Physics · Conservation and dissipation of energy
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A rechargeable hand warmer is tested using 100 J of energy supplied to its electrical heater. The energy transferred to the thermal energy store of the hand warmer is 65 J. The energy transferred to the surroundings by sound is 5 J. Explain, using these figures, how the results demonstrate conservation and dissipation of energy. Include what happens to the remaining energy and why this energy is less useful.
Level 3 (5-6): Relevant points (reasons/causes) are identified, given in detail and logically linked to form a clear account. Level 2 (3-4): Relevant points (reasons/causes) are identified, and there are attempts at logically linking. The resulting account is not fully clear. Level 1 (1-2): Points are identified and stated simply, but their relevance is not clear and there is no attempt at logical linking. No relevant content (0).
The heater is given 100 J and 65 J makes the hand warmer hot. Another 5 J is sound. The rest is probably wasted, so not all the energy warms the hand warmer.
What this answer missed
- •calculate the remaining energy as 100 J - 65 J - 5 J = 30 J
- •state that the 30 J has been transferred to the surroundings, so the total energy is still accounted for and has not been destroyed
- •explain that dissipated energy is spread out into the surroundings and is therefore harder to use for further useful energy transfers
100 J is supplied, but only 65 J is transferred to the hand warmer's thermal energy store. 5 J goes as sound, so 30 J is wasted or lost to the surroundings.
What this answer missed
- •the energy has not been destroyed; when the 65 J, 5 J and 30 J transfers are included, the total remains 100 J
- •the 30 J is dissipated, usually through unwanted heating or other unwanted energy transfers
- •dissipated energy is spread out into the surroundings, so it becomes harder to use for further useful transfers
The missing energy is 100 - 65 - 5 = 30 J. This 30 J is transferred to the surroundings as unwanted energy, such as heating. This shows conservation because the energy transfers add up to the 100 J supplied, although not all of it is transferred usefully.
What this answer missed
- •The dissipated energy becomes spread out into the surroundings, so it is harder to use for further useful energy transfers.
The 100 J supplied is accounted for by 65 J transferred to the hand warmer's thermal energy store, 5 J transferred to the surroundings as sound and the remaining 30 J transferred to the surroundings, for example by unwanted heating. Since 65 + 5 + 30 = 100, energy is conserved. The sound and other unwanted transfers are dissipated, becoming spread through the surroundings and harder to use for useful heating.
- •The 100 J supplied is fully accounted for by 65 J transferred to the hand warmer's thermal energy store, 5 J transferred as sound and 30 J transferred to the surroundings.
- •The calculation 65 + 5 + 30 = 100 demonstrates conservation of energy.
- •The remaining 30 J is transferred to the surroundings, for example by unwanted heating, rather than being destroyed.
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