IP Physics Notes (Upper Secondary, Year 3-4): 15) Electromagnetism

Quick recap -- Electric currents create magnetic fields, and those fields can push on other currents or moving charges. Use the right-hand grip to sketch fields and Fleming's left hand to predict the resulting forces.

Magnetic Effect of a Current

• Straight wire: concentric circular field lines; use right-hand grip rule (thumb = current, curled fingers = field direction).
• Flat coil: field resembles that of a bar magnet; centre behaves like the interior of a solenoid.
• Solenoid: produces a strong, nearly uniform field inside; right-hand grip rule gives poles (thumb points to north when fingers curl with current).
• Field strength increases with higher current, more turns, or inserting soft iron core.

Applications of Current-Produced Fields

• Electric bell: current energises an electromagnet which attracts an armature; contact opens, circuit breaks, spring restores armature, cycle repeats.
• Relays: low-current control coil closes or opens a separate high-current circuit.
• Circuit breaker: surge current magnetises a coil strongly enough to trip a latch, opening the live conductor.
• Magnetic levitation, door buzzers rely on similar on/off electromagnet actions.

Force on a Current-Carrying Conductor

• When a conductor carrying current sits within external magnetic field \( \vec{B} \), it experiences a sideways force.
• Direction determined by Fleming's left-hand rule:
  • First finger: field (\( \vec{B} \), north to south).
  • Second finger: current (\( \vec{I} \), positive to negative).
  • Thumb: resultant force.
• Reversing current or field reverses the force; reversing both keeps same direction.
• Parallel currents attract if they flow the same way; repel if opposite directions (think of each wire's field pushing on the other).

Force on Moving Charges

• Charged particles entering a magnetic field deflect at right angles to both velocity and field.
• Direction again from left-hand rule (treat motion as "current direction").
• Uniform fields cause circular/spiral paths depending on entry angle.

DC Motor Essentials

• Rectangular coil between magnetic poles; current causes opposite forces on either side -> turning effect.
• Split-ring commutator reverses current every half turn so torque keeps same rotational direction.
• Brushes provide sliding contact with supply.
• Increasing number of turns, current, magnetic flux, or adding soft iron armature boosts torque.

Worked Example: Predicting Motor Rotation

Coil side on left has current flowing upward in a uniform field from left (north) to right (south). Fleming's left hand -> thumb points towards you; so that side experiences forward force, right side backward, producing clockwise rotation (viewed from commutator end).

Key Takeaways

• Apply the right-hand grip rule for magnetic field direction around wires/solenoids.
• Fleming's left hand connects field, current, and force; practise enough that the mapping is instant.
• Electromagnetic devices depend on switching magnetisation rapidly; permanent magnets alone can't provide repetitive motion.
• DC motors require commutation to keep torque in one direction; exam questions often ask you to identify supply polarity or motion given a diagram.