A) The light bulb used in an overhead projector has a resistance of 48 Ohm. What is the current through the bulb when it is operating on 120 V?
B) Which costs more, running a 1,200 W hair dryer for 3 min or leaving a 60 W lamp on overnight (1h)?
A) The Ohm law says that the resistance of a device is equal to the voltage (on that device) over the current (through the devive).
$R = U/I$, $I = U/R = 120/48 = 2.5 A$
B) One is paying the energy nedded to that particular device. (Power = Energy/time)
$E1 = P1*t1 =1200*3*60 =216000 J$
$E2 =P2*t2 =60*1*60*60 =216000 J$
The cost is the same.