Magnetism
Senior 2 · Physics
Magnetism
Lesson 2 of 4
22 min
A compass works in the middle of a forest in Nyungwe with no signal and no batteries, because the whole Earth is surrounded by a magnetic field. Engineers who design motors, loudspeakers and MRI scanners in Kigali's hospitals think about magnetic fields every day. A field is invisible, but you can map it, and the map tells you both the direction and the strength of the magnetic force.
A magnetic field is the region around a magnet in which a magnetic force acts on magnetic materials and on other magnets. We draw it using magnetic field lines.
With iron filings. Lay a sheet of paper over a bar magnet, sprinkle iron filings thinly on it and tap the paper gently. Each filing becomes a tiny induced magnet and turns to lie along the field, so the filings trace out the pattern of the lines.
With a plotting compass. Place a small compass near the N pole and mark a dot at the tip of its needle. Move the compass so that the tail of the needle sits on that dot, and mark a new dot at the tip. Repeat until you reach the S pole, then join the dots and add arrows pointing from N to S. The compass method shows the direction of the field; the filings show the pattern more quickly but not the direction.
The Earth behaves as if it had a giant bar magnet inside it. A compass needle's N pole points towards the geographic North Pole, and unlike poles attract, so the magnetic pole near the geographic North Pole must be a magnetic south pole. The Earth's field lines leave the southern hemisphere, travel north over the surface and enter the ground in the far north. Near the poles the lines point steeply into or out of the ground; near the Equator, as in Rwanda, they are much closer to horizontal, which is where a compass works best.
The magnetic poles are not exactly at the geographic poles, so a compass does not point exactly to true north. The angle between magnetic north and true north is called the declination, and map-makers print it on their maps.
The strength of a magnetic field, B, is measured in tesla (T). One tesla is a very strong field, so small fields use the millitesla (1 mT = 10⁻³ T) and the microtesla (1 μT = 10⁻⁶ T).
| Source | Typical field |
|---|---|
| Earth's field at the surface | 25 to 65 μT (about 50 μT is a useful round figure) |
| Fridge magnet, near its surface | about 5 mT |
| Strong neodymium magnet, at its surface | about 1 T |
| Hospital MRI scanner | 1.5 to 3 T |
Worked example. How many times stronger is a 5.0 mT fridge-magnet field than a 50 μT Earth field?
Put both in tesla: 5.0 mT = 5.0 × 10⁻³ T; 50 μT = 50 × 10⁻⁶ T = 5.0 × 10⁻⁵ T.
Ratio = (5.0 × 10⁻³) ÷ (5.0 × 10⁻⁵) = 100 times.
Open Magnet and Compass. Drag the compass slowly around the bar magnet and watch which way its needle points: that is the field direction at each place. The small needles in the background show the field everywhere at once; notice they are brightest (strongest) near the poles. Click Flip Polarity and watch every needle turn round.
Tip
Three field-line rules for every diagram: (1) arrows go N → S outside the magnet; (2) lines never cross; (3) crowded lines = strong field. Examiners take marks for missing arrows and for lines that touch or cross.
Earth's trick: the compass's N pole points north, so the Earth's magnetic pole in the north is a south pole.
State three properties of magnetic field lines.
Solution: They point from N to S outside a magnet; they never cross; they are closer together where the field is stronger. (Also: they form closed loops.)
Two bar magnets lie in a straight line with their N poles facing each other, 10 cm apart, and the magnets are identical. Where is the neutral point, and why?
Solution: Midway between them, 5 cm from each N pole. There the two equal fields point in opposite directions and cancel.
Express the Earth's field of 50 μT in tesla, in standard form.
Solution: 50 μT = 50 × 10⁻⁶ T = 5.0 × 10⁻⁵ T.
An MRI scanner has a field of 3.0 T. How many times stronger is it than a 50 μT Earth field?
Solution: 3.0 ÷ (5.0 × 10⁻⁵) = 60 000 times.
Why is the Earth's magnetic pole in the Arctic called a south pole?
Solution: The N pole of a compass needle points towards it. Unlike poles attract, so the pole it is attracted to must be a magnetic south pole.
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