4.3 Electric Circuits
Know the Electrical Components
- Diode — used for rectification.
- Centre-zero galvanometer — detects tiny currents and polarity.
Series Circuits
- Current: same at every point →
I = I₁ = I₂ = I₃. - Potential difference: shares add →
V = V₁ + V₂ + …. - Combined resistance: adds →
R = R₁ + R₂ + …. - Sources in series: e.m.f.s add algebraically (mind polarity) — e.g. three 12 V cells → 36 V.
Parallel Circuits
- Voltage: same across each branch = supply e.m.f.
- Current: splits & recombines (junction rule). Example set:
I₁ = I₂ + I₃,I₆ = I₄ + I₅→I₂ = I₄,I₃ = I₅. - Resistance:
1/R = 1/R₁ + 1/R₂ (+ …). - Advantages (lamps): same brightness; one failure doesn’t black out others.
Potential Divider
A potential divider uses series resistors so the supply e.m.f. is shared in proportion to resistances. A potentiometer (variable divider) has a sliding contact to vary the output p.d.; more wire in circuit → larger share of p.d.
Input Transducers — Thermistor & LDR
- Thermistor: temperature ↑ → resistance ↓ → its share of p.d. ↓ → p.d. across the fixed series resistor ↑ (output ↑).
- LDR: light level ↑ → resistance ↓ → its share of p.d. ↓ → p.d. across series resistor ↑ (output ↑).
Relay (Switching a Large Current Safely)
A relay uses a coil to attract an iron armature and close contacts when a small current flows. Typical use: car ignition — small control current switches a high motor current.
Diode & Rectification
- Action: conducts in forward direction (very low R), blocks in reverse (very high R).
- Rectifiers: half-wave (single diode) and bridge (full-wave) to convert a.c. → d.c.
Quick Rules — Series & Parallel
| Idea | Expression | Comment |
|---|---|---|
| Series current | I same everywhere | One path only |
| Series p.d. | V = ΣVᵢ | Shares add to supply |
| Series resistance | R = ΣRᵢ | Add straight |
| Parallel voltage | V_branch = V_supply | Same across branches |
| Parallel current | I_total = ΣI_branch | Kirchhoff junction rule |
| Parallel resistance | 1/R = Σ(1/Rᵢ) | Less than the smallest |
Exam Tips
- Ammeter in series, voltmeter in parallel with component.
- In dividers with sensors: explain how R changes → V_out changes.
- For bridge questions, draw the conventional current path for each half-cycle.
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