What Is Energy? — Question 3

Question 3

Steady energy does not mean no energy transfer

For this introductory problem use the supplied accounting rule $\Delta E=E_{\mathrm{in}}-E_{\mathrm{out}}$, with all quantities in joules. A lamp receives 600 J electrically during an observation interval. It emits 120 J as light and transfers 480 J thermally to its surroundings. Treat the lamp as the system.

1. Find its total energy change.

2. Explain why a zero change does not show the lamp is isolated.

3. If the light output stays 120 J but thermal transfer falls to 450 J, what additional change or transfer is required?

Question 3 - Solution

Steady energy does not mean no energy transfer

(a) Take the signed difference.

The governing relation is $$\boxed{\Delta E=E_{\mathrm{electrical}}-E_{\mathrm{light}}-E_{\mathrm{thermal}}}$$

$$\Delta E=600-120-480=0\ \mathrm J.$$ The system can have a steady energy store while energy flows through it.

(b) Distinguish isolation from balance.

An isolated system exchanges no energy with its surroundings. Here 600 J enters and 600 J leaves. Equal nonzero transfers are not absent transfers.

(c) Close the revised ledger.

With no other transfers, $$\Delta E=600-120-450=30\ \mathrm J.$$ Alternatively, a further 30 J must leave if the lamp energy is to remain unchanged.