Topic 1 of 6

Account for energy stores and transfers

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

Change in total energy of the system = energy transferred in - energy transferred out

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.

320 J input = 200 J gravitational increase + 120 J internal increase

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.

The 320 J input is accounted for by two store increases. The 200 J of mechanical work and the 200 J gravitational increase describe the same transfer and its result; they are not two separate outputs.

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.