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Electromagnets at work - Physics narrative
Physics Narrative for 11-14
A Physics Narrative presents a storyline, showing a coherent path through a topic. The storyline developed here provides a series of coherent and rigorous explanations, while also providing insights into the teaching and learning challenges. It is aimed at teachers but at a level that could be used with students.
It is constructed from various kinds of nuggets: an introduction to the topic; sequenced expositions (comprehensive descriptions and explanations of an idea within this topic); and, sometimes optional extensions (those providing more information, and those taking you more deeply into the subject).
The ideas outlined within this subtopic include:
- Making electromagnets
- Separating electrical from magnetic loops
- Electromagnetic devices.
What do electric bells, scrap-yard cranes and the central-locking in cars have in common? They all work using magnets, but not the permanent magnets of door catches and children's toys. The bells, cranes and car locks are based on electromagnets.
What is an electromagnet?
All electromagnets work on the principle that an electric current in a wire produces a magnetic field. In fact it is remarkably straightforward to make an electromagnet. Simply coil a length of wire round a piece of iron, such as a long iron nail, and pass an electric current through the wire. When the current flows a magnetic field is created and the iron becomes magnetised.
Linking electrical current to magnetism
Physics Narrative for 11-14
Current moving magnets
The connection between electric currents and magnetic fields was first demonstrated in 1820 by a Danish scientist named Hans Christian Oersted. Oersted showed that a compass needle (a small pivoted magnet) moves whenever an electric current passes through a nearby wire.
This simple demonstration can be replicated in the classroom to great effect, making the link from electricity to magnetism:
- Everyone knows that wires connected to batteries are
- Everyone knows that compass needles are
- Switch on the electric current and the compass needle moves.
- You have witnessed, at first hand, the link between electricity and magnetism!
An early view of the connection between electric currents and magnetism
The French scientist Andre-Marie Ampere considered that the phenomenon of magnetic fields being produced by electric currents is a fundamental feature of magnetism. In fact, he suggested that all magnetism, including that associated with permanent magnets, is due to electric currents. But how can electric currents be responsible for the behaviour of permanent magnets, when there seems to be no electric current?
Ampere's reasoning went something like this:
A permanent magnetic effect in a piece of iron or steel is due to the circulation of electric charge (imagine the motion of electrons around the nucleus of each atom). This moving charge constitutes a minute electric current and so all magnetic effects are ultimately due to electric currents.
The magnetic fields produced by electric currents
Physics Narrative for 11-14
Fields, current-carrying wires, current-carrying coils
A clue as to the shape of the field due to a single current-carrying wire: when a compass is placed above the wire and the electric current switched on, the needle deflects at right angles to the wire. When the compass is placed below the wire, and the electric current switched on, the needle deflects in the opposite direction.
In fact, the field around the wire is circular in shape; the needles of the plotting compasses form a continuous loop around the wire. These circles stretch out along the wire, forming cylinders.
If an electric current is passed through a long coil of wire (a
solenoid) a magnetic field is produced which is the same shape as that of a bar magnet.
How is it that the bar magnet field shape is produced around the solenoid?
We can answer this question by starting from what we already know, first remembering the field around the long straight wire and now imagining the wire is coiled up. Adding the field due to all of the coils of wire amounts to a field shape exactly the same as that of a bar magnet.
This level of detail in thinking about magnetic fields is not part of the 11–14 curriculum, but it is helpful to have these more advanced ideas in mind when teaching the principles of magnetic fields.
Electromagnets in everyday use
Why use electromagnets?
Why might we want to use an electromagnet rather than a permanent bar magnet? There are two obvious advantages to using electromagnets.
Firstly they can be switched on and off. Complete an electric circuit and a current passes to produce a magnetic field. Switch off the current and the magnetism disappears (provided the iron forms a temporary magnet).
Also, their magnetic strength can be changed. The strength of the magnetic field around the solenoid can be increased by:
- Increasing the number of coils (or turns) of wire.
- Increasing the electric current through the coil.
- Placing a magnetic material inside the solenoid coil.
Car scrap-yards use huge electromagnets to lift heaps of crumpled iron and steel. Switch off the current and the object crashes to the ground.
In the home, by far the most common use of electromagnets is in electric motors. Think of all of those bits of electrical equipment with some kind of electric motor: vacuum cleaners, refrigerators, washing machines, tumble driers, food blenders, fan ovens, microwaves, dish-washers, hair driers.
The list is a long one, and when you start thinking more widely about electric motors in cars, lawn-mowers and a whole host of industrial applications, it becomes obvious that this application of electromagnets is extensive and extremely important to our daily lives. The question of how electric motors work builds on the basics of magnetism introduced here, and is usually worked on in later years.
Bells, relays and motors
Electromagnetic door bells are
make and break devices which work via an electromagnet. There is one electric circuit containing two switches. One is a conventional push-button switch. The second has two parts, a spring and an electromagnet. The alternating action of the spring and the electromagnet makes and breaks the circuit for as long as the push-button switch is pressed.
An electromagnetic relay consists of two circuits. The first circuit contains a simple electromagnet which requires a relatively small current to make it work. When the switch is closed, there is an electric current through the coil of wire and the iron rocker arm is attracted to the electromagnet. The arm rotates about the pivot and closes a switch to complete the second circuit and the motor starts up – the motor requires a much larger current. When the switch in the first circuit is opened the electromagnet releases the rocker arm and the switch springs open again. The motor circuit is now broken.
The motor is more complex than either, but included here for completeness.
Electromagnets and what can be done with them
The interactions of magnets serve as a template for the interactions of electromagnets. The similarity of the field around a bar magnet to the field around a coil of wire in which there is an electric current is one building block. The details of the interactions of fields are dealt with in the SPT: Electricity and energy topic, so a rather functional approach to the electromagnetic devices is probably sufficient here.