Series and parallel circuits - Teacher Slides.pptx

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Series and parallel circuits

Electricity

Lesson 4 of 10

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Warm-up

Electricity

Lesson 4 of 10

Answer each one, then check.

1

What is the same everywhere in a series loop?

2

What does an ammeter measure?

3

How is a voltmeter connected?

4

What is the equation linking V, I and R?

5

Add 4 + 8.

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Warm-up - Answers

Electricity

Lesson 4 of 10

1

What is the same everywhere in a series loop?

The current

2

What does an ammeter measure?

Current

3

How is a voltmeter connected?

In parallel

4

What is the equation linking V, I and R?

V = IR

5

Add 4 + 8.

12

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Learning Objectives

Electricity

Lesson 4 of 10

1

Describe the differences between series and parallel circuits.

2

Use R_(total) = R₁ + R₂ for resistors in series.

3

Calculate currents, pds and resistances in series circuits.

4

Explain why adding resistors in series increases and in parallel decreases the total resistance.

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Electricity

Lesson 4 of 10

SERIES AND PARALLEL

Series: same current, pd shared, R_(total) = R₁ + R₂. Parallel: same pd, currents add, R_(total) is less than the smallest resistor.

You are not required to calculate the total resistance of two resistors in parallel.

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Series and Parallel

Electricity

Lesson 4 of 10

One path or several paths for the current.

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Series or Parallel?

Electricity

Lesson 4 of 10

SERIES

• One loop for the current.

• The same current through each component.

• The supply pd is shared between components.

• Total resistance = R1 + R2.

PARALLEL

• More than one loop.

• The same pd across each branch.

• The currents in the branches add to the total current.

• Total resistance is less than the smallest resistor.

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Series Circuit

Electricity

Lesson 4 of 10

A 12 V battery is connected in series with resistors of 4.0 Ω and 8.0 Ω. Calculate the current and the pd across each resistor.

1

Total resistance

R = 4.0 + 8.0 = 12 Ω

2

Current

I = V ÷ R = 12 ÷ 12 = 1.0 A

3

Pd across 4.0 Ω

V = IR = 1.0 × 4.0 = 4.0 V

4

Pd across 8.0 Ω

V = 1.0 × 8.0 = 8.0 V

5

Check

4.0 + 8.0 = 12 V

ANSWER

Current 1.0 A; pds 4.0 V and 8.0 V.

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Parallel Circuit

Electricity

Lesson 4 of 10

Two lamps are connected in parallel to a 6.0 V supply. A1 reads 1.2 A and A2 reads 0.80 A. What does A3 in the main wire read?

1

Currents in the branches add

I_(total) = 1.2 + 0.80

2

Answer

I_(total) = 2.0 A

ANSWER

2.0 A; each lamp has 6.0 V across it.

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Why Resistance Changes

Electricity

Lesson 4 of 10

Explain it in words.

In series

The current has to pass through each resistor one after another, so the total opposition is greater.

In parallel

There are more paths for the current, so more charge can flow: the total resistance is less than any single resistor.

Uses of series circuits

Measurement and testing, for example a circuit with a thermistor and an ammeter.

Uses of parallel circuits

House lighting: each lamp has the full supply pd and can be switched on separately.

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Key Terms

Electricity

Lesson 4 of 10

Series

Components connected one after another in a single loop.

Parallel

Components connected on separate branches.

Equivalent resistance

The single resistance that could replace the combination.

Branch

One of the paths in a parallel circuit.

Total resistance

The resistance of the whole circuit.

Shared pd

The supply pd divides between series components.

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Electricity

Lesson 4 of 10

YOUR TASK

Circuit Puzzle

12 minutes

A 9.0 V cell is connected to two resistors in series, 6.0 Ω and 12 Ω. (a) Find the total resistance. (b) Find the current. (c) Find the pd across the 12 Ω resistor.

1

Add the resistances.

2

Find I, then V across each.

WHAT A GOOD ANSWER SHOWS

(a) 18 Ω (b) 9.0 ÷ 18 = 0.50 A (c) 0.50 × 12 = 6.0 V

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Can I...?

Electricity

Lesson 4 of 10

Describe series and parallel circuits.

Add resistors in series.

Find the current in a series circuit.

Find the pd across each resistor.

Add branch currents in parallel.

State that pd is the same across parallel branches.

Explain why total resistance changes.

Draw a circuit diagram.

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Summary & Exam Focus

Series: R_(total) = R₁ + R₂, same current, shared pd.

Parallel: same pd, currents add, total resistance less than the smallest.

Ammeters in series, voltmeters in parallel.

Household circuits use parallel wiring.

EXAM FOCUS

A 12 V battery is connected in series to resistors of 4.0 Ω and 8.0 Ω. Calculate the current. (3 marks)

Add the resistances first, then use V = IR.

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Exam Practice: Series and parallel circuits

Electricity

Lesson 4 of 10

Answer all questions. Use the mark allocation as a guide to how much to write. · 19 minutes

Question 1 · 3 marks · Calculate

The diagram shows a 12 V battery connected to two resistors in series. Calculate (a) the total resistance and (b) the current in the…

Question 2 · 3 marks · Calculate

Use your answer to the previous question to calculate the potential difference across the 8.0 Ω resistor. The current is 1.0 A.

Question 3 · 3 marks · Use the diagram

The diagram shows two lamps connected in parallel to a 6.0 V supply. Ammeter A1 reads 1.2 A and A2 reads 0.80 A. (a) Calculate the reading…

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Exam Practice: Series and parallel circuits (cont.)

Electricity

Lesson 4 of 10

Question 4 · 2 marks · Explain

Explain why the lights in a house are connected in parallel rather than in series.

Question 5 · 3 marks · Explain

Explain, in terms of the flow of current, why adding a second resistor in series increases the total resistance but adding a second…

Question 6 · 4 marks · Calculate

A 9.0 V cell is connected to a 6.0 Ω resistor and a 12 Ω resistor in series. Calculate the potential difference across the 12 Ω resistor.

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Question 1 · 3 marks · Calculate

Electricity

Lesson 4 of 10

The diagram shows a 12 V battery connected to two resistors in series. Calculate (a) the total resistance and (b) the current in the circuit. (3 marks)

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Question 1 · mark scheme

Electricity

Lesson 4 of 10

3 marks available. Award a mark for each point made.

Adds resistances

1 mark

12 Ω

1 mark

1.0 A

1 mark

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Question 2 · 3 marks · Calculate

Electricity

Lesson 4 of 10

“

Use your answer to the previous question to calculate the potential difference across the 8.0 Ω resistor. The current is 1.0 A.

HOW TO ANSWER IT

Command word: Calculate. Worth 3 marks, so plan before writing.

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Question 2 · mark scheme

Electricity

Lesson 4 of 10

3 marks available. Award a mark for each point made.

Correct equation

1 mark

Correct substitution

1 mark

8.0 V

1 mark

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Question 3 · 3 marks · Use the diagram

Electricity

Lesson 4 of 10

The diagram shows two lamps connected in parallel to a 6.0 V supply. Ammeter A1 reads 1.2 A and A2 reads 0.80 A. (a) Calculate the reading on A3. (b) State the pd across each lamp. (3 marks)

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Question 3 · mark scheme

Electricity

Lesson 4 of 10

3 marks available. Award a mark for each point made.

Adds the branch currents

1 mark

2.0 A

1 mark

6.0 V

1 mark

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Question 4 · 2 marks · Explain

Electricity

Lesson 4 of 10

“

Explain why the lights in a house are connected in parallel rather than in series.

HOW TO ANSWER IT

Command word: Explain. Worth 2 marks, so plan before writing.

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Question 4 · mark scheme

Electricity

Lesson 4 of 10

2 marks available. Award a mark for each point made.

Each lamp gets the full supply pd

1 mark

Lamps can be switched independently or one failing does not stop the others

1 mark

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Question 5 · 3 marks · Explain

Electricity

Lesson 4 of 10

“

Explain, in terms of the flow of current, why adding a second resistor in series increases the total resistance but adding a second resistor in parallel decreases the total resistance.

HOW TO ANSWER IT

Command word: Explain. Worth 3 marks, so plan before writing.

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Question 5 · mark scheme

Electricity

Lesson 4 of 10

3 marks available. Award a mark for each point made.

Series: current passes through both, so resistance is greater

1 mark

Parallel: two paths

1 mark

More current for the same pd, so total resistance is less

1 mark

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Question 6 · 4 marks · Calculate

Electricity

Lesson 4 of 10

“

A 9.0 V cell is connected to a 6.0 Ω resistor and a 12 Ω resistor in series. Calculate the potential difference across the 12 Ω resistor.

HOW TO ANSWER IT

Command word: Calculate. Worth 4 marks, so plan before writing.

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Question 6 · mark scheme

Electricity

Lesson 4 of 10

4 marks available. Award a mark for each point made.

Total resistance 18 Ω

1 mark

Current 0.50 A

1 mark

Correct substitution

1 mark

6.0 V

1 mark

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