Electrical energy and power
- Two heaters A and B have power rating of 1 kW and 2 kW, respectively. Those two are first connected in series and then in parallel to a fixed power source. The ratio of power outputs for these two cases is: Answer: 3. 2 : 9 Key idea: P=V²/R , so R∝1/P at fixed voltage. Take R_A=2 units (1 kW) and R_B=1 unit (2 kW). Series: R_s=3 , so P_s=V²/3 .... 1
- Two resistors of resistance, 100 Ω and 200 Ω are connected in parallel in an electrical circuit. The ratio of the thermal energy developed in 100 Ω to that in 200 Ω in a given time is : Answer: 3. 2 : 1 In a parallel connection both resistors have the same voltage, so heat produced in a given time depends inversely on resist... 1
- Six similar bulbs are connected as shown in the figure with a DC source of emf E, and zero internal resistance.The ratio of power consumption by the bulbs when (i) all are glowing and (ii) in the situation when two from section A and one from section… Answer: 2. 9 : 4 From the figure, the three bulbs of section A are in parallel with each other, as are the three bulbs of section B. The two... 1
- Which of the following acts as a circuit protection device ? Answer: 4. fuse A fuse contains a thin wire that melts when the current exceeds a safe limit, breaking the circuit and protecting the applia... 1
- A filament bulb (500 W, 100 V) is to be used in a 230 V main supply. When a resistance R is connected in series, it works perfectly and the bulb consumes 500 W. The value of R is Answer: 2. 26 Ω The bulb rated 500 W at 100 V must carry its rated current for full power. Rated current I=P/V=500/100=5 A. Supply 230 V; bu... 1
- The charge flowing through a resistance R varies with time t as Q = at - bt^2, where a and b are positive constants. The total heat produced in R is : Answer: 1. a³R/6b Key idea: current I=dQ/dt ; heat H=∫ I²R dt . I=a-2bt , which becomes zero at t=a/2b . H=R∫₀^(a/2b)(a-2bt)²dt=R[-(a-2bt)³/... 1