how much energy is used by a 75-W bulb that has been left on for 4 hours?
To calculate the amount of energy used by a bulb, you need to use the formula: Energy (in kilowatt-hours) = Power (in kilowatts) * Time (in hours).
In this case, the power of the bulb is given as 75 watts. To convert watts to kilowatts, divide by 1000. So, the power of the bulb is 75/1000 = 0.075 kilowatts.
The time the bulb has been left on is given as 4 hours.
Now, substituting the values into the formula: Energy = 0.075 kilowatts * 4 hours = 0.3 kilowatt-hours.
Therefore, a 75-W bulb that has been left on for 4 hours would use 0.3 kilowatt-hours of energy.