GCSE Physics Circuits revision

In a metal wire the charge carriers are free electrons drifting through the lattice of positive ions. Electric current is the rate of flow of charge: a current of 1 ampere means 1 coulomb of charge passes each second, so I = Q ÷ t. The ammeter that measures it goes in series — the same current must pass through meter and component. Potential difference (p.d.) is the energy transferred per unit charge passed: a 12 V supply gives every coulomb 12 J of energy to deliver around the circuit, and V = I R ties it to current and resistance. The voltmeter measures it in parallel across the component, comparing the energy before and after.

• Current rule: there is only one path, so the current is the same at every point — ammeters anywhere in the loop read the same.
• P.d. rule: the supply p.d. is shared between the components in proportion to their resistances: 12 V across 2 Ω and 4 Ω in series splits as 4 V and 8 V.
• Resistance rule: total resistance = sum of the separate resistances: 2 Ω + 3 Ω + 5 Ω = 10 Ω.

Adding a resistor in series always increases the total resistance, so the current from the same cell falls. Two lamps in series share the p.d., so each is dimmer than one lamp alone. Series circuits fail all at once: one broken lamp breaks the whole loop — the reason old-style fairy lights went dark from a single failure.

• P.d. rule: each branch is connected straight across the supply, so every branch gets the full supply p.d.
• Current rule: the supply current splits between branches; the branch currents add back to the total — 2 A + 1 A + 4 A means the supply delivers 7 A.
• Resistance rule: the total resistance of resistors in parallel is less than the smallest single resistance, because each branch is another path for charge.

For two resistors in parallel the product-over-sum shortcut works: R = (R₁ × R₂) ÷ (R₁ + R₂). Two 6 Ω resistors in parallel give 36 ÷ 12 = 3 Ω; 4 Ω with 12 Ω gives 48 ÷ 16 = 3 Ω. Why does adding a resistor lower total resistance? Each extra branch gives electrons another path — more total charge flows per second, which is exactly what a smaller total resistance means. This is why houses wire everything in parallel: every appliance gets 230 V and works independently, and switching one off does not turn the house dark.

Reading graphs is examinable two ways: describe the shape, and calculate R at a point (R = V ÷ I from the point, not the gradient unless the graph is a straight line through the origin). A lamp at low current behaves almost ohmically; as the filament glows brighter its resistance climbs — that self-regulation is why the lamp does not simply burn out at once. The diode is a one-way valve, which is exactly what rectifying circuits and LEDs exploit.

A thermistor (in GCSE circuits, the NTC type) has a resistance that falls as temperature rises — warmer frees more charge carriers. Put it in a potential divider and its p.d. share signals temperature: a thermostat switches heating off when the room is warm enough. An LDR (light-dependent resistor) has a resistance that falls as light intensity rises — brighter light liberates more charge carriers. It is the sensor in automatic night lights and street lamps: dark → high LDR resistance → the LDR takes a bigger p.d. share → the sensing circuit switches the lamp on.

Both devices earn their marks in explanations: name the resistance change, then link it through V = I R to the p.d. share in the divider, then to what the circuit does.

1. 1Resistance of a wire — method

   Clip a resistance meter (or ammeter + voltmeter) across a test wire; record R at several lengths measured from the same end, e.g. every 20 cm up to 1 m.

2. 2Resistance of a wire — controls and risks

   Keep the wire material, thickness and temperature constant (use a small current so it does not warm; switch off between readings). Currents are small, but wires can get hot — let them cool.

3. 3Resistance of a wire — result

   R is proportional to length: a straight-line graph through the origin. Doubling length doubles the collisions electrons make — the longer wire is the resistors in series.

4. 4Resistor combinations — method

   Connect two known resistors in series, then in parallel; measure the total p.d. and current (or read R directly) for each arrangement, using several p.d. values and averaging.

5. 5Resistor combinations — result

   Series total equals the sum (e.g. 2 Ω + 3 Ω = 5 Ω); the parallel total is less than either alone (2 Ω and 3 Ω in parallel give 1.2 Ω).

6. 6I–V characteristics — method

   Use a variable resistor (or a potential divider) to sweep the p.d. across the component from negative to positive; record I at each step with an ammeter and voltmeter; repeat for a fixed resistor, a filament lamp and a diode.

7. 7I–V characteristics — variables and care

   Independent: the p.d.; dependent: the current; controls: temperature (take readings quickly, use low currents), same component, same meters. Reverse the connections to trace the negative side and the diode’s blocking behaviour.

Two 2 Ω and 3 Ω resistors: in series the measured total is 5 Ω; in parallel, predicted (2 × 3) ÷ (2 + 3) = 1.2 Ω — an easy number to check against your practical. In the I–V practical the fixed resistor gives the straight line, the lamp gives the S-curve, and the diode gives nothing backwards then a steep rise forward past 0.7 V.