7.2 - Energy Transfer Examples

Energy transfers in everyday devices

Every energy transfer can be described by naming three things: the store the energy starts in, the pathway it travels by, and the store it ends up in.

Examples of transfers in devices

• Electric iron - Transfers energy electrically from the mains to the thermal energy store of its metal plate.
• Hair dryer - Transfers energy electrically from the mains to the kinetic energy store of its fan and the thermal energy store of its heating element, with some energy carried away by sound.
• Battery-powered heater - Transfers energy electrically from the chemical energy store of the battery to its thermal energy store, then by heating to the surroundings.
• Mains lamp - Transfers energy electrically from the mains, with the useful output carried away by radiation as light and the rest wasted by heating.
• Motor in a power tool - Transfers energy electrically from the mains to the kinetic energy store of its moving parts, doing work mechanically on whatever it turns.

Energy transfers in everyday situations

The same four transfer pathways describe what happens in everyday situations. Working through a few common ones makes it easier to analyse any new situation.

Throwing, rolling and dropping

• Ball thrown upwards - Chemical store (person) to kinetic and gravitational potential stores (ball), by doing work.
  • The arm does work on the ball, transferring energy from the chemical energy store of the person to the kinetic and gravitational potential energy stores of the ball. As the ball rises and slows, energy transfers mechanically from its kinetic store to its gravitational potential store.
• Ball rolling up a slope - Kinetic store to gravitational potential store (ball), mechanically.
  • The ball does work against gravity, transferring energy mechanically from its kinetic energy store to its gravitational potential energy store. Friction transfers some energy mechanically to the thermal energy stores of the ball and slope, then by heating to the surroundings.
• Ball or rock dropped from a height - Gravitational potential store to kinetic store (object), by gravity doing work.
  • Gravity does work on the object as it falls, transferring energy from its gravitational potential energy store to its kinetic energy store.

Stopping and hitting

• Bat hitting a ball - Kinetic store (bat) to kinetic store (ball), mechanically.
  • Energy is transferred mechanically from the kinetic store of the bat to the kinetic store of the ball. The rest is wasted: transferred mechanically to the thermal stores of the bat and ball, and carried away by sound.
• Car braking - Kinetic store (car) to thermal store (brakes and surroundings), by friction doing work.
  • Friction between the brakes and wheels does work, transferring energy from the kinetic energy store of the car to the thermal energy store of the brakes and surroundings, with some carried away by sound.