Cambridge Lower Secondary CheckpointStage 8

Physics: Electricity and magnetism

Science Stage 8 Chapter Notes

What this chapter covers

Physics: Electricity and magnetism
ShareWhatsAppPost
Physics: Electricity and magnetism notes

Unable to load PDF

The notes viewer could not load. Please refresh the page.

Read online free. Download a watermarked copy with a free account.

Read the notes

The full Physics: Electricity and magnetism notes as text: skim, search, and jump between subtopics.

~11 min read

In this chapter

  1. 1Magnetic Fields and Materials
  2. 2Making an Electromagnet
  3. 3Investigating Electromagnet Strength
  4. 4Applications of Electromagnets

1. Magnetic Fields and Materials

A magnet is an object that produces a magnetic field. Every magnet has two poles: a North pole and a South pole. The fundamental law of magnetism is that like poles repel (e.g., North repels North) and unlike poles attract (e.g., North attracts South). A magnetic field is the region around a magnet where a magnetic force can be detected. We can visualise these invisible fields using magnetic field lines, which are drawn from the North pole to the South pole. The closer the lines are, the stronger the magnetic field. Materials that are attracted to a magnet, such as iron, steel, nickel, and cobalt, are called 'magnetic materials'. A 'magnetised' material is one that has been turned into a magnet itself and has its own North and South poles.

Key term

Magnetic Field: A region of space around a magnet or a current-carrying wire where a magnetic force can be experienced.

Examiner insight

Examiners reward clear diagrams of magnetic fields. Always include arrows on your field lines pointing from the North pole to the South pole.

Common pitfall

Confusing the terms 'magnetic' and 'magnetised'. An iron nail is magnetic, but it is not magnetised until it is turned into a magnet.

Worked example 12 marks

Two bar magnets are placed near each other as shown below. Magnet 1 is free to move, while Magnet 2 is fixed. Will Magnet 1 be attracted or repelled by Magnet 2? Explain your answer. [Magnet 1: S-N] [Magnet 2: S-N]

  1. 1

    Step 1: Identify the poles facing each other. The North pole (N) of Magnet 1 is facing the South pole (S) of Magnet 2.

  2. 2

    Step 2: Recall the law of magnetism. Unlike poles attract each other.

  3. 3

    Step 3: Conclude the outcome. Therefore, Magnet 1 will be attracted to Magnet 2 and will move towards it.

Worked example 23 marks

Explain the difference between a 'magnetic' material like a paperclip and a 'magnetised' material like a bar magnet.

  1. 1

    Step 1: Define a magnetic material. A magnetic material (like an iron paperclip) is a material that is attracted to a magnet, but does not have its own permanent magnetic field or poles.

  2. 2

    Step 2: Define a magnetised material. A magnetised material (like a bar magnet) has been turned into a magnet. It produces its own persistent magnetic field and has its own North and South poles.

  3. 3

    Step 3: State the key difference. The key difference is that a magnetised material can attract and repel other magnets, whereas a magnetic material can only be attracted.

Recap

  • Magnets have a North pole and a South pole.
  • Like magnetic poles repel each other, and unlike poles attract.
  • A magnetic field is the area around a magnet where a magnetic force is felt.
  • Magnetic field lines show the direction and strength of a field, running from North to South.
  • Magnetic materials (iron, steel, nickel, cobalt) are attracted to magnets.
  • A magnetised material is a magnet itself.

Quick check

  1. Which of these is not a magnetic material: iron, aluminium, cobalt, steel?1 mark
  2. What happens when two North poles are brought close together?1 mark

2. Making an Electromagnet

An electromagnet is a temporary magnet that is created using electricity. The principle is simple: when an electric current flows through a wire, it generates a magnetic field around it. To create a useful, strong magnet, the wire is wrapped into a tight coil, called a solenoid. The magnetic field inside this coil is strong and uniform. To make the electromagnet much stronger, a piece of 'soft' iron is placed inside the coil. This is called the core. The soft iron core becomes strongly magnetised by the field from the coil, but crucially, it loses most of its magnetism as soon as the current is switched off. A simple electromagnet circuit consists of a power source (like a cell), a switch, the coil of insulated wire, and the soft iron core.

Key term

Electromagnet: A temporary magnet made by passing an electric current through a coil of wire, often wrapped around a soft iron core.

Common pitfall

Forgetting that the wire in the coil must be insulated. If a bare wire is wrapped around the iron core, the current will short-circuit through the core instead of flowing through the coil.

Fun fact

The first practical electromagnet was invented in 1824 by British scientist William Sturgeon. It could lift about 4 kg, which was 20 times its own weight.

Worked example 14 marks

Draw a labelled diagram to show how an electromagnet can be made using a cell, a switch, a coil of insulated wire, and an iron nail as the core. Use standard circuit symbols for the cell and the switch.

  1. 1

    Step 1: Draw the iron nail.

  2. 2

    Step 2: Draw a wire wrapped tightly around the nail to form a coil. Ensure the wire is shown as insulated from the nail.

  3. 3

    Step 3: Draw the rest of the circuit connecting the ends of the wire. This should include the correct symbol for a single cell (one long line, one short line).

  4. 4

    Step 4: Include the correct symbol for an open switch in series with the cell and the coil.

  5. 5

    Step 5: Add labels for all four components: 'Iron core (nail)', 'Coil of insulated wire', 'Cell', and 'Switch'.

Recap

  • An electric current in a wire creates a magnetic field.
  • Wrapping the wire into a coil (solenoid) concentrates the magnetic field.
  • Placing a soft iron core inside the coil makes the electromagnet much stronger.
  • An electromagnet is temporary; it is only a magnet when the current is flowing.
  • The main components are a power source, switch, insulated coil, and a soft iron core.

Quick check

  1. What is the purpose of the soft iron core in an electromagnet?1 mark
  2. What happens to the electromagnet when the switch in the circuit is opened?1 mark

3. Investigating Electromagnet Strength

The strength of an electromagnet is not fixed; it can be changed. There are three main factors that affect its strength. To investigate these factors, we need a way to measure the magnet's strength, for example, by counting the maximum number of identical paperclips it can hold. The factors are: 1. The size of the current: Increasing the current flowing through the coil increases the strength of the magnetic field. 2. The number of turns in the coil: Increasing the number of turns of wire in the coil makes the electromagnet stronger. 3. The type of core: Using a soft iron core makes the electromagnet much stronger than using an air core (no core) or a core made of a non-magnetic material like wood or plastic.

Key term

Soft Iron: A type of iron that is easily magnetised when a current flows but also loses its magnetism quickly when the current is switched off, making it ideal for electromagnet cores.

Examiner insight

When asked to describe an investigation, always clearly state the independent, dependent, and control variables. This shows you understand the principles of a fair test.

Worked example 15 marks

A student wants to investigate how the current affects the strength of an electromagnet. Describe the experiment she should carry out. You should state the variables she needs to measure and control.

  1. 1

    Step 1: Set up the apparatus. Create an electromagnet with an iron core, a coil, a switch, and a variable power supply (or ammeter and variable resistor) to change the current. Have a supply of identical paperclips.

  2. 2

    Step 2: Identify variables. The independent variable (what you change) is the current in the coil. The dependent variable (what you measure) is the strength of the magnet, measured by the number of paperclips it can pick up and hold.

  3. 3

    Step 3: Identify control variables. To make it a fair test, the number of turns in the coil and the iron core material must be kept the same throughout the experiment.

  4. 4

    Step 4: Describe the method. Set the current to a specific value using the ammeter. Switch on the electromagnet and count the maximum number of paperclips it can hold. Record the current and the number of paperclips. Repeat this for several different values of current.

  5. 5

    Step 5: State the expected conclusion. You would expect that as the current increases, the number of paperclips the electromagnet can hold also increases.

Worked example 22 marks

An electromagnet is made with 100 turns and a 2A current. State two separate changes you could make to increase its strength.

  1. 1

    Change 1: Increase the number of turns in the coil to a value greater than 100, while keeping the current at 2A.

  2. 2

    Change 2: Increase the current to a value greater than 2A, while keeping the number of turns at 100.

Recap

  • The strength of an electromagnet can be varied.
  • Increasing the current in the coil increases the strength.
  • Increasing the number of turns in the coil increases the strength.
  • Using a soft iron core dramatically increases the strength.
  • To investigate one factor, the other factors must be kept constant (controlled).

Quick check

  1. List the three factors that can be changed to alter the strength of an electromagnet.3 marks

4. Applications of Electromagnets

The most important property of an electromagnet is that it can be switched on and off. This makes it useful in a huge range of devices where a permanent magnet would be unsuitable. A key application is the scrap yard crane. A powerful electromagnet is lowered onto a pile of scrap, switched on to pick up iron and steel items, moved to a new location, and then switched off to drop the load. Another clever use is in an electric bell or buzzer. When the bell push is pressed, it completes a circuit, turning on an electromagnet. This pulls a metal hammer to strike the bell. However, this very movement breaks the circuit, switching the electromagnet off. A spring then pulls the hammer back, which completes the circuit again, and the cycle repeats rapidly, causing the continuous ringing sound. Electromagnets are also used in relays, which are switches that use a small, safe current to control a separate, high-power and potentially dangerous circuit.

Key term

Relay: An electrically operated switch that uses a small current in an electromagnet to switch on or off a much larger current in a separate circuit.

Common pitfall

Simply stating an application without explaining *why* the switchable nature of the electromagnet is essential for that application.

Fun fact

Magnetic Resonance Imaging (MRI) scanners in hospitals use extremely powerful, superconducting electromagnets to create detailed images of soft tissues inside the human body.

Worked example 13 marks

For a scrap yard crane that lifts cars, explain why an electromagnet is used instead of a strong permanent magnet.

  1. 1

    Step 1: State the function of the crane. The crane needs to be able to pick up the scrap metal (cars) and then release it in a different location.

  2. 2

    Step 2: Describe how an electromagnet achieves this. An electromagnet can be switched on to create a strong magnetic field to lift the iron/steel car parts.

  3. 3

    Step 3: Explain the release mechanism. Once the car is moved to the desired location, the current to the electromagnet can be switched off. This causes it to lose its magnetism, and the car is released.

  4. 4

    Step 4: Explain the problem with a permanent magnet. A permanent magnet would be able to lift the car, but it would not be able to release it easily, making it useless for this task.

Worked example 23 marks

Fire doors in a school corridor are held open by electromagnets connected to the fire alarm system. Explain how this system works as a safety feature.

  1. 1

    Step 1: Describe the normal state. During normal operation, current flows to the electromagnet, which holds the heavy fire door open against a closing mechanism.

  2. 2

    Step 2: Describe the event of a fire. When the fire alarm is triggered, the power supply to the electromagnet is cut.

  3. 3

    Step 3: Explain the result. The electromagnet switches off and loses its magnetism, releasing the door.

  4. 4

    Step 4: State the safety outcome. The door's closing mechanism then automatically shuts it, which helps to prevent the spread of fire and smoke through the building.

Recap

  • The key advantage of electromagnets is that they can be switched on and off.
  • Scrap yard cranes use electromagnets to pick up and drop metallic waste.
  • Electric bells use a self-interrupting circuit with an electromagnet to create a continuous ringing sound.
  • Relays use a small current in an electromagnet to safely switch a high-current circuit.
  • Safety devices like fire doors use electromagnets that release the door when power is cut.

Quick check

  1. State the main reason an electromagnet is used in a device instead of a permanent magnet.1 mark
  2. Name two devices that use electromagnets.2 marks

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. List two magnetic materials and two non-magnetic metals.2 marks
  2. Explain the difference between a permanent magnet and an electromagnet in terms of their magnetic field and construction.3 marks
  3. A student builds an electromagnet that can pick up 12 identical paperclips. State two different ways they could modify the electromagnet to make it pick up approximately 24 paperclips.2 marks
  4. Draw a diagram of the magnetic field around a single bar magnet. Your diagram should include at least four field lines and arrows to show the direction of the field.3 marks
  5. A scrap yard uses a large electromagnet to sort metals. Explain how it can separate iron and steel from a pile of mixed scrap that also contains aluminium and copper.3 marks
  6. A student is investigating how the number of turns on a coil affects the strength of an electromagnet. Identify the independent, dependent, and one control variable for this experiment.3 marks
  7. An electric bell uses an electromagnet to ring. Explain why the bell stops ringing immediately if a permanent magnet is used to replace the electromagnet.3 marks
  8. Describe, with the aid of a simple diagram, the operation of a relay switch. Explain why a relay is a useful device.5 marks
  9. An electromagnet is made with a 200-turn coil and a current of 3 A. A second electromagnet is made with a 600-turn coil and a current of 1 A. Both have identical soft iron cores. Compare the likely strengths of the two electromagnets, giving a reason for your answer.3 marks
  10. Some materials are 'magnetically hard' (like steel), meaning they are difficult to magnetise but retain their magnetism. Other materials are 'magnetically soft' (like iron), meaning they are easy to magnetise and demagnetise. Explain which type of material is suitable for i) a permanent magnet and ii) the core of an electromagnet, justifying your choices.4 marks

Go deeper

Practise and revise with member-only material for this chapter.

Workbook

Exam-style practice questions with full worked solutions.

Unlock with a plan

Chapter at a Glance

A single-page summary of every key idea for fast revision.

Unlock with a plan

Free notes are just the start.

Unlock every Workbook and Chapter at a Glance, and generate your own worksheets and predicted papers.

Explore plans

Related chapters

Thinking and Working Scientifically: Models and representationsThinking and Working Scientifically: Scientific enquiry: purpose and planningThinking and Working Scientifically: Carrying out scientific enquiryThinking and Working Scientifically: Scientific enquiry: analysis, evaluation and conclusionsBiology: Structure and functionBiology: Life processes