Electricity: Magnetic and Heating Effects · Lesson 1 of 7
Electric Current and Magnetic Fields
“Discover how a simple compass reveals the magnetic field produced by electric current.”
• Explain how a magnetic compass can reveal that current is flowing in a nearby wire. • Define magnetic field and the magnetic effect of electric current. • Describe what changes when current is switched ON and OFF. • Connect Oersted's observation with the idea that electricity and magnetism are linked.
A Current-Carrying Wire Can Affect a Compass
Imagine a circuit that has a cell, connecting wires and a switch, but no lamp. At first, it may seem difficult to tell whether current is actually flowing. A magnetic compass gives us another clue. Because the compass needle is itself a tiny magnet, its movement can reveal a magnetic effect produced near the wire.
To investigate this, a straight section of wire is placed just above a magnetic compass and connected to a cell through a switch. First observe the compass with the switch OFF. Then close the switch and observe the needle again. Repeating the ON–OFF change makes the pattern easier to notice.
What the Deflection Means
When the switch is ON and current flows through the wire, the compass needle deflects from its original direction. When the switch is turned OFF, the current stops and the needle returns to its original direction. The repeated change shows that the deflection is connected with the flow of current.
A compass needle normally responds to magnetic influences. Its deflection therefore tells us that the current-carrying wire produces a magnetic effect around itself. The effect caused by that current disappears when the current stops.
The region around a magnet or a current-carrying conductor where its magnetic effect can be detected is called a magnetic field.
When electric current flows through a conductor, it produces a magnetic field around the conductor. This is called the magnetic effect of electric current.
The wire shows a magnetic effect while current is flowing. When the current stops, the magnetic field produced by that current disappears.
Oersted's Observation
In 1820, Hans Christian Oersted observed that a compass needle near an electrical circuit moved when the circuit was closed or opened. He investigated the effect and concluded that electric current produces a magnetic field. This observation provided clear evidence that electricity and magnetism are connected.
The magnetic effect of current is used in devices such as electromagnets, electric bells, motors, fans and loudspeakers. The next lessons focus on one especially useful application: the electromagnet.
Problem
A compass is placed below a wire in a simple circuit. The needle points in its usual direction when the switch is open, but turns when the switch is closed. What can you conclude?
- 1.Closing the switch completes the circuit, so current can flow through the wire.
- 2.A compass responds to magnetic effects.
- 3.The deflection therefore shows that the current-carrying wire is producing a magnetic field around it.
Quiz
What observation is the clearest evidence of the magnetic effect of current?
What happens to the magnetic field produced by the wire when current stops?
Why is a compass useful in this investigation?
Which statement best describes a magnetic field?
Oersted's observation helped show that
Practice Problems
- Describe the compass needle's behaviour when the switch is moved from OFF to ON and then back to OFF.
- Explain why compass deflection can be used as evidence that current is flowing in a nearby wire.
- Define magnetic field in your own words and give one way it can be detected in this lesson.
- A student says, 'The wire becomes a permanent magnet after current passes through it.' Explain why this is not supported by the investigation.
- Name two devices mentioned in the chapter that make use of the magnetic effect of electric current.
Key Takeaways
• A current-carrying conductor produces a magnetic field around it. • A nearby compass can reveal this magnetic effect by deflecting. • The magnetic field produced by the current disappears when the current stops. • Oersted's observation established an important link between electricity and magnetism.
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Next · Lesson 2
Making an Electromagnet