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A history of magnetism. Magnetic materials. gold (Au). cobalt (Co). iron (Fe). iron (Fe). nickel (Ni). cobalt (Co). nickel (Ni). copper (Cu). polythene. magnesium (Mg). polythene. copper (Cu). aluminium (Al). wood. gold (Au). aluminium (Al). magnesium (Mg). wood.
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Magnetic materials gold (Au) cobalt (Co) iron (Fe) iron (Fe) nickel (Ni) cobalt (Co) nickel (Ni) copper (Cu) polythene magnesium (Mg) polythene copper (Cu) aluminium (Al) wood gold (Au) aluminium (Al) magnesium (Mg) wood Which of the materials below aremagnetic?
Can magnetism be blocked? A fridge magnet may be coated in plastic but will stick to a metal fridge. Can magnetic forces act through any material? • Attach a paperclip to a piece of thread and tape the other end of the thread to a work bench. • Hold a magnet above the paperclip to check that the paperclip is attracted. • Try inserting thin sheets of different materials between the magnet and the paperclip. Note the effects of each material.
What is a magnetic field? magnetic field The region around a magnet where it has a magnetic effect is called itsmagnetic field. When a magnetic material is placed in a magnetic field it will experience a force. The iron filings feel the effect of the magnetic field and line up along the direction of the forces in this region.
Magnetic field lines The shape of a magnetic field can be shown by drawing magnetic field lines. These always point from the north pole of a magnet to the south pole, along the direction of the magnetic forces in each region. Where is the magnetic field strongest? strongest field at poles The closer together the magnetic field lines, the stronger the magnetic field. weakest field further away from poles
Viewing magnetic fields: N poles together Bring the north poles of two bar magnets together. What happens to the magnets? Next, bring the two north poles as close to each other as possible and place a piece of paper on top of the magnets. • Carefully scatter iron filings onto the paper. • Draw the pattern created by the iron filings.
Magnetic field pattern: N poles together What do you notice about the pattern of the lines of force in the region between the two north poles?
Viewing magnetic fields: N and S poles together Bring the north and south poles of two bar magnets together. What happens to the magnets? Next, put the north and south poles close to each other, without letting them touch, and place a piece of paper on top. • Carefully scatter iron filings onto the paper. • Draw the pattern created by the iron filings.
Magnetic field pattern: N and S poles together What do you notice about the pattern of the lines of force in the region between the north and south poles? How does this pattern compare with the pattern between the two north poles?
The Earth’s magnetic field People have used magnetism to navigate for hundreds of years. The Earth’s iron core creates a magnetic field. The north poles of magnets, such as compass needles, are attracted to the North Pole of the Earth. But if like poles repel, why is the north pole of a magnet attracted to the North Pole of the Earth? These poles were named before magnetism was properly understood. The ‘Magnetic North Pole’ of the Earth is really a magnetic south pole. Do you think that you would be able to use a compass to navigate on other planets, or on the moon?
Making a magnet A magnet can be made by magnetizing a material that is attracted to a magnet, e.g. a paper clip. There are three methods that can be used to make a magnet: • Stroke a magnet along the paper clip from one end to the other and then, starting from the same place, repeat the movement. The more times this is done, the more magnetic the paper clip becomes. • Hold a nail in a magnetic field and hit it with a hammer. • Put a magnetic material in a strong magnetic field.
Investigating electromagnets The strength of an electromagnet depends on whether it has a core of iron. Two experiments can be carried out to investigate the other factors that can affect the strength of an electromagnet: • Investigate how the number of coilsaffects the number of drawing pins attracted to an electromagnet – keep the current the same in this experiment. • Investigate how the size of the current affects the number of drawing pins attracted to an electromagnet –keep the number of coils the same in this experiment.
Investigating electromagnets How could you use the apparatus below to investigate the effects of changing the current and the number of coils on the strength of an electromagnet?
Investigating electromagnets – results number of drawing pins attracted number of drawing pins attracted number of coils current (A) 0 0 0 0 20 8 1 12 40 18 2 23 60 31 3 38 80 46 4 49 Here are some example results.
Investigating electromagnets – analysis 50 40 30 number of drawing pins attracted 20 10 0 0 20 40 60 80 100 number of coils How did the number of coils affect the number of drawing pins attracted to the electromagnet?
Investigating electromagnets – analysis 60 50 40 30 number of drawing pins attracted 20 10 0 0 1 2 3 4 5 current (A) How did the size of the current affect the number of drawing pins attracted to the electromagnet?
Using electromagnets – recycling A large electromagnet is used on a recycling plant conveyor belt to pick up and move metal cans. Which metals would the electromagnet attract? What advantages does an electromagnet have over a permanent magnet?
Uses of electromagnets – electric bell The circuit for a door bell includes an electromagnet. When the circuit is closed, the electromagnet pulls the armature towards it, causing the hammer to strike the bell. The movement of the armature breaks the circuit, and the hammer returns to its original position. This sequence repeats, causing the bell to sound continuously.
Using electromagnets – the relay Lifts, cars and other large electrical machines use high currents. Relaysuse an electromagnet to allow a small current in one circuit to control a large current in another circuit. Can you predict what will happen when the switch is closed in this circuit?
