EXERCISE
MULTIPLE CHOICE QUESTIONS
Choose the best possible answer:
1.A current is flowing North in a magnetic field that points West. It experiences.
a. a force down b. a force up
c. a force west d. no force
2. Two long, straight wires have currents flowing in them in the same direc tion, the force between the wires is
a. attractive b. repulsive
C. zero d. infinite
3. One weber is equal to
a. N A2/m c. NA/m
b. N m²/A d. N m/A
4. Magnetic flux will be maximum if the angle between magnetic field strength and vector area is
a. 0° b. 45°
c. 60° d. 90°
5. Ampere's law is applied over any
a. surface b. closed surface
d. closed path c. path
6. When the number of turns in a solenoid is doubled without any change in the length of the solenoid its self induction will be
a. Four times
b. Doubled
c. Halved
d. One forth.
7. If the charge is at rest in magnetic field, then force on charge is
a. qvB b. qvB close
c. qvB sine d. zero
8. Work done on a charge particle moving in a uniform magnetic field is
a. maximum b. minimum c. zero
d. infinite
9. The Earth northern magnetic poles acts like:
a. the north pole of a magnet c. it has a positive charge
b. the south pole of a magnet d. it has a negative charge
10. The pole pieces of the magnet in a galvanometer are made concave to make the field
a. Radial b. Stronger
c. Weaker d. Both A & B
11. When a small-resistance is connected parallel to galvanometer, the resulting circuit behaves as
a. Voltmeter b. Ammeter
c.Velocity Selector d. AVO meter
12. To measure the voltage, the voltmeter is connected with the circuit in
a. series
b. parallel
c. Perpendicular
d. Straight Line
13. The resistance of ideal voltmeter is
a. Small
b. Large
c. Zero
d. Infinite
14. The torque in the coil can be increased by increasing
a. Number of turns
b. Current and magnetic field
c. Area of coil
d. All of these
CONCEPTUAL QUESTIONS
Give a short response to the following questions:
1. A compass needle is deflected when a charge plastic rod is held near it. What is the origin of the force that produces the deflection?
Ans. The origin of the force that produces deflection in the compass needle is the electrostatic interaction.
Explanation: Since compass needle is made of magnet and the magnet is metal. It has positive and negative mobile charges. When metal (any metal, not just a mag net) comes near a charged object, charges in the metal will rearrange themselves. Opposite charges attract and like charges repel. If the metal approaches a rod with positive charge then negative charge will move toward the rod and positive charge will move away. The result will be an attractive electric force.
2. What is the difference between permeability and permittivity?
Ans. In electromagnetic theory, there are two concepts known as the permittivity
and permeability. The concept related to the formation of an electric field is known as permittivity, and the one related to the magnetic field is known as permeability.
Permeability: It is the property of the material which supports the formation of
magnetic flux when passed through a magnetic field and is denoted by μ. Permittivity: It is the measurement of resistance to the expansion of electric field
lines in a medium and is denoted by E.Difference between Permeability and Permittivity: Some of the differences be tween permeability and permittivity are given below. The ability of a material to allow magnetic lines to conduct through it isknown as permeability, whereas the obstruction produced by the material to the formation of electric field is known as permittivity.Permeability is epresented by u and permittivity by ε. Farad/meter is the SI unit of permeability, and Henry/meter is for permittivity.
3. Can the objects in rotational motion create a magnetic field? Provide an example.
Ans. Yes, the objects in rotational motion can create a magnetic field.
Explanation: As we know that whenever a charge particle is in motion, it creates magnetic field around itself. Since rotation is also a type of motion, therefore a charge object in rotational motion will also create magnetic field around itself. Example: The spin magnetic field of an electron is due to the rotation of electron about its own axis. It is also believed that the earth's magnetic field is due to the spinning of earth about its axis. The outer core of earth is mostly consisting of molten iron and nickel. When the earth rotates about its axis, an electrical current is setup by motion of these metallic fluids, which results in the for mation of magnetic field around the earth's sur face. This effect is also known as dynamo-effect.
4. Electron and proton are projected with the same velocity normal to the magnetic field. Which one will suffer greater deflection? Why?
Ans. Electron will be deflected more as it is less massive than the proton.
Explanation: Since electron and proton are charged particle having same charge, therefore by projecting them normal to the magnetic field, they will adopt a cir cular path. The centripetal force required for the circular motion is provided by the magnetic force. Therefore;
F - = F -> (1)
Since
F_{c} = (m * v ^ 2)/r and F = qvB therefore Eq. (1) becomes;
m * v ^ 2 = qvB
Solving for r, we get;
r = (vm)/(Bq)
V/(8a) :
constant
r-constantm
Thus it is clear that radius of the path is directly proportional to the mass of the particle. Since proton is 1836 times massive than electron, therefore electron will deflect more as compared to proton.
5. A charged particle moves in a straight line through a particular region of space. Could there be a non-zero magnetic field in this region?
Ans. No, we cannot say for sure whether the magnetic field could be non-zero in this region. It is also possible that the magnetic field is zero.
Explanation: When a charged particle moves in a magnetic field, it experiences a
force, whose magnitude is given by the equation;
F= qvBsine
There are three possibilities existed for a charged particle moving straight in a re
1) There is no magnetic field in the region.
gion of space.
2) There is a magnetic field such that the direction of motion of the charged particle and the magnetic field are same. In such a case the angle 0=0°.
Since sin 0° 0, therefore, the magnetic force on the charged particle would be zero and the particle would be moving in a straight line. 3) If there is an electric field adjusted such that its force equals to the mag netic field but having opposite direction. In such a case both force will be cancelling each other.
Conclusion: So it is not possible to say with certainty that there could be a non-zero magnetic field in the region.
6. If a current is passed through an unstretched spring, will the spring contract
or expand?
Ans. The spring will contract if a current is passed through an unstretched spring. Explanation: It must be noted that when two wires are placed parallel to each other and current flows in the same direction, then the wires attract each other.
Now consider the case of spring by passing current through it. The current will flow through all the turns in the same direction as shown in the figure. On account of force of attraction between two adjacent turns car rying current in the same direction, the spring will contract or compress.
7. How can neutrons be accelerated in a cyclotron?
Ans. No, neutrons cannot be accelerated in a cyclotron.
Explanation: Cyclotron is a type of particle accelerator in which charge particle is accelerated under the action of magnetic field towards a target. The frequency at which charge particles revolve in a cyclotron is given by;
f=
2rtm
Since neutrons have no charge, therefore they cannot be accelerated.
8. A current carrying loop, free to turn, is placed in a uniform magnetic B. What will be its orientation relative to B, in the equilibrium state?
Ans. In equilibrium state, the normal to the current carrying loop will be parallel to the magnetic field.
Explanation: Since torque acting on a current carrying loop is given by; T=NIBAsine In equilibrium state as shown in the figure, the net torque acting on the current carrying coil is zero, which in this case is possible only if the angle be tween the normal to the plane of the coil and the magnetic field is zero, i.e. 0=0° T=NIBAsino
sin0° = 0
Ï€=0
9. How does a current carrying coil behave like a bar magnet?
Ans. The magnetic field of a current carrying coil resembles that of a bar magnet therefore, current carrying coil behaves like a bar magnet..
Explanation: A bar magnet is a permanent magnet, having North and South Poles. Magnetic field lines originate from North Pole and ends on South Pole as shown in the figure (a).
Bar Magnet (a)
Current Carrying Coil(b)
Now consider a coil having N number of turns. When current passes through the coil, magnetic field is setup around it as shown in figure (b). Direction of this field can be determined by Right Hand Rule III, which states that "grab the current car rying conductor in your right hand in the direction of current, then the thumb will point in the direction of North Pole and other side will behave like a South Pole". By comparing both the fields, it is clear that both are alike. Therefore, a current carrying coil behaves like a bar magnet.
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