Chapter 2 Quizzes | Topic 2 Electricity

Quiz 1 — Units, charge, $E=QV$, a.c./d.c.

1. A current of $200\,\text{mA}$ flows for $40\,\text{s}$. The charge transferred is A. $8000\,\text{C}$ B. $0.005\,\text{C}$ C. $8.0\,\text{C}$ D. $5.0\,\text{C}$

$I=0.200\,\text{A}$, $Q=It=8.0\,\text{C}$. A forgot to convert mA.

2. $5.0\,\text{C}$ of charge passes through a $9.0\,\text{V}$ lamp. Energy transferred is A. $1.8\,\text{J}$ B. $45\,\text{J}$ C. $0.56\,\text{J}$ D. $14\,\text{J}$

$E=QV=45\,\text{J}$.

3. Mains electricity in the home is A. alternating current (a.c.) B. direct current from a cell C. always $12\,\text{V}$ d.c. D. a flow of positive ions in the wires

2.6: mains is a.c.; cells/batteries are d.c. Current in metals is electrons.

4. In a solid metal wire the current is a flow of A. positive protons B. positive ions C. both electrons and protons D. negatively charged electrons

Specification 2.16.

5. One volt is equal to A. one coulomb per joule B. one joule per coulomb C. one watt per second D. one ampere per ohm

2.20: the volt is a joule per coulomb.

Quiz 2 — Mains safety

1. The fuse in a plug should be connected in the A. earth wire B. live wire C. neutral wire only D. cable grip

The fuse must break the live supply.

2. A double-insulated appliance A. must have an earth wire B. has a metal case that is always live C. has a plastic case and does not need an earth wire D. cannot have a fuse

Square-in-square symbol: plastic case, no earth needed.

3. If the live wire touches an earthed metal case A. a large current flows in the earth wire and the fuse melts B. the earth wire becomes the live supply permanently C. nothing happens because earth is an insulator D. the voltage of the mains falls to zero everywhere

Earth path → large $I$ → fuse melts → case isolated from live.

4. A circuit breaker is useful because it A. increases the current in a fault B. stores charge for later C. converts a.c. to d.c. D. trips quickly and can be reset

Unlike a fuse, it does not need replacing.

5. Plastic insulation on a cable protects the user because plastic A. is a good electrical conductor B. is an electrical insulator C. melts the fuse D. is connected to earth

You cannot get a shock by touching the insulation.

Quiz 3 — Power, energy and fuses

1. A $690\,\text{W}$ appliance on $230\,\text{V}$ draws a current of A. $3.0\,\text{A}$ B. $0.33\,\text{A}$ C. $460\,\text{A}$ D. $13\,\text{A}$

$I=P/V=690/230=3.0\,\text{A}$.

2. For that $3.0\,\text{A}$ appliance the most appropriate fuse is A. $3\,\text{A}$ B. $5\,\text{A}$ C. $13\,\text{A}$ only D. $1\,\text{A}$

Next standard rating above $3.0\,\text{A}$ is $5\,\text{A}$. A $3\,\text{A}$ fuse may melt in normal use.

3. A $2.0\,\text{kW}$ heater runs for $5.0\,\text{minutes}$. Energy transferred is A. $10\,\text{J}$ B. $600\,\text{J}$ C. $6.0 \times 10^5\,\text{J}$ D. $2.0 \times 10^3\,\text{J}$

$P=2000\,\text{W}$, $t=300\,\text{s}$, $E=Pt=6.0\times 10^5\,\text{J}$.

4. A fuse rating that is much too large is dangerous because A. a large fault current may not melt it before the cable overheats B. the appliance will not switch on C. it converts a.c. to d.c. D. earth is disconnected

The fuse must melt before the flex does.

5. A current in a resistor raises the temperature because A. charge is created in the resistor B. the voltage becomes zero C. electrons stop moving D. electrical energy is transferred to thermal energy in the resistor

2.3: collisions transfer energy; temperature rises. Used in kettles and heaters.

Quiz 4 — Series, parallel and $V=IR$

1. In a series circuit the current A. is used up by each component B. is the same at every point C. splits equally at every component D. is zero after the first lamp

Charge is conserved. Current is not “used up”.

2. Two resistors $4.0\,\Omega$ and $8.0\,\Omega$ in series on $12\,\text{V}$. The current is A. $1.0\,\text{A}$ B. $3.0\,\text{A}$ C. $0.50\,\text{A}$ D. $12\,\text{A}$

$R_{\text{T}}=12\,\Omega$, $I=12/12=1.0\,\text{A}$.

3. The voltage across two components in parallel is A. always zero B. shared in the ratio of their currents only C. the same D. larger on the smaller resistor only if they are in series

2.18: voltage across parallel branches is the same.

4. House lamps are wired in parallel so that A. if one lamp fails, they all go out B. each lamp can have the full mains voltage and can stay on if another fails C. the current is the same through every lamp always D. no fuse is needed

Specification 2.7.

5. Increasing the resistance in a series circuit (same supply voltage) A. increases the current B. does not change the current C. increases the supply voltage D. decreases the current

$I=V/R$ (2.10).

Quiz 5 — I–V graphs, LDR, thermistor

1. A straight I–V line through the origin means the component is A. ohmic — $R$ is constant B. a diode in reverse bias C. a filament lamp at high temperature only D. broken

Wire or fixed resistor at constant temperature.

2. The I–V graph of a filament lamp curves because A. current is used up in the filament B. resistance falls as the lamp heats C. resistance rises as the filament temperature rises D. the lamp only works on a.c.

Hotter metal → more ion vibration → larger $R$ → graph less steep.

3. A diode A. has the same $R$ in both directions B. allows a significant current in one direction only C. is used only as an earth wire D. has a linear I–V graph through the origin

One-way device; reverse current ≈ 0.

4. When a room gets darker, the resistance of an LDR A. increases B. decreases C. stays exactly $0$ D. becomes negative

$R$ decreases as illumination increases — so $R$ increases in the dark.

5. As a thermistor is heated, its resistance A. increases, so the current falls if $V$ is fixed B. is unchanged C. decreases, so the current rises if $V$ is fixed D. becomes infinite

2.11: thermistor $R$ falls as temperature rises. $I=V/R$ then rises.