A hair dryer with a resistance of 9.6 ohms operates at 120 volts for 2.5 minutes. The total electrical energy used by the dryer during this time interval is

(1) 2.9 * 103J

(2) 3.8 * 103J

(3) 1.7 * 105J

(4) 2.3 * 105J

For the answer to the question above,

hm's law : U=RI, and : Energy=UIΔt

then : I=U/R (we don't have, the current); then : Energy=E=U²Δt/R

U is voltage in volts (V), Δt is the time of plugging in hours (h), R is resistance in ohms (Ω)

we have 30 days in seconds : Δt = 10 min per day * 30 days = 1/6 hour per day * 30 days = 5 hours

U=120 V; R=9.6 Ω

then : E=120²*5/9.6 = 7500 Wh = 7.5 kWh