Topic 6: Magnetism and Electromagnetism

Lessons:

Syllabus Statements:

recall that magnets repel and attract other magnets and attract magnetic substances (6.2 )
recall the properties of magnetically hard and soft materials (6.3 )
understand the term ‘magnetic field line' (6.4 )
understand that magnetism is induced in some materials when they are placed in a magnetic field ) (6.5 )
sketch and recognise the magnetic field pattern for a permanent bar magnet and that between two bar magnets (6.6 )
know how to use two permanent magnets to produce a uniform magnetic field pattern (6.7 )
recall that an electric current in a conductor produces a magnetic field round it (6.8 )
describe the construction of electromagnets (6.9 )
sketch and recognise magnetic field patterns for a straight wire a flat circular coil and a solenoid when each is carrying a current (6.10 )
appreciate that there is a force on a charged particle when it moves in a magnetic field as long as its motion is not parallel to the field (6.11 )
recall that a force is exerted on a current-carrying wire in a magnetic field and how this effect is applied in simple d.c. electric motors and loudspeakers (6.12 )
predict the direction of the resulting force when a wire carries a current perpendicular to a magnetic field (6.13 )
recall that the force on a current-carrying conductor in a magnetic field increases with the strength of the field and with the current (6.14 )
recall that a voltage is induced in a conductor when it moves through a magnetic field or when a magnetic field changes through a coil; also recall the factors which affect the size of the induced voltage (6.15 )
describe the generation of electricity by the rotation of a magnet within a coil of wire and of a coil of wire within a magnetic field; also describe the factors which affect the size of the induced voltage (6.16 )
recall the structure of a transformer, and understand that a transformer changes the size of an alternating voltage by having different numbers of turns on the input and output sides (6.17 )
explain the use of step-up and step-down transformers in the large-scale generation and transmission of electrical energy (6.18 )
recall and use the relationship between input (primary) and output (secondary) voltages and the turns ratio for a transformer (6.19 )
recall and use the relationship input power = output power: VPxIP = VSxIS for 100% efficiency (6.20 )

 

Magnetism and Magnetic Fields

Question:

What is a magnet? Why does it happen?

Because electricity and magnetism are the same thing (Electromagnetism!) We need to understand both of them a little more before we can link them together.

Task:

Experiment to plot the magnetic field of a bar magnet – Easy.

We have plotted a picture of the magnetic field of a bar magnet. But can you explain what a magnetic field is, and what the lines represent?

A field is an area within which an object with a particular property experiences a force.

  • In a gravitational field an object with mass experiences a force (gravity).
  • In a magnetic field an object with magnetism experiences a force (magnetism).
  • In an electric field an object with charge experiences a force (electric force)

The field lines represent the direction that the force would act on a magnetized object if it were placed in the field at that point. The density of the field lines (how close together they are) represents how strong the field is.

Task:

Now use a Hall Probe to determine the magnetic field strength at different places around your bar magnet. Do the results of this measurement agree with the diagram above? Justify your answer.

Question:

Why do like poles attract, and unlike poles repel?

How do these field diagrams explain the phenomenon?

How can you arrange two permanent magnets to get a 'uniform' magnetic field?

So why are some materials magnetic and others not?

In each atom negatively charged electrons orbit a nucleus. The movement of charge brings about an electric current. As we will see next lesson all electric currents have a magnetic effect.

In magnets all the atoms are lined up in the same way so that the magnetic effect of each on adds up, and an overall magnetic effect is created.

In non-magnets the atoms are not lined up, but orientated at random, so that their magnetic effects effectively cancel each other out.

But we can also make make some materials into magnets

All we have to do is align the atoms. This is possible in magnetic materials, but not in non-magnetic materials .

By stroking a magnetic material such as steel with one end of a magnet it is possible to turn that material into a magnet. What you are doing is using the magnetic field of the magnet to push all of the atoms in the steel pin into line, and thus turning the steel pin into a magnet.

Task:

Do it! Then verify that it is done using the Hall probe to measure the magnetic field strength of one of the poles of your magnet.

Questions:

  • How does the field strength change as you stroke the steel pin more with the magnet?
  • Why do you think this is?
  • How can you demagnetise something?

Extension: Different Types of Magnetic Materials

 

Physics for You pp284-291, questions pp291

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Magnetic Fields due to Electric Currents- Electromagnetism

Electricity and magnetism are really consequences of the same thing...

The movement of charge.

If charges move then we have an electrical current, BUT we also get a magnetic field.

Demonstrations of magnetic field near a current carrying wire.

1) A straight wire.

2) A soleniod.

The Earth has a magnetic field...

Question:

Where is the movement of charge that can make the Earth a magnet?

Experiment:

Design, carry out and analyse an experiment to investigate the strength of an electromagnet. You may use the following apparatus...

thin green electrical wire

digital voltmeter

Hall probe

plotting compasses

low voltage powerpacks (12V)

steel paper clips

permanent bar magnet

 

digital ammeter

small rods of different materials

clamp stands

 

Consider the following:

  • What are the variables? Which will be your independent and dependent variables? Which ones will you control? How?
  • How will you carry out enough measurements to look for a trend? How will you record your results?
  • How will you make sure your experiment is safe?

You should write up your experiment as a lab report, so as to meet the criteria in the KS4 rubric. You will mark each others experiments next lesson.

Task:

Uses of electromagnets. Re-arrange the sentences for the following devices to explain how they work.

Physics for You pp286-289

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Electric Currents in Magnetic Fields- The Electric Motor

Electricity can cause magnetism and magnetism can cause electricity- The two things interact.

Demonstration

Conducting metal pin in a magnetic field.

You will notice that the direction the pin moves, the direction of the current and the direction of the magnetic field lines are all at right angles. This is Flemmings left hand rule , which we use to predict the effects of electromagnetic interactions.

Experiment:

You can now use this rule to predict and test the direction of the magnetic field causing current carrying aluminium foil to levitate...

The Electric Motor

So using this rule, you should be able to explain the steps of operation of the electric motor...

Task:

You must use the equipment and instruction sheet to construct your own working motor and complete the worksheet. Careful, it's fiddly!

Can you explain how it works? Another good applet

Physics for You pp286-293, questions pp294-295

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Electromagnetic Induction- Generators

Possibly the best thing about electromangetism is the ability it gives us to generate electricity, and transfer it from one place to another efficiently.

This relys on alternating current , which is the type of electric current we get from generators.

So far we have looked at how we use the interaction between electricity and magnetism to get movement. But we can also use the interaction between movement and magnetism to get electricity.

To do this we need either movement of a conductor through a magnetic field, or movement of a magnetic field through a conductor...

Either way field lines are 'cut' and electric current is produced...

Demo AC on CRO

Task:

Complete the worksheet to consolidate your understanding of generators

Physics for You pp296-301

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Transformers

In the DC transformer a bulb lights when current switched on or off. This is because a changing magnetic field causes an electric current to flow- magnetic field lines 'cut' a conducting wire.

The same demo with AC gives us a constant current. This means there must be a constantly changing magnetic field.

How can we make the induced voltage bigger?

More turns of wire

Transformers are devices that do this for us, to create useable voltages for our electronic devices.

Tasks:

Complete the paragraph...

A step up transformer has more turns on its secondary coil than on its primary. This means the voltage increases.A step down transformer has...

Draw transformer from pp302 (PfY)

Complete the class experiment to determine the transformer equation

N Primary N Secondary V Primary V Secondary I Primary I Secondary
1200 300        
1200 600        
300 1800 2      

 

Notice the relationship between N and V...

What do you notice about the way V and I are related?

VPxIP = VSxIS

Extension Question:

How is VPxIP = VSxIS a consequence of energy conservation?

 

Task:

Work through the worksheet

Physics for You pp296-304, questions pp305

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anrophysics 2007