OxfordAQA International A Level Physics
OxfordAQA International A Level Physics 3.13 Energy Sources Exam Questions
Free OxfordAQA International A Level Physics revision for 3.13 Energy Sources: what you need to know, common mistakes and exam-style questions with answers. Covers rotational motion, moment of inertia, angular momentum, wind power, solar cells and intensity, hydroelectric power and pumped storage systems.
Practise 3.13 Energy Sources with past papers
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About OxfordAQA International A Level Physics 3.13 Energy Sources
Energy sources is the final content section of Unit 4 (PH04), Energy and energy resources, in the OxfordAQA International A2 year, and it is distinctive to this specification. It starts with rotational dynamics, including moment of inertia, torque, angular momentum and rotational kinetic energy, which you need to understand turbines and flywheels. It then applies physics to wind energy, solar energy and hydroelectric power with pumped storage. It links back to circular motion, work and power, and electrical circuits. Examiners like calculations of power available from wind or falling water, the characteristics of solar cells from the required practical, and evaluations of how each source meets base-load and peak demand.
What you need to know
- 3.13.1 Rotational motion Moment of inertia I = sum of mr^2 and depends on how mass is distributed about the axis. Torque T = I alpha. Rotational kinetic energy = 1/2 I omega^2 and angular momentum = I omega, which is conserved when no external torque acts. The angular equivalents of the suvat equations apply for constant angular acceleration.
- Wind energy Kinetic energy of air passing through blade area A per second gives power = 1/2 rho A v^3. Not all of this can be extracted because the air must keep moving behind the turbine. Turbines in a wind farm are spaced to reduce turbulence from neighbouring turbines, and environmental factors such as noise and visual impact matter.
- Solar energy Solar intensity is power per unit area; from a point source it follows the inverse-square law. A solar cell's V-I characteristic shows maximum power at an optimum load. Cells are combined in series and parallel in arrays to supply the required voltage and current.
- Hydroelectric power Power = mass flow rate x g x height = rho Q g h for volume flow rate Q. Turbines convert gravitational potential energy of water into kinetic then electrical energy.
- Pumped storage Water is pumped uphill when demand is low and released quickly when demand peaks, giving fast back-up to base-load stations. Overall efficiency is below 100% because of losses in pumping and generation.
- Base-load and back-up Base-load stations run continuously; back-up stations must start quickly to meet sudden surges in demand. Wind and solar output vary with weather, so storage or back-up is needed.
Common mistakes in 3.13 Energy Sources
- Writing wind power as proportional to v^2. Mass flow rate is also proportional to v, so power is proportional to v^3.
- Using the blade length as the area. The swept area is pi r^2, where r is the blade length.
- Assuming a solar cell gives maximum power at maximum voltage or maximum current. Maximum power occurs at the knee of the I-V curve, at an optimum load resistance.
- Forgetting that moment of inertia depends on the axis of rotation, not just the mass.
- Claiming pumped storage is a source of energy. It is a store that returns less energy than was used for pumping.
Try three 3.13 Energy Sources exam-style questions
1. A wind turbine has blades 40 m long. Calculate the power in the wind passing through the blades when the wind speed is 10 m s-1. Density of air = 1.2 kg m-3. (3 marks)
Show the mark scheme answer
Area = pi x 40^2 = 5.0 x 10^3 m^2; power = 1/2 rho A v^3 = 0.5 x 1.2 x 5030 x 10^3; power = 3.0 x 10^6 W.
2. Water flows through a hydroelectric turbine at 50 m^3 s-1 from a height of 80 m. Calculate the maximum power available. Density of water = 1000 kg m-3, g = 9.81 m s-2. (2 marks)
Show the mark scheme answer
Mass flow rate = 50 x 1000 = 5.0 x 10^4 kg s-1; power = 5.0 x 10^4 x 9.81 x 80 = 3.9 x 10^7 W.
3. A flywheel has a moment of inertia of 2.0 kg m^2 and rotates at 50 rad s-1. Calculate its rotational kinetic energy and its angular momentum. (2 marks)
Show the mark scheme answer
Ek = 1/2 I omega^2 = 0.5 x 2.0 x 50^2 = 2.5 x 10^3 J; angular momentum = I omega = 2.0 x 50 = 100 kg m^2 s-1.
3.13 Energy Sources FAQs
What does OxfordAQA International A Level Physics 3.13 Energy Sources cover?
It covers rotational motion including moment of inertia, torque and angular momentum, wind energy, solar energy and solar cells, and hydroelectric power with pumped storage.
Which unit is 3.13 Energy Sources examined in?
It is examined in Unit 4 (PH04), Energy and energy resources.
Why does wind power depend on v cubed?
The kinetic energy of each kilogram of air depends on v^2, and the mass of air passing through the blades each second is rho A v. Multiplying gives power = 1/2 rho A v^3, so doubling the wind speed gives eight times the power.
