How many electrons flow through a filament of a \(220V\) and \(100W\) electric bulb per second? Given: electronic charge,\(e=1.6\times10^{-19}C\)
Ans: \(\left(2.84\times 10^{18}\right)\).
The maximum power rating of a \(50V\) d.c supply draws a current of \(12A\). If, the efficiency of motor is \(30%\), estimate the resistance of the winding of the motor.
Ans: \(R=2.6\Omega\).
A \(500W\) heating unit is designed to operate from a \(200V\) line. By what percentage will its heat output drop if the line voltage drops to \(160V\)?
Ans: \(36\%\).
We have \(30W\), \(6V\) bulb which we want to glow by a supply of \(120V\). What will have to be done for it?
Ans: \(22.8\Omega\) resistance must be connected in series.
A heater coil is rated \(100W\) and \(200V\). It is cut into two identical pieces. Both pieces are connected together in parallel, to the same source of \(200V\). Calculate: the energy liberated per second in the new combination?
Ans: \(400J\).
An electric bulb rated for \(500W\), \(100V\) is used in a circuit having a \(200V\) supply. Calculate: the resistance R that must be put into a series connection with the bulb, so that the bulb delivers \(500W\).
Ans: \(R=20\Omega\).
An electric heater is rated \(800W\). If the supply voltage is \(200V\); (a) What is the current rating? (b) How much time would be required to heat \(1lt\) of water from \(200^{o}C\) to \(1000^{o}C\)?
Ans:(a) \(4A\). (b) \(t=1h10min\).
A thin metallic wire of resistance \(200\Omega\) is immersed in calorimeter containing \(250g\) of water at \(10^{o}C\) and a current of \(0.5Ampere\) is passed through it for half an hour. If, the water equivalent of the calorimeter is \(10g\), find: the rise in temperature.
Ans: \(+\Delta T=82.4^{o}C\).
Two heaters are marked \(200V\), \(300W\) and \(200V\), \(600W\). If the heaters are connected in series and the combination is connected to a \(200V\) d.c supply, which heater will produce more heat?
Ans: \(Heater_{1^{st}}\).
There are two electric bulbs rated \(60W\), \(110V\) and \(100W\), \(110V\). They are connected is series with a \(220V\) d.c supply. Will any bulb fuse? What will happen if they are connected in parallel with same supply?
Ans: \(\).
In a part of the circuit shown in below figure. The rate of heat dissipation in \(4\Omega\) resistor is \(100\frac{J}{s}\). Calculate; the heat dissipated in the \(3\Omega\) resistor in \(10seconds\)?
Ans: \(Heat=3000J\).
Three resistors \(R_{1}\),\(R_{2}\) and \(R_{3}\) are connected to a battery of \(12V\) as shown in below figure. Calculate: the heat produced in each of three resistors in 2 minutes?
Ans: \(H_{1}=320J\), \(H_{2}=640J\) and \(H_{3}=1920J\).
The current is drawn from a cell of e.m.f \(E\) and internal resistance \(r\) connected to the network of resistors each of resistance \(r\). Obtain the expression for; (i) The current drawn from the cell? (ii) The power consumed in the network?
Ans: \(P_{consumed}=\frac{3E^{2}}{4r}\).
A heater is designed to operate with a power of \(1000W\) in a \(100V\) connection. It is connected to two resistance of \(10\Omega\) and \(R\Omega\). If, now the heater is operating with a power of \(62.5W\), Calculate: the value of \(R\)?
Ans: \(R=5\Omega\).
4 cell’s of identical e.m.f. \(E\), internal resistance \(r\), are connected in series to a variable resistor. The following graph shows the variation of the terminal voltage of the combination with the current output. (i) What is the e.m.f. of the each cell? (ii) For, what current from the cell does maximum power of \(20W\), otherwise it will melt. What maximum power will the whole circuit dissipates? (iii) Calculate: the internal resistance of the each cell?
Ans:(a) \(e.m.f=1.4V\), (b) \(I_{max}=1A\). and (c) \(r=0.7\Omega\).
Two batteries, each of e.m.f. \(E\), and internal resistance \(r\), are connected in parallel. If we take current from this combination in an external resistance \(R\), then for what value of \(R\) maximum power will be obtained? What will be this power?
Ans: \(P_{max}=\frac{E^{2}}{2r}\).
Two wires made of tinned copper having identical cross section \(\left(10^{6}m^{2}\right)\) and length \(15cm\) are to be used as fuses. Show that the fuses will melt at the same value of current in each case?
Ans: \(h=\frac{I^{2}\rho}{2\pi^{2}r^{3}}\).
A series of batteries of \(6\) lead accumulator each of e.m.f. \(2.0V\) and internal resistance \(0.50\Omega\), is charged by a \(100V\) source. What resistor must be used in series in the charging circuit in order to limit the current to \(8.0A\)? Using required resistor, obtain: (i) The power supplied by the d.c source. (ii) The power dissipated as heat and (iii) Chemical energy stored in the battery for \(15min\).
Ans:(i) \(P=800W\). (ii) \(P_{heat}=704W\) and (iii) \(E_{battery}=86400J\).
Calculate: the ratio of the heat produced in the four arm of wheatstone bridge as shown figure:
Ans: \(H_{AB}:H_{BC}:H_{AD}:H_{DC}=6:15:4:1\).
Two bulbs are rated \(\left(P_{1},V\right)\) and \(\left(P_{2},V\right)\). If, they are connected (i) In series. (ii) In parallel across a supply V. Find; the power dissipated in two combinations in terms \(P_{1}\) and \(P_{2}\).
Ans:(i) \(\frac{1}{P_{s}}=\frac{1}{P_{1}}+\frac{1}{P_{2}}\). (ii) \(P_{p}=P_{1}+P_{2}\).
