Question

A 75W outdoor lamp is connected to 240V source and left to turn on continuously for an average 14 hours per day. Determine:i) the current through the lamp when it is in operation.ii) the cost of operating the light for one year (365 days) if electricity costs in Malaysia is RM0.22 per kWh

Answer 1

We are given that a 75W lamp is connected to a 24V source. To determine the current passing through the lamp we will use the following formula:

`P=VI`

Where:

`\begin{gathered} P=\text{ electric power} \\ V=\text{ voltage} \\ I=\text{ current} \end{gathered}`

Now we plug in the given values:

`75W=(240V)I`

Now we divide both sides by 240 to solve for "I":

`(75W)/(240V)=I`

Solving the operation we get:

`0.31A=I`

Therefore, the current passing is 0.31 Ampere.

Now we will calculate the cost of operating the lamp for one year. To do that we will calculate first the number of hours the lamp will be connected for the year. Since the lamp is connected 14 hours a day, this means that in 365 days the number of hours is:

`14(hours)/(day)*365day=5110hours`

Now we determine the kWh. To do that we will multiply the number of hours by the electrical power and we will divide the result by 1000 to convert the W into kW. We get:

`75W*(1kW)/(1000W)*5110h=383.25kWh`

Now we determine the cost by multiplying the kWh of the lamp by the cost per kWh:

`0.22(RM)/(kWh)*383.25kWh=84.32RM`

Therefore, the cost of operating the lamp for a year is 84.32 RM: