Electricity Topic

Electromagnet

This science revision note aims to help your child

– understand what happens when the switch is closed (You need to understand the sequence of events that will happen when the switch is closed)

– How does the iron rod become an electromagnet?

– proper scientific keywords to explain the concepts clearly.

But before you read on, you might want to download this entire revision notes in PDF format to print it out for your child, or to read it later.

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Question

Study the diagram below that shows a circuit breaker that acts as a switch to protect an electric circuit from damage due to a sudden increase in electric current flowing through it.

It is connected to an electric circuit at points X and Y. The iron arm is held by a spring to ensure that the metal contacts are not separated. However, the metal contacts will separate when there is too much electric current flowing through the circuit.

(a) State a relevant physical property of the iron arm for the circuit breaker to work.

Step1: Recall

Identify the keywords in the question:

Electric circuit
Iron arm
Circuit breaker

The topic tested here is electricity. How does a circuit breaker work?

When there is too much electric current flowing through the circuit, the metal cylinder becomes a stronger electromagnet.
The electromagnet is strong enough to attract the iron arm.
Metal contacts separate and the circuit is open. The electric current no longer flows through the circuit.

What property of the iron arm allows the iron arm to be attracted by the electromagnet (metal cylinder)?

Step 2: What science topics are tested?

Identify which are magnetic materials.

Cobalt
Iron
Steel
Nickel

Refer to this video:

Step 3: Piecing your answer together.

(a) State a relevant physical property of the iron arm for the circuit breaker to work.

Correct answer:

The iron arm is a magnetic material.

Common mistakes:

Wrong answer: The iron arm must be a conductor of electricity.

Imagine this: When the arm is a conductor of electricity but NOT a magnetic conductor, a closed circuit is formed and electric current flows through the circuit. However, when there is too much current and the metal cylinder becomes an electromagnet, the electromagnet is unable to attract the arm and the circuit remains closed. The metal contacts will not separate. That means the circuit breaker has failed to work.

(b) Explain how the metal contacts separate when there is too much electric current flowing through to prevent damage to the electric circuit.

Step 1 – Recall

Topic tested here is on electricity. How does a circuit breaker work?

Refer to part (a)

Step 2: What science topics are tested?

Understand how electromagnets are formed: When electric current flows through the coils of wire around the magnetic material, the magnetic material becomes an electromagnet.

Step 3: Piecing your answer together.

(b) Explain how the metal contacts separate when there is too much electric current flowing through to prevent damage to the electric circuit.

Correct Answer:

When too much electric current flows through the circuit, the metal cylinder becomes an electromagnet that exerts a stronger magnetic force of attraction on the magnetic iron arm. The circuit is now opened, and electric current can no longer flow through the circuit.

Step 1: Recall

Identify the keywords:

Not able to hold the iron arm

Topic tested here is forces.

Identify the forces acting on the iron arm:

Elastic spring force
Magnetic force

Step 2: What Science topics are tested?

Comparing forces
- When two forces of unequal strength are acting on an object, the object will move in the direction of the stronger force

Refer to this video:

Step 3: Piecing your answer together

Correct answer:

When too much electric current flows in the circuit, the magnetic force of attraction between the electromagnet and the iron arm is stronger than the elastic spring force exerted by the spring on the iron arm. The iron arm swings towards the electromagnet.

Let’s Try Another Example!

Question

Ben sets up an electrical circuit as shown below.

When Ben closes the switch, the hammer hits the gong. Explain why.

Note: You need to understand the sequence of events.

Step 1: Recall

Identify the keywords in the question:

Electric circuit
Iron rod

The topic tested here is electricity.

How does the circuit work?

When the switch is closed, the circuit is closed, so electric current can flow through the circuit. The iron rod becomes an electromagnet.
The electromagnet is strong enough to attract the soft iron strip.
The metal contact is separated from the soft iron strip. The electric current no longer flows through the circuit.

Step 2: What Science topics are tested?

Understand how electromagnets are formed : When electric current flows through the coils of wire around the magnetic material, the magnetic material becomes an electromagnet.

Step 3: Piecing your answer together.

When Ben closed the switch, the hammer hits the gong. Explain why.

Correct Answer:

When the switch is closed, the circuit is closed, so electric current can flow through the circuit. The iron rod becomes an electromagnet. The electromagnet attracts the soft iron strip. This causes the hammer to hit the gong.

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