Question

3. Calculate the resistance of a 250V immersion heater required
to raise the temperature of 115 kg of water from 15°C to 85°C
in 1 hour.
(Ans. 6.65 92)

Answer 1

Given : A 250 volts immersion heater supplies heat to 115kg water to raise the temperature from 15° C to 85°C in an hour.

To find : Find the resistance of immersion heater.

solution : let R is the resistance of heater.

power of heater = V²/R

= (250)²/R watt

energy gained by water , E = ms∆T

= 115kg × 4200 J/kg/°C × (85°C - 15°C)

= 115 × 4200 × 70 J

power = energy/time taken

= 115 × 4200 × 70/3600 watt

= 9391.67 watt

now power of heater = power used by water

⇒(250)²/R = 9391.67

⇒R = (250)²/9391.67 = 62500/9391.67

⇒R = 6.65 Ω

Therefore the resistance of heater is 6.65 Ω