Energy Stores and Transfers

Energy is never used up

It is tempting to talk about energy being made, or used up, or turned into a different kind. None of that is quite right, and getting the words right is most of this topic. Energy sits in an energy STORE, and it only ever MOVES from one store to another. It is never created and never destroyed. ⭐⭐ There are a handful of stores to know: the kinetic store of a moving object, the gravitational store of something lifted up, the elastic store of a stretched spring, the thermal store of a warm object, and the chemical store in fuel or food. Naming the STORE, not a type of energy, is what examiners want. ⭐ Between stores, energy moves along a transfer PATHWAY. It can be transferred mechanically when a force does work, electrically by a current, by heating from a hotter to a cooler object, or by radiation such as light. So a full description names the store it left, the store it reached, and the pathway between. ⭐⭐ Because energy is conserved, every question is really BOOKKEEPING. Which store lost energy, which store gained it, and how much? The equations exist only to answer that last part, putting a number on the energy in a store or moved along a pathway. This module is the stores, the pathways and the counting. How efficiently a machine keeps energy useful, and where wasted energy goes, is a different module. ⚠️ Carry one question the whole way through: which store filled, which store emptied, and by how much?

Stores, pathways, and counting

The first column is where energy sits, the second is how it moves, and the third is how you count how much.

Tap the two energy stores

Tap the TWO that name an energy STORE, rather than a way energy is transferred.

• The kinetic store of a rolling ball.
• The gravitational store of a book on a high shelf.
• Transfer by heating from a flame to a pan.
• Transfer electrically along a wire.

The falling ball

A ball is dropped and speeds up as it falls. Which store empties, and which fills?

• The gravitational store empties and the kinetic store fills
• The kinetic store empties and the gravitational store fills
• The chemical store empties and the thermal store fills
• No stores change, because energy is destroyed

The language of energy

Six terms this topic turns on. The first two are the core idea; the rest are stores and the counting.

Match each to what it means

Two stores, not pathways

Select the TWO that are energy STORES, rather than transfer pathways.

• The elastic store of a stretched spring
• The chemical store in a fuel
• Transfer by radiation, such as light
• Transfer mechanically by a force

Describing energy for marks

Energy questions are marked on precise language and tidy bookkeeping. ⭐⭐ NAME THE STORE AND THE PATHWAY. Say the energy left the chemical store, was transferred mechanically, and arrived in the kinetic store. Vague phrases like the energy was used up, or heat energy, are marked wrong; energy sits in stores and moves along pathways. ⭐ DO THE BOOKKEEPING. Because energy is conserved, if one store loses a certain amount, another store gains the same amount, unless some was dissipated. Start every answer by naming which store empties and which fills. ⭐⭐ PICK THE RIGHT EQUATION. Kinetic energy is one half times the mass times the speed squared, so remember to square the speed. Gravitational is mass times gravitational field strength times height. Specific heat is mass times specific heat capacity times temperature change. Power is the energy transferred divided by the time. KEEP THE UNITS STRAIGHT. Mass in kilograms, energy in joules, power in watts, and convert first if a value is given in grams or minutes. A right method with the wrong units still loses the mark. ⚠️ A wording habit. Never write heat energy; write thermal store, or energy transferred by heating. It is one of the most common ways to drop an easy mark. Keep the one question to hand: which store filled, which store emptied, and by how much?

Order the energy calculation

Put the steps of working out an energy transfer into a sensible order.

• Identify the store that empties
• Identify the store that fills
• Choose the equation for that store
• Put the values in, with the right units
• State the energy transferred, in joules

Work out the kinetic energy

A trolley of mass 4 kilograms moves at a speed of 3 metres per second. Its kinetic energy is one half times the mass times the speed squared. What is its kinetic energy, in joules?

The energy run

Five quick decisions about energy stores and transfers. Three lives.

Stock moving around a warehouse

Leave physics aside for a screen. Picture a big warehouse with several rooms, holding a fixed amount of goods, where nothing is delivered in and nothing is sold out during the day. ⭐ Boxes are moved between the rooms all day long, by a forklift, or a trolley, or along a conveyor. The rooms fill and empty as the day goes on, but the total number of boxes in the whole warehouse never changes, because none is made and none leaves. ⭐⭐ So if the manager wants to understand the day, the question is always the same bookkeeping: which room lost boxes, which room gained them, and how many. Answer that and you have described what happened, without ever needing a box to appear or vanish. And it is worth naming HOW the boxes moved, too. Saying they went by forklift, or by conveyor, tells you more than just saying they moved, because the method is part of the story. ⚠️ Hold that picture. A room is an energy store, moving boxes between rooms is a transfer, the forklift and conveyor are the different pathways, and the fixed total of boxes is energy that is never made or lost. Every energy question is the manager question: which room emptied, which filled, and by how much?

Finish each energy line

Energy sits in an energy _____, such as a kinetic or a gravitational one. Energy moves between stores by an energy _____, such as by heating or mechanically. The energy in the store of a moving object, found from its mass and speed, is _____ energy. The energy transferred each second, measured in watts, is the _____. The rule that energy is never created or destroyed is its _____.

Three energy descriptions

Three situations arise. For each, choose the response that names stores and pathways correctly.

• A rollercoaster car is released from the top of a high drop and speeds up as it comes down.
• A student writes that a kettle "gives the water heat energy" and asks whether that is the right wording.
• A moving car brakes to a stop, and a student says its kinetic energy was simply destroyed.

Describe an energy transfer

Describe the energy changes when a ball is thrown straight up and then falls back down, using the correct language of stores and pathways. Explain what is conserved, and name an equation you could use to find an energy in the process.

• Name the store the energy starts in and the pathway that launches the ball
• Describe the store changes on the way up and on the way down
• State that energy is conserved throughout
• Name one energy equation you could use and what it would tell you
• Avoid the phrase heat energy and use stores and pathways instead