# Physics Questions (Electrical Energy)

• Aug 24th 2009, 11:28 PM
Supaweak
Physics Questions (Electrical Energy)
Hi there I just have a series of questions which i've answered but im thinking that ive got most of them wrong/ i dont know what the heck im doing

Here it is:
A 120 ohm heating element was connected to a 240 Volt power supply for 15 minutes. Determine -
a. current through the element during the 15 minutes
b. amount of charge which flowed in the 15 minutes
c. number of electrons which passed through th element in 15 minutes. The charge on one electron is 1.6x10^-19 C
d. power dissipated by the heating element
e. amount of electrical energy converted in the 15 minutes

a. 2 amps
b. 1800 coulombs or 2 coulombs (something def wrong here)
c. 1.25x10^19 (got no clue)
d. 57120 watts
e. 432000 joules

hope you can help, thanks!
• Aug 25th 2009, 01:11 AM
Kiwi_Dave
By my reckoning you have a, b (1800) and e correct.

The answer to c is simply the amount of charge calculated in b) divided by the charge of a single electron. I expect that you can probably work out part d).
• Aug 25th 2009, 02:29 AM
Supaweak
thanks for the reply! yeah i think i get the other questions but im still kind of lost on part d)

• Aug 25th 2009, 09:21 AM
Greengoblin
For d, remember the formulas for electrical power:

$P=IV=I^2R=\frac{V^2}{R}$

Which can be transposed from:

$I=\frac{Q}{t}$

$V=\frac{E}{Q}$

$P=\frac{E}{t}$

$V=IR$ (Ohms law)