Question

As shown in the figure above A and B are two lamps. Lamp A is rated at 12 V, 24W. Lamp B is rated at 6.0 V. When lamp B operates at its rated voltage, the current in it is 3.0 A. The values of R1 and R2 are chosen so that both lamps operate at their rated voltages. Based on the information given, answer the following. 

(i) Calculate the current in Lamp A. 

(ii) State and give reason for the reading of the Voltmeter. 

(iii) Calculate the resistance of R2. 

(iv)Find the value of the resistance R1.

Answer 1

(i) Use of P = IV 

I = P ÷ V = 24 W ÷ 12V 

Current in lamp A – 2 A 

(ii) Voltmeter reading = 12 V

Lamp A and Lamp B are in parallel. 

Hence p.d. across the arm containing A = p.d. across the arm containing B 

= 12 V (from a) 

(iii) p.d. across R₂ + p.d. across B = 12 V.  

p.d. across B = 6 V (given) Hence p.d. across R₂ = 12 V - 6 V = 6V 

Current through R₂ = Current through B = 3A (given)

Use of R = V/I R₂ =6V/3A = 2Ω 

(iv) Current through R₁ = Total Current = 3A+2A = 5A 

p.d. across R₁ = 15V -12 V = 3V 

R₁ = 3V/5A = 0.6 Ω