Where does energy go

Part 1 of 8Useful and wasted energy

• Useful energy energy changed into the form that is wanted
• Wasted energy energy changed into forms that are not wanted
• Always some wasteevery time energy is changed or transferred some of it is wasted

Part 2 of 8Motorcycle example

• Usefulchemical energy in the fuel is changed to kinetic energy to move the motorcycle and rider
• Wastedchemical energy is also changed into thermal energy and sound
• Efficiencyonly about one quarter (25%) of the chemical energy is used for movement
• Wastethe other three quarters (75%) is wasted

Part 3 of 8Dissipation

• Dissipated energy energy that spreads out where there is no use for it
• Cannot be recoveredthermal and sound energy cannot be gathered and brought back into one place to be stored or used
• Thermal energysome thermal energy is wasted every time energy is changed or transferred
• Alwaysfor everything that uses an energy change or transfer, some energy will always be dissipated

Part 4 of 8Comparing two lamps

	Lamp A	Lamp B
Electrical energy changed to light	About 15%	About 50%
Wasted as thermal energy	About 85%	About 50%
Light	Same brightness	Same brightness
Waste	More wasted	Less wasted

Part 5 of 8Heating water on a fire

• Energy changechemical energy from wood is changed to thermal energy by burning
• Usefulthe thermal energy that heats the water
• Wastedthermal energy also heats the rocks, the metal container and the air around it
• Escapingsome thermal energy leaves in the steam
• Lightthe fire also changes energy into light, which is wasted

Part 6 of 8Cooling of hot tea

1. Measure the temperature of very hot tea in a cup.
2. Stir the tea with a spoon before each reading so the temperature is the same throughout.
3. Record the time and temperature every minute.
4. Plot a line graph with time on the horizontal axis and temperature on the vertical axis.
5. Conclude that the temperature falls quickly at first and then more slowly, as thermal energy dissipates to the surroundings.

Part 7 of 8What the cooling shows

• Thermal energythe higher the temperature of the tea, the more thermal energy it has
• Where it goesthe energy dissipates to the cup, the air and the spoon
• Slower coolinga lid or insulated cup makes the tea cool more slowly
• Faster coolinga wide cup or stirring makes it cool more quickly
• End pointthe tea stops cooling when it reaches the temperature of its surroundings

Part 8 of 8Distance from a hot object

• Pattern — as you move away from a hot object you feel less heat
• Graph shape — the temperature goes down as distance increases