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Energy is conserved: a transfer changes where energy is stored, rather than creating or destroying it. Stores include kinetic, thermal, chemical, gravitational and elastic. Energy can be transferred mechanically, electrically, by heating or by radiation. Choose a system boundary before describing the changes. Energy dissipated to the surroundings is still conserved but becomes less useful for the intended task. Avoid treating electrical energy as a substance that disappears inside an appliance.
Video by Cognito. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Describe energy transfers using stores and pathways
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What happens to kinetic energy if speed doubles?
It quadruples. Speed is squared in the equation.
After watching 2: What does one watt mean?
One joule per second. Power is a rate of energy transfer.
What you'll be able to do
Energy is conserved: a transfer changes where energy is stored, rather than creating or destroying it. Stores include kinetic, thermal, chemical, gravitational and elastic. Energy can be transferred mechanically, electrically, by heating or by radiation. Choose a system boundary before describing the changes. Energy dissipated to the surroundings is still conserved but becomes less useful for the intended task. Avoid treating electrical energy as a substance that disappears inside an appliance.
Kinetic energy is and gravitational potential-energy change is near Earth’s surface. Use kg, m, m s⁻¹ and the supplied gravitational field strength to obtain joules. Doubling speed quadruples kinetic energy; doubling mass doubles it. Work done by a force along a displacement is W = Fs. Changes in stored energy can be connected to mechanical work where the model’s assumptions apply.
Power is energy transferred per second, P = E/t, measured in watts. Efficiency is useful energy output divided by total input, or the equivalent power ratio, optionally multiplied by 100%. Compare like quantities. A powerful device is not necessarily efficient: it may transfer energy rapidly while dissipating a large fraction. Insulation and lubrication can reduce unwanted transfers, but their value depends on the process and the energy saved.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: A motor takes 200 J and gives 150 J useful output in 5.0 s. Find efficiency and useful power.
75% efficient; useful power 30 W.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.