the potential difference across it.

1

Write the equation

V = IR

2

Substitute

V = 0.50 × 20

3

Answer

V = 10 V

ANSWER

10 V

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Calculating Resistance

Electricity

Lesson 2 of 10

A voltmeter reads 6.0 V and an ammeter reads 20 mA. Calculate the resistance.

1

Convert the current

20 mA = 0.020 A

2

Rearrange

R = V ÷ I

3

Substitute

R = 6.0 ÷ 0.020

4

Answer

R = 300 Ω

ANSWER

300 Ω

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Getting Good Results

Electricity

Lesson 2 of 10

Why we switch off between readings.

Heating

A current makes the wire hotter, and a hotter metal wire has a higher resistance.

Constant temperature

Use a low current and switch off between readings.

Zero errors

Check the meters read zero before starting.

Repeat

Repeat and average to reduce random errors.

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Key Terms

Electricity

Lesson 2 of 10

Resistance

How much a component opposes the flow of current.

Ohm

The unit of resistance (Ω).

Potential difference

The energy transferred per coulomb; measured in volts.

Voltmeter

Measures pd; connected in parallel.

Constant temperature

Kept the same so the resistance does not change for other reasons.

Directly proportional

Doubling one quantity doubles the other.

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Electricity

Lesson 2 of 10

YOUR TASK

Predict the Graph

10 minutes

A student measures the resistance of 20 cm, 40 cm, 60 cm, 80 cm and 100 cm of a wire. What does the graph of resistance against length look like? A 30 cm length has a resistance of 1.8 Ω; what is the resistance of 90 cm?

1

Sketch the axes.

2

Use the ratio of lengths.

WHAT A GOOD ANSWER SHOWS

A straight line through the origin: resistance is directly proportional to length. 90 cm is three times as long, so R = 5.4 Ω.

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Can I...?

Electricity

Lesson 2 of 10

Recall V = IR.

Rearrange for I or R.

Convert mA to A.

Describe the resistance of wire method.

Explain why the wire is switched off between readings.

Draw a graph of R against length.

Say what happens to resistance in series and parallel.

Use the correct units.

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Summary & Exam Focus

V = IR.

Ammeter in series, voltmeter in parallel.

Longer wire means greater resistance.

Switch off between readings to keep the temperature constant.

EXAM FOCUS

A potential difference of 9.0 V is applied across a resistor and the current is 0.30 A. Calculate the resistance. (2 marks)

State the equation, substitute and give the unit Ω.

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