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Understand Uses of Electromagnetism

Worksheet Overview

QUESTION 1 of 10

Electromagnets are more useful than permanent magnets because they can be switched on and off; the strength of the magnetic field can be altered; they can easily be given the desired shape; and they are cheaper to make. They are also much cooler, allowing us to make floating trains and other sci-fi stuff like that! You are living in the future now!

 

General uses

Huge electromagnets are used to lift cars and other heavy metal objects. The objects are dropped to the desired location by switching off the electric current and subsequently the magnet.

Electric motors (see image below) use the motor effect to spin a coil of wire inside a magnetic field. The speed of the motor can be changed by altering the strength of the magnet, adding more loops and changing the electric current. The direction of the motor can be changed by altering the direction of the current.

 

An image of a motor

 

In loudspeakers (see diagram below) an alternating current that flows through the coil represents the sound wave. The force produced makes the coil move, which pulls the cone in one direction. As the direction of the current changes, the direction of the force also changes and the cone is pulled the opposite way. The forward and backward motion of the cone produces a sound wave in the air.

The pitch and loudness of the sound depend on the frequency and amplitude of the electrical signal, which affects the size of the force. 

 

An image of a transducer powering speakers.

 

Relays work by using a small current circuit to activate an electromagnet -  this pushes the soft iron armature which brings the contacts of the large current circuit together, thus creating the output effect.

 

Generating electricity

Electricity can be generated in two ways:

 

1) Moving a wire in a magnetic field. The direction and speed of the wire determines the direction and size of the current, respectively.

2) Moving a magnet in a coil of wire. The direction and position of the poles determine the direction of the current. The speed of the magnet determines the size of the current.

 

Industrial processes of generating electricity involve spinning a coil of wire in a magnetic field. The current produced is called induced current. During each rotation, each side of the coil moves up and down. The change in direction results in the change of the current's direction, so the current is called an alternating current (AC).

Transformers are used in the National Grid to change the voltage of alternating current to reduce energy waste and to produce current safe for use at home. The magnetic field around the alternating current in the primary coil changes, which induces an alternating current and voltage in the secondary coil. The number of loops in the coil determines the size of the voltage (see diagram).

 

An image of a step-down transformer

Step-up and step-down wave-forms for AC current.

 

The size of the output voltage can be calculated with the formula in the diagram above.

 

Phew - that's a lot of information to take in! Are you ready to put some of this to the test in a set of questions? Good luck!

Tick any reasons why electromagnets are more useful than permanent magnets.

They cannot be switched off

They can be switched on and off

They have a fixed shape

They can be shaped according to the purpose of their use

Their magnetic field is fixed

They are expensive

Why are electromagnets used to lift heavy objects?

The object stays fixed to the magnet

The objects are heavy and only electromagnets can lift them

The objects can be dropped to the desired location when the electromagnet is switched off

How can the speed of an electric motor be increased?

Use a weaker magnet

Use a stronger magnet

Add loops to the coil of wire

Reduce the number of loops in the wire

In a loudspeaker, what does the alternating current represent?

A sound wave

A magnetic force

A magnetic field

How is a sound wave produced in loudspeakers?

By the backward motion of the cone

By the forward motion of the cone

By the backward and forward motion of the cone

What is the purpose of an electromagnet in relays?

When it is activated, it brings together the contacts in the small circuit

When it is activated, it brings together the contacts in the large circuit

It creates a magnetic field

How can electricity be generated.

By moving a wire in a magnetic field

By moving a magnet in a magnetic field

By moving a magnet in a coil of wire

By moving a transformer in a relay

What is the current generated in the electricity industry called?

Study the diagram about the direction of induced current:

 

RIght Hand Rule for Induced Current

A conductor in motion through a magnetic field will generate an induced current in the conductor.

Example: AC generator.


Felming right hand law

 

LeFt Hand Rule for Force

A conductor carrying an electric current in a magnetic field will move because the magnetic field of the magnet will interact with the magnetic field of the current in the wire.

Example: electric motor.

 

Fleming left hand law

 

Tick the option that states the correct relationship between the magnetic field and the induced current.

The induced current flows in the opposite direction to the magnetic field

The induced current flows at a right angle to the direction of the magnetic field

The direction of the magnetic field is shown by the second finger

What determines the size of the voltage in a transformer?

Alternating current

Primary current

Number of loops

  • Question 1

Tick any reasons why electromagnets are more useful than permanent magnets.

CORRECT ANSWER
They can be switched on and off
They can be shaped according to the purpose of their use
EDDIE SAYS
There were two correct options to tick here. Did you get them both? Electromagnets are more useful than permanent magnets because they can be switched on and off and they can easily be given the desired shape. Two other advantages to them, not mentioned above, are that the strength of the magnetic field can be altered and that they are cheaper to make. You've made a good start with this activity - let's move on to the next question.
  • Question 2

Why are electromagnets used to lift heavy objects?

CORRECT ANSWER
The objects can be dropped to the desired location when the electromagnet is switched off
EDDIE SAYS
The reason for lifting heavy objects is to move them to a specified place. It would be impossible to do that with a permanent magnet, as the objects would stay firmly attracted to it. With an electromagnet, magnetism can be switched on and off by switching the electric current on and off.
  • Question 3

How can the speed of an electric motor be increased?

CORRECT ANSWER
Use a stronger magnet
Add loops to the coil of wire
EDDIE SAYS
Two pairs of opposite answers here - did you select the correct one each time!! Both answers were to do with getting bigger in some way, which makes sense if you want to increase the speed. The speed of an electric motor increases when a stronger magnet is used, or when adding more loops to the coil of wire. This increases the artificial magnetic field in them making them more repulsive to one another. This, in turn, speeds everything up!
  • Question 4

In a loudspeaker, what does the alternating current represent?

CORRECT ANSWER
A sound wave
EDDIE SAYS
The alternating current in loudspeakers represents sound waves. If it were a direct current, then all you would get is one chirp when you turned it on and one chirp when you turned it off. Not a lot of use really!
  • Question 5

How is a sound wave produced in loudspeakers?

CORRECT ANSWER
By the backward and forward motion of the cone
EDDIE SAYS
How did you get on with this one? In loudspeakers, an alternating current that flows through the coil represents the sound wave. The force produced makes the coil move, which pulls the cone to one direction. As the direction of the current changes, the direction of the force also changes and the cone is pulled the opposite way. The forward and backward motion of the cone produces a sound wave in the air.
  • Question 6

What is the purpose of an electromagnet in relays?

CORRECT ANSWER
When it is activated, it brings together the contacts in the large circuit
EDDIE SAYS
Don't worry if you found this one a bit trickier - some of this topic is rather complicated. You can always go back to check out the Introduction if you're not sure about any part of it. When the electromagnet in the small relay circuit is activated, it brings together the contacts in the large circuit by pushing the soft iron armature and creates a magnetic field.
  • Question 7

How can electricity be generated.

CORRECT ANSWER
By moving a wire in a magnetic field
By moving a magnet in a coil of wire
EDDIE SAYS
Another tricky question this, but an important one to sort out, so a quick read of the Introduction might be a good idea if you found this hard. Electricity can be generated by moving a wire in a magnetic field or by moving a magnet in a coil of wire. This is done in every way of generating electricity apart from solar.
  • Question 8

What is the current generated in the electricity industry called?

CORRECT ANSWER
Induced
Induced current
EDDIE SAYS
Ouch, this was difficult wasn't it? Particularly since there was no multiple choice option to help you out. How did you do with it? Current produced in the electricity industry is induced current. It means that there is not really meant to be a current there, but we are making one by moving the coil of wires in the magnetic field.
  • Question 9

Study the diagram about the direction of induced current:

 

RIght Hand Rule for Induced Current

A conductor in motion through a magnetic field will generate an induced current in the conductor.

Example: AC generator.


Felming right hand law

 

LeFt Hand Rule for Force

A conductor carrying an electric current in a magnetic field will move because the magnetic field of the magnet will interact with the magnetic field of the current in the wire.

Example: electric motor.

 

Fleming left hand law

 

Tick the option that states the correct relationship between the magnetic field and the induced current.

CORRECT ANSWER
The induced current flows at a right angle to the direction of the magnetic field
EDDIE SAYS
Did you have a go at holding your hands up to make these diagrams? It's a good plan to do this because it will really help you to picture what's going on. Fleming's rule for induced current and magnetic fields states that the induced current flows at a right angle to the direction of the magnetic field. Could you see that from the diagrams above?
  • Question 10

What determines the size of the voltage in a transformer?

CORRECT ANSWER
Number of loops
EDDIE SAYS
The last question - phew! The number of loops in the coil determines the size of the voltage. If there are more coils on the secondary coil, it means it is a step-up transformer and fewer coils means that it is a step-down transformer. Well done for getting to the end of this challenging activity - if you're still finding it rather confusing, it might be a good idea to go back and read the Introduction again to help you get to grips with the basics.
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