# Homework Help: Current & Power

1. Aug 10, 2009

### exitwound

Problem 1

Just starting on the Current & Power chapter and I'm a little confused. I know they're just basic definitions and concepts, but I'm having trouble wrapping my head around it. Easy questions follow, but I'm lost.

1. The problem statement, all variables and given/known data

How many electrons flow past a point in a wire each second if the wire has a current of 1.00 Amperes?

2. Relevant equations

1 Ampere = 1 Coulomb/1 second

3. The attempt at a solution

I don't know where to start.

Problem 2

1. The problem statement, all variables and given/known data
A lamp draws 0.50 A from a 120 V generator. (a) How much power does the generator deliver to the lamp? (b) How much electric energy does the lamp convert to light and heat in a period of 5.0 minutes?

2. Relevant equations

P=Energy/time or dU/dt or current*Voltage.

3. The attempt at a solution

a.) P=current*Voltage
P= (.5 Amps)(120 Volts)= 60 Watts

Maybe I'm just missing some key concepts, or something just isn't sticking in my stupid head due to the way it's been explained. I don't know.

Any help?

2. Aug 10, 2009

### queenofbabes

1) How much charge is flowing through the wire per second if the current is 1 ampere? What is the charge of an electron?

2b) The bulb converts electrical energy to light and heat, right? How much much energy does the bulb use in 5 mins?

3. Aug 10, 2009

### exitwound

1.) I get the right answer if I divide 1/1.6e-19 but I'm not entirely sure why.

2b.) P=dU/dt
dU=P*dt
dU=60*300=18kJ which is right.

I didn't realize it said five MINUTES.

4. Aug 10, 2009

### bm0p700f

Each electron has a fixed charge of -1.6e-19 C.
So with 1C/s it follows that there are

Units in brackets
electrons/second = 1 (C/s)/1.6e-19 (C) = 6.25E18 electrons per second.

It bit like saying a brick cost 25p, you have £1000 how many bricks can you get £1000/£0.25= 4000 bricks.

5. Aug 10, 2009

### exitwound

Gotcha. As soon as I started reading your second sentence, it hit me. :)

Thanks guys. or girls. or dogs. or whatever you might be.

6. Aug 10, 2009

### bm0p700f

None of the above I am actually a fish.