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A-Level Energy stores and transfers, work, kinetic and potential energy, fields, power and efficiency
What the A-Level syllabus expects for Energy stores and transfers, work, kinetic and potential energy, fields, power and efficiency, and how to practise it.
What the syllabus expects
- Recognise that energy resides in stores within a system and can pass from one store into another
- Name assorted energy stores and pathways, and lean on energy conservation when solving problems
- See work as energy moved mechanically, and define it as force multiplied by the displacement along that force's line, then use this
- Combine the definition of work with the straight-line constant-acceleration equations to arrive at Ek = ½mv2
- Apply Ek = ½mv2 in calculations from memory
- Understand a field as a region of space where an object can feel a force belonging to that field
- Define gravitational field strength as the gravitational pull per unit mass felt by a test mass at the point, and electric field strength as, for a positive test charge placed there, the electric pull it feels divided by its charge
- Map both gravitational and electric fields with field lines, covering uniform and radial layouts, and explain how equipotential surfaces relate to those lines
- Grasp that the force on a mass in gravity, or on a charge in an electric field, runs along the field lines, and that the field's work in shifting it equals minus the change in potential energy
- Keep the ideas of gravitational, electric and elastic potential energy separate from one another
- Remember that a stretched material's stored elastic energy shows up as the area beneath its force–extension curve, and use this in problems
- Define power as how quickly energy gets transferred
- Understand mechanical power as force times the velocity component along that force
- Appreciate what wasted energy means for real machines, and compute efficiency as the useful output divided by the total input
How it's examined
Questions on this topic most often ask you to find, determine, state. About 5% of the past-paper style questions in Rae's bank for this subject sit in this topic.
Worked examples
Example 1 (2 marks)
The apparatus shown is designed to launch a ball of mass 40 g straight upward. A spring with force constant 60 N m⁻¹ sits underneath a platform, and the ball rests on the platform at the spring's top. Take the spring and platform to have no significant mass, and ignore air resistance. (a) The spring is squashed down by a distance of 0.12 m. Find the force the compressed spring pushes on the ball with the moment it is let go.
Show the worked answer
At the instant of release the spring is compressed by x = 0.12 m, so by Hooke's law the force it exerts on the ball is: F = k x = 60 x 0.12 = 7.2 N.
Example 2 (2 marks)
(a) In place of a petrol engine, a prototype car uses a flywheel. The flywheel is spun up to a high speed before the journey begins. To move off, the flywheel is coupled to the drive wheels through a gear system, and the rotational energy it stores is used to drive the car forward. The flywheel has a moment of inertia of 80 kg m². Ahead of each journey it is spun up until its angular speed reaches 320 rad s⁻¹. (i) State the factors that determine the moment of inertia of the flywheel.
Show the worked answer
The moment of inertia of a body about an axis depends on: 1. The total mass of the flywheel. 2. The distribution of that mass relative to the axis of rotation (i.e. how far the mass lies from the axis – its radius/shape and the position of the axis). Mass placed farther from the axis contributes more to the moment of inertia.
Example 3 (3 marks)
This petrol car holds 0.055 m³ of fuel (1.87 GJ) and delivers 25% of that as useful work. Given 0.086 GJ of useful energy covers each 100 km, obtain the car's driving range. [3]
Show the worked answer
Principle: efficiency tells you what fraction of the energy stored in the fuel actually ends up doing useful work. Range is then found by seeing how many times the useful energy available covers the energy needed for a fixed distance. Step 1 - find the useful energy from a full tank. useful energy = efficiency × total energy in the fuel useful energy = 0.25 × 1.87 GJ useful energy = 0.4675 GJ (the other 75 per cent, about 1.40 GJ, is wasted mostly as heat and sound) Step 2 - use the energy cost per 100 km. The car needs 0.086 GJ of useful energy for every 100 km travelled. So the number of 100 km stretches the tank can cover is number of stretches = useful energy / energy per stretch number of stretches = 0.4675 GJ / 0.086 GJ number of stretches = 5.44 (both energies are in GJ, so the units cancel and this is a pure number - no conversion to joules is needed) Step 3 - convert that into a distance by multiplying by 100 km. range = 5.44 × 100 km range = 544 km The car's range is about 544 km. (The tank volume 0.055 m³ is not needed here, because the energy it holds, 1.87 GJ, has already been given.)
More worked questions on this topic
- A top-selling electric car measures 1.62 m high by 1.88 m wide, has drag coefficient 0.29 and p (3 marks)
More A-Level H2 Physics topics
Physical quantities, units, measurement uncertainty and vector basics · Types of force, turning effects and conditions for equilibrium · Kinematics, uniformly accelerated motion, momentum and Newton's laws · Falling freely, the gravitational potential energy of a uniform field, and how air resistance changes the motion · Impulse and the conservation of momentum and energy · Kinematics of uniform circular motion and centripetal acceleration · all of A-Level H2 Physics