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Class 9–10 · Physics
Magnetic effects of current
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Magnetic effects of current
Lessons in this chapter
Lessons
1Magnets and fields
Like poles repel and unlike attract. Magnetic field lines form closed loops, travelling N to S outside a magnet. Cutting a magnet gives smaller dipoles rather than isolated poles. A compass aligns with the local field.
Key relationship
B unit tesla (T)
Worked example
Move compass around bar magnet ⇒ needle orientation changes.
Next lesson →2Field around a straight wire
A straight current-carrying wire produces circular field lines. Point the right thumb with conventional current; curled fingers show field direction. Field increases with current and decreases with distance.
Key relationship
long wire: B ∝ I/r
Worked example
Double current at same distance ⇒ double field.
Next lesson →3Solenoids and electromagnets
Coil turns reinforce their fields. A long solenoid has an approximately uniform internal field. Increase current, turns per length or suitable core permeability to strengthen it. Soft iron retains relatively little magnetism after current stops.
Key relationship
ideal air core: B ≈ μ₀(N/L)I
Worked example
Double N at fixed length and current ⇒ approximately double B
Next lesson →4Magnetic force and motors
A magnetic field exerts force on a current-carrying wire. Opposite forces on coil sides produce torque. A DC motor's split-ring commutator reverses coil current each half-turn to maintain rotation.
Key relationship
F = BIL sinθ
Worked example
B = 0.2 T, I = 3 A, L = 0.5 m, θ = 90° ⇒ F = 0.3 N
Next lesson →5Induction and Lenz's law
Changing magnetic flux induces emf; a closed path permits current. Change field, area, angle or relative position. Lenz's law gives the direction opposing the change, consistent with energy conservation.
Key relationship
Φ = BA cosθ; ε = −NΔΦ/Δt
Worked example
N = 100, flux change per turn 0.02 Wb in 0.5 s ⇒ magnitude ε = 4 V
Next lesson →6Generators, AC and DC
A generator converts mechanical energy into electrical energy through changing flux. Slip rings support alternating output. AC reverses direction periodically; DC retains direction but may vary in magnitude. Fleming's right-hand rule gives generator directions.
Key relationship
motor: electrical → mechanical; generator: mechanical → electrical
Worked example
Flowing water turns a turbine that drives a generator.
Next lesson →7Transformers and transmission
Changing primary current induces secondary emf through a shared core. Voltage ratio follows turns ratio. Ideal input and output powers match. High-voltage transmission reduces current for a given power and lowers resistive losses.
Key relationship
Vs/Vp = Ns/Np; ideal VpIp = VsIs
Worked example
Np = 1000, Ns = 100, Vp = 200 V ⇒ Vs = 20 V
Next lesson →Check your understanding
Try answering first, then open the question to compare your reasoning.
1. Reverse wire current: field direction?
Reverses, by the right-hand rule
2. Main energy conversion in a motor?
Electrical → mechanical
3. What must change to induce emf?
Magnetic flux linking the circuit
4. Turns ratio 2, input 100 V: ideal output?
Vs = 200 V
5. Why not operate a normal transformer with steady DC?
Steady flux gives no continuous induced secondary emf
Sources and scope
Original GetReady teaching notes. The 13 core chapters follow the existing site sequence. Physics to save life is an explicitly labelled older-edition supplement. Check your current book and board instructions for examination scope. Illustrations are schematic. Unless stated otherwise, use g = 9.8 m/s². A physics teacher should review content and edition coverage before it is relied on.