Orientation
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.
Core Content
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 |
Concept Check
-
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.
-
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.
-
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.
Applied Practice
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.
Deep Practice And Writing
Prompt: explain how an electromagnet can be strengthened and why each change affects the magnetic field.
Maintenance Loop
Retrieve field pattern around a wire, solenoid field direction, and the condition for maximum magnetic force.