Question

two bulbs A of 20W, 100V and B 40W, 100V are joined in parallel across a 100V source. calculate the total current drawn from the voltage source​

Answer 1

Given :

• Two bulbs connected in parallel .
• For bulb A : 20 W , 100 V
• For bulb B : 40 W , 100 V
• Voltage source = 100 V

To find :

• Current drawn

Solution :

The formula of Power is given by :

Power = Potential difference × Current

P = V I

P = V × V/R ______ [ as I = V/R]

P = V²/R

R = V²/ P

For bulb A : V = 100 V , P = 20 W

R (A) = V (A) ² / P(A)

= 100 × 100 / 20

= 500 Ω

For bulb B : V = 100 V , P = 40 W

R (B) = V (B)² / P(B)

= 100 × 100 / 40

= 250 Ω

• Given : Bulbs are connected in parallel

Therefore equivalent resistance R is :

1/R = 1/ R(A) + 1/ R (B)

1/ R = 1/ 500 + 1/ 250

1/R = 1 + 2/ 500

1/ R = 3/ 500

R = 500 / 3 Ω

Current drawn is given by :

• I = V/ R

Given : V = 100 , R = 500 / 3

I = $\dfrac{100}{\dfrac{500}{3}}$

I = 100 × 3 / 500

I = 3/5

I = 0.6 A

Hence the current drawn from the source will be 0.6 A .