How a compass works
Definition
Magnetic compass
A device containing a small magnetised needle, free to turn on a pivot, whose north-seeking pole lines up with the magnetic field it is in.
- A magnetic compass contains a small, lightweight magnetised needle balanced on a pivot so that it can turn freely.
- The needle is a tiny permanent magnet, so when it sits in a magnetic field it turns until it lines up with the field.
- The end of the needle that points along the field is its north-seeking pole, usually painted red or marked with an arrow.
- A compass responds to the strongest field around it, so a nearby bar magnet or steel object can pull the needle away from its normal direction.
- With no magnets close by, a compass anywhere on Earth still settles in one particular direction, with its north-seeking pole pointing roughly towards geographic north.
- A needle can only turn to a definite direction if a magnetic force acts on it, so this behaviour is the first piece of evidence that the Earth has its own magnetic field.
The Earth's magnetic field
Definition
Earth's magnetic field
The magnetic field surrounding the Earth, whose pattern is similar to the field of a giant bar magnet inside the planet.
- The pattern of the Earth's magnetic field is very similar to the pattern around a bar magnet, as if a giant bar magnet were buried inside the planet.
- The field lines come out of the ground in the Southern Hemisphere, loop around the planet and go back into the ground in the Northern Hemisphere.
- The north-seeking pole of a compass is pulled towards the Arctic, and only unlike poles attract, so the magnetic pole near geographic north must be a magnetic south pole.
- In the same way, the magnetic pole near geographic south is a magnetic north pole.
- The Earth's magnetic poles lie close to the geographic poles but not exactly on them, so a compass points slightly away from true north.
- The Earth's field is weak compared with the field near a school bar magnet, which is why a compass is so easily pulled off course by a magnet on the desk.
Evidence from a dipping compass
Definition
Dipping compass
A compass with a magnetised needle pivoted to turn in a vertical plane, so that it shows how steeply a magnetic field points into or out of the ground.
Definition
Angle of dip
The angle between the direction of the Earth's magnetic field and the horizontal at a particular place.
- An ordinary compass only turns horizontally, so it shows the direction of the field across the ground but not whether the field also points up or down.
- A dipping compass has its needle mounted on a horizontal axle so that it can swing up and down in a vertical plane.
- When the dipping compass is turned to face along the magnetic north to south direction, its needle lines up with the Earth's field in three dimensions and shows how steeply the field enters or leaves the ground.
- Near the magnetic equator, the needle lies almost horizontal, with an angle of dip close to 0∘0^\circ0∘.
- Moving towards either magnetic pole, the needle tilts more and more steeply.
- In the UK, the north-seeking end of the needle dips down towards the ground at roughly 66∘66^\circ66∘ to the horizontal.
- Close to a magnetic pole, the needle points almost vertically, with an angle of dip close to 90∘90^\circ90∘.
- A dipping needle moved along a bar magnet behaves in exactly the same way, lying flat beside the middle of the magnet and tilting steeply near each pole.
- The Earth's field lines therefore run parallel to the surface near the equator and plunge steeply into or out of the ground near the poles, which shows that they come from deep inside the Earth rather than from anything on the surface.
- A field with this shape needs a large magnetic source at the centre of the planet, so the behaviour of compasses is evidence that the core of the Earth must be magnetic.
- The core is made mostly of iron and nickel, which are both magnetic materials, so this conclusion fits with what the core is made of.
Practical
Modelling the Earth's field with a dipping needle
- Aim: to show how the angle of a dipping needle changes along a bar magnet, and to compare the pattern with dipping compass readings across the Earth.
- Apparatus: a bar magnet, a small dipping needle on a stand (or a magnetised sewing needle hung by its centre from a thread), a sheet of stiff card, a protractor, a ruler, and a polystyrene ball with a slot cut through its centre.
- Method:
- Lay the bar magnet flat on the bench, cover it with the card and mark the positions of the magnet's middle and its two poles on the card.
- Hold the dipping needle a few millimetres above the card over the middle of the magnet, with its axle at right angles to the magnet so that the needle swings in the plane along the magnet's length.
- Let the needle settle, then use the protractor to measure the angle between the needle and the horizontal card.
- Move the needle in steps of 2 cm2\ \text{cm}2 cm towards one pole, measuring the angle at each position, and then do the same towards the other pole.
- Record each angle against its distance from the middle in a table, noting which end of the needle points downwards.
- Push the magnet into the slot in the polystyrene ball to make a model Earth, and repeat the readings at its equator and at points moving towards each end.
- Results: the needle lies nearly flat over the middle of the magnet, tilts more steeply towards either end and stands almost upright over each pole, with the downward-pointing end swapping over at the centre.
- Conclusion: the pattern matches real dipping compass readings from the equator to the poles, which supports the idea that the Earth's field comes from a magnetic core.
- Watch out: other magnets and steel objects nearby will turn the needle, and a needle that is still swinging gives a false angle, so wait until it is completely still.
- Safety: a magnetised sewing needle is sharp, so handle it by its thread.
Example
Interpreting dip angles around the world
- Dipping compass readings are taken at three places: 3∘3^\circ3∘ near the magnetic equator, 66∘66^\circ66∘ in the UK and 88∘88^\circ88∘ close to a magnetic pole.
- The angle increases steadily as the readings move from the equator towards the pole.
- A small angle means the field runs almost parallel to the ground, and a large angle means the field plunges steeply into the ground.
- This is the same change seen when a dipping needle is moved from the middle of a bar magnet to one of its poles.
- The readings therefore fit a field produced by a large magnet deep inside the Earth, which is evidence that the core is magnetic.
Example
Naming the Earth's magnetic poles
- The north-seeking pole of a compass needle points towards the Arctic.
- A north pole is only attracted to a south pole, because unlike poles attract.
- The magnetic pole in the Arctic is therefore a magnetic south pole, even though it lies near the geographic North Pole.
- The Earth's field lines point into the ground in the Arctic, which is why the north-seeking end of a dipping needle tips downwards in the Northern Hemisphere.
Exam technique
Explaining the evidence for a magnetic core
- Begin with the observation, stating that a compass needle lines up in a fixed direction and that a dipping needle tilts more steeply nearer the poles.
- Explain the observation by saying that the needle is a magnet that lines up with the Earth's magnetic field.
- Finish with the conclusion, stating that the field has the same pattern as a bar magnet's field, so the core of the Earth must be magnetic.
- An answer that only says a compass points north usually loses marks, because it gives no reason why this counts as evidence.
Common Mistake
- Do not say that a compass points north because of gravity, because gravity pulls the needle downwards and cannot turn it to face north.
- Do not call the magnetic pole in the Arctic a magnetic north pole, because it attracts the north-seeking end of a compass and so behaves as a south pole.
- Do not confuse an ordinary compass with a dipping compass, because only a dipping compass shows the vertical tilt of the field.
Self review
- Why does a compass needle line up with a magnetic field?
- What does a dipping compass show that an ordinary compass cannot?
- Describe how the angle of dip changes from the magnetic equator to a magnetic pole.
- What type of magnetic pole lies near the geographic North Pole?
- Explain how dipping compass readings provide evidence that the Earth's core is magnetic.
