4PH1

Magnetism & Electromagnetism

Magnetism & Electromagnetism · 1 question type

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 9% of your exam marks.

stable

Medium

Stable9%

Magnetic field patterns, the motor effect and Fleming's Left-Hand Rule tested in most series.

Use a small plotting compass to map out the magnetic field around (a) a single bar magnet and (b) a pair of bar magnets, and produce labelled diagrams of the field-line patterns

Equipment	Purpose
Two bar magnets	Provide the fields to be mapped
Plotting compass (small compass on a clear plastic base)	Reads the direction of the field at each point
Sheet of plain A3 paper	Surface to draw the field lines on
Sharp pencil and ruler	Marks accurate dots and joins them into smooth curves

Lay the bar magnet flat on the centre of the paper. Draw round its outline with the pencil so you can replace it accurately if it gets knocked
Place the plotting compass near one pole of the magnet (say the N pole) with one end of the needle touching the magnet
The compass needle aligns with the field at that point. Pencil a small dot at the far end of the needle (the end pointing away from the magnet)
Slide the compass forwards so the other end of the needle now sits on the dot you just drew. The needle realigns. Pencil a fresh dot at the far end again
Continue this leapfrog motion until you have a chain of dots that reaches all the way to the S pole of the magnet
Lift the compass off and join the dots smoothly with a pencil line; this is one field line. Add an arrowhead pointing from N to S
Repeat steps 2–6 starting from different points around the magnet so you build up about 6–8 field lines in total
Two-magnet variant: replace the single magnet with two bar magnets laid end-to-end with a 5 cm gap. Do the procedure first with opposite poles facing (you should get smooth lines crossing the gap) and again with like poles facing (you should see a neutral point and lines bending away)

Random error: read the compass needle from directly above so it isn't viewed at a slanted angle; let the needle settle for a couple of seconds before marking each dot; keep iron objects (scissors, pencils with steel ferrules) away from the apparatus so they don't deflect the compass
Systematic error: avoid using a magnetised steel pencil-tip near the compass; use a sharp graphite pencil for the markings only
Safety: this is a low-risk practical with no hot or electrical equipment; the only precaution is to keep the magnets away from pacemakers, credit cards and computer hard drives