Lesson 1 of 6
Account for energy stores and transfers
I can explain that physical systems store energy and that energy can transfer between stores.Syllabus
Syllabus 8867, 4(a). Explain that physical systems store energy and that energy can transfer between stores.
I can give examples of stores and transfers and apply conservation of energy to problems.Syllabus
Syllabus 8867, 4(b). Give examples of stores and transfers and apply conservation of energy to problems.
Make a guess. You are not marked.
A box slides to rest on a rough floor. Where has its kinetic energy gone?
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Into internal energy of the box and floor, which warm slightly.
Friction does work on the sliding surfaces. The kinetic energy is transferred to the internal energy of the box, the floor and the air, plus a little sound.
Start by naming the system: the objects you include in the account. Then identify which energy stores change and which transfers cross its boundary.
A moving object has kinetic energy. Interacting objects can have gravitational or electric potential energy, and a deformed material can store elastic potential energy. Chemical and internal stores also matter when explaining batteries, fuels, warming and friction.
- Kinetic store
- Associated with motion, such as a moving trolley. Its kinetic energy depends on its mass and speed.
- Potential-energy stores
- Associated with an interaction or configuration: the separation of a load and Earth, the positions of interacting charges, or the deformation of a spring.
- Chemical store
- A battery or fuel can supply energy through changes in its chemical system.
- Internal store
- Associated with microscopic motion and interactions within matter. A rise in temperature is one possible sign of an increase in internal energy.
A transfer is a process, not another store. Mechanical work transfers energy when a force acts through a displacement. Electrical transfer can carry energy from a battery to a motor. Radiation can transfer energy to an object and warm it. Heating is transfer associated with a temperature difference. Power describes how quickly energy transfers.
For example, a battery-driven lifting motor draws on the battery's chemical store. Energy transfers electrically to the motor and mechanically to the lifted load. The gravitational store belongs to the load-Earth system, because it depends on their separation.
Use conservation with a stated boundary
This account applies to a fixed collection of matter. If no net energy crosses its boundary, its total energy stays constant. Its kinetic-plus-potential energy need not stay constant: some energy may become internal energy.
Energy dispersed into a motor and its surroundings is still conserved. It is less useful for lifting the load. The intended task determines usefulness: warming water is useful for a heater, while unwanted warming reduces a lifting motor's useful output.
Worked energy account
A steady lift
A motor raises a load of weight 40.0 N through 5.00 m during a steady 4.00 s interval. The measured electrical input for that interval is 320 J. The load's speed does not change.
The lifting force is 40.0 N, so its work is force × upward displacement = 40.0 × 5.00 = 200 J. This transfer raises the load-Earth gravitational store by 200 J. The stated model assigns the remaining 320 - 200 = 120 J to internal energy of the motor and surroundings.
For this account, include the motor, load-Earth system and warmed surroundings, and exclude the electrical supply. No other energy change is included in the model.
Name the system and account for the same 320 J once
The bar widths share one energy scale. Lifting work transfers 200 J into the load-Earth gravitational store; do not count that transfer and the store increase twice.
Changing the boundary can change which terms are transfers and which are internal changes. Keep one boundary throughout a calculation, and count each amount once. The power comparison uses this same lift to distinguish an energy total from a rate.