Energy transfers questions

Energy transfers syllabus

Questions on energy transfers can test recall, calculations, explanations, diagrams, data handling and practical skills. You should be able to:

• 4.2 describe energy transfers involving energy stores: energy stores: chemical, kinetic, gravitational, elastic, thermal, magnetic, electrostatic, nuclear energy transfers: mechanically, electrically, by heating, by radiation (light and sound)
• 4.3 use the principle of conservation of energy
• 4.4 know and use the relationship between efficiency, useful energy output and total energy output: efficiency = useful energy output / total energy output × 100%
• 4.5 describe a variety of everyday and scientific devices and situations, explaining the transfer of the input energy in terms of the above relationship, including their representation by Sankey diagrams
• 4.6 describe how thermal energy transfer may take place by conduction, convection and radiation
• 4.7 explain the role of convection in everyday phenomena
• 4.8 explain how emission and absorption of radiation are related to surface and temperature
• 4.9 practical: investigate thermal energy transfer by conduction, convection and radiation
• 4.10 explain ways of reducing unwanted energy transfer, such as insulation

How to answer energy transfers questions

1. Name the energy stores at the start and end and state the pathway that transfers energy.
2. Separate useful output from wasted output when calculating or explaining efficiency.
3. Show the equation, substitution and unit for work, power and energy calculations.
4. When evaluating resources, compare reliability, environmental effects, cost and response time.

Energy transfers example questions and worked answers

These examples show how information in a diagram, graph or experimental context becomes part of a complete exam answer.

Example 1: Following energy through a system

June 2015 (R) · Paper 1P · Question 11

Question 1

A student investigates how the energies of a ball and spring change when the ball and spring vibrate together.

The diagrams and bar charts show how the energies of the ball and spring vary with the position of the ball.

The ball has a mass of 1 kg.

GPE = gravitational potential energy of the ball (zero at ground level)

KE = kinetic energy of the ball

EPE = elastic potential energy of the spring

Use information from the diagrams and the bar charts to describe what happens to the energy, speed and position of the ball as it vibrates on the spring.

Final answer

As the ball moves from the top to the bottom of the spring, its GPE decreases and the spring’s EPE increases. At the top, the ball has 25 J of GPE, 1 J of EPE and zero KE. In the middle, its KE and speed are greatest: its KE is 4 J and its speed is 2.8 m/s. At the bottom, it has 5 J of GPE, 21 J of EPE and zero KE, so it is momentarily stationary at both ends of its motion. The total energy remains constant at 26 J. As the ball moves back up, the changes in the energy stores reverse.

Mark scheme points

Any six valid points are needed for full marks. This answer uses these six:

1. M1 KE is zero at the top and bottom, and greatest (4 J) in the middle.
2. M2 GPE is greatest (25 J) at the top and least (5 J) at the bottom.
3. M3 EPE is least (1 J) at the top and greatest (21 J) at the bottom.
4. M4 Total energy is constant in all three charts (26 J).
5. M5 The ball’s speed is greatest in the middle.
6. M6 The ball’s speed in the middle is 2.8 m/s.

Explanation

The ball is at its highest position at the top, so its GPE is greatest there. As it moves down, the spring stretches and stores more EPE. At the bottom, the ball is momentarily stationary, so its KE is zero; it is also momentarily stationary at the top.

The ball’s KE is greatest in the middle, so its speed is greatest there. Use the kinetic energy equation to find the speed:

KE = ½mv²
4 J = ½ × 1 kg × v²
v = √(8 m²/s²) ≈ 2.8 m/s

The charts show that although the amounts in the energy stores change as the ball moves, their total remains 26 J.

Common mistakes

• Do not confuse the ball’s GPE with the spring’s EPE: GPE is greatest at the top, while EPE is greatest at the bottom.
• Do not say the ball is fastest at an endpoint; its KE and speed are greatest in the middle.
• Do not say the total energy changes: the charts show it stays constant while energy shifts between stores.

Example 2: Sketching cooling curves for huddled penguins

2017 specimen · Paper 1P · Question 12(a)(iv)

Question 2

Penguins are adapted to survive in cold conditions.

The adaptations help them to maintain a constant body temperature of 39 °C.

Penguins also crowd together in groups of many penguins.

A student wants to investigate how the temperature of a penguin is affected when they crowd together in groups.

She uses this apparatus.

Each test tube represents a penguin.

Draw a sketch graph of the results you predict the student will obtain.

Label and use the axes below.

Final answer

Label the horizontal axis time and the vertical axis temperature. Draw both lines starting at the same temperature and decreasing as time increases. The line for the single test tube is below the line for the huddle after the common starting point.

Mark scheme points

1. M1 Label the x-axis time and the y-axis temperature.
2. M2 Show both lines starting at the same temperature on the y-axis.
3. M3 Show both temperatures decreasing as time increases.
4. M4 Show the single test tube line below the huddle line throughout after the common starting point.

Explanation

The graph compares temperature with time, so time belongs on the horizontal axis and temperature on the vertical axis. Both test tubes start at the same temperature. Their temperatures then fall, but the huddled group stays warmer than the single test tube, so its line is higher.

Common mistakes

• Swapping the axes or leaving them without labels.
• Starting the lines at different temperatures, or drawing either temperature rising with time.
• Putting the single test tube line above the huddle line; the single test tube should be cooler after the shared starting point.

Practise Energy transfers questions

Build a focused practice set from this part of the Edexcel IGCSE Physics syllabus.