Physics · Grades 9–12
Fields, motors and induced effects
Connect magnetic fields with currents without treating field lines as wires.
Learn the idea
Field model
Magnetic field lines are a representation of field direction and relative strength, not physical strings.
Current and force
A current-carrying wire can experience a magnetic force; direction depends on current and field.
Induction
Changing magnetic flux through a circuit can induce an emf. A static situation does not automatically do so.
Lenz's law
The induced effect opposes the change that produces it. This is consistent with energy accounting.
Worked examples
What are magnetic field lines?
Field lines help describe direction and relative field strength.
A representation of a field
A 0.2 m wire carries 5 A perpendicular to a 0.4 T field. Find force magnitude.
F = BIL = 0.4 × 5 × 0.2 = 0.4 N.
0.4 N
Check your understanding
Which change can induce an emf in a coil?
- Changing magnetic flux through it
- Only leaving all magnetic conditions unchanged
- Renaming the coil
Why does an induced current not provide free energy from nothing?
A coil has 100 turns and flux per turn changes by 0.02 Wb in 0.5 s. Find average emf magnitude.
Answers and marking notes
Answers and marking notes
- Changing magnetic flux through it
Induction depends on flux change, not a label.
- Mechanical or other input is required to drive the change, and energy transfers between systems.
Credit work against induced effects or a coherent energy balance.
- 4 V
NΔΦ/Δt = 100 × 0.02/0.5 = 4 V.
Further reading and review references
These are reference links, not copied exercises, institutional endorsement or proof of independent review.
Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.