Flashcards · 4PH1 · Topic 2

Electricity flashcards

22 cards on Topic 2, subtopics 2a to 2d. Pick a subtopic or study them all.

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Units of charge and current: Charge: coulomb (C). Current: ampere (A). 1 A = 1 C/s.
Units of voltage and resistance: Voltage: volt (V); 1 V = 1 J/C. Resistance: ohm (Ω).
Units of energy and power: Energy: joule (J). Power: watt (W); 1 W = 1 J/s. Time in seconds (s).

a.c. vs d.c.: Mains is alternating current (a.c.), reversing direction periodically. Cells and batteries supply direct current (d.c.), flowing one way only.
How do a fuse and a circuit breaker protect a circuit?: Both cut off the current if it becomes too large. A fuse melts; a circuit breaker switches off and can be reset.
Earthing vs double insulation: Earthing: a metal case is connected to earth, so a fault sends a large current to earth and the fuse blows. Double insulation: the case is non-conducting, so it cannot become live.
Why does a current in a resistor heat it, and where is this useful?: The moving electrons collide with the ions of the resistor, transferring energy so its temperature rises. Used in heaters, kettles, toasters and irons.
A kettle draws 10 A from 230 V. Power, and the best fuse from 3 A, 5 A and 13 A?: P = IV = 10 × 230 = 2300 W. The fuse must be just above the normal current: 13 A.
The same kettle (10 A, 230 V) runs for 3 minutes. Energy transferred?: E = IVt = 10 × 230 × 180 = 414 000 J (414 kJ).

Series circuit rules: The current is the same everywhere; the voltage is shared between the components. Resistances add.
Parallel circuit rules, and why house lighting is parallel: The voltage is the same across every branch; the current divides between branches. Each lamp gets full mains voltage and one failing does not switch off the others.
A 6.0 V battery drives 0.50 A through a resistor. Resistance?: V = IR, so R = 6.0 ÷ 0.50 = 12 Ω.
Current–voltage graphs: resistor, filament lamp, diode: Resistor (constant temperature): straight line through the origin. Filament lamp: curve, as resistance rises with temperature. Diode: conducts one way only; almost no current in reverse.
LDR and thermistor: LDR: resistance falls as light intensity increases. Thermistor: resistance falls as temperature increases.
Current and charge. 15 C passes a point in 5.0 s: current?: Current is the rate of flow of charge: Q = It, so I = 15 ÷ 5.0 = 3.0 A.
What is the current in a metal, and what happens at a junction?: A flow of electrons. Current is conserved at a junction: current in = current out.
Voltage in terms of energy and charge: Voltage is the energy transferred per unit charge: E = QV. 1 V = 1 J/C.
4 Ω and 8 Ω resistors in series across 6 V. Current, and voltage across the 8 Ω?: Total R = 12 Ω, so I = 6 ÷ 12 = 0.5 A. V = IR = 0.5 × 8 = 4 V (and 2 V across the 4 Ω).

Conductors and insulators: Conductors (metals) allow charge to flow; insulators (plastics) do not. (Paper 2 only)
How does rubbing two insulators charge them?: Electrons are transferred. The surface that loses electrons becomes positive; the one that gains electrons becomes negative. Charge is not created. (Paper 2 only)
Forces between charges: Like charges repel; unlike charges attract. (Paper 2 only)
Uses and dangers of electrostatic charge: Uses: photocopiers, inkjet printers. Danger: sparks while fuelling aircraft or tankers, so fuelling equipment is designed to dissipate static. (Paper 2 only)