Question

An electric iron is marked 400 W and 220 V. What is the resistance when iron is hot? How long could it be used for ₹3 of electrical energy that costs 15 paise per unit.​

Answer 1

Given, P = 400 W and V = 220 V

Using, P = V²/R

⇒ R = V²/P = 220²/400 = 121 Ω

So, resistance when iron is hot = 121 Ω

Now, rate of electrical energy = 15 paise per unit

and total money = ₹3 = 300 paise

So, electrical energy consumed, E = 300/15 = 20 unit = 20 kWh = 20000 Wh

Using, E = P × t

⇒ t = E/P = 20000/400 = 50 h

So, It can be used for 50 hours.

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Answer 2

Answer:

Resistance = 121 ohm

Time = 50 hours

Explanation:

Given, P = 400 W and V = 220 V

Using, P = V²/R

⇒ R = V²/P = 220²/400 = 121 Ω

So, resistance when iron is hot = 121 Ω

Now, rate of electrical energy = 15 paise per unit

and total money = ₹3 = 300 paise

So, electrical energy consumed, E = 300/15 = 20 unit = 20 kWh = 20000 Wh

Using, E = P × t

⇒ t = E/P = 20000/400 = 50 h

So, It can be used for 50 hours.

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