Magnetic Effect of Electric Current: Field Rules, Electromagnets and the Motor
This domain tests Oersted's discovery, the rules for magnetic field lines, the right-hand thumb rule, electromagnet design and the action of an electric motor.
Field lines and rules
- Field lines leave the north pole and enter the south pole outside a magnet, form closed loops, and never intersect.
- Around a straight conductor the field lines are concentric circles; the right-hand thumb rule gives their direction - thumb along the current, curled fingers along the field.
- Inside a solenoid the field is uniform and strong, and the solenoid behaves like a bar magnet. Its polarity is found by gripping it with the right hand, fingers along the current, thumb pointing to the north pole.
Electromagnet and motor
An electromagnet uses a soft-iron core: it becomes strongly magnetised while current flows and loses its magnetism when the current stops. The field strength increases with more turns, a larger current and a core of high permeability.
A motor works on the force exerted on a current-carrying conductor in a magnetic field, with direction given by Fleming's left-hand rule: forefinger field, middle finger current, thumb force. The split-ring commutator reverses the current in the coil every half rotation so that rotation continues in the same direction.
Worked example
A wire carries current vertically upward and lies in a magnetic field directed into the page. Forefinger into the page, middle finger upward; the thumb points to the left. The wire is pushed left, perpendicular to both the current and the field.
Traps that cost marks
- Using Fleming's left-hand rule for a generator; that needs the right-hand rule.
- Saying magnetic field lines can cross one another.
- Believing an electromagnet keeps its field after the current is switched off.
- Forgetting that the force is maximum when the conductor is perpendicular to the field.