Magnetism

Lesson goal: describe magnetic fields and field patterns around magnets, straight wires, coils, solenoids, and electromagnets.

Direction rules matter. A correct answer should state both the rule used and the resulting field direction.

Magnetic fields exert forces on magnetic materials and moving charges. Around a current-carrying wire, the magnetic field forms circular loops. Around a solenoid, the field resembles a bar magnet and is strengthened by more turns, greater current, and a suitable core.

Useful force relations:

$$F = BIL\sin\theta$$

$$F = qvB\sin\theta$$

Situation	Direction rule
straight current-carrying wire	right-hand grip rule
solenoid	curled fingers show current, thumb points north pole
force on current in field	relevant motor-effect hand rule if introduced

1. Around a straight current-carrying wire, magnetic field lines are:

   • A. circular around the wire
   • B. straight away from the wire only
   • C. absent
   • D. always vertical

   Answer: A.

2. A solenoid's field is strengthened by:

   • A. reducing current to zero
   • B. increasing turns or current
   • C. removing all coils
   • D. using no core under any condition

   Answer: B.

3. Magnetic force on a moving charge is greatest when velocity is:

   • A. parallel to the field
   • B. perpendicular to the field
   • C. zero
   • D. unrelated to field direction

   Answer: B.

A 0.30 m wire carries 4.0 A perpendicular to a 0.20 T magnetic field. Find the magnetic force.

$$F = BIL\sin\theta = 0.20\times4.0\times0.30\times\sin90^\circ$$

$$F = 0.24\ \text{N}$$

Final answer: $0.24\ \text{N}$; force is maximum because the wire is perpendicular to the field.

Prompt: explain how an electromagnet can be strengthened and why each change affects the magnetic field.