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TL;DR Two experiments, one theme: electromagnetism in action. In the electromagnet experiment, wind insulated wire around a soft-iron nail and show that more turns or more current picks up more paperclips. In the motor effect demo, place a current-carrying conductor between magnet poles and watch it jump - use Fleming's left-hand rule (thuMb = Motion, First finger = Field, seCond finger = Current) to predict which way. Both setups test the same underlying physics: a current produces a magnetic field, and a magnetic field exerts a force on a current.
For marking priorities and examiner expectations, pair this walkthrough with the Paper 3 Marking Guide.
1 | Two Experiments, One Theme
The O-Level Physics 6091 syllabus covers electromagnetism across two practical contexts:
Electromagnet experiment - investigate the factors that affect the strength of an electromagnet (number of turns, current, core material).
Motor effect demonstration - observe the force on a current-carrying conductor in a magnetic field, and use Fleming's left-hand rule to predict the direction of that force.
Both experiments connect to the same foundational idea: when a current flows through a conductor, it produces a magnetic field around it. In the electromagnet, that field magnetises a core. In the motor effect, that field interacts with an external field to produce a force.
2 | Experiment 1 - Factors Affecting the Strength of an Electromagnet
Apparatus
Item
Purpose
Iron nail or soft-iron rod (core)
Acts as the ferromagnetic core that the coil magnetises
Test objects to measure the picking-up strength of the electromagnet
Connecting wires
Completes the circuit
What you are investigating
The strength of an electromagnet depends on three factors:
Number of turns of wire wound around the core
Current flowing through the coil
Core material - soft iron produces a much stronger electromagnet than an air core or a steel core
The experiment typically tests one of the first two factors while keeping the others constant.
Method - varying the number of turns
Wind 10 turns of insulated copper wire around the iron nail. Connect the wire, ammeter, switch, and power supply in a series circuit.
Set the power supply so that the ammeter reads a fixed current (e.g. 1.0 A). This is your controlled variable.
Close the switch and bring the end of the nail close to a pile of paperclips. Count the number of paperclips picked up. Record this as trial 1.
Open the switch. Remove the paperclips from the nail (tap the nail gently if any cling due to residual magnetism). Repeat the pick-up test two more times (trial 2 and trial 3) at the same number of turns and current.
Increase the number of turns to 20, keeping the current the same. Repeat step 3 and step 4.
Continue for 30, 40, and 50 turns.
For each row, calculate the mean number of paperclips.
Method - varying the current
If you prefer to investigate current instead, keep the number of turns fixed (e.g. 30 turns) and vary the current using the variable power supply. Record the ammeter reading and the number of paperclips picked up at each setting.
Controlled experiment checklist
Independent variable: number of turns (or current)
Dependent variable: number of paperclips picked up
Controlled variables: current (or number of turns), same core, same type and size of paperclips, same end of the nail used for pick-up
Repeat each measurement at least three times and calculate the mean.
3 | Data Table Template - Electromagnet Experiment
Varying number of turns (current fixed at 1.0 A)
Number of turns
Trial 1 / paperclips
Trial 2 / paperclips
Trial 3 / paperclips
Mean / paperclips
10
20
30
40
50
Varying current (number of turns fixed at 30)
Current / A
Trial 1 / paperclips
Trial 2 / paperclips
Trial 3 / paperclips
Mean / paperclips
0.5
1.0
1.5
2.0
2.5
Expected results
More turns at the same current produces a stronger electromagnet (more paperclips).
Higher current at the same number of turns produces a stronger electromagnet.
A soft-iron core is much stronger than an air core because iron is ferromagnetic and is easily magnetised by the field of the solenoid.
4 | Experiment 2 - Motor Effect Demonstration
Apparatus
Item
Purpose
Horseshoe magnet or two slab magnets with a yoke
Provides a uniform magnetic field between the poles
Copper rod or stiff copper wire
The current-carrying conductor placed in the field
Aluminium cradle or rails
Supports the copper rod so it is free to move
Power supply (variable d.c.)
Provides current through the conductor
Switch
Controls the circuit
Ammeter
Monitors the current
Connecting wires
Completes the circuit
Method
Place the two magnets so that their poles face each other with a gap between them (N facing S). If using a horseshoe magnet, the field already runs from N to S across the gap.
Rest the copper rod on the cradle or rails so that it lies perpendicular to the magnetic field lines between the poles.
Connect the rod, ammeter, switch, and power supply in a series circuit.
Close the switch. The rod should roll or jump in a direction perpendicular to both the current and the field.
Note the direction of motion. Then use Fleming's left-hand rule to verify that the observed direction matches the prediction.
Reverse the current (swap the connections on the power supply). Observe that the rod now moves in the opposite direction.
Reverse the magnetic field instead (flip the magnets so that the poles swap). Observe the rod moves in the opposite direction again.
This confirms that the force depends on both the direction of the current and the direction of the magnetic field.
5 | Fleming's Left-Hand Rule
Fleming's left-hand rule predicts the direction of the force on a current-carrying conductor in a magnetic field. Hold your left hand with the thumb, first finger, and second finger all at right angles to each other:
ThuMb = direction of Motion (the force on the conductor)
First finger = direction of the magnetic Field (from N to S)
SeCond finger = direction of the Current (conventional current, from + to -)
How to apply it
Point your first finger in the direction of the magnetic field (N to S).
Point your second finger in the direction of the conventional current (+ to -).
Your thumb now points in the direction of the force on the conductor.
If only two of the three directions are known, the rule gives you the third. This is how exam questions typically test the concept - they give the field and current directions and ask for the force, or give the force and field and ask for the current direction.
Remember that it is the left hand - the right-hand rule is used for generators (the dynamo effect), not for motors.
6 | Factors Affecting the Force
The force on a current-carrying conductor in a magnetic field depends on three quantities:
Magnetic field strength - a stronger magnet produces a larger force.
Current - increasing the current increases the force.
Length of conductor in the field - a longer section of wire inside the field experiences a greater force.
For reference, the relationship is:
F=BIL
where:
F = force on the conductor (N)
B = magnetic flux density (T)
I = current (A)
L = length of conductor in the field (m)
This equation is not required at O-Level, but it is useful to understand why the three factors matter. The force is zero if the conductor is parallel to the field, and maximum when the conductor is perpendicular to the field.
7 | Applications of the Motor Effect
DC motor
A rectangular coil of wire is placed between the poles of a magnet. When current flows, each side of the coil experiences a force (one side up, the other side down, by Fleming's left-hand rule). This pair of forces creates a turning effect (torque) that rotates the coil. A split-ring commutator reverses the current direction every half-turn so that the coil continues to rotate in the same direction.
Loudspeaker
A coil of wire is attached to a paper cone and sits inside a permanent cylindrical magnet. When an alternating current (the audio signal) flows through the coil, the motor effect pushes the coil back and forth. The cone vibrates, producing sound waves.
Moving-coil galvanometer
A small coil is suspended between the poles of a magnet. When current passes through the coil, the motor effect causes it to rotate. A spring provides a restoring force, so the coil deflects by an amount proportional to the current. A pointer attached to the coil moves across a scale.
8 | Sources of Error
Electromagnet experiment
Paperclips sticking together - one paperclip may hang from another, giving an overcount. Tap the nail gently after each trial to remove loosely held clips, and count only the clips directly attracted to the nail.
Residual magnetism in the core - if the core retains magnetism after the current is switched off, subsequent readings are higher than they should be. Use a soft-iron core (not steel), which demagnetises almost completely when the current stops.
Heating of the wire - at high currents, the coil wire heats up and its resistance increases, causing the actual current to drop below the set value. Keep the switch closed for the shortest time needed to take each reading.
Inconsistent winding - if the turns are loosely or unevenly wound, the field inside the solenoid is weaker. Keep the turns tight and closely packed.
Motor effect demonstration
Friction on the rails - if the copper rod does not move freely on the cradle, the force may not be large enough to produce visible motion. Use smooth, clean rails and a light copper rod.
Weak magnets - old or weak magnets may produce too small a force to see. Use strong ceramic or neodymium magnets placed as close together as possible.
Current too low - increase the current (within safe limits) to produce a more obvious force.
9 | Common Exam Mistakes
Confusing the motor effect with the dynamo effect. The motor effect is about a force on a current-carrying conductor in a field. The dynamo effect (electromagnetic induction) is about generating a voltage by moving a conductor through a field. They use different hand rules (left for motor, right for dynamo).
Getting Fleming's left-hand rule fingers wrong. The most common mix-up is swapping the first finger (field) and second finger (current). Use the mnemonic: thuMb = Motion, First = Field, seCond = Current.
Using the right hand instead of the left. Fleming's left-hand rule is for the motor effect. The right-hand rule is for generators. If the question is about a force on a current-carrying conductor, use the left hand.
Confusing soft iron and steel. Soft iron is easily magnetised and easily demagnetised - it is used for electromagnet cores, transformer cores, and relay armatures. Steel is hard to magnetise but retains its magnetism - it is used for permanent magnets. Using a steel core in the electromagnet experiment introduces residual magnetism that corrupts results.
Forgetting that reversing current OR field reverses the force. If only one of the two is reversed, the force direction flips. If both are reversed simultaneously, the force direction stays the same. Examiners test this frequently.
Omitting repeat readings. In the electromagnet experiment, always repeat the paperclip count at least three times and record the mean. A single trial is not reliable because the number of paperclips picked up varies from attempt to attempt.
Not identifying controlled variables. When varying the number of turns, state explicitly that the current, core material, and type of paperclips are kept constant. Marks are awarded for naming the controlled variables.
10 | Frequently Asked Questions
Why must the core be soft iron, not steel?
Soft iron is easily magnetised when the current is on and loses its magnetism almost immediately when the current is off. This makes the electromagnet controllable - it can be switched on and off. Steel retains its magnetism after the current stops, which means the electromagnet stays partially magnetised. In the experiment, residual magnetism from a steel core would carry over between trials and make later readings unreliable.
What happens if I increase both the number of turns and the current at the same time?
The electromagnet gets much stronger, but you cannot tell which factor caused the improvement. This is why a controlled experiment changes only one variable at a time. If both change together, it is an unfair test and the results are not valid for drawing conclusions about either factor individually.
Can Fleming's left-hand rule be used for an electron beam in a magnetic field?
Yes, but with a twist. Conventional current flows from positive to negative, which is the opposite direction to electron flow. When applying Fleming's left-hand rule to an electron beam, point the second finger (current) in the direction opposite to the electron's velocity. The thumb then gives the direction of the force on the electron.
Why does the copper rod move perpendicular to both the current and the field?
The force produced by the motor effect is always at right angles to both the current direction and the magnetic field direction. This is a fundamental property of the interaction between moving charges and magnetic fields. If the current ran parallel to the field, there would be no force at all. The maximum force occurs when the current is perpendicular to the field, and this is the arrangement used in the demonstration.
How is the motor effect different from the electromagnet experiment?
In the electromagnet experiment, a current creates a magnetic field around a coil and that field magnetises a core. There is no external magnetic field involved - the coil is the source of the field. In the motor effect, an external magnetic field is already present (from permanent magnets), and a current-carrying conductor placed in that field experiences a force. The electromagnet is about creating a field; the motor effect is about a force arising from the interaction of a current with an existing field.
