# Homework Help: No Idea how to work this out! - Charging and discharging an iPod

1. Jul 6, 2012

### andyb1990

No Idea how to work this out! -- Charging and discharging an iPod

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

It takes about 5 hours for a fully charged ipod nano to run out of power when its screen is left on and all other functions are switched off. Subsequently, it takes about 90 minutes to recharge the ipod from a charging unit with a 5V and 500mA output. The same ipod takes about 24hours to run out of power when only the audio function is used

a) calculate the power consumption of the screen and audio output seperately

b) calculate when the power will run out if the user switches on the screen for 10 seconds every 3mins (i.e. about once every track) assuming the ipod is fully charged at the start.

2. Relevant equations

3. The attempt at a solution

2. Jul 6, 2012

### Staff: Mentor

Re: No Idea how to work this out! -- Charging and discharging an iPod

You are given complete information about the charging situation. How much power is used to charge the iPod? How much energy is stored in the battery during the charging time?

3. Jul 6, 2012

### andyb1990

Re: No Idea how to work this out! -- Charging and discharging an iPod

is the power of the iPod

90*60 = 5400joules?

4. Jul 6, 2012

### Staff: Mentor

Re: No Idea how to work this out! -- Charging and discharging an iPod

What is that? 90 minutes * 60 seconds/minute ?

What is the equation for power P in terms of voltage V and current I ?

What is the equation for Energy E in terms of power P and time t ?

5. Jul 7, 2012

### andyb1990

Re: No Idea how to work this out! -- Charging and discharging an iPod

so the formula for power

P= I . V ?? ----> 5 x 0.5 = 2.5 watts?

E= P . t -----> 2.5 x (5*60*60) = 45000 ?

then I'm unsure on what to do?

6. Jul 9, 2012

### Staff: Mentor

Re: No Idea how to work this out! -- Charging and discharging an iPod

Good! Be sure to carry units along in your equations, so your E = 45000J.

So what is the discharge power if it takes 24 hours to discharge that energy?

7. Aug 9, 2012

### andyb1990

Re: No Idea how to work this out! -- Charging and discharging an iPod

would it be

45000J/ 24*60*60??