9.4
Magnetic fields can apply a force from a distance In this topic, you will learn that:
• the further away an object moves from the magnet, the weaker the field.
How compasses work A compass needle is a weak magnet. When a compass is placed near a strong magnet, the compass needle points in the direction of the field. You can see this by moving a compass around the sides and ends of a bar magnet. The north pole of a compass always points to the south pole of a magnet. Iron fi lings and iron powder are tiny bits of iron. If you put them near a strong magnet, they become temporary magnets. They line up like tiny compass needles around the strong magnet. You can draw this pattern and make a map of the magnetic field. There is a large magnetic field around the Earth. A compass needle will line up with the Earth’s magnetic field. The part of the compass needle with the ‘N’ on it points to the north magnetic pole of the Earth. It is important to note that the ‘geographic’ North Pole of the Earth is not the same as the magnetic north pole. They are both in the Arctic Circle but hundreds of kilometres apart. The North Pole, also known as the geographic North or true North Pole, is the northernmost point of Earth. If you tunnelled through the Earth from the North Pole in a straight line, you would come out the other side at the South Pole. The magnetic north pole is quite different. The magnetic north pole is not a fi xed point – it moves about according to the magnetic field of the Earth and has done so for hundreds of years. This movement is caused by the Earth’s magnetic field. The magnetic south pole does not always line up with the magnetic north pole.
c
D
Figure 1 Magnetic fields: a around a single bar magnet; b between two attracting bar magnets; c between two repelling magnets magnetic pole (here) each of the points near the extremities of the axis of rotation of the Earth or another body where a magnetic needle dips vertically
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Axis of rotation
Magnetic north pole
Geographic North Pole
AF T
a
b b
c
• a magnetic field cannot be seen, but we can see the way it interacts with other objects
R
a
• a magnetic field is the area around a magnet where a magnetic force is experienced
Magnetic south pole
The Earth’s magnetic field
S
N
Geographic South Pole
Figure 2 The Earth’s geographic poles are not in the same place as its magnetic poles.
How turtles use the Earth’s magnetic field When a turtle hatches, it crawls down the beach to the water and swims out to the brightest light on the horizon, which is usually the Moon. For the next 30 years, it will swim in the fastflowing sea currents around the world. When it is ready, the turtle is able to detect the magnetic field around the Earth. It can measure the direction of the magnetic field (just like a compass) and how strong it is. All it needs to do is follow the magnetic field back to exactly the same beach where it hatched. Once there, it will mate and lay eggs, completing the cycle of life once again.
OXFORD UNIVERSITY PRESS OXFORD SCIENCE 7: VICTORIAN CURRICULUM No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means.
