Power

Q. If the voltage across a bulb rated $220V-100W$ drops by $2.5\%$ of its rated value, the percentage of the rated value by which the power would decrease is

1. $20\%$
2. $2.5\%$
3. $5\%$
4. $10\%$

Q. The value of power dissipated across the Zener diode $(V_z=15\text{V})$ connected in the circuit as shown in the figure is $x\times {10}^{-1}\text{watts}$.


The value of $x$ to the nearest integer is

Q. When a $24\text{V}$ battery is connected to a resistor, the current in it is $0.4\text{A}$. What will be the current through it when it is connected to a battery of $6\text{V}?$

1. $0.4\text{A}$
2. $0.1\text{A}$
3. $0.2\text{A}$
4. $0.3\text{A}$

Q. The switch $S$ has been closed for a long time and the electric circuit shown carries a steady current. Let $C_1=3.0\mu \text{F}$, $C_2=6.0\mu \text{F}$, $R_1=4.0\Omega$ and $R_2=7.0\Omega$. The power dissipated in $R_2$ is $2.8\text{kW}$. Then:

1. The power dissipated in the resistor $R_1$ is $1.6\text{kW}$.
2. The charge on capacitor $C_1$ is $240\mu \text{C}$.
3. The charge on capacitor $C_2$ is $440\mu \text{C}$.
4. When the switch is opened after long time, the charge on $C_1$ is $660\mu \text{C}$.

Q.

One Dioptre is equal to ____________

Q. If the length of the filament of a heater is reduced by $10\%$, the power of the heater will

1. increase by about $9\%$
2. increase by about $11\%$
3. increase by about $19\%$
4. decrease by about $10\%$

Q. A zener diode having zener voltage $8\text{V}$ and power dissipation rating of $0.5\text{W}$ is connected across a potential divider arranged with maximum potential drop across zener diode is as shown in the diagram. The value of protective resistance $R_p$ is ______$\Omega$.

Q. 12.Two resistance 2 and 4 ohm are connected in series in an electric circuit. Ratio of thermal power dissipated in each resistance is

Q. Two zener diodes ($A$ and $B$) having breakdown voltages of $6\text{V}$ and $4\text{V}$ respectively are connected as shown in the circuit below. The output voltage $V_0$ variation with input voltage linearly increasing with time, is given by:
($V_{input}=0\text{V}$ at $t=0$)
(figures are qualitative)

1. 
2. 
3. 
4. 

Q. ten 60W, 220 V bulbs are connected in series to220V supply power consumed in circuit i

Q. 15.Power Dissipated in an LCR series circuit connected to an a.c. source of emf E is ?

Q. A series $LCR$ circuit of $R=5\Omega ,L=20\text{mH}$ and $C=0.5\mu \text{F}$ is connected across an $\text{AC}$ supply of $250\text{V}$, having variable frequency. The power dissipated at resonance condition is _____ $\times {10}^2\text{W}$.

Q. An electric bulb, rated as $200\text{W}$ at $100\text{V}$, is used in a circuit having a $200\text{V}$ supply. The resistance $R$, that must be put in series with the bulb, so that the
bulb delivers the same power is $\Omega$)

Q. Photodiode can be used as -

1. Switch
2. Anode
3. Photodetector
4. Cathode

Q. A battery is charged at a potential of $15V$ for $8h$ when the current flowing is $10A$. The battery on discharge supplies a current of $5A$ for $15h$. The mean terminal voltage during discharge is $14V$. The watt-hour efficiency of the battery is

1. $80\%$
2. $82.5\%$
3. $90\%$
4. $87.5\%$

Q. $\text{A, B, C}$ are identical bulbs. How does the brightness of $\text{A}$ and $\text{B}$ change when the switch S is closed ?

1. The brightness of $\text{A}$ increases and that of $\text{B}$ decreases.
2. The brightness of $\text{A}$ does not change and that of $\text{B}$ decreases.
3. The brightness of both $\text{A}$ and $\text{B}$ decreases.
4. The brightness of $\text{A}$ increases and that of $\text{B}$ does not change.

Q. An electrical bulb rated $220\text{V}$, $100\text{W}$, is connected in series with another bulb rated $220\text{V},60\text{W}$. If the voltage across combination is $220\text{V}$, the power
consumed by the $100\text{W}$ bulb will be about ____ $\text{W}$.

Q. Prove that in series combination of electric appliances, the reciprocal of total power consumption is equal to the sum of the reciprocals of the powers of individual appliances

Q.

L, C and R represent the physical quantities inductance, capacitance and resistance respectively. Which of the following combinations have dimensions of frequency?

1. $\frac{1}{RC}$

2. $\frac{R}{L}$

3. $\frac{1}{\sqrt{LC}}$

4. $\frac{C}{L}$

Q. For the given circuit, the average power developed is -

1. $50\sqrt{2}\text{W}$
2. $200\text{W}$
3. $50\sqrt{2}\text{W}$
4. $200\sqrt{2}\text{W}$

Q. Two thin lenses of focal length $f_1=10\text{cm}$ and $f_2=20\text{cm}$ are kept as shown. The resultant power of combination will be

1. $-10\text{D}$
2. $0$
3. $10\text{D}$
4. $5\text{D}$

Q. Two insulated metallic spheres of \(3 \mu F\) and \(5 \mu F\) capacitance are charged to \(300 V\) and \(500 V\) respectively. The energy loss, when they are connected by a wire is

Q. A convergent lens of power $16\text{D}$ is used as a simple microscope. The magnification produced by this lens, when the final image is formed at least distance of distinct vision is

1. $4$
2. $5$
3. $6$
4. $7$

Q. An engine develops $20\text{kW}$ of power. How much time will it take to slowly lift a mass of $100\text{kg}$ to a height of $20\text{m}$?

$(g=10{\text{m/s}}^2)$

1. $4\text{s}$
2. $5\text{s}$
3. $1\text{s}$
4. $10\text{s}$

Q. A thin symmetrical double convex lens of power $P$ is cut into three parts, as shown in the figure. Power of $A$ is:

1. $2P$
2. $\frac{P}{2}$
3. $\frac{P}{3}$
4. $P$

Q. Four identical bulbs each rated $100\text{watt}$ , $220\text{volts}$ are connected across a battery as shown. The total electric power consumed by the bulbs (in $\text{watt}$) is:

Q. A man carrying a bag of mass $25\text{kg}$ climbs up to a height of $10\text{m}$ in $50\text{s}$. The power delivered by him to the bag is
$[\text{Take}g=9.8{\text{m/s}}^2]$

1. $64\text{W}$
2. $36\text{W}$
3. $49\text{W}$
4. $23\text{W}$

Q. N-m/sec is unit of ? a.work b.pressure c.power d.energy

Q. The circuit shown below contains two ideal diodes, each with a forward resistance of $50\Omega$. If the battery voltage is $6\text{V}$, the current through the $100\Omega$ resistance (in Amperes) is :

1. $0.027$
2. $0.030$
3. $0.020$
4. $0.036$

Q. A condenser of capacity $C_1$ is charged to a potential $V_0$. The electrostatic energy stored in it is $U_0$. It is connected to another uncharged condenser of capacity $C_2$ in parallel. The energy dissipated in the process is

1. $\frac{C_1}{C_1+C_2}{U}_0$
2. $\frac{C_2}{C_1+C_2}{U}_0$
3. $\left(\frac{C_1-C_2}{C_1+C_2}\right)U_0$
4. $\frac{C_1{C}_2}{2(C_1+C_2)}{U}_0$