9.4 Magnetic fields can apply a force from a distance

9.4

Magnetic fields can apply a force from a distance

In this topic, you will learn that:

• a magnetic field is the area around a magnet where a magnetic force is experienced • a magnetic field cannot be seen, but we can see the way it interacts with other objects • the further away an object moves from the magnet, the weaker the field.

a

b

b

c

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

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 fi eld. 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 fi eld. There is a large magnetic fi eld around the Earth. A compass needle will line up with the Earth’s magnetic fi eld. 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 fi eld of the Earth and has done so for hundreds of years. This movement is caused by the Earth’s magnetic fi eld. The magnetic south pole does not always line up with the magnetic north pole.

Axis of rotation Magnetic north pole

Geographic North Pole The Earth’s magnetic field

S

N

Magnetic south pole 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 fastfl owing sea currents around the world. When it is ready, the turtle is able to detect the magnetic fi eld around the Earth. It can measure the direction of the magnetic fi eld (just like a compass) and how strong it is. All it needs to do is follow the magnetic fi eld back to exactly the same beach where it hatched. Once there, it will mate and lay eggs, completing the cycle of life once again.

Figure 3 What do magnets and turtles have in common? Magnets create a magnetic field, and turtles use the magnetic field to find their way back to the same beach where they hatched.

Flipping the magnetic poles

Throughout history, the magnetic north and south poles have flipped upside down every now and then. The last flip happened 780 000 years ago. The flip can take a few thousand years to complete. While this happens, the poles become very disordered and a magnetic north or south pole can appear anywhere. How will this affect the turtles being able to find their beach?

Figure 4 The Earth’s magnetic poles between reversals and during a reversal

Bankcards and magnets

You use magnetic fields in your own life. The black strip on the back of a bankcard has a series of small, magnetised zones separated by demagnetised zones. You can see these zones if you sprinkle fine iron filings on them. The iron filings arrange themselves according to the magnetic field surrounding the magnetic zones, which look a bit like a bar code. When the card is swiped through a card reader, the magnetic bar code is read and the person’s name, bank and account number are decoded.

The information on the black strip on a bankcard can be changed if it is put next to a strong magnet. This includes the magnetic clasps on a purse, or wallet. Some stores also attach magnetic security devices to their stock to protect against theft. They remove these using a demagnetiser near the cash register. Leaving a bankcard on a store demagnetiser will also change the magnetic strips on the card.

Figure 5 The magnetic strip on a bankcard contains zones of magnetised and demagnetised areas.

9.4 Check your learning

Remember and understand

1 Describe a magnetic field. 2 Explain how you could map the magnetic field around a magnetic nail. 3 Describe in words the shape of the magnetic field when two magnets are: a attracting b repelling. 4 Describe how you could decide which magnet was stronger by looking at the magnetic fields made by different magnets. 5 Draw the magnetic field around a broken magnet: a that has been re-joined b with the two pieces 10 cm apart c with the two pieces 1 cm apart.

Apply and analyse

6 Explain how a compass works. 7 Explain why you should never leave a library card on the demagnetising panel of a shop.