# Homework Help: Headlight Power Question

1. Aug 24, 2013

### FredericChopin

1. The problem statement, all variables and given/known data
The headlights on a car operate typically at 60 W and the parking lights typically at 5 W. Assuming that there are two main headlights and four parking lights, what length of time will it take to discharge a 60 Ah battery, if the lights are left on?

2. Relevant equations
I = Q/t

V = W/Q

P = W/t

P = I2*R

P = V2/R

P = IV

3. The attempt at a solution
I have gotten no where with this question.

All that I could do was find that the total power consumed by the lights:

PT = (2*10) + (4*5) = 140 W

, and the charge of the battery:

1 Ah = 3600 C
∴ 60 Ah = 216,000 C

, and that's it.

I have tried simultaneous equations by substitution and elimination but that doesn't work because all of the equations are derived from the base equations I = Q/t, V = W/Q and P = W/t, so I end up getting P = P all the time.

Hint's anybody?

Thank you.

2. Aug 24, 2013

### TSny

Not sure, but maybe you are suppose to assume that the voltage of the battery is the typical voltage for a car battery.

3. Aug 24, 2013

### ehild

You need the voltage of the car battery to find the current. Look at Automotive battery - Wikipedia, the free encyclopedia, or just see the battery in your car...

ehild

4. Aug 25, 2013

### FredericChopin

Hmm... It seems like it... Based on this link, which is the worked solution to the question.

Thank you TSny and ehild.

So much for "complex, challenging and novel"...