Energy
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Forms and Sources of Energy 20 questions
Different objects and materials store or use energy in different forms.
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A skateboarder of mass $40\,\text{kg}$ moves at a speed of $3\,\text{m/s}$. Calculate her kinetic energy. State the formula you use, show your substitution, and give your answer with the correct unit.
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Step 1 — State the formula:
$$ E_k = \tfrac{1}{2} m v^2 $$
Step 2 — Substitute the values:
$$ E_k = \tfrac{1}{2} \times 40 \times 3^2 $$
Step 3 — Calculate:
$$ E_k = \tfrac{1}{2} \times 40 \times 9 = 180 $$
Answer: $E_k = 180\,\text{J}$
A book of mass $1.5\,\text{kg}$ sits on a shelf $2\,\text{m}$ above the floor. Take $g = 10\,\text{N/kg}$. Calculate the gravitational potential energy (GPE) of the book relative to the floor.
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Step 1 — State the formula:
$$ E_p = m g h $$
Step 2 — Substitute:
$$ E_p = 1.5 \times 10 \times 2 $$
Answer: $E_p = 30\,\text{J}$
A trampolinist of mass $45\,\text{kg}$ bounces up and down on a trampoline.
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A crane lifts a steel beam of mass $250\,\text{kg}$ to a height of $12\,\text{m}$ above the ground. Take $g = 10\,\text{N/kg}$.
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An $800\,\text{kg}$ roller-coaster car sits at the top of a hill, $20\,\text{m}$ above the lowest point of the track. Take $g = 10\,\text{N/kg}$. Calculate the gravitational potential energy of the car at the top of the hill. State the formula, show your substitution, and give the answer with the correct unit.
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$$ E_p = mgh = 800 \times 10 \times 20 $$
Answer: $E_p = 160\,000\,\text{J} = 160\,\text{kJ}$
A ball of mass $0.4\,\text{kg}$ is thrown and, at a certain instant, has $32\,\text{J}$ of kinetic energy.
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Answer: $v = 12.6\,\text{m/s}$ (3 s.f.)
A student wants to compare the amount of chemical energy stored in two snack foods, a peanut and a crisp, by burning a sample of each under a test tube of water and measuring the temperature rise of the water.
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A student wants to investigate how the distance between a lamp and a solar panel affects the electrical energy the panel produces, measured using a voltmeter connected to the panel.
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In the food-energy investigation above (burning a food sample to heat a test tube of water), a large amount of heat escapes into the surrounding air rather than heating the water, making the results unreliable.
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| Box | Mass (kg) | Height lifted (m) | GPE gained (J) |
|---|---|---|---|
| A | 2 | 5 | 100 |
| B | 4 | 3 | ? |
The table shows two boxes lifted onto a shelf. Take $g = 10\,\text{N/kg}$. Calculate the GPE gained by Box B.
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$$ E_p = mgh = 4 \times 10 \times 3 $$
Answer: $E_p = 120\,\text{J}$
| Toy car | Mass (kg) | Speed (m/s) | Kinetic energy (J) |
|---|---|---|---|
| 1 | 0.5 | 2 | 1 |
| 2 | 0.5 | 4 | 4 |
| 3 | 0.5 | 6 | 9 |
| 4 | 0.5 | 8 | 64 |
| 5 | 0.5 | 10 | 25 |
All five toy cars have the same mass. The kinetic energy for each was calculated using $E_k = \tfrac{1}{2}mv^2$.
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The bar chart shows the approximate energy released when $1\,\text{kg}$ of each fuel is completely burned.
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Answer: natural gas releases about 3.4 times as much energy per kilogram as wood.
| Speed (m/s) | 0 | 10 | 20 | 30 |
|---|---|---|---|---|
| Kinetic energy (kJ) | 0 | 50 | 200 | 450 |
The table shows the kinetic energy of a $1000\,\text{kg}$ car at different speeds.
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| Hiker | Mass (kg) | Height climbed (m) | GPE gained (J) |
|---|---|---|---|
| Amir | 60 | 100 | 60 000 |
| Beth | 40 | 100 | ? |
| Chen | 60 | 150 | 90 000 |
Three hikers climb the same hill via different routes, reaching different heights. Take $g = 10\,\text{N/kg}$.
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| Mass of wax burned (g) | 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|
| Energy released (kJ) | 40 | 80 | 120 | 95 | 200 |
A student burns different masses of the same candle wax and measures the energy released using a data logger.
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Answer: mean energy released $= 40\,\text{kJ per gram}$ of wax.
| Student | 1 | 2 | 3 | 4 |
|---|---|---|---|---|
| Temperature rise (°C) | 18 | 42 | 19 | 20 |
Four students each measured the temperature rise of $100\,\text{g}$ of water after a $5\,\text{g}$ peanut sample was burned beneath it. Use the formula: $$\text{energy transferred to water (J)} = \text{mass of water (g)} \times 4.2 \times \text{temperature rise (}^\circ\text{C)}$$
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(This is much lower than a peanut's true energy content because a lot of heat escapes into the surrounding air rather than reaching the water.)
Energy drinks contain large amounts of sugar, which provides the body with a large amount of chemical energy that can be used quickly.
Discuss one benefit and one drawback of regularly consuming high-energy drinks, using ideas about energy.
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Benefit: The large amount of chemical energy in sugar can be quickly converted for use by the body, giving a fast source of energy for short bursts of intense activity — useful, for example, for athletes needing rapid fuel during exercise.
Drawback: If the chemical energy taken in is greater than the energy the body actually uses, the excess is stored as fat, which over time can contribute to health problems such as obesity. The high sugar content can also damage teeth, and regular high-energy drinks often contain caffeine, which can affect sleep and cause dependency.
Nuclear power stations release very large amounts of energy from a small mass of uranium fuel, without burning any fuel or producing greenhouse gases during operation.
Discuss one benefit and one drawback of using nuclear fuel as an energy source.
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Benefit: A very small mass of uranium releases an enormous amount of energy compared with the same mass of a fossil fuel, and nuclear power stations do not release carbon dioxide or other greenhouse gases while generating electricity, making them useful for providing large, reliable amounts of energy without directly worsening climate change.
Drawback: Nuclear power produces radioactive waste that remains hazardous for many thousands of years and must be stored extremely carefully, at high cost, to avoid harming people and the environment. There is also a small but serious risk of accidents releasing dangerous radiation over a wide area.
Some countries have very few natural energy sources of their own — no fossil fuel reserves, and limited sunshine or wind for renewable generation — so they must import fuel from other countries to generate electricity.
Evaluate the benefit and the drawback of a country relying heavily on imported energy sources.
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Benefit: Importing fuel allows a country without its own energy resources to still access a reliable energy supply, supporting homes, industry, and economic activity that would otherwise be impossible without local coal, oil, gas, or strong renewable conditions.
Drawback: Depending on other countries for energy makes the supply vulnerable to events outside the importing country's control — for example price rises, political conflicts, or disruption to shipping and pipelines in the exporting country — all of which threaten energy security. Transporting fuel over long distances also adds cost and, if the fuel is a fossil fuel, additional emissions from transport.
Energy Transfers and Transformations 20 questions
Electrical appliances transform electrical energy into other, more useful forms.
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An electric motor is supplied with $100\,\text{J}$ of electrical energy. It usefully transforms $70\,\text{J}$ of this into kinetic energy; the rest is wasted as heat and sound. Calculate the efficiency of the motor as a percentage.
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Step 1 — State the formula:
$$ \text{efficiency} = \frac{\text{useful energy output}}{\text{total energy input}} \times 100\% $$
Step 2 — Substitute:
$$ \text{efficiency} = \frac{70}{100} \times 100\% $$
Answer: efficiency $= 70\%$
A lamp is supplied with $60\,\text{J}$ of electrical energy and produces $9\,\text{J}$ of useful light energy. Calculate the energy wasted, and state the form this wasted energy mostly takes.
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Wasted energy $= 60 - 9 = 51\,\text{J}$.
This wasted energy is mostly transferred as heat (thermal energy) to the surroundings.
A washing machine motor is supplied with $500\,\text{J}$ of electrical energy each cycle. It usefully transfers $350\,\text{J}$ of this to kinetic energy of the drum.
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A wind-up toy car has its internal spring wound up, then is released so it drives across the floor, gradually slowing down and stopping due to friction.
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A solar panel has an efficiency of $18\%$. It receives $2000\,\text{J}$ of light energy from the Sun.
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Answer: $360\,\text{J}$
An LED bulb transforms electrical energy into light with an efficiency of $80\%$. It produces $40\,\text{J}$ of useful light energy every second.
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This $10\,\text{J}$ per second is mainly transferred as heat to the surroundings.
A student wants to compare how much energy is wasted as heat by an LED bulb and a filament bulb of the same power rating, by measuring the temperature rise of the air just above each bulb after it has been switched on for a fixed time.
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A student wants to investigate whether the efficiency of a small electric motor changes as it lifts different masses. The motor lifts each mass through a fixed height using a string over a pulley, while a joulemeter measures the electrical energy supplied.
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In the motor-lifting investigation above, the calculated efficiency turned out to be much lower than the motor's stated (manufacturer's) efficiency.
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| Appliance | Energy input (J) | Useful energy output (J) |
|---|---|---|
| Kettle | 1000 | 900 |
Calculate the efficiency of the kettle.
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$$ \text{efficiency} = \frac{900}{1000} \times 100\% $$
Answer: efficiency $= 90\%$
| Bulb type | Energy input (J) | Useful light output (J) |
|---|---|---|
| Filament | 100 | 5 |
| LED | 100 | 80 |
The table compares two bulbs supplied with the same electrical energy.
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The bar chart shows the approximate efficiency of different types of power station at converting their energy source into useful electrical energy.
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An incandescent (filament) lamp is supplied with $100\,\text{W}$ of electrical power. It produces $5\,\text{W}$ of useful light and $95\,\text{W}$ of heat.
A student claims: “Since $95\,\text{W}$ is 'wasted', this lamp must be broken and unsafe to use.”
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| Hour | 1 | 2 | 3 | 4 |
|---|---|---|---|---|
| Energy input from water (MJ) | 1000 | 1000 | 1000 | 1000 |
| Electrical energy output (MJ) | 900 | 910 | 400 | 895 |
A hydroelectric power station's energy input and output were recorded over four hours.
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| Vehicle | Energy input per km (MJ) | Useful kinetic energy output per km (MJ) |
|---|---|---|
| Petrol car | 3.6 | 0.9 |
| Electric car | 1.2 | 0.9 |
The table compares a petrol car and an electric car travelling the same $1\,\text{km}$ distance, producing the same amount of useful kinetic energy.
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| Trial | 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|
| Electrical energy input (J) | 50 | 50 | 50 | 50 | 50 |
| GPE gained by mass (J) | 21 | 19 | 8 | 22 | 20 |
A student repeated the motor-lifting experiment five times, each time supplying $50\,\text{J}$ of electrical energy.
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Many countries require new appliances, such as fridges and washing machines, to display an energy efficiency label (for example, a rating from A to G) showing how efficiently they use electricity.
Discuss one benefit and one drawback of these labelling schemes.
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Benefit: Efficiency labels help consumers make informed choices by clearly showing which appliances waste less energy, encouraging people to buy more efficient models. This reduces household electricity bills over the appliance's lifetime and lowers the amount of energy wasted nationally, which can reduce the fossil fuels burned (and emissions produced) at power stations.
Drawback: More efficient appliances are often more expensive to buy, which can be a barrier for lower-income households, even though they would save money on bills over time. Rating scales can also change or be updated over the years, which can confuse consumers comparing an older label to a newer one.
Replacing old filament light bulbs with LED bulbs across a whole country could hugely reduce national electricity use, since LEDs waste far less energy as heat than filament bulbs do.
Discuss one benefit and one drawback of a national scheme to replace all filament bulbs with LEDs.
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Benefit: Since LEDs waste far less energy as heat, a nationwide switch would significantly reduce the total electricity demand for lighting. This lowers household running costs and reduces the amount of fuel burned (and greenhouse gases released) at power stations that generate that electricity.
Drawback: The upfront cost of replacing bulbs and fittings across an entire country could be expensive, particularly for households and businesses that cannot easily afford it. Throwing away large numbers of still-working filament bulbs also creates waste, and manufacturing millions of new LED bulbs has its own environmental impact (energy use, materials, and eventual disposal).
No machine can ever be $100\%$ efficient — some energy is always transformed into non-useful forms, such as heat and sound, which spread out into the surroundings.
Evaluate the importance of continuing to research and develop more efficient technologies (such as engines, motors, and power stations), discussing both a benefit and a limitation of this effort.
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Benefit: Even a small efficiency improvement, applied to technology used on a massive scale (such as car engines or power stations), can save an enormous total amount of energy across a country or the world. This reduces the fossil fuels burned, lowers greenhouse gas emissions, and reduces running costs for individuals and businesses over time.
Limitation: Because no device can ever reach $100\%$ efficiency, technologies that are already quite efficient can only be improved by smaller and smaller amounts — often at increasing cost, complexity, and research time, a pattern known as diminishing returns. Resources spent chasing very small further efficiency gains might sometimes achieve more if instead directed at reducing overall energy demand or switching to renewable energy sources altogether.
Conservation of Energy 20 questions
The law of conservation of energy is one of the most important ideas in science.
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A ball of mass $2\,\text{kg}$ is released from rest at the top of a frictionless hill $5\,\text{m}$ high. Take $g = 10\,\text{N/kg}$. Using the law of conservation of energy, calculate the kinetic energy of the ball when it reaches the bottom of the hill.
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Since the hill is frictionless, all of the ball's GPE at the top transforms into KE by the time it reaches the bottom (no energy is transformed into other, non-useful forms).
$$ E_p(\text{top}) = mgh = 2 \times 10 \times 5 = 100\,\text{J} $$
Answer: $E_k(\text{bottom}) = 100\,\text{J}$
A trolley of mass $4\,\text{kg}$ has $72\,\text{J}$ of kinetic energy. Calculate its speed.
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Step 1 — Rearrange the kinetic energy formula:
$$ E_k = \tfrac{1}{2}mv^2 \;\Rightarrow\; v = \sqrt{\frac{2E_k}{m}} $$
Step 2 — Substitute:
$$ v = \sqrt{\frac{2 \times 72}{4}} = \sqrt{36} $$
Answer: $v = 6\,\text{m/s}$
A stone of mass $0.5\,\text{kg}$ is dropped from a height of $20\,\text{m}$ and falls freely, with air resistance small enough to ignore. Take $g = 10\,\text{N/kg}$.
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Answer: $v = 20\,\text{m/s}$
A pendulum bob of mass $0.2\,\text{kg}$ is pulled to one side and released from rest. At its highest point it has $4\,\text{J}$ of gravitational potential energy relative to its lowest point. Assume there is no friction at the pivot and no air resistance.
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Answer: $v = 6.32\,\text{m/s}$ (3 s.f.)
A skateboarder has $800\,\text{J}$ of GPE at the top of a ramp. At the bottom of the ramp, she has $650\,\text{J}$ of kinetic energy.
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A $2\,\text{kg}$ ball is released from rest at a height of $8\,\text{m}$ on a frictionless track. It rolls down, then up a second frictionless hill. Take $g = 10\,\text{N/kg}$.
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Energy transformed to non-useful forms $= 160 - 130 = 30\,\text{J}$ (mainly to heat and sound, due to friction and air resistance).
A student wants to investigate how the height a ball is dropped from affects the height it rebounds to, in order to explore how energy is transformed during the bounce.
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A student wants to investigate how the surface material of a ramp (for example, smooth plastic compared with rough carpet) affects how much of a trolley's starting GPE is converted into KE by the bottom of the ramp. A light gate connected to a timer is used to measure the trolley's speed as it crosses the bottom of the ramp.
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In the ball-drop investigation above, measuring the exact rebound height by eye is difficult, because the ball reaches its highest point only briefly before falling again.
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| Position | Height (m) | GPE (J) | KE (J) |
|---|---|---|---|
| Top | 10 | 200 | 0 |
| Middle | 5 | 100 | ? |
| Bottom | 0 | 0 | 200 |
The table shows the GPE and KE of a $2\,\text{kg}$ ball at three points as it falls freely (ignore air resistance). Using the law of conservation of energy (total mechanical energy $=$ GPE $+$ KE $=$ constant, since there is no air resistance), calculate the missing KE value at the middle point.
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At the top, all the energy is GPE, so the total mechanical energy is $200 + 0 = 200\,\text{J}$. This total stays constant throughout the fall.
At the middle point: $$ \text{KE} = 200 - 100 = 100\,\text{J} $$
| Point | Height (m) | GPE (J) | KE (J) |
|---|---|---|---|
| A (start) | 20 | 2000 | 0 |
| B (bottom) | 0 | 0 | 1800 |
| C (top of 2nd hill) | 15 | 1500 | 200 |
A $10\,\text{kg}$ cart on a roller-coaster track is measured at three points, A, B and C.
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| Drop height (cm) | 100 | 50 | 25 |
|---|---|---|---|
| Rebound height (cm) | 64 | 32 | 16 |
The table shows the drop and rebound heights of a bouncy ball, all measured on the same surface.
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| Position | Height above lowest point (m) | GPE (J) | KE (J) |
|---|---|---|---|
| Start (released) | 0.2 | 1 | 0 |
| Lowest point | 0 | 0 | 1 |
A pendulum bob of mass $0.5\,\text{kg}$ is released from rest, as shown in the table. Take $g = 10\,\text{N/kg}$. Assume no friction or air resistance.
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Answer: $v = 2\,\text{m/s}$
| Ramp | Height (m) | Length of ramp (m) | Speed at bottom (m/s) |
|---|---|---|---|
| Steep | 5 | 6 | 10 |
| Shallow | 5 | 12 | 10 |
Two frictionless ramps, one steep and one shallow, both start at the same height. A ball of the same mass is released from rest at the top of each.
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| Point | Height (m) | GPE (J) | KE (J) |
|---|---|---|---|
| A (start) | 10 | 2000 | 0 |
| B | 4 | 800 | 1050 |
| C (end, flat ground) | 0 | 0 | 1600 |
A $20\,\text{kg}$ skateboarder is measured at three points, A, B and C, in a skate park.
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| Trial | 1 | 2 | 3 | 4 |
|---|---|---|---|---|
| Release height (cm) | 80 | 80 | 80 | 80 |
| Height reached on far ramp (cm) | 68 | 52 | 70 | 69 |
A student releases a trolley from rest on a curved track and measures the height it reaches on a second ramp on the far side, repeating the test four times from the same $80\,\text{cm}$ release height.
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In a normal car, the brakes convert the car's kinetic energy into heat through friction, which spreads into the surroundings and cannot be reused. Many electric and hybrid cars instead use "regenerative braking", where the motor runs in reverse to convert kinetic energy back into electrical energy, stored in the battery for later use.
Discuss one benefit and one drawback of regenerative braking systems.
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Benefit: Regenerative braking significantly improves the overall efficiency of a vehicle by reusing energy that would otherwise be transformed into wasted heat, instead storing it as useful electrical energy in the battery. This extends the vehicle's driving range and reduces the total energy (and fuel or electricity) it needs.
Drawback: Regenerative braking systems add extra cost, weight, and complexity to a vehicle, since the motor must be able to work in reverse as a generator as well as drive the wheels. They also typically cannot recover all of the car's kinetic energy — some is still transformed into heat and sound — and under heavy or emergency braking, conventional friction brakes are usually still needed alongside the regenerative system for full stopping power.
Occasionally, inventors claim to have built a "perpetual motion machine" — a device that, once started, keeps producing energy forever without any energy input, or that produces more useful energy than is put into it.
Discuss one way the law of conservation of energy helps protect people from such claims, and one drawback that arises when this scientific reasoning is ignored.
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How the law helps: Because energy cannot be created, only transformed, the law of conservation of energy immediately tells scientists and engineers that a machine which produces more energy output than input, indefinitely, is impossible — some energy is always transformed into other forms (such as heat and sound, through friction and air resistance) in any real machine. This lets people quickly and confidently identify a claimed "free energy" device as impossible, without needing to test it in detail.
Drawback when ignored: Despite this, people offering "perpetual motion" or "free energy" devices have, throughout history, defrauded investors and members of the public out of large amounts of money, because sound scientific reasoning based on the conservation of energy is sometimes ignored in favour of exciting but impossible promises — a clear example of how understanding a fundamental scientific law helps protect people from being misled.
Pumped-storage hydroelectric power stations pump water uphill into a reservoir using electrical energy when electricity demand (and price) is low, then release the water back downhill through turbines to generate electricity again when demand is high.
Evaluate the benefit and the limitation of this technology, using ideas about conservation of energy.
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Benefit: This allows excess electrical energy — for example, from wind turbines generating more electricity than is needed overnight — to be stored, by transforming it into gravitational potential energy of the pumped water, rather than being wasted. This stored energy can then be converted back into electricity within minutes when it is needed most, helping match a variable electricity supply (especially from renewable sources) to variable demand.
Limitation: By the law of conservation of energy, no energy transformation is ever $100\%$ efficient — pumping the water uphill wastes some energy as heat and sound in the pumps and pipes, and generating electricity again as the water flows back down wastes further energy in the turbines and generators. Overall, a pumped-storage system typically returns noticeably less electrical energy than was originally used to pump the water uphill (a round-trip efficiency of roughly $70$–$80\%$), so it should be understood as an efficient way to store energy for later use, not a way to create extra energy.
Work and Power (basic) 20 questions
A worker pushes a crate with a steady force of $20\,\text{N}$, moving it $5\,\text{m}$ in the direction of the force. Calculate the work done on the crate.
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Step 1 — State the formula:
$$ W = F \times d $$
Step 2 — Substitute:
$$ W = 20 \times 5 $$
Answer: $W = 100\,\text{J}$
An electric motor transfers $500\,\text{J}$ of energy in $10\,\text{s}$. Calculate its power output.
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Step 1 — State the formula:
$$ P = \frac{W}{t} $$
Step 2 — Substitute:
$$ P = \frac{500}{10} $$
Answer: $P = 50\,\text{W}$
In physics, work is only done on an object when a force causes it to move some distance in the direction of that force.
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A forklift truck exerts an upward force of $4000\,\text{N}$ to lift a pallet, doing $12\,000\,\text{J}$ of work in the process.
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Answer: $d = 3\,\text{m}$
An electric winch does $6000\,\text{J}$ of work lifting a load in $15\,\text{s}$.
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A weightlifter lifts an $80\,\text{kg}$ barbell a height of $2\,\text{m}$. Take $g = 10\,\text{N/kg}$.
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A pump does $90\,000\,\text{J}$ of work raising water in $2.5$ minutes.
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Answer: $80\%$
A student wants to measure her own power output by timing how long it takes her to climb a flight of stairs of known height, then calculating the work done against gravity and her power output.
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A student wants to investigate how carrying an additional load (a rucksack) affects the power she needs to develop to climb the same flight of stairs at her fastest safe pace.
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In the stair-climbing power investigation, the student repeated the climb five times at each load, and noticed her power readings became gradually lower with each repeat.
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| Box | Force (N) | Distance moved (m) | Work done (J) |
|---|---|---|---|
| A | 50 | 4 | 200 |
| B | 30 | 6 | ? |
Calculate the work done moving Box B.
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$$ W = F \times d = 30 \times 6 $$
Answer: $W = 180\,\text{J}$
| Appliance | Power (W) | Time used (hours) |
|---|---|---|
| Toaster | 1000 | 0.1 |
| Hairdryer | 1500 | 0.2 |
| Phone charger | 5 | 3 |
The table shows three appliances and how long each was used for on a particular day.
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The claim is incorrect: even over $3$ hours, the phone charger only uses $54\,\text{kJ}$, far less than the $360\,\text{kJ}$ used by the toaster in a single $6$-minute use. This is because total energy used depends on both the power rating and the time used — the toaster's much higher power ($1000\,\text{W}$ vs $5\,\text{W}$) far outweighs its much shorter usage time.
The bar chart shows the approximate average power output of a person doing different activities.
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This is $\dfrac{36\,000}{3000} = 12$ times more work than walking for the same $30\,\text{s}$, matching the fact that the sprint's power output ($1200\,\text{W}$) is exactly $12$ times greater than walking's ($100\,\text{W}$).
| Load carried (kg) | 0 | 5 | 10 |
|---|---|---|---|
| Total work done (J) | 2400 | 3000 | 3600 |
| Time taken (s) | 12 | 14 | 15 |
A student climbs the same flight of stairs carrying different loads, and the work done and time taken are recorded.
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| Crate | Force needed (N) | Height lifted (m) | Work done (J) |
|---|---|---|---|
| 1 | 200 | 3 | 600 |
| 2 | 200 | 5 | 1000 |
| 3 | 200 | 4 | 1200 |
| 4 | 200 | 6 | 1200 |
A crane lifts four crates using the same $200\,\text{N}$ force, to different heights. The work done for each was calculated using $W = F \times d$.
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| Trial | Mechanical work done cranking (J) | Time (s) | Electrical energy produced (J) |
|---|---|---|---|
| 1 | 1200 | 20 | 900 |
| 2 | 1500 | 25 | 1125 |
| 3 | 1000 | 20 | 300 |
A hand-crank generator's mechanical work input and electrical energy output were measured over three trials.
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| Student | Mass (kg) | Height of stairs (m) | Time taken (s) |
|---|---|---|---|
| A | 50 | 6 | 12 |
| B | 65 | 6 | 15 |
| C | 80 | 6 | 12 |
Three students each climb the same flight of stairs, and their mass and time taken are recorded. Take $g = 10\,\text{N/kg}$.
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Fast electric vehicle chargers can deliver very high power (for example, $150\,\text{kW}$) to recharge a car's battery in a fraction of the time taken by a standard home charger (about $7\,\text{kW}$).
Discuss one benefit and one drawback of installing many high-power fast chargers.
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Benefit: Much shorter charging times make electric vehicles far more convenient for long journeys, removing one of the main barriers to owning one. This can encourage more people to switch away from petrol and diesel cars, potentially reducing local air pollution and emissions from transport.
Drawback: Delivering very high power to many vehicles at once places a large demand on the local electricity grid, which may require expensive upgrades to cables and substations to cope. If that electricity is generated from fossil fuels, fast charging does not necessarily reduce overall emissions as much as expected, and the high currents involved can also cause faster degradation of a car's battery over time.
In some areas without reliable access to mains electricity, hand-crank or pedal-powered generators are used to charge radios, lights, and phones, or to pump water, using a person's own muscular effort to develop power.
Discuss one benefit and one drawback of these human-powered devices.
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Benefit: These devices let people without access to mains electricity or fuel still generate small amounts of usable power for essential needs — such as lighting, charging a phone, or pumping water — using only their own effort, with no fuel costs or dependence on grid infrastructure. This is especially useful in remote areas or during power cuts.
Drawback: The human body can only sustainably develop a fairly small amount of power for extended periods (a fit adult can typically sustain roughly $75$–$100\,\text{W}$), so these devices can only provide small amounts of energy compared with mains electricity, and require ongoing physical effort and cause fatigue in the user — limiting how much they can practically be relied upon.
Machines such as engines and motors can sustain far higher continuous power output than any human, which is a major reason machines have replaced manual human labour in many industries, such as agriculture and construction.
Evaluate the benefit and the drawback of this widespread mechanisation, using ideas about power.
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Benefit: Because machines can sustain far higher power output than a human for far longer without tiring, tasks such as ploughing fields, lifting heavy loads, or manufacturing goods can be completed vastly faster and more efficiently than by manual labour. This has massively increased productivity and removed much of the physically exhausting, and sometimes dangerous, work that used to be required of people.
Drawback: Widespread mechanisation has eliminated many jobs that used to be done by people, causing unemployment and economic hardship in communities that relied on that manual work. In addition, the machines themselves usually require a continuous supply of fuel or electricity to sustain their high power output, and producing that energy can have its own significant environmental and economic costs.
Renewable vs Non-Renewable Resources 20 questions
Energy resources can be classified as renewable or non-renewable.
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Define what it means for an energy resource to be described as "renewable".
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A renewable energy resource is one that is naturally replenished (restored) at a rate similar to, or faster than, the rate at which it is used — so it will not run out on a human timescale. Examples include solar, wind, and hydroelectric power.
A $3\,\text{kW}$ electric heater is switched on for $4$ hours. Calculate the energy it uses, in kWh.
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Step 1 — State the formula:
$$ E = P \times t $$
Step 2 — Substitute:
$$ E = 3\,\text{kW} \times 4\,\text{h} $$
Answer: $E = 12\,\text{kWh}$
A wind turbine has a power output of $2\,\text{MW}$ when operating at full capacity. It operates at full capacity for $6$ hours on a particularly windy day.
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A solar farm has a maximum power output of $50\,\text{MW}$, but because of clouds and night-time, it only generates electricity at close to this maximum rate for an average of $5$ hours per day.
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A small hydroelectric generator on a stream has a constant power output of $15\,\text{kW}$.
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Answer: $t = 12\,\text{hours}$
A wind farm consists of $40$ turbines, each with a power output of $2.5\,\text{MW}$ when running at full capacity. On average, due to varying wind speeds, the wind farm actually operates at only $35\%$ of this full-capacity output over the course of a year (its overall capacity factor). There are $8760$ hours in a year.
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Answer: $306\,600\,\text{MWh}$
A student builds a model wind turbine using a small electric motor as a generator, with blades attached to its shaft. She wants to investigate how the number of blades affects the voltage produced when a desk fan blows air onto it at a fixed speed.
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A student wants to investigate how the angle between a solar panel and a fixed lamp affects the voltage the panel produces, using a protractor to set each angle and a voltmeter to measure output.
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In the model wind turbine investigation above, the student noticed the voltmeter reading fluctuated noticeably (going up and down) even while testing a single number of blades at a constant fan setting.
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| Source | Coal | Gas | Renewables | Nuclear |
|---|---|---|---|---|
| Percentage of electricity generated | 20% | 30% | 40% | 10% |
A country generates a total of $300\,\text{TWh}$ of electricity in a year, split between sources as shown. Calculate how much electricity (in TWh) was generated from renewable sources.
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$$ 300 \times 0.40 $$
Answer: $120\,\text{TWh}$
The bar chart shows the percentage of a country's electricity generated from renewable sources over several years.
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| Source | CO? emissions (g per kWh) | Electricity generated (kWh) |
|---|---|---|
| Coal | 900 | 1000 |
| Gas | 400 | 1000 |
| Wind | 10 | 1000 |
The table shows typical carbon dioxide emissions per unit of electricity generated by three sources.
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| Time | 8am | 10am | 12pm | 2pm | 4pm |
|---|---|---|---|---|---|
| Solar panel output (kW) | 0.5 | 2.0 | 3.5 | 1.0 | 2.5 |
The table shows the power output of a solar panel measured at different times on a partly cloudy day.
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| Installation | Number of turbines | Homes powered per turbine | Total homes powered |
|---|---|---|---|
| Wind farm A | 25 | 800 | ? |
| Wind farm B | 40 | 800 | 32 000 |
The table compares two wind farms, where each turbine on average powers the same number of homes.
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| Year | Fossil fuels (%) | Renewables (%) | Nuclear (%) |
|---|---|---|---|
| 2000 | 65 | 18 | 17 |
| 2023 | 55 | 30 | 15 |
The table shows the approximate global share of electricity generation from different sources in 2000 and 2023. A news headline claims: “Renewables have already overtaken fossil fuels as the world's main source of electricity.”
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| Year | Rated capacity (MW) | Actual annual energy generated (GWh) |
|---|---|---|
| 2019 | 100 | 300 |
| 2020 | 100 | 310 |
| 2021 | 100 | 90 |
| 2022 | 100 | 295 |
| 2023 | 100 | 305 |
A wind farm's rated capacity and actual annual energy generated are shown for five years. A source's capacity factor is calculated as: $$ \text{capacity factor} = \frac{\text{actual annual energy generated}}{\text{maximum possible annual energy at full rated capacity}} \times 100\% $$ where the maximum possible annual energy $=$ rated capacity (MW) $\times\ 8760$ hours (converted to GWh by dividing by $1000$).
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Solar panels and wind turbines produce no direct CO? emissions while generating electricity, but manufacturing them requires large amounts of energy and raw materials, including some rare metals, and they eventually need replacing or recycling.
Discuss one benefit and one drawback of a country switching rapidly to wind and solar power.
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Benefit: A rapid switch dramatically cuts the ongoing CO? emissions and air pollution produced while generating electricity, helping combat climate change and improving air quality and public health over time. It also relies on free, effectively endless energy sources (sunlight and wind), rather than a finite fuel that must be continually mined, extracted, and burned.
Drawback: Manufacturing solar panels and wind turbines requires large amounts of energy, raw materials, and mining — including some rare or limited metals — meaning there is a significant upfront environmental impact and cost before any "clean" electricity is even generated. Responsibly disposing of and recycling old panels and turbine blades at the end of their working life is also a growing challenge, since recycling infrastructure for these specific materials is still limited in many places.
Because wind and solar power output depends on the weather, a country relying heavily on them needs a way to supply electricity during periods of little wind or sun — for example, using large-scale battery storage that stores excess electricity generated on windy or sunny days for use later.
Discuss one benefit and one drawback of countries building large-scale battery storage to support renewable energy.
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Benefit: Large-scale storage lets a country continue to rely mainly on renewable, low-emission electricity even when generation temporarily drops, by storing excess energy from windy or sunny periods for use later — improving the overall reliability of a renewable-heavy electricity grid without needing to burn fossil fuels as backup.
Drawback: Mining the raw materials needed for large-scale batteries (such as lithium and cobalt) has its own significant environmental impact, and in some cases raises human-rights concerns in the regions where they are mined. Batteries are also very expensive to build at a national scale, and typically only store enough energy for hours to a few days — not necessarily enough to cover longer periods of low wind or sun that can sometimes last for weeks.
Some developing countries and remote communities without access to a national electricity grid have started installing small-scale solar and wind systems directly in their villages, rather than waiting for large fossil-fuel power stations and long-distance transmission cables to reach them.
Evaluate the benefit and the challenge of this approach for these communities.
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Benefit: Small-scale solar and wind systems can be installed locally, relatively quickly, and often more cheaply than building a large power station and hundreds of kilometres of transmission cables. This gives communities access to electricity — for lighting, refrigeration, communication, and healthcare equipment — far sooner than waiting for national grid expansion, and without the cost and supply difficulties of transporting fossil fuels to remote areas.
Challenge: Small-scale renewable systems still depend on the weather, and without a large national grid to share power with other regions, or large-scale storage, communities can face periods without reliable electricity during poor weather. The technology can also be expensive to install and maintain, with specialist parts and expertise sometimes not locally available — meaning ongoing funding and support are needed for the systems to remain reliable in the long term.