Originally Posted by

**looi76** **Question:**

(a) The current in a Led is $\displaystyle 20mA$ when it has a p.d of $\displaystyle 2V$ across it. Calculate the resistance.

(b) A $\displaystyle 240V$ heater takes a current of $\displaystyle 4.2A$. 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) $\displaystyle I = 20 \times 10^{-3} A \ , \ V = 2V$

$\displaystyle R = \frac{V}{I} = \frac{2}{20 \times 10^{-3}} = 100\Omega$

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

(i) $\displaystyle Q = It$

$\displaystyle Q = 4.2 \times 180$

$\displaystyle Q = 756 C$

(ii) $\displaystyle W = QV$

$\displaystyle W = 756 \times 240$

$\displaystyle W = 3.15J$

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

Can you please correct my mistakes?

Thnx in advance