# Homework Help: Ohm's Law and Power ?

1. Jan 31, 2010

### 2slowtogofast

A bicycle LED headlight requires 5 V, 250 mA to operate. Using Ohm’s law, determine the
equivalent resistance of the light. What is the power consumption rate in W? How many
electrons pass through per second? It is powered by four AA rechargeable battery in series. Each battery provides 1.25 V and has a specification of 2300 mA-Hour. How many second would the battery last?

The first part i used R = v/I

The second part I know P = VI but am not sure what to use for time because it does't give one.

The third part i dont know about. I am not looking for the answer but help on how to get there thank you

2. Jan 31, 2010

### 2slowtogofast

can somebody give me an idea

3. Jan 31, 2010

### LCKurtz

Isn't 1 watt = one joule per second?

Maybe better: 1 ampere = 6.242*1018 electrons/second

Last edited: Jan 31, 2010
4. Jan 31, 2010

### vk6kro

Each battery provides 1.25 V and has a specification of 2300 mA-Hour. How many second would the battery last?

This means each battery would last for an hour if it was supplying 2300 mA. It would last for 10 hours if it was supplying 230 mA.

So product of (current * time) = 2300 mAH
or
time = 2300 mAH / current

So, how long would it last if it was supplying 250 mA?