Electric and magnetic forces: electromagnets, motors and generators
Week 9 · Lesson 3 Electric and Magnetic Forces: the magnetic effect of current, the electromagnet and its strength, force on a current-carrying wire, the electric motor and its torque, the electric generator · 3 lessons
1
A current makes a magnet: Oersted and the electromagnet
Science Prep 3 Term 1, Unit 2: pp. 54–63
GoalDescribe Oersted’s discovery that a current makes a magnetic field, build an electromagnet in thought, and say what makes it stronger.
New words
magnetic field · the space around a magnet or a current in which a magnetic force actselectromagnet · a coil of insulated wire round a soft iron core that is a magnet only while a current flowssoft iron core · a piece of soft iron inside a coil that is magnetised by the current and loses its magnetism when the current stops
Explanation
The Danish scientist Hans Oersted noticed that a compass needle near a wire turns when a current flows in the wire. So a current has a magnetic effect: while it flows, a magnetic field surrounds the wire. An electromagnet is made by coiling insulated copper wire round a soft iron core. The current magnetises the core, and when the current stops, the magnetism goes: it is a temporary magnet that can be switched on and off. This is why it is used in scrap-yard cranes, in sorting metal for recycling and in electric bells. The strength grows with more turns of wire, a bigger current and the soft iron core, and the number of paper clips it lifts is a way to measure it. In an electric bell, the electromagnet pulls a springy strip away from a contact, which breaks the circuit; the magnet then switches off, the strip springs back, the circuit closes again, and the cycle repeats and the hammer rings the bell. Careful: an electromagnet is not a permanent magnet, a soft iron core is not a steel one, and a compass is not “attracted” by the wire: it turns because of the magnetic field.
current in the wire→magnetic field around the wire→compass needle turns
Worked examples
Maya makes an electromagnet with a long iron nail and 10 turns of wire on one AA cell. It lifts 3 paper clips. With 20 turns it lifts 7, and with 30 turns it lifts 11 (sample results, yours will differ). More turns, stronger magnet: the count goes up each time.
Switching an electromagnet on and off: a scrap crane picks up a steel car body when the current is on, and drops it when the current is off, because the core loses its magnetism. A permanent magnet could not be switched off like this.
An electric bell: the electromagnet pulls the strip, the contact opens, the current stops, the strip springs back, the contact closes, the current flows again. The hammer therefore strikes the gong again and again while the button is pressed.
Class activity
Compass and electromagnet, with a grown-up. (1) An adult strips the ends of a wire. Lay it over a compass, touch the ends to a 1.5 V cell for one second only and watch the needle. Do not keep it connected. (2) Wind 10 turns of thin insulated wire round a long iron nail, connect it to one AA cell for a few seconds and count how many paper clips it lifts. Repeat with 20 turns. The wire and the cell warm up, so switch off between tests. Use ordinary bar magnets only, never small strong magnets: they are a swallowing hazard.
Practice
1
What did Oersted observe?
A compass needle near a wire turned when a current flowed in the wire.
2
Name the parts of an electromagnet.
Insulated copper wire coiled round a soft iron core, joined to a source.
3
Give three ways to make an electromagnet stronger.
More turns of wire, a larger current and a soft iron core.
4
Why does an electromagnet drop a load when the current is switched off?
The soft iron core loses its magnetism when the current stops.
