# Q. The magnetic field associated with a light wave is given, at the origin, by $B = B_0 [\sin(3.14 \times 10^7)ct + \sin(6.28 \times 10^7)ct]$. If this light falls on a silver plate having a work function of 4.7 eV, what will be the maximum kinetic energy of the photo electrons ? $(c = 3 \times 10^{8} ms^{-1}, h = 6.6 \times 10^{-34} J-s)$

JEE Main JEE Main 2019

Solution:

## $B = B_0 \sin (\pi \times 10^7C)t + B_0 \sin (2 \pi \times 10^7C)t$ since there are two EM waves with different frequency, to get maximum kinetic energy we take the photon with higher frequency $B_{1} = B_{0} \sin \left(\pi \times10^{7} C\right)t v_{1} = \frac{10^{7}}{2} \times C$ $B_{2} = B_{0} \sin \left(2 \pi \times10^{7} C\right)t v_{2} = 10^{7} C$ where C is speed of light $C =3 \times10^{8}$ m/s $v_2 > v_1$ so KE of photoelectron will be maximum for photon of higher energy. $v_2 = 10^7 C$ Hz $v = \phi + KE_{max}$ energy of photon $E_{ph} = h_{V} = 6.6 \times10^{34} \times10^{7} \times3 \times10^{9}$ $E_{ph} = 6.6 \times3 \times10^{-19 }J$ $= \frac{6.6 \times3 \times10^{-19}}{1.6 \times10^{-19}} eV = 12.75 eV$ $KE_{max } = E_{ph} - \phi$ = 12.375 - 4.7 = 7.675 eV $\approx$ 7.7 eV

You must select option to get answer and solution

## 1. An AC ammeter is used to measure current in a circuit. When a given direct current passes through the circuit, the AC ammeter reads 3 A. When another alternating current passes through the circuit, the AC ammeter reads 4 A. Then the reading of this ammeter, if DC and AC flow through the circuit simultaneously, is

AIIMS 2010 Alternating Current

## 2. Assertion : Perspiration from human body helps in cooling the body Reason : A thin layer of water on the skin enhances its emissivity.

AIIMS 2006 Thermal Properties Of Matter

## 3. The speed of sound through oxygen at T K is v m$s^{-1}$. As the temperature becomes 2T and oxygen gas dissociates into atomic oxygen, the speed of sound

Manipal 2003 Electromagnetic Waves

## 4. If voltage across a bulb rated 220 volt-100 watt drops by $2.5\%$ of its rated value, the percentage of the rated value by which the power would decrease is

AIPMT 2012 Current Electricity

## 5. If the dimensions of a physical quantity are given by $\ce{M^aL^bT^c}$, then the physical quantity will be

AIPMT 2009 Physical World, Units and Measurements

## 6. There are 26 tuning forks arranged in the decreasing order of their frequencies. Each tuning fork gives 3 beats with the next. The first one is octave of the last. What is the frequency of 18th tuning fork?

BHU 2005 Electromagnetic Waves

## 7. The electric resistance of a certain wire of iron is $R$. If its length and radius are both doubled, then

JIPMER 2015 Current Electricity

## 8. A quarter horse power motor runs at a speed of 600 rpm. Assuming 40% efficiency, the work done by the motor in one rotation will be

Manipal 2009 Work, Energy And Power

## 9. A cannon of mass I 000 kg, located at the base of an inclined plane fires a shelI of mass 100 kg in a horizontal direction with a velocity 180 $kmh^{-1}$. The angle of inclination of the inclined plane with the horizontal is 45$^{\circ}$. The coefficient of friction between the cannon and the inclined plane is 0.5. The height, in metre, to which the cannon ascends the inclined plane as a result of the recoiI is (g = 10 $ms^{-1})$

EAMCET 2004 Laws Of Motion

## 10. Two charges $q_1$ and $q_2$ are placed 30 cm apart, as shown in the figure. A third charge $q_3$ is moved along the arc of a circle of radius 40cm from C to D. The change in the potential energy of the system is $\frac{q_3}{ 4 \pi \varepsilon_0}$ k. where k is

AIPMT 2005 Electrostatic Potential And Capacitance