Energy can't be created or destroyed

Science · Energy · ages 11-12

Name ______________________ Date ____________

In an isolated system with no energy entering or leaving, the total energy changes whenever its energy stores change.
Circle one: True False
What does the law of conservation of energy say?
- Energy can't be created or destroyed, only transferred
- Energy can be created by machines but never destroyed
- Energy is destroyed when it spreads into cooler surroundings
What is it called when energy spreads into thermal stores in the surroundings and becomes less available for a job?
- Energy storage
- Dissipation
- Energy creation
Why can't a fuel-burning car engine turn all of its fuel's energy into useful motion?
- Some energy dissipates as heat
- Some energy is destroyed by the engine
- Fuel contains energy only after the engine starts
A blender's motor becomes warm while running. Where does this thermal energy come from?
- Thermal energy appears without any energy transfer
- The motor stores all electrical energy without warming
- Some supplied energy warms the motor's parts
In a charging model, 100 J enters. Exactly 8 J transfers to surroundings; the rest increases only the battery's chemical store. What is that increase in J?
Answer: ______________
Squeezing a bike's brakes slows the bike down, and the brake pads get hot. What best explains this?
- The brakes create thermal energy without using the bike's kinetic energy
- The bike's kinetic energy is destroyed instead of transferred to the brakes
- Friction transfers kinetic energy to thermal energy
Mia says: 'A bouncing ball gets lower each bounce because its energy is being destroyed.' What's the mistake in her thinking?
- Energy is created at each bounce to replace destroyed energy
- Energy transfers to sound and heat; none is destroyed
- Energy is destroyed when the ball makes sound and warms the floor

Answer key

For grown-ups. Fold this page away before handing over the rest.

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False · No energy crosses the boundary, so the total stays constant even when individual stores change.
Energy can't be created or destroyed, only transferred · Energy cannot be created or destroyed. It can remain stored, transfer between stores or change form. Conservation does not require continual change.
Dissipation · Dissipation spreads energy into thermal stores. It reduces availability for the intended job without destroying energy.
Some energy dissipates as heat · Energy warms engine parts and exhaust gases, and friction adds thermal transfers. That energy is not useful motion, but it still exists.
Some supplied energy warms the motor's parts · Electrical resistance and friction can increase the motor's thermal store. Warmer parts then transfer energy to the surroundings.
92 · The model accounts for input as a transfer out plus a store increase: 100 minus 8 = 92 J.
Friction transfers kinetic energy to thermal energy · Friction between the brakes and wheel turns the bike's kinetic energy into heat, dissipating it into the surroundings.
Energy transfers to sound and heat; none is destroyed · Each bounce, some energy transfers away as sound and heat, so less is left as movement, but none of it vanishes.