Physics

11.4 Wavelength of the monochromatic light, $\lambda =632.8nm$ = 632.8 $*{10}^{-9}$ m

Power emitted by laser, P = 9.42 mW = 9.42 $*{10}^{-3}$ W

Planck's constant, h = 6.626 $*{10}^{-34}$ Js

Speed of light, c = 3 $*{10}^8$ m/s

Mass of hydrogen atom, m = 1.66 $*{10}^{-27}$ kg

The energy of each photon is given as, $E$ = $\frac{hc}{\lambda }$ = $\frac{6.626\mathrm{}*{10}^{-34}*3\mathrm{}*{10}^8}{632.8*{10}^{-9}}$ J = 3.141 $*{10}^{-19}$ J

The momentum of each photon is given by $p$ = $\frac{h}{\lambda }$ = $\frac{6.626\mathrm{}*{10}^{-34}}{632.8*{10}^{-9}}$ = 1.047 $*{10}^{-27}$ kg-m/s

Number of photons arriving per second at a target irradiated by the beam = n

Assume that the beam has a uniform cross-section that is less than the target area.

Hence, the equation for power can be written as, $P=nE$

n = $\frac{P}{E}$ = $\frac{9.42*{10}^{-3}}{3.141*{10}^{-19}}$ = 3.0 $*{10}^{16}$

Momentum of the hydrogen atom = momentum of the proton = 1.047 $*{10}^{-27}$ kg-m/s

The momentum of the hydrogen atom , $p=mv$ , where m = mass of hydrogen atom and v = velocity

Hence, v = $\frac{p}{m}$ = $\frac{1.047*{10}^{-27}}{1.66\mathrm{}*{10}^{-27}}$ = 0.631 m/s

11.2 Work function of cesium metal, ${\varnothing }_0$ = 2.14 eV

Frequency of light, $\nu$ = 6 $*{10}^{14}$ Hz

The maximum kinetic energy is given by the photoelectric effect, $K$ = $h\nu -{\varnothing }_0$ ,

Where $h$ = Planck's constant = 6.626 $*{10}^{-34}$ Js, 1 eV = 1.602 $*{10}^{-19}$ J

$K$ = $\frac{6.626*{10}^{-34}*6*{10}^{14}}{1.602*{10}^{-19}}-2.14$ = 0.345 eV

$\mathrm{H}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{}\mathrm{m}\mathrm{a}\mathrm{x}\mathrm{i}\mathrm{m}\mathrm{u}\mathrm{m}\mathrm{}\mathrm{k}\mathrm{i}\mathrm{n}\mathrm{e}\mathrm{t}\mathrm{i}\mathrm{c}\mathrm{}\mathrm{e}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{y}\mathrm{}\mathrm{o}\mathrm{f}\mathrm{}\mathrm{t}\mathrm{h}\mathrm{e}\mathrm{}\mathrm{e}\mathrm{m}\mathrm{i}\mathrm{t}\mathrm{t}\mathrm{e}\mathrm{d}\mathrm{}\mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{n}\mathrm{}\mathrm{i}\mathrm{s}\mathrm{}0.3416\mathrm{}\mathrm{e}\mathrm{V}$

For stopping potential $V_0$ , we can write the equation for kinetic energy as:

$K$ = $V_0$ , where e = charge of an electron = $1.6*{10}^{-19}$

or $V_0=\frac{K}{e}$ = $\frac{0.345*1.602*{10}^{-19}}{1.6*{10}^{-19}}$ = 0.345 V

Hence, the stopping potential is 0.345 V

Maximum speed of the emitted photoelectrons = v

Kinetic energy K = $\frac{1}{2}$ m $v^2$ , where m = mass of the electron = 9.1 $*{10}^{-31}$ kg

Hence, v = $\sqrt{\frac{2K}{m}}$ = $\sqrt{\frac{2*0.345*1.602*{10}^{-19}}{9.1\mathrm{}*{10}^{-31}}}$ = 348.5 $*{10}^3$ m/s = 346.8 km/s

Therefore, the maximum speed of the emitted photoelectrons is 346.8 km/s.

7.26 The rating of step-down transformer = 40000 V – 220 V

Hence, the input voltage,  $V_1$ = 40000 V

Output voltage,  $V_2$ = 220 V

Total electric power required, P = 800 kW = 800 $*{10}^3$ W

Source potential, V = 220 V

Voltage at which electric plant generates power, V' = 440 V

Distance between the town and power generating station, d = 15 km

Resistance of the two wires lines carrying power = 0.5 Ω /km

Total resistance of the wire, R = 2 $*15*0.5\Omega$ = 15 Ω

rms current in the wire lines is given as

I = $\frac{P}{V_1}$ = $\frac{800\mathrm{}*{10}^3}{40000}$ = 20 A

Line power loss = $I^{2}R$ = ${20}^2*$ 15 = 6 $*{10}^3$ W = 6 kW

Since the leakage power loss is negligible, the total power to be supplied by plant = 800 + 6 = 806 kW

Voltage drop in the transmission line = IR = 20 $*15$ = 300 V. Hence, the total voltage transmitted from plant = 300 + 40000 = 40300 V

Since the power generated is 440V, the transformer rating at the plant should be 440 V – 40300 V

Hence, power loss during transmission = $\frac{6}{806}$ $*100$ = 0.744 %

In the previous exercise, power loss during transmission = $\frac{600}{1400}$ $*100$ = 42.86 %

Since the power loss is less for a high voltage transmission, high voltage transmission is preferred.

7.25 Total electric power required, P = 800 kW = 800 $*{10}^3$ W

Supply voltage, V = 220 V

Electric plant generating voltage, V' = 440 V

Distance between the town and power generating station, d = 15 km

Resistance of the two wires lines carrying power = 0.5 Ω /km

Total resistance of the wire, R = 2 $*15*0.5\Omega$ = 15 Ω

Step-down transformer rating 4000 – 220 V, hence

Input voltage to the transformer,  $V_1$ = 4000 V

Output voltage from the transformer,  $V_2$ = 220 V

rms current in the wire lines is given as

I = $\frac{P}{V_1}$ = $\frac{800\mathrm{}*{10}^3}{4000}$ = 200 A

Line power loss = $I^{2}R$ = ${200}^2*$ 15 = 600 $*{10}^3$ W = 600 kW

Since the leakage power loss is negligible, the total power to be supplied by plat = 800 + 600 = 1400 kW

Voltage drop in the transmission line = IR = 200 $*15$ = 3000 V. Hence, the total voltage transmitted from plant = 3000 + 4000 = 7000 V

Since the power generated is 440V, the transformer rating at the plant should be 440 V – 7000 V

7.22 (a) It is true that in any AC circuit, the applied voltage is equal to the average sum of instantaneous voltages across the series elements of the circuit. However, this is not true for rms voltages because voltage across different elements may not be in phase.

(b) A capacitor is used in the primary circuit of an induction coil. This is because when the circuit is broken, a high induced voltage is used to charge the capacitor to avoid sparks.

(c) The dc signal will appear across capacitor C because for dc signals, the impedance of an inductor (L) is negligible while the impedance of a capacitor © is very high (almost infinite). Hence, a dc signal appears across C

For an AC signal of high frequency, the impedance of L is high and that of C is very low. Hence, an AC signal of high frequency appears across L.

(d) If an iron core is inserted in the choke coil (which is in series with a lamp connected to AC line), then the lamp will glow dimly. This because the choke coil and the iron core increase the impedance of the circuit.

(e) A choke coil is needed in the use of fluorescent tubes with AC Mains because it reduces the voltage across the tube without wasting much power. An ordinary resistor cannot be used instead of choke coil for this purpose because it wastes power in the form of heat.