P3 Electricity and Magnetism
Current, voltage and resistance
Electric current (I) is the rate of flow of charge:
Q = It
where Q = charge C, I = current A, t = time (s).
Potential difference (p.d. / voltage, V) is the energy transferred per unit charge:
V = W / Q (energy per coulomb)
Resistance (R, ohms Ω) opposes current:
V = IR (Ohm's law)
Series and parallel circuits
Series: same current throughout; p.d.s add; resistances add (R_total = R₁ + R₂).
Parallel: p.d. same across each branch; currents add; 1/R_total = 1/R₁ + 1/R₂.
IV characteristics
PAG P3.1 (OCR): Use ammeters and voltmeters to plot I–V graphs for:
- Ohmic resistor: straight line through origin — resistance constant.
- Filament lamp: curve (S-shape) — resistance increases as temperature rises.
- Diode: conducts in one direction only; very high resistance in reverse direction.
- Thermistor: resistance decreases as temperature increases (NTC).
- LDR: resistance decreases as light intensity increases.
Electrical power and energy
P = IV = I²R = V²/R
E = Pt = QV
Units: power in watts (W), energy in joules (J), time in seconds (s).
Mains electricity: 230 V AC in UK, 50 Hz. DC has constant direction; AC alternates.
Electromagnetism — motor effect
A current-carrying conductor in a magnetic field experiences a force (Fleming's left-hand rule):
- First finger → magnetic Field direction (N to S)
- seCond finger → Conventional current direction
- Thumb → motion (force) direction
F = BIl (Higher)
where B = magnetic flux density (T), I = current A, l = length of conductor in field (m).
Electric motor: uses the motor effect to convert electrical energy → kinetic energy.
Electromagnetic induction
When a conductor moves through a magnetic field (or the field changes), an EMF is induced (Faraday's law). This is the principle of the generator.
Lenz's law (Higher): the induced current opposes the change causing it (conservation of energy).
Factors increasing induced EMF: stronger magnet, faster movement, more turns of wire.
Alternator: rotates coil in field → AC output. Dynamo: uses split-ring commutator → DC output.
Transformers
A transformer changes AC voltage. It has a primary and secondary coil on an iron core.
Vₛ/Vₚ = Nₛ/Nₚ (turns ratio = voltage ratio)
For an ideal (100% efficient) transformer:
VₚIₚ = VₛIₛ (power in = power out)
Step-up transformer: Nₛ > Nₚ → Vₛ > Vₚ. Step-down transformer: Nₛ < Nₚ → Vₛ < Vₚ.
The National Grid uses step-up transformers to transmit at high voltage (low current → less energy lost as heat, since P_loss = I²R).
PAG summary (P3)
- PAG P3.1: IV characteristics using variable resistor, ammeter, voltmeter.
- PAG P3.2: Resistance of a wire (vary length/thickness, plot R vs length).
⚠Common mistakes
- Ammeter in parallel / voltmeter in series: always reverse — A in series, V in parallel.
- Parallel resistors: students add rather than using 1/R formula.
- Power formula: using P = IV but substituting wrong values (use resistor p.d., not supply p.d., for a component).
- Transformer and DC: transformers do NOT work with DC — they need changing flux AC.
- Left-hand rule direction: forgetting conventional current (+ to −) not electron flow.
AI-generated · claude-opus-4-7 · v3-ocr-physics