Flashcards · 9702 · AS Topic 9

Electricity (AS Level) flashcards

16 cards on AS Topic 9, subtopics 9.1 to 9.3. Pick a subtopic or study them all.

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Electric current: The rate of flow of charge: I = Q ÷ t, so Q = It. Unit A; one coulomb is the charge passing when 1 A flows for 1 s.
What does it mean that charge is quantised?: Charge on charge carriers comes only in whole-number multiples of the elementary charge e = 1.60 × 10⁻¹⁹ C.
A current of 2.0 A flows for 3.0 minutes. How much charge passes?: Q = It = 2.0 × 180 = 360 C.
I = Anvq: what does each symbol mean?: A: cross-sectional area. n: number density of charge carriers (per m³). v: mean drift velocity. q: charge on each carrier.
The same current flows through a thick and a thin section of a wire. Where is the drift velocity higher?: In the thin section: with I and n fixed, a smaller A means a larger v (I = Anvq).

Potential difference: The energy transferred (work done) per unit charge as charge passes between two points: V = W ÷ Q. 1 V = 1 J C⁻¹.
Three expressions for electrical power: P = VI = I²R = V² ÷ R.
Electrical energy transferred in time t: W = VIt (from W = VQ and Q = It).
A 12 V supply drives 2.0 A for 5.0 minutes. How much energy is transferred?: W = VIt = 12 × 2.0 × 300 = 7200 J.
A 60 W lamp runs on 230 V. Find the current.: I = P ÷ V = 60 ÷ 230 = 0.26 A.

Resistance: The ratio of potential difference to current: R = V ÷ I. Unit ohm (Ω = V A⁻¹).
Ohm’s law: The current in a conductor is directly proportional to the potential difference across it, provided physical conditions, especially temperature, stay constant.
Filament lamp I–V characteristic: A curve whose gradient falls as V rises. The filament heats up, lattice ions vibrate more, electrons collide more often, so resistance increases.
Semiconductor diode I–V characteristic: Almost zero current until about 0.6 V in forward bias, then a steep rise. Negligible current in reverse bias.
NTC thermistor and LDR: how does each one’s resistance change?: Thermistor: resistance falls as temperature rises. LDR: resistance falls as light intensity rises. In both, more charge carriers are released.
Resistivity equation and unit: R = ρL ÷ A, where ρ is the resistivity of the material. Unit Ω m (not Ω/m). Resistivity is a property of the material, not of the wire’s shape.