HomeWork Help: What is the current in the bulb..

Q) A 100 W lightbulb is plugged into a standard 120 V outlet. (a) How much does it
cost per 31-day month to leave the light turned continuously? Assume electrical
energy costs US$0.06/kWh. (b) What is the resistance of the bulb? (c) What is the current in the bulb?

Sol:

First let us find the energy used in 31 days is(in kWh) and then let go for calculating the cost.

a) U = P * t = 0.100kW * 31days * 24hrs/day

       = 74.4kWh

Cost per 31 day month = 74.4kW h * $0.06/kW h

  = $4.464

b) The resistance of the bulb is given by,

P = V²/R

R = V²/P

  = (120V)²/100W

=144Ω

c) The current in the bulb is given by,

P = VI

I = P/V

     = 100W/120V

 = 0.833 A