Question

A hotplate uses 1200 W for 15 minutes. How much energy does it use?The efficiency of real-world systems is always between 0% and 100%.

Answer 1

The value of the power given is,

$P=1200\text{ W}$

The hotplate is used for the time t is,

$\begin{gathered} t=15\text{ minutes} \\ t=15\times60 \\ t=900\text{ s} \end{gathered}$

Thus, the energy used is,

$\begin{gathered} E=P\times t \\ E=1200\times900 \\ E=1080\times10^3\text{ J} \\ E=1080\text{ kJ} \end{gathered}$

Hence, the energy used is 1080 kJ.