# Math Help - Electricity Questions

1. ## Electricity Questions

Question:
(a) The current in a Led is $20mA$ when it has a p.d of $2V$ across it. Calculate the resistance.
(b) A $240V$ heater takes a current of $4.2A$. Calculate:
(i) The charge that passes through the heater in 3 minutes
(ii) The heat energy produced per second by the heater
(iii) The resistance of the heater.

Attempt:

(a) $I = 20 \times 10^{-3} A \ , \ V = 2V$
$R = \frac{V}{I} = \frac{2}{20 \times 10^{-3}} = 100\Omega$

(b) $V = 240V \ , \ I = 4.2A \ , \ t = 3 \times 60 = 180s$

(i) $Q = It$
$Q = 4.2 \times 180$
$Q = 756 C$

(ii) $W = QV$
$W = 756 \times 240$
$W = 3.15J$

(iii) $R = \frac{V}{I} = \frac{240}{4.2} = 57\Omega$

Can you please correct my mistakes?

2. Originally Posted by looi76
Question:
(a) The current in a Led is $20mA$ when it has a p.d of $2V$ across it. Calculate the resistance.
(b) A $240V$ heater takes a current of $4.2A$. Calculate:
(i) The charge that passes through the heater in 3 minutes
(ii) The heat energy produced per second by the heater
(iii) The resistance of the heater.

Attempt:

(a) $I = 20 \times 10^{-3} A \ , \ V = 2V$
$R = \frac{V}{I} = \frac{2}{20 \times 10^{-3}} = 100\Omega$

(b) $V = 240V \ , \ I = 4.2A \ , \ t = 3 \times 60 = 180s$

(i) $Q = It$
$Q = 4.2 \times 180$
$Q = 756 C$

(ii) $W = QV$
$W = 756 \times 240$
$W = 3.15J$

(iii) $R = \frac{V}{I} = \frac{240}{4.2} = 57\Omega$

Can you please correct my mistakes?
I'm not seeing any mistakes. You are saying there's something wrong somewhere?

-Dan

3. Originally Posted by topsquark
I'm not seeing any mistakes. You are saying there's something wrong somewhere?

-Dan
I think part (b)(ii) is wrong

4. Originally Posted by looi76
I think part (b)(ii) is wrong
Yes it is wrong:

Since you want heat energy per second,

Energy = $W = QV$

But Energy per second = $\frac{W}{t} = \frac{Q}{t}\cdot V = IV$ = ?

5. I'll just say "Oops!" about part b) and move on. Thanks for the catch Isomorphism.

-Dan