Calculating Talk Time for a 1050mAh Mobile Phone Battery Pack

[Travian]

Homework Statement

A primary energy source is able to provide 0.7 Watts power in order to charge a mobile telephone battery pack. The telephone operates at 3.5 V, and the mobile telephone’s 3.5 V battery pack has a capacity of 1050mAh. In “talk” the mobile phone draws an average current from its battery of 100mA.

(i) How many minutes of talk time will this battery pack provide, at the average current for “talk”?

Homework Equations

P=0.7
I=100mA
V=3.5V

The Attempt at a Solution

Now I'm not sure what formula to use in order to find time. Can someone help me?

 

[nasu]

The capacity of the battery is given in mAh. This means units of current (amps) times units of time. So what would be the formula that defines the capacity of the battery?

 

[Travian]

This formula.
(Cb) = I x t

then
time = C / I
?
I need to find time

 

[nasu]

This formula.
(Cb) = I x t

Yes, this is it. It is actually electric charge (Q) but in battery industry they call it capacity.

then
time = I / (Cb)
?
I need to find time

No, this is not correct. Watch your algebra.
You need to find time and you can do it from the formula above (Cb=I x t). Solve for t.

 

[Travian]

Sorry i changed it to time=Cb/I

How can i use this formula (Cb=I x t) if i don't know time?
Shouldn't i use this? t=Cb/I (t = 1.05Ah / 0.1A = 10.5)

 

[nasu]

Sorry i changed it to time=Cb/I

How can i use this formula (Cb=I x t) if i don't know time?
Shouldn't i use this? t=Cb/I (t = 1.05Ah / 0.1A = 10.5)

Yes, now it's OK. You use Cb=I*t to calculate the time, t= Cb/I, as you wrote above.
First time you wrote t=I/Cb which is not correct.

 

[Travian]

so the answer i suppose is 10.5 hour = 650 minutes.

 

[Travian]

There are more to this exercise.

(ii)How much energy (in Joule units) is stored in the battery pack, when fully charged?

I used this formula:

E = Cb x V
E = 1.05Ah x 3.5V
E = 3.675 (what units?)

 

[nasu]

There are more to this exercise.

(ii)How much energy (in Joule units) is stored in the battery pack, when fully charged?

I used this formula:

E = Cb x V
E = 1.05Ah x 3.5V
E = 3.675 (what units?)

If you convert your inputs in SI units (something you did not do here) then the answer should be in SI units of energy, as required in the question.

 

[Travian]

SI units.. Ah to mAh and V to mV?