1 \begin{problem*}{27.49}
2 A car owner forgets to turn off the headlights of his car while it is
3 parked in his garage. If the $12.0\U{V}$ battery in his car is rated
4 at $90.0\U{A$\cdot$h}$ and each headlight requires $36.0\U{W}$ of
5 power, how long will it take the battery to completely discharge?
9 The current drawn by each headlight is given by
12 I_1 &= \frac{P_1}{V} = 3.00\U{A} \;.
14 So the total current drawn from the battery is
16 I_t = 2I_1 = 6.00\U{A} \;.
18 This will deplete the battery in
20 \Delta t = \frac{\Delta Q}{\deriv{t}{Q}} = \frac{\Delta Q}{I_t}
21 = \frac{90.0\U{A$\cdot$h}}{6.00\U{A}}
22 = \ans{15.0\U{hours}} \;.
24 If you want, you could plug in symbolically for $I_t$, giving
26 \Delta t = \frac{\Delta Q V}{2P_1} \;.