An electric kettle takes 4 amperes when put on a circuit of 250 volts. calculate the time required to prepare two cups calculate the cost at the rate of 20 p per unit

Solution:

Mass of water = 450 g

Rise in temp = 100 – 20 = 800C

Heat produced = 450 × 1 × 80 = 36000 cal

E = 250 volts

= 4 A

t = ?

$$\begin{array}{l}H=frac{Etimes Itimes t}{4.2}\end{array}$$
$$\begin{array}{l}36000=frac{250times 4times t}{4.2}\end{array}$$
$$\begin{array}{l}t=frac{36000times 4.2}{250times 4}=151.2s\end{array}$$
$$\begin{array}{l}t=frac{151.2}{3600}hour\end{array}$$

Energy consumed

$$\begin{array}{l}=frac{Etimes Itimes t}{1000}kWh\end{array}$$
$$\begin{array}{l}=frac{250times 4times 151.2}{1000times 3600}=0.042kWh\end{array}$$

Rate = 20 P a unit

Cost = 0.042 × 20 = 0.84 P