Electromagnetic Induction & Induced Voltage Study Notes

Apparatus described
- Circular copper coil connected to a sensitive galvanometer (zero-center scale).
- No external power source; initial galvanometer reading 0\,\text{A}.
- Bar magnet with marked North (N) and South (S) poles.
Observations
- Moving N-pole toward coil ⇒ galvanometer deflects (current induced).
- Moving N-pole away ⇒ deflection in opposite direction (current reverses).
- Repeating with S-pole shows the same pattern but signs are reversed.
- Keeping magnet stationary (no relative motion) ⇒ no deflection.
- Moving the coil instead of the magnet produces identical effects → only relative motion matters.
Terminology
- Current produced by motion ⇒ induced current.
- Driving “pressure” that creates it ⇒ induced electromotive force (EMF).
- Entire phenomenon ⇒ electromagnetic induction.

Relative Motion Requirement: Either conductor moves in a magnetic field or the field moves relative to the conductor.
Induced Current: Charge flow resulting from an induced EMF in a closed circuit.
EMF (E): Energy per unit charge supplied by the induction process.
Free / Delocalised Electrons: Weakly bound electrons in metals that can move between atoms; crucial for conductivity.
Charge Separation: Motion-induced forces push electrons to one end, leaving the other end electron-deficient (positive).
Fleming’s Rules
- Teacher mentions “right-hand FBI” (physics convention) & “left-hand slap rule”.
- Determines the direction of magnetic force \mathbf{F} on a moving charge q: \mathbf{F}=q\,\mathbf{v}\times\mathbf{B}.

Example described
- Magnetic field \mathbf{B} into the page.
- Metal bar moved toward a fan (rightward in teacher’s sketch).
- Consider a single electron in the bar.
- Using hand rule ⇒ force on electron points downward.
Consequences
- All mobile electrons experience the same downward force, accumulate at the bar’s lower edge.
- Lower edge becomes negative; upper edge left positive (electron deficit).
- Creates an internal electric field / potential difference (voltage).

Formula provided repeatedly: {\text{EMF}} = B\,v\,l where
- B = magnetic flux density (tesla, \text{T}),
- v = speed of the conductor relative to the field (metre per second, \text{m\,s}^{-1}),
- l = active length of conductor cutting the field lines (metres, \text{m}).
Clarifications
- Only the portion of the bar inside the field contributes to l.
- Units: always convert speed to \text{m\,s}^{-1} before substitution.
- EMF is sometimes labelled \varepsilon or \mathcal{E}; teacher uses “induced voltage”.

If the bar (or coil) is part of a closed external circuit:
1. Charge separation raises potential energy (voltage).
2. Electrons have a pathway to escape repulsion → drift round the circuit.
3. Continuous motion keeps replenishing separated charge, sustaining current.
Use Ohm’s Law to predict current magnitude:
{I}=\frac{E}{R} where R is total resistance of the external circuit.
Teacher nickname “disc-urrent”: electrons “hate” crowding and “escape” along wires.

For a loop to sustain current, at least one segment must be outside the magnetic field.
- Inside segments experience equal & opposite EMFs that can cancel.
- Outside segment sees no magnetic force → allows potential difference to drop along the rest of circuit.
If entire loop resides in a uniform field and moves rigidly with it:
- Forces on opposite sides are equal & opposite vectors.
- Net EMF for the whole loop =0.
- Electrons are pushed but can’t produce a continuous potential difference around loop → no current.

“Top part is positive because protons moved” → Wrong. Only electrons move.
Voltage is a vector quantity in this context; opposite contributions cancel.
Must identify which particle moves when describing charge separation.
Don’t forget conversion of units (cm → m, km/h → m/s, etc.).

Rod length l = 24\,\text{cm} = 0.24\,\text{m}.
Displacement mentioned 30\,\text{m} (context: distance moved) – speed given separately.
Magnetic field example value “that value” (not specified in transcript) labelled B.
Steps to answer exam-style question:
1. Convert all lengths to metres, speed to \text{m\,s}^{-1}.
2. Compute E = B v l.
3. If asked for current, apply I = \dfrac{E}{R} using the circuit’s resistance.

Device that “generates electrical current from kinetic energy” ⇒ inductor / generator principle.
Airplane wings example often appears in exams (large metal span moving through Earth’s magnetic field; can induce tiny voltages).
Links to earlier lessons on
- Static electricity (role of delocalised electrons).
- Work & Energy (potential energy difference).
- Right-hand rule practice in earlier magnetism lectures.
Ethical / Practical considerations (implied): Energy harvested must come from mechanical work; nothing “free”.

Determine whether any circuit segment is outside the magnetic field.
State clearly: “Electrons accumulate on ____ side, leaving the opposite side electron-deficient (positive).”
Use correct sign convention when reversing magnet poles or motion.
Distinguish “induced voltage” vs “induced current” — voltage can exist without current if circuit is open.
Include reasoning about vector cancellation for loops entirely inside a uniform field.
Always quote equations: E=B v l and I=\dfrac{E}{R}.
Show unit conversions and label answers with units (V, A).