Question

A certain amount of water boils in 10 minutes in an
electric kettle. If same amount of water has to be
boiled by another coil having double resistance,
then time taken by the coil is (using same supply)
41
(1) 20 minute
(2) 5 minute
(3) 10 minute
(4) 30 minute​

Answer 1

2) 5 minute

Explanation:

We know form the Joule's Law of electrical heat:

$Q=i^2.R.t$

where:

$i=\ current$

$R=\ resistance$

$t=\ time\ duration$

So, time:

$t=\frac{Q}{i^2.R}$

• When the resistance is doubled, time t':

$t'=\frac{Q}{2\times R.i^2}$

i.e.

$t'=\frac{t}{2}$

Given that, $t=10\ min$

Therefore, $t'=5\ min$

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TOPIC: Joule's Heating

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Answer 2

Answer:

20 minutes.

Explanation:

Heat required to boil a fixed quantity of water would be same.... whatever may be the resistance.

"Using Same Supply " means Voltage would remain same . V = IR

Initially,

H= i^2Rt. ------------------equation 1

or H= i^2R×10

NOW,

Let the new time be " p" minute.

when resistance is doubled , ( voltage must remain constant.) in order to keep voltage same current (. i ) must get halved.

in other words V= ( i/2) ×2R

H = ( i/2)^2 ×2R × p -------------------- equation 2

Equating both equation 1 and 2 we get

p = 20 minute.