6.1.4 Core Practical: Investigating Magnetic Fields · 6. Magnetism and Electromagnetism | Edexcel International GCSE Science Double Award 4SD0: Physics

Section 1 of 9Aim and apparatus

The Edexcel core practical asks you to investigate the magnetic field pattern of a bar magnet and of combinations of magnets, and to show how the field changes with distance.

Aim and apparatus

The apparatus needed: a bar magnet (or two), a plotting compass, sheets of plain paper, a pencil, a ruler, and iron filings in a shaker (optional).

For field strength measurements an alternative is to use a second magnet and measure the force,

or to use a small piece of magnetic material such as a steel paper clip hung from a newton meter.

Section 2 of 9Method for plotting field lines

1

Place the bar magnet in the centre of a sheet of paper and draw round it. Mark the poles.

2

Place the plotting compass so that its needle is next to the north pole. Mark two dots at the ends of the needle.

3

Move the compass so that the tail of the needle sits on the head dot. Mark a new dot at the head of the needle.

4

Continue until the line reaches the south pole (or the edge of the paper),

then join the dots with a smooth curve and add an arrow in the direction the north end of the needle pointed.

5

Start again from a different point on the pole to draw 8–10 lines.

6

Repeat with two magnets (N facing S, and N facing N).

Section 3 of 9Method with iron filings

Place a sheet of paper or transparent plastic over the magnet.

Method with iron filings

Sprinkle iron filings thinly and evenly from a height, tapping the paper gently.

Each filing becomes an induced magnet and lines up along the field, forming a pattern of lines that cluster near the poles.

The filings give a quick overall picture but do not show the direction; a compass is needed for that.

Section 4 of 9Investigating strength and distance

To investigate how the field strength changes with distance, fix a bar magnet on a bench with one pole pointing at a compass.

1

The Earth's field acts as a reference: place the compass at a measured distance d from the magnet,

and notice how far the needle is deflected from its normal north-south direction.

2

A larger deflection means a stronger field from the magnet.

3

Record the deflection for distances such as 5, 10, 15, 20 and 25 cm and plot the result.

4

The field falls rapidly with distance, so the deflection decreases quickly.

Section 5 of 9Accuracy, safety and improvements

1

Keep magnets and compasses away from iron or steel objects, other magnets and mobile phones, which distort results.

2

Keep the compass small and in the same orientation; mark dots precisely and at the needle ends.

3

Take repeated readings and calculate means.

4

Do not drop magnets, as they can lose strength; wash hands after using iron filings and do not blow them into the air; wear eye protection.

Section 6 of 9Recording and presenting results

Draw the field lines in pencil on a copy of the paper with the magnet outline clearly shown.

1

Label the poles and add arrows.

2

When comparing the two arrangements (N facing S and N facing N), note where the lines are most crowded and where there is a neutral point.

3

For the distance investigation, tabulate distance d in cm against the deflection in degrees, repeat each reading three times and calculate the mean.

4

A graph of the mean deflection against distance should be a smooth curve decreasing towards zero; do not join the points with straight segments.

Evaluate the experiment

sources of error include the Earth's magnetic field acting on the compass (which is why the starting needle direction should be noted), the thickness of the compass, parallax in reading the angle and nearby iron objects.
Using a larger sample of distances and a protractor with finer divisions improves the precision.

Section 7 of 9Worked examples

A compass 5 cm from a magnet is deflected by 70°, 10 cm gives 35° and 20 cm gives 10°. What conclusion do you make?
Why is a compass needed in addition to iron filings when plotting a field?

Section 8 of 9Key terms

Term	Meaning
Plotting compass	Small compass used to find the direction of a field at a point.
Iron filings	Small iron fragments that line up along field lines.
Deflection	Angle through which the needle turns.
Repeat readings	Taking measurements again to reduce random error.

Section 9 of 9Exam technique

1

Starting the new compass position anywhere other than at the previous dot.

2

Forgetting the arrows to show direction.

3

Placing steel items or other magnets near the apparatus.

4

Thinking that the field lines on the paper are physical lines.

Core Practical: Investigating Magnetic Fields