Chapter 13: ELECTROMAGNETISM (Short Questions/Answers)

 

Chapter no 13: 

ELECTROMAGNETISM 

13.1: what is the force that a conductor of length L carrying a current I, experiences when placed in a  magnetic field B? What is the direction of the force? 

Ans: When a current carrying conductor is placed in a magnetic field then it experiences a magnetic  force. This force is the result of interaction of magnetic field due to current carrying conductor and the  applied field which is given as: 

F = I(L × B) or F= ILBSinθ 

According to right hand rule, the direction of is perpendicular to the plane containing L and B.  13.2: what is the nature of the force between two parallel current carrying wires ( in same direction )? 

Ans: When two current carrying conductors in which same current in same direction is flowing then  both the conductors attract each other. It is because the net field b/w the conductors are weaker as  compared to outer sides. So conductors are pulled from stronger to weaker region. 

13.3: what t is the magnitude of the force on a charge q moving with a velocity v in a magnetic field B? 

Ans: when a charge particle is moving with a velocity v in a magnetic field B then it will experience a  magnetic force which is given as: 

F = q (v × B) or F= qvBSinθ 

If v is parallel or anti parallel to B then θ= 0 or θ= 180 

F= 0 

If v is perpendicular to B then θ=90  

F=qvB = Maximum 

13.4: in a uniform magnetic field B , an electron beam enters with velocity v . Write the expression for  the force experienced by the electrons? 

Ans: When an electron is moving with a velocity v in a magnetic field B then it will experience a magnetic  force which is given as: 

F = q (v × B) or F= qvBSinθ 

If electron is moving parallel or anti parallel to magnetic field then θ=0 or θ=180, then no force acts on  the electron but if it moves perpendicular to magnetic field θ=90, then maximum force acts on it.  

13.5: what will be the path of a charged particle moving in a uniform magnetic field at any arbitrary  angle with the field?

Ans: When charge particle moves in a magnetic field with 

a). perpendicular to B then it follows a circular path 

b). Parallel or anti parallel to B then it moves in a straight line without deviation. c). 0<θ<90 then it follows helical path. 

13.6: an electron does not suffer any deflection while passing through a region. Are you sure that  there is no magnetic field? 

Ans: When a charge particle is moving through some region of space without deflection then magnetic  field in that region may or may not be zero because no net force acts on it. If magnetic field is zero(B=0)  then F= qv(0)=0 

If charge particle is moving parallel to magnetic field then θ=0 

So F=qvB Sin0 =0 

13.7: an electron beam passes through a region of crossed electric and magnetic fields of intensity E  and B respectively. For what value of the electron speed the beam will remain un-deflected? 

Ans: when an electron beam passes through a region where electric and magnetic fields are  perpendicular then speed of beam of electron is so adjusted that it will experience no net force. 

Fe =FB or qE = qvB 

Or E/B =v or v = E/B 

In this value of speed electron beam will remain un-deflected. 

13.8: uniform electric and magnetic field are produced pointing in the same direction. An electron is  projected in the direction of the field. What will be the effect on the kinetic energy of the electron due  to the two fields? 

Ans: In such case electron will experience only electric force because magnetic field exerts no force  when electron is moving parallel to B. hence the net force on electron is: 

F = Fe + FB = qE+ 0= qE 

So the particle will accelerates due to this net force and K.E will continue to increase.  

13.9: what is the cyclotron frequency of a charged particle of mass m, charge q moving in a magnetic  field B? 

Ans: When an electron is moving with a velocity v in a magnetic field B then it will experience a  magnetic force due to which it moves in a circular path with a particular frequency which is called 

frequency of cyclotron. During its circular motion magnetic force provides a required centripetal force  which is given as: Fc = FB or mv2/ r =qvB 

V/r = qB/m or rω/r = qB/m 

ω = qB/m or 2πf = qB/m 

f =1/2π (qB/m) 

13.10:can neutrons be accelerated in a cyclotron? Give reasons. 

Ans: Neutrons cannot be accelerated in a cyclotron because in cyclotron, charge particles are  accelerated in a magnetic field. But neutrons are neutral, so no magnetic force acts on it therefore it  cannot be accelerated in cyclotron. 

13.11: a current carrying loop, free to turn , is placed in a uniform magnetic field B, what will be its  orientation relative to B, in a equilibrium state? 

Ans: As torque acts on a current carrying loop is 

τ = BINACosθ 

If current carrying loop is oriented perpendicular in a magnetic field i;e θ=90 

So τ = BINACos90 =0 

Hence current carrying loop is in equilibrium. 

13.12: How does a current carrying coil behave like a bar magnet? 

Ans: As we know that due to flow of current in a coil magnetic field is produced inside it in form of  straight lines that resembles the magnetic field of a bar magnet. According to right hand rule “ hold the  coil in your right hand in such a way that curling of fingers show direction of current whereas thumb  indicate the direction of north pole .