Hello, hello? My phone's almost dead...drat.

Electricity and Magnetism Level 2

Cell phone batteries are rated by their voltage and total ability to output current, usually in units of milliamperes $\times$ hours, for which we use the abbreviation mA-h. If a battery has a value of $1000~\mbox{mA-h}$ , this means it can deliver $1000~\mbox{mA}$ of current for one hour. The cell phone battery in this question has a voltage rating of $3.7~\mbox{V}$ and current rating of $1100~\mbox{mA-h}$ . How much energy in Joules does the battery store when fully charged?

Details and assumptions

Assume the voltage remains constant as the battery discharges. The voltage of real batteries decreases as they discharge, but we'll ignore that effect for simplicity.
Hint: Try converting the rating to Amp-seconds and think how you could use that unit with Volts to get an energy.

The answer is 14652.

4 solutions

Noor Muhammad Malik
Nov 25, 2013

Formula for energy is:
$E = VIt$
putting the values:
$E = (3.7)(1100*10^{-3})(3600)$
$E=\boxed{14652J}$

thumbs up

ashutosh mahapatra - 7 years, 2 months ago

Abubakarr Yillah
Nov 25, 2013

First, convert the rating to Amp-seconds. i.e 1100 mA-h becomes: 1100 0.001 3600 = 3960 As power or energy = IV = 3960 * 3.7 = 14652 Joules

Muhammad Arifur Rahman
Feb 2, 2015

Its a piece of cake to you. You know, $P=VI$ So let's multiply the two sides with time- $Pt=VIt$ As you know, $Energy, E=Pt$ So You get $E=VIt$ Then its just putting the values-

$V=3.7V$ $It=1.1*3600C=3960C$

So you get E= $\boxed{14652J}$

Its really simple, isn't it!

Nowfel Mashnoor
Nov 26, 2013

Here, 1100 mA-h = 1.1 A-h we know, P=VI, V=3.7, I=1.1 so, P = 4.07 again, W=Pt t = 1 h = 3600 sec so, W = 4.07*3600 = 14652 J

Hello, hello? My phone's almost dead...drat.

The answer is 14652.

4 solutions

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