Chapter 4 Quizzes | Topic 4 Energy

Quiz 1 — Stores, pathways, conservation

1. Which of these is an energy store? A. electrical B. heating C. gravitational D. radiation

Specification 4.2. Electrical, heating and radiation are pathways, not stores.

2. Energy is transferred from a cell to a resistor in a circuit A. electrically B. from an electrical store to a chemical store C. by convection only D. mechanically from a nuclear store

Chemical store of the cell → transferred electrically → thermal store of the resistor.

3. Conservation of energy means that energy A. can be created in a power station but not destroyed B. cannot be created or destroyed, only transferred C. is destroyed when it is wasted D. only exists in kinetic and thermal stores

Specification 4.3. Wasted energy still exists — usually in the thermal store of the surroundings.

4. A falling apple (no air resistance) mainly empties its A. chemical store, filling an electrical store B. elastic store, filling a nuclear store C. thermal store, filling a magnetic store D. gravitational store, filling its kinetic store

Transferred mechanically. With drag, some energy would also fill the thermal store of the air.

5. Light leaving a lamp is a transfer A. by radiation B. from a light store C. mechanically D. by convection through the vacuum in the bulb

Radiation includes light and sound. There is no “light store.”

Quiz 2 — Efficiency and Sankey diagrams

1. A kettle is supplied with $2000\,\text{J}$. $1600\,\text{J}$ is useful. The efficiency is A. $20\%$ B. $80\%$ C. $125\%$ D. $1600\%$

$1600/2000\times 100\%=80\%$. C inverted the fraction.

2. A lamp: $60\,\text{J}$ in, $12\,\text{J}$ useful light. Wasted energy is A. $12\,\text{J}$ B. $72\,\text{J}$ C. $48\,\text{J}$ D. $5\,\text{J}$

Input = useful + wasted, so wasted $=48\,\text{J}$. Efficiency $=20\%$.

3. On a Sankey diagram the width of an arrow is A. proportional to the energy it represents B. proportional to the time taken C. the same for useful and wasted energy D. proportional to the temperature

Specification 4.5. Input width equals useful plus wasted.

4. A motor: $800\,\text{J}$ in, $600\,\text{J}$ useful $KE$. Efficiency as a decimal is A. $75$ B. $1.33$ C. $0.25$ D. $0.75$

$600/800=0.75$ (or $75\%$). A forgot it was a decimal. B inverted.

5. Efficiency can never be greater than $100\%$ because A. wasted energy is not real B. energy cannot be created — useful cannot exceed the input C. Sankey arrows would be too wide D. $g$ is $10\,\text{N/kg}$

Conservation (4.3). If you get over $100\%$, you swapped useful and total.

Quiz 3 — Conduction, convection, radiation, surfaces

1. Metals are the best conductors mainly because they have A. free electrons that transfer energy quickly B. convection currents in the lattice C. no particles D. a vacuum between the atoms

Lattice vibrations happen in all solids; free electrons make metals much better conductors.

2. Convection can take place in A. solids only B. a vacuum C. liquids and gases only D. metals better than in air

Fluids only. Solids cannot flow; a vacuum has no particles to circulate.

3. In a convection current, warm fluid rises because it is A. denser than the surrounding fluid B. less dense after expanding C. attracted to the ceiling D. transferred by free electrons

Heat → expand → less dense → rise. Cooler fluid sinks.

4. The best emitter and absorber of infrared is A. shiny silver B. white gloss paint C. a polished metal mirror D. dull black

Specification 4.8. Shiny silver is the worst emitter/absorber and the best reflector.

5. Thermal radiation can travel through space because it is A. infrared electromagnetic radiation and needs no medium B. a convection current of photons in air C. conduction by free electrons in the vacuum D. sound from the Sun

Only radiation crosses a vacuum. Sound cannot travel in space.

Quiz 4 — Insulation and reducing wasted energy

1. Double glazing reduces unwanted transfer mainly because A. glass is a better conductor than copper B. trapped air between the panes is a poor conductor C. convection currents in the glass carry energy back in D. the vacuum in the glass stops radiation

Trapped air (or argon) is the insulator. Glass is a solid — no convection in the pane.

2. The vacuum in a flask reduces A. conduction and convection B. radiation only C. the mass of the drink D. infrared completely, so silvering is unnecessary

No particles → no conduction or convection. Radiation still crosses a vacuum — that is why the walls are silvered.

3. Shiny foil behind a radiator is there to A. start a convection current in the wall B. absorb infrared so the wall heats faster C. reflect infrared back into the room D. conduct energy through the bricks more quickly

Shiny silver is a good reflector and a poor absorber of infrared.

4. Cavity-wall foam reduces wasted energy because it A. is a metal, so it conducts energy back into the room B. creates a vacuum in the bricks C. is dull black and emits more radiation D. traps air, which is an insulator, and stops convection in the cavity

Trapped air is the key phrase for loft insulation, cavity foam and double glazing.

5. The plastic stopper on a vacuum flask mainly A. reduces convection of air out of the neck and is a poor conductor B. reflects all infrared from the Sun C. creates the vacuum D. increases the gravitational store of the drink

Vacuum, silvering and stopper — one feature, one process.

Quiz 5 — $W=Fd$, $mgh$, $\tfrac{1}{2}mv^2$, $P=W/t$

1. A $40\,\text{N}$ force pushes a box $3.0\,\text{m}$. The work done is A. $13\,\text{J}$ B. $120\,\text{J}$ C. $43\,\text{J}$ D. $120\,\text{W}$

$W=Fd=120\,\text{J}$. D used the power unit.

2. A $2.0\,\text{kg}$ brick is lifted $5.0\,\text{m}$. $g=10\,\text{N/kg}$. $GPE$ gained is A. $100\,\text{J}$ B. $10\,\text{J}$ C. $25\,\text{J}$ D. $100\,\text{N}$

$mgh=2.0\times 10\times 5.0=100\,\text{J}$. D is a force, not energy.

3. $m=1.2\,\text{kg}$, $v=5.0\,\text{m/s}$. Kinetic energy is A. $3.0\,\text{J}$ B. $6.0\,\text{J}$ C. $15\,\text{J}$ D. $30\,\text{J}$

Square $v$ first: $\tfrac{1}{2}\times 1.2\times 25=15\,\text{J}$. A forgot the square.

4. $GPE=100\,\text{J}$ at the top. Friction does $20\,\text{J}$ of work. $KE$ at the bottom is A. $120\,\text{J}$ B. $100\,\text{J}$ C. $20\,\text{J}$ D. $80\,\text{J}$

$mgh=KE+W_f$, so $KE=80\,\text{J}$. B ignored friction.

5. A motor does $2400\,\text{J}$ of work in $8.0\,\text{s}$. Its power is A. $300\,\text{W}$ B. $300\,\text{J}$ C. $19\,200\,\text{W}$ D. $0.0033\,\text{W}$

$P=W/t=2400/8.0=300\,\text{W}$. B used the energy unit. C multiplied instead of dividing.