SITEMAP   School Physics Notes: Electricity-magnetism 9.2 Plotting magnetic fields

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Magnetism: 9.2 Magnetic fields - plotting them with a compass and drawing the field lines pattern

Doc Brown's Physics exam study revision notes

There are various sections to work through.

9.2 Magnetic fields - plotting them, drawing them

You can map out the magnet field around a bar magnet by using a small plotting compass.

A magnetic field is the region around a magnetic (magnetised) object where a force acts on another magnet or another magnetisable object/material.

represent compass plotting positions on the diagram below (N=>S).

The magnetic field lines should indicate that the 'magnetic flux' runs from the north pole to the south pole.

Inside the plotting compass a tiny bar magnet (compass needle) aligns itself so that its north pole is attracted to the south pole of the magnet it is near.

The same thing happens with a navigation compass, which aligns itself with the Earth's magnetic field.

Your plotting compass does the same thing when well away from the magnet - which shows the Earth creates a magnetic field - because the Earth has an iron core and this swirling mass of mainly molten iron generates a magnetic field.

Plotting! On a sheet of white paper, place a bar magnet in the middle and draw a rectangle around it, so you can always make sure it is in the same place when plotting the magnetic field.

You carefully select a starting point close to a pole and mark on the paper a dot by the end of the compass needle arrow closest to the compass (* →).

Then mark another dot at the other end of the compass needle arrow (→ *).

Then, move the plotting compass around, making a dot adjacent to each end of the compass needle as you move from one position to another.

Gradually a line a force can be drawn by joining up the points all the way round from one end to the other end of the magnet.

You just repeat the procedure from a different starting point from the north pole.

How to plot the magnetic field of a bar magnet

The results of plotting a magnetic field around a permanent bar magnet.

By convention, the lines of force run around from the north pole to the south pole.

The lines of force show which way a force would act on a north pole if it was placed at that point in the field - see how the compass needle aligns itself along the line of force.

The closer the lines of force are together, the stronger the magnetic field.

The lines of force are closer together nearest the poles - this means the strength of the magnetic field is strongest close to the poles.

The further you get away from a magnet, the weaker the magnetic field.

The strength of the magnetic field is called the magnetic flux density and is measured in teslas (T).

For on magnetic field density see Electromagnetism, solenoid coils, uses of electromagnets

B is the magnetic field lines diagram for the attraction of unlike poles

Diagrams A and B show the magnetic field lines around a permanent bar magnet.

How to plot this was described above with the lines of force running from north to south.

You can clearly show this pattern using iron filings.

You place a permanent bar magnet under a sheet of paper or plastic.

You then sprinkle iron filings randomly on top of the sheet.

Then, tap the sheet gently and repeatedly until the pattern of the magnetic field emerges.

The permanent magnet induces magnetism in the iron filings and converts them into temporary magnets which all line up - N - S - N - S - etc. along the lines of force of the magnetic flux.

Its quicker than doing the compass plot method, but you don't end up with a plot unless you photograph it!

Diagram C shows the magnetic field lines when unlike poles of two bar permanent bar magnets are brought close together and attracted.

When a N-S or S-N attraction happens the lines of force join up from one magnet to another in a uniform pattern and the lines of force still run from a north pole to a south pole.

D and E show the magnetic field lines diagram for the repulsion of like poles

Diagrams D and E show when like poles of two permanent magnets are brought close together and repulsion takes place.

In this case the lines of force do NOT join together from one magnet to the other, but they still run from a north pole to a south pole.

Keywords, phrases and learning objectives on magnetism

Be able to do, and describe with diagrams, the method of how to plot and draw a magnet field of a bar magnet using a plotting compass.

Be able to describe and sketch magnetic fields when like poles are brought close together (repulsion) and unlike poles are brought close together (attraction).

All the field line diagrams should be clear and dealing with the magnetic fields of permanent bar magnets.

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