Science questions, chapter 7

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Electricity and magnetism

44 marks. Use Practice to check each answer as you go, or Exam to answer everything against the clock first.

Section A: objective questions

13 marks

Choose the best answer for each question.

Question 1

What is the S.I. unit of energy?

Watt
Joule
Newton
Ampere

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Energy is the ability to do work and its S.I. unit is the joule (J).

For the teacher: Remembering, low difficulty, standard 7.1.1

Textbook p. 142

Question 2

A polythene rod becomes negatively charged when it is rubbed with a woollen cloth. Which statement explains this?

Protons move from the cloth to the rod
Protons move from the rod to the cloth
Electrons move from the cloth to the rod
The rod gains protons and loses electrons

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Only electrons move when objects are rubbed. The object that gains electrons is negatively charged. Protons never move.

For the teacher: Understanding, low difficulty, standard 7.1.2

Textbook p. 145

Question 3

A positively charged strip is touched to the metal cap of an electroscope and the gold leaf diverges. Why does the gold leaf diverge?

The gold leaf and the stem have the same charge, so they repel each other
The gold leaf and the stem have opposite charges, so they attract each other
The gold leaf becomes neutral and falls away
The metal cap pulls the gold leaf upwards

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The charges spread to the stem and the gold leaf. Same charges repel, so the leaf diverges. The more it diverges, the larger the quantity of charge.

For the teacher: Understanding, low difficulty, standard 7.1.2

Textbook p. 146

Question 4

Why is a lightning conductor installed on a tall building?

To attract more lightning to the building
To store the electric charges from the clouds
To make the clouds lose their negative charges
To provide a path for the electric charges to flow into the ground

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The conductor gives the charges an easy path to the ground, so the building is not struck.

For the teacher: Understanding, low difficulty, standard 7.1.3

Textbook p. 146

Question 5

Which of the following defines electric current?

The potential difference between two points
The rate of flow of electric charges through a conductor
The ability of a conductor to resist the flow of charges
The energy stored in a dry cell

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Electric current is the rate of flow of electric charges. The first option is voltage and the third is resistance.

For the teacher: Remembering, low difficulty, standard 7.1.4

Textbook p. 148

Question 6

A bulb has a current of 0.3 A when it is connected to a 9 V supply. What is the resistance of the bulb?

30 Ω
2.7 Ω
0.03 Ω
3 Ω

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R = V ÷ I = 9 ÷ 0.3 = 30 Ω.

For the teacher: Applying, low difficulty, standard 7.1.6

Textbook p. 152

Question 7

Diagram 1 shows the graph of current against voltage for a resistor. What is the current when the voltage is 5 V?

Straight line through the origin passing through 4 V and 0.8 A, and 6 V and 1.2 A — Diagram 1

0.8 A
0.9 A
1.0 A
1.2 A

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The graph is a straight line, so R = 4 ÷ 0.8 = 5 Ω. At 5 V, I = 5 ÷ 5 = 1.0 A.

For the teacher: Analysing, medium difficulty, standard 7.1.6

Textbook p. 152

Question 8

Which statement about a series circuit is correct?

The current is different through each bulb
The other bulbs keep lighting if one bulb is removed
The voltage across each bulb is the same as the voltage of the cell
The voltage of the cell is shared among the bulbs

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In a series circuit the current is the same everywhere and V = V₁ + V₂. If one bulb is removed the circuit is broken and all bulbs go out.

For the teacher: Understanding, low difficulty, standard 7.2.1

Textbook p. 154

Question 9

Two resistors of 6 Ω each are connected in parallel. What is the effective resistance?

12 Ω
3 Ω
6 Ω
0.33 Ω

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1/R = 1/6 + 1/6 = 2/6 = 1/3, so R = 3 Ω. (12 Ω would be the series answer.)

For the teacher: Applying, low difficulty, standard 7.2.1

Textbook p. 156

Question 10

Diagram 2 shows a parallel circuit. What is the reading of ammeter A₃?

Parallel circuit with a 6 V cell, ammeter A3 in the main wire and ammeters A1 = 0.8 A and A2 = 0.3 A in the two branches — Diagram 2

0.5 A
0.8 A
1.1 A
1.6 A

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In a parallel circuit I = I₁ + I₂ = 0.8 + 0.3 = 1.1 A.

For the teacher: Analysing, medium difficulty, standard 7.2.1

Textbook p. 156

Question 11

The lamps in a house are connected in parallel. What is the main reason?

Each lamp gets the full voltage and can be switched on or off separately
The effective resistance of the lamps is larger
The current through every lamp is smaller
Less electricity is used than in a series circuit

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In a parallel circuit every branch has the same voltage as the supply, and each branch has its own switch. The effective resistance is smaller, not larger.

For the teacher: Evaluating, high difficulty, standard 7.2.1

Textbook p. 156

Question 12

Which statement about magnetic field lines is correct?

They begin at the south pole and end at the north pole
They cross each other at the centre of the magnet
They are the same distance apart everywhere
They are closer together where the magnetic field is stronger

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Lines begin at the north pole and end at the south pole, never cross, and are closer where the field is stronger.

For the teacher: Remembering, low difficulty, standard 7.3.1

Textbook p. 159

Question 13

Which change will NOT make an electromagnet stronger?

Increasing the number of turns of the coil
Reversing the direction of the current
Increasing the current in the coil
Adding one more dry cell in series

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Reversing the current only reverses the poles. The strength depends on the current and the number of turns.

For the teacher: Analysing, medium difficulty, standard 7.3.3

Textbook p. 161

Section B: structured questions

31 marks

Answer all questions. Write your answers in the spaces provided.

Question 14

Choose the form of energy for each situation.

[1 mark] (a)A lighted bulb

ALight energyBElastic potential energyCGravitational potential energy

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Light energy

[1 mark] (b)A stretched rubber band

ALight energyBElastic potential energyCGravitational potential energy

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Elastic potential energy

For the teacher: Remembering, low difficulty, standard 7.1.1

Textbook p. 143

Question 15

A balloon is rubbed on a student's hair and then held near small pieces of paper. The pieces of paper jump towards the balloon.

[2 marks] (a)The balloon becomes negatively charged. State the charge on the hair and explain your answer.

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Positive1
The hair has lost electrons (to the balloon)1

Tick each point you got.

[1 mark] (b)Explain why the pieces of paper are attracted to the balloon.

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The negative charges on the balloon attract the positive charges on the pieces of paper (opposite charges attract)1

Tick each point you got.

For the teacher: Understanding, low difficulty, standard 7.1.2

Textbook p. 145

Question 16

Lightning is an example of electrostatic charges in nature.

[1 mark] (a)What type of charge is found on the bottom part of a storm cloud?

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Negative charge1

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[2 marks] (b)Explain how lightning occurs.

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Friction between clouds and air charges the cloud (negative charges at the bottom)1
The negative charges in the cloud are attracted to the positive charges on the ground and discharge (flow) to the earth1

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[1 mark] (c)State one way to protect a tall building from lightning.

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Install a lightning conductor (to carry the charges safely to the ground)1

Tick each point you got.

For the teacher: Applying, low difficulty, standard 7.1.3

Textbook p. 146

Question 17

Diagram 3 shows two resistors connected in series to a 12 V cell.

A 12 V cell in series with an 8 ohm resistor R1 and a 4 ohm resistor R2 — Diagram 3

[1 mark] (a)Calculate the effective resistance.

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R = 8 + 4 = 12 Ω1

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[1 mark] (b)Calculate the current in the circuit.

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I = V ÷ R = 12 ÷ 12 = 1 A1

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[1 mark] (c)Calculate the voltage across R₁.

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V₁ = I R₁ = 1 × 8 = 8 V1

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[1 mark] (d)Calculate the voltage across R₂.

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V₂ = 1 × 4 = 4 V (or 12 − 8 = 4 V)1

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[1 mark] (e)R₁ is removed from the circuit. What happens to the current? Give a reason.

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The current becomes zero because the circuit is broken (only one path)1

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For the teacher: Applying, low difficulty, standard 7.2.1

Textbook p. 156

Question 18

Diagram 4 shows two resistors connected in parallel to a 6 V cell.

A 6 V cell with an ammeter in the main wire and resistors R1 = 12 ohm and R2 = 4 ohm in parallel — Diagram 4

[1 mark] (a)What is the voltage across R₂?

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6 V (same as the cell)1

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[1 mark] (b)Calculate the current through R₁.

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I₁ = 6 ÷ 12 = 0.5 A1

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[1 mark] (c)Calculate the current through R₂.

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I₂ = 6 ÷ 4 = 1.5 A1

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[1 mark] (d)What is the reading of the ammeter?

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I = 0.5 + 1.5 = 2 A1

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[2 marks] (e)Calculate the effective resistance of the two resistors.

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1/R = 1/12 + 1/4 = 4/12 (correct substitution)1
R = 3 Ω (answer with unit)1

Tick each point you got.

For the teacher: Analysing, high difficulty, standard 7.2.1

Textbook p. 157

Question 19

Diagram 5 shows an iron nail wound with insulated copper wire and connected to a dry cell through a switch. Some pins are placed below the nail.

An iron nail wound with insulated copper wire, connected to a 3.0 V cell and a switch, with three pins below the nail — Diagram 5

[1 mark] (a)What happens to the pins when the switch is turned on?

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The pins are attracted to the iron nail1

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[1 mark] (b)Name the type of magnet formed.

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Electromagnet1

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[2 marks] (c)What happens to the pins when the switch is turned off? Explain.

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The pins fall off1
An electromagnet is only magnetic while current flows (its magnetism is temporary)1

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For the teacher: Applying, medium difficulty, standard 7.3.3

Textbook p. 161

Question 20

A shopkeeper wants to install three lights in his shop. He can connect them in series or in parallel.

[1 mark] (a)Which connection is better?

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Parallel1

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[2 marks] (b)Give two reasons for your answer.

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Any 2 points

Each light receives the full voltage, so all are equally bright1
If one light is damaged the others still work1
Each light can be switched on or off separately1

Tick each point you got.

For the teacher: Evaluating, high difficulty, standard 7.2.1

Textbook p. 157

Question 21

Aina wants to investigate whether the number of turns of a coil affects the strength of an electromagnet. She uses an iron rod, copper wire, a power supply and some pins.

[1 mark] (a)State the manipulated variable.

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Number of turns of the coil1

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[1 mark] (b)State the responding variable and how it is measured.

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Strength of the magnetic field, measured by the number of pins attracted1

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[1 mark] (c)State one constant variable (a variable that is kept the same).

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Any 1 points

Current (setting of the power supply)1
Type / size of the iron rod1
Length / thickness of the copper wire1

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[1 mark] (d)Construct a hypothesis for this investigation.

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The more the number of turns of the coil, the stronger the electromagnet (more pins attracted)1

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For the teacher: Creating, high difficulty, standard 7.3.3

Textbook p. 161

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