Energy Stores & Transfers
Energy is never made or lost: only moved. Name the stores it sits in, the pathways it moves along, and calculate the energy in a moving, lifted or heated object.
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Energy Stores & Transfers
Energy cannot be created or destroyed, only transferred from one store to another. That single idea, conservation of energy, runs through all of physics. A store is where energy is held, and every object with energy has it in one or more of these:
Match each object to its main energy store
- A car driving along
- A stretched spring
- A book on a high shelf
- A lump of coal
- Kinetic store
- Elastic potential store
- Gravitational potential store
- Chemical store
Which are stores?
Pick the TWO energy STORES from this list.
- Kinetic
- Chemical
- Heating
- Electrical
Energy transfers
Energy moves between stores along pathways: the four transfers: mechanical (a force doing work), electrical (a current), heating, and radiation (e.g. light). ⚠️ Use the store and transfer names precisely. Banned phrases like "heat energy" lose marks: say the thermal store and the heating transfer instead.
Store or transfer?
Which of these is an energy TRANSFER (a pathway), not a store?
- Heating
- Chemical
- Kinetic
- Elastic
Kinetic energy
A moving object has energy in its kinetic store. You must recall this equation: Ek = ½ m v² (energy in joules, mass in kg, speed in m/s). The speed is squared, so doubling the speed gives four times the kinetic energy.
Kinetic energy
A 2 kg ball moves at 3 m/s. Work out its kinetic energy using Ek = ½ m v². Give your answer in joules (J).
Gravitational potential energy
Lifting an object stores energy in its gravitational potential store. Recall: Ep = m g h (mass in kg, gravitational field strength g = 9.8 N/kg, height in m). Raise the mass or the height and the stored energy goes up in proportion.
Gravitational PE
A 2 kg book is lifted to a shelf 5 m high. Using Ep = m g h with g = 9.8 N/kg, how much gravitational potential energy does it gain (in J)?
Order the method
Put the steps for a physics energy calculation in order (worked for kinetic energy).
- Write the equation: Ek = ½ m v²
- Check the units (mass in kg, speed in m/s) and convert if needed
- Substitute the values: ½ × 2 × 3²
- Calculate and add the unit: ½ × 2 × 9 = 9 J
Recall it, or look it up?
Some energy equations are printed on the exam sheet and some are not. Knowing which is which is worth revision time on its own: memorising a given equation wastes effort, and expecting a recalled one to appear costs you the question.
Heating something up
Using ΔE = m c Δθ, how much energy is needed for a 3 kg block with c = 100 J/kg°C to rise by 4 °C? Give your answer in J.
Power
Power is the rate of energy transfer: how much energy is moved each second. Recall: P = E ÷ t (power in watts, energy in joules, time in seconds). One watt is one joule per second. A more powerful device transfers the same energy in less time.
Work out the power
A device transfers 100 J of energy in 20 seconds. What is its power in watts (W)? (P = E ÷ t.)
Pick the equation
Four questions, none of which names an equation. Choose the one you would reach for.
- A 1200 kg car is travelling at 15 m/s. How much energy is in its kinetic store?
- A 20 kg box is lifted 3 m onto a shelf. How much energy is now in its gravitational store?
- How much energy is needed to raise 2 kg of water by 30 °C?
- A kettle transfers 96 000 J in 80 seconds. What is its power?
The energy rules
Energy is held in energy _____ (such as kinetic and chemical) and moves between them by energy _____ (such as heating and electrical). Power is the rate of energy transfer, measured in _____.