Electricity: Magnetic and Heating Effects · Lesson 3 of 7
Controlling and Using Electromagnets
“Learn how the strength and poles of an electromagnet can be controlled and put to practical use.”
• Identify the two poles of an electromagnet using a magnetic compass. • Explain how current and the number of coil turns affect electromagnet strength. • Predict what happens to the poles when the direction of current is reversed. • Explain how lifting electromagnets can pick up and release iron or steel objects. • Relate the chapter's magnetic ideas to Earth's magnetic field and other electrical devices.
Electromagnets Have Two Poles
A bar magnet has a North pole and a South pole. A current-carrying coil also behaves like a magnet, so it is natural to ask whether its two ends act as magnetic poles. A compass provides a practical way to identify them.
Place a compass near one end of the electromagnet and note which pole of the compass is attracted. Because unlike magnetic poles attract, the compass tells us the polarity of that end. Repeating the test at the other end shows that the two ends have opposite poles.
Changing the Strength
The strength of an electromagnet is not fixed. The chapter investigates two controllable factors: the amount of current through the coil and the number of turns in the coil. If a larger current flows through the same coil, its magnetic field becomes stronger. Increasing the number of turns also makes the coil a stronger magnet.
| Change made | Observed effect | Conclusion |
|---|---|---|
| Use more cells with the same coil | Greater compass deflection and more clips can be attracted | More current gives a stronger magnetic effect |
| Increase the number of turns with the same supply | The coil acts as a stronger magnet | More turns strengthen the electromagnet |
| Stop the current | Magnetic effect disappears | The electromagnet can be switched off |
A useful extension is to make coils with 25, 50, 75 and 100 turns, connect each to the same cell, and keep the compass at the same position. The comparison is fair only when the other conditions are kept the same. The pattern in compass deflection can then be used to judge how the number of turns affects strength.
Reversing the Poles
The poles of an electromagnet depend on the direction of current through the coil. If the battery connections are reversed, the direction of current reverses. The North and South poles of the electromagnet then exchange positions.
Using more cells can make the magnetic effect stronger. Reversing the connections changes the direction of current and reverses the poles. These are different changes.
Lifting Electromagnets
A lifting electromagnet uses the ability to control magnetism with a switch. When current is switched ON, the electromagnet attracts iron or steel objects. When current is switched OFF, the magnetic field produced by the current disappears and the objects are released. This makes lifting electromagnets useful for moving and sorting heavy metal objects in factories and scrap yards.
Problem
A crane must lift a group of iron objects and then drop them at a chosen place. Why is an electromagnet more convenient than a permanent magnet for this task?
- 1.The operator can switch current ON to produce a strong magnetic effect and lift the objects.
- 2.The crane can move the objects while the current remains ON.
- 3.Switching the current OFF removes the electromagnet's magnetic effect, allowing the objects to be released when required.
Earth's Magnetic Field and a Wider Connection
Earth also has a magnetic field. The chapter explains that movement of liquid iron deep inside Earth creates electric currents that generate this field. Many migratory birds, fish and other animals use Earth's magnetic field while navigating, and the field also helps shield Earth from harmful particles arriving from space.
The connection between electricity and magnetism extends beyond electromagnets. The chapter previews that a moving magnet can also lead to electric current, an idea studied in greater depth later. This electricity–magnetism link is important in devices such as electric motors and power generators.
Quiz
How can you identify the pole at one end of an electromagnet?
What generally happens when more cells are used with the same electromagnet coil?
What is the effect of increasing the number of turns in the coil?
What happens to the poles when the direction of current through the coil is reversed?
Why are lifting electromagnets useful in scrap yards?
Practice Problems
- Describe how you could use a compass to determine the polarity of both ends of an electromagnet.
- Predict how compass deflection would change if the same coil were connected first to one cell and then to a battery with more cells.
- Explain why testing coils of 25, 50, 75 and 100 turns with the same cell is a useful investigation.
- What happens to an electromagnet's poles if the battery terminals are reversed? Explain the cause of the change.
- Explain the complete ON–lift–move–OFF–release sequence of a lifting electromagnet.
Key Takeaways
• An electromagnet has North and South poles like a magnet. • Increasing current or increasing the number of coil turns can strengthen an electromagnet. • Reversing current reverses the poles of the electromagnet. • Lifting electromagnets work because their magnetic effect can be controlled with a switch. • Earth's magnetic field is another example of a magnetic field linked to electric currents.