First+law+of+thermodynamics

The **first law of thermodynamics** states that energy is conserved (never lost). It can be summarised by the equation:

Δ**U =** Δ**Q +** Δ**W**

delta **Δ** means 'change in'

So the equation can be written as: Increase in internal energy = energy gained by heating + energy gained by working

1. A 100 W filament lamp takes 200ms to reach full brightness. Show how the first law of themodynamics may be applied to the filament (a) during the first 0.001ms after it is switched on and (b) after it has been switched on for some time.
 * Questions**

answers

(a) The filament takes 200ms to reach full brightness. In 0.001ms the temperature rises a little but insufficient to heat its surroundings. During this time the power supply is working on the filament because a current is flowing through the filament so ΔW is positive. The filament is still very nearly the same temperature as its surroundings so no heat is being lost, therefore ΔQ is zero. This means that the work done by the power supply is used to raise the internal energy: ΔU = ΔW.

(b) After the lamp has been switched on for some time it has reached its steady operating temperature. The filament's temperature is not changing, which means its internal energy is not changing, ΔU = 0. This means that

0 = ΔQ + ΔW. Remeber that ΔW must be positive because there is a current running through the filament, doing work on it. This means that ΔQ must be negative - which means that the filament is losing heat. The energy gained by the filament is negative; so the filament is losing heat to the surroundings, because at its operating temperature the filament is hotter thant the surroundings.