17.4 – Magnetic Fields | Physics with Mr Shone

Definition: Magnetic Field
Magnetic field is a regions around a magnet where a magnetic material will experience a force.

Seeing the shape of a magnetic field

There are two common ways to view the magnetic field shape directly:

1. Using Iron Filings

Iron shavings, or filings, are tiny pieces of iron. When allowed to fall near a magnet they will orientate themselves to the direction of the field showing the shape of the field when they land.

Iron filings can be sprinkled from a pepper pot like this:

Alternatively, in the physics lab you will find iron filings suspended in oil.

iron filings suspended in oil

If this is shaken and then placed on magnets the filings will show the field pattern as seen below:

VIDEO: iron filings in oil needs to be vigorously shaken first

iron filings showing magnetic field pattern around a single magnet

VIDEO: single magnet

magnetic field pattern around a pair of magnets
clearly showing field shape between unlike poles

VIDEO: like poles

magnetic field pattern around a pair of magnets
clearly showing field shape between like poles

VIDEO: unlike poles

2. Using Plotting Compasses

Recall a compass needle is in fact a tiny magnet, so placed in a magnet field (ie near a magnet) it will align with the magnetic field.

Plotting compasses are very small and weak compass needles that are used for determining the direction of a magnetic field at a point.

several plotting compasses showing the direction (shape) of the field around the north pole of a magnet

using a plotting compass to trace out magnetic field lines

Magnetic Field Lines

Magnetic Field Lines
Magnetic field lines or magnetic lines of force show the shape of a magnetic field around a magnet. They have the following properties: They attempt to form closed loops from pole to pole. They never cross one another as they are mutually repulsive and have lateral tension. Their density (field strength) decreases with increasing distance from the poles. The direction is from the north pole to south pole (outside the material)

Magnetic field lines or magnetic lines of force show the shape of a magnetic field around a magnet.

They have the following properties:

They attempt to form closed loops from pole to pole.
They never cross one another as they are mutually repulsive and have lateral tension.
Their density (field strength) decreases with increasing distance from the poles.
The direction is from the north pole to south pole (outside the material)

The following are some common magnetic field shapes that we should be familiar with:

magnet field lines around a bar magnet

magnetic field lines between two unlike poles

magnetic filed lines between two like poles

Between the two N poles in the above example is a null point (marked “X”). Here the two fields from the two magnets perfectly cancel out leaving no magnetic field at that point.

Example 1
A bar magnet has four plotting compasses placed next to it. In which direction would the plotting compasses point? Imagine the shape of the magnetic field around the magnet: All compasses will simply point in the direction of the magnetic field line that passes through them: Thus the four compass needles would look like this: Compare to a photograph of a real magnet and compasses:

Example 1

A bar magnet has four plotting compasses placed next to it.

In which direction would the plotting compasses point?

Imagine the shape of the magnetic field around the magnet:

All compasses will simply point in the direction of the magnetic field line that passes through them:

Thus the four compass needles would look like this:

Compare to a photograph of a real magnet and compasses:

Example 2
A small compass is aligned to the Earth’s magnetic field as shown: Next a powerful magnet, AB, is brought next to the compass (without moving it).(a) What is the polarity of end A of the magnet? It is a north pole as the north of the compass (red end of the needle) points away from it. (b) The powerful magnet is slowly moved far away from the compass. Why does the compass needle no longer point in the same direction? In this position the compass is far from AB so the magnetic force on the compass needle from AB will be weaker. It will also be experiencing a force from the Earth’s magnetic field. Thus the resultant of two forces acting on the compass needle will be somewhere between the top two diagrams.

Example 2

A small compass is aligned to the Earth’s magnetic field as shown:
Next a powerful magnet, AB, is brought next to the compass (without moving it).(a) What is the polarity of end A of the magnet?

It is a north pole as the north of the compass (red end of the needle) points away from it.

(b) The powerful magnet is slowly moved far away from the compass. Why does the compass needle no longer point in the same direction?

In this position the compass is far from AB so the magnetic force on the compass needle from AB will be weaker. It will also be experiencing a force from the Earth’s magnetic field. Thus the resultant of two forces acting on the compass needle will be somewhere between the top two diagrams.

Earth’s Magnetic Field
In most questions we will not be concerned by the Earth’s magnetic field. The left diagram shows the magnetic field around a strong bar magnet. The right diagram shows the Earth’s magnetic field (a uniform field). If we look at the resultant field produced by these two fields we get something complex like this: You can see that near the strong magnet the field is unchanged. Most questions will assume that magnetic fields are only due to the magnet shown (that we are close to the magnet and that it is considerably stronger than the Earth’s magnetic field at that point. https://youtu.be/HcFvegnQpPo

Earth’s Magnetic Field

In most questions we will not be concerned by the Earth’s magnetic field.

The left diagram shows the magnetic field around a strong bar magnet. The right diagram shows the Earth’s magnetic field (a uniform field).

If we look at the resultant field produced by these two fields we get something complex like this:

You can see that near the strong magnet the field is unchanged. Most questions will assume that magnetic fields are only due to the magnet shown (that we are close to the magnet and that it is considerably stronger than the Earth’s magnetic field at that point.

https://youtu.be/HcFvegnQpPo

Cause of Earth’s Magnetic Field
The Earth’s magnetic field occurs due to the motion of molten iron at the Earth’s core. However, the field produced were as if there were a very large bar magnet in the Earth as shown below. (Note this giant bar magnet is entirely imaginary – the magnetic field is very real.)

Cause of Earth’s Magnetic Field

The Earth’s magnetic field occurs due to the motion of molten iron at the Earth’s core. However, the field produced were as if there were a very large bar magnet in the Earth as shown below. (Note this giant bar magnet is entirely imaginary – the magnetic field is very real.)

Example 3

Why do magnetic objects fall down rather than be attracted to the Earth’s magnetic poles?

The gravitational force acting on the object will be much greater than the magnetic force acting on the object.

Caution
We must compare the forces acting on the object. Do not say that “the Earth’s magnetic field is weaker than its gravitational field”. Magnetic field strength and gravitational field strength are different physical quantities, with different units, so their numerical strengths cannot be directly compared.

Links
Magnetism Notes 17.4 – (Mr Shone Explains)
Drawing Field Lines (YouTube Video)

Seeing the shape of a magnetic field

1. Using Iron Filings

2. Using Plotting Compasses

Magnetic Field Lines

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