Chapter – 14
Q.1: What is flux density and how is it related to the number of lines of induction expressed in Webers?
Ans: Magnetic flux density is the magnetic flux per unit area . The unit of flux density is Weber per (or tesla, ). Magnetic flux is the total number of magnetic lines of induction passing perpendicularly through an area . Its SI unit is ‘Weber’ . Hence magnetic flux density refers to the number of lines of induction (in Webers) per square meter.
Q.2: Charged particles fired in a vacuum tube hit a fluorescent screen. Will it be possible to know whether they are positive or negative?
Ans: Yes, the charge on particles in motion can be found by applying a magnetic field perpendicular to the motion of the charges and by observing the deflection. A positive charge in an inward perpendicular magnetic field is deflected upward. In an electric field, a positive charge will be deflected towards the negative side (plate).
Q.3: Beams of electrons and protons are made to move with the same velocity at right angles to a uniform magnetic field of induction. Which of them will suffer a greater deflection? What will be the effect on the beam of electrons if their velocity is doubled?
Ans: The radius of the circular path of a particle moving in a magnetic field is . Thus is proportional to , but deflection is proportional to . Thus, the electron, being lighter, will be deflected more than the proton.
Since , if velocity is doubled, the radius will also be doubled; but deflection is halved.
Q.4: A circular loop of wire hangs by a thread in a vertical plane. An electric current is maintained in the loop anti-clockwise when looking at the front face. To what direction will the front face or the coil turn in the earth’s magnetic field?
Ans: Toward the geographic north pole.
Q.5: Imagine that the room in which you are seated is filled with a uniform magnetic field pointing vertically upward. A loop of wire, which is free to rotate about the horizontal axis is placed through its center parallel to its length, has its plane horizontal. For what direction of current in the loop, as viewed from above, will the loop be in a stable equilibrium with respect to forces and torque of magnetic origin?
Ans: Anti-clockwise.
Q.6: Two identical loops, one of copper and the other of aluminum, are similarly rotated in a magnetic field of induction. Explain the reason for their different behavior. Is an electric generator a ‘generator of electricity’? Where is the electricity before it is generated? What do such machines generate?
Ans: Since the conductivities of copper and aluminum are different, they show different behavior with the induced e.m.f. As the conductivity of copper is higher than that of aluminum, a copper loop will have a greater induced current than an identical aluminum loop moving with the same speed in the same magnetic field. An electric generator is not a generator of electricity (i.e., quantity of charge). Electricity is present in the conducting coil of the generator before it was driven in an electrical circuit. A generator provides e.m.f. to drift the haphazardly moving electrons in the conducting coil. In fact, a generator converts mechanical energy into energy of moving charges.
Q.7: A loosely wound helical spring of a stiff wire is mounted vertically with the lower end just touching mercury in a dish. When a current is started in the spring, it excretes a vibratory motion with its lower end jumping out and into mercury. Explain the reason for this behavior?
Ans: When a current is passed through a helical spring, whose one end is just above a mercury pool, a magnetic field is produced. The current through all the loops is in the same direction. This produces attraction between them, so its length decreases. The dipping and moving out of the mercury causes the circuit to break. Due to elasticity, the helix regains its original length. The electrical contact is established again. The process is repeated, so the helix vibrates up and down.
Q.8: What is the mechanism of transfer of energy between the primary and secondary windings of a transformer? A certain amount of power is to be transferred over a long distance. If the voltage is stepped up 10 times, how is the transmission line loss reduced?
Ans: Electromagnetic induction is the phenomenon responsible for the transfer of energy between the primary windings (one circuit) to the secondary windings by means of a changing magnetic field which links the two coils. The mutual induction transforms the voltage or e.m.f. of large or similar value due to a different number of turns in the primary and secondary coils.
Suppose a power line has input power . The same power can be carried at low current if the voltage is made high. Input current . If voltage is stepped up 10 times:
When the current is 10 times smaller, the power loss as heat in the wires is , i.e., 100 times smaller.
Q.9: What is the difference between magnetic field a.c. generators? What is meant by the frequency of alternating current?
Ans: An alternating current generator that uses a permanent magnet to provide the magnetic flux rather than an electromagnet is called ‘magnetic’. It is used in systems like petrol engines, motorbikes, and motorboats. The a.c. generator that employs electromagnets is called “alternate.” It has a rotating field magnet (called rotor) and a stationary armature (called stator) or vice versa.
Alternating current (a.c.) is produced by a voltage source whose terminal polarity reverses with time. The number of cycles per second made by an a.c. is called its frequency . Its unit is hertz (Hz). We have , and an a.c. reverses its polarity times per second. An a.c. with a frequency of 50 Hz has a time period of seconds. This a.c. reaches zero every 0.01 seconds.
Q.10: In what direction are the magnetic field lines surrounding a straight wire carrying current that is flowing directly towards you?
Ans: Anti-clockwise (using the right-hand rule).
Q.11: What kind of field or fields does or do surround a moving electric charge?
Ans: When an electric charge is in motion, it is surrounded by an electrostatic field as well as a magnetic field.
Q.12: Can an electron at rest be set in motion with a magnet?
Ans: No. When an electron is at rest, it has no magnetic field . So, in the absence of any magnetic field of its own, it cannot interact with a magnet.
Q.13: A beam of electron is directed towards a horizontal wire in which the current flows from left or right. In what direction is the beam deflected?
Ans: If the beam is parallel to the wire, it will follow a spiral path; and if it is perpendicular to the wire, it will adopt a circular path.
Q.14: A charged particle is moving in a circle under the influence of a uniform magnetic field. If an electric field is turned on at that point along the same direction as the magnetic field, what path will the charged particle take?
Ans: When a charged particle is moving in a circle under the influence of a uniform magnetic field; and if an electric field is applied along the same direction, it will exert lateral force on the charged particle. Consequently, the charged particle will move in a cyclic path in the form of a spiral (called helix).
Q.15: A loop of wire is suspended between the poles of a magnet with its plane parallel to the pole faces. What happens if direct current is passed through the coil? What will happen if an alternating current is passed instead?
Ans: When d.c. passes through the loop such that its magnetic field is: (i) opposite the direction of the field of the magnet, the coil will turn round through 180° and then will stay in equilibrium. (ii) Along the field of the magnet, the coil will stay in equilibrium. However, when an a.c. is passed through the loop, it will remain in its initial position (with slight vibration).
Q.16: A current-carrying wire is placed in a magnetic field. How must it be oriented so that the force acting on it is zero or is maximum?
Ans:
- Force will be zero if theta is equal to zero (parallel to B);
- Force will be maximum if theta is equal to 90° (perpendicular to B).
Q.17: Why is the magnetic field strength greater inside a current-carrying loop of wire?
Ans: In a loop of wire, the direction of current in the opposite sides of the loop is opposite to each other. This is analogous to two parallel conductors carrying current in opposite directions. The directions of both the magnetic fields are along the same direction in the loop. This increases the strength of the field.
Q.18: What exactly does a transformer transform?
Ans: A transformer transforms the magnitude of alternating voltage and current.
Q.19: Can an efficient transformer step up energy? Explain.
Ans: Transformers cannot charge energy. In an ideal transformer, the power remains constant, i.e., power input equals power output . Thus it cannot step up energy.
Q.20: In what three ways can a voltage be induced in a wire?
Ans:
- By moving a wire in a magnetic field.
- By moving a magnet near it.
- By changing current through a circuit near it.
Q.21: Does the voltage output of a generator change if its speed of rotation is increased?
Ans: Yes, because induced e.m.f. = . Thus, an e.m.f. increases if the speed of rotation “Omega” is increased.
Q.22: When a beam of electrons is shot into a certain region of space, the electrons travel a straight line through the region. Can we conclude that in the region there is no electric field? No magnetic field?
Ans: There are two possibilities:
- No electric or magnetic field is present.
- The electric and magnetic fields are at right angles to each other, and their strengths are exerting equal but opposite forces on the electron beam.
Q.23: A copper ring is placed above a solenoid with an iron core to increase its field. When the current is turned on in the solenoid, the copper ring moves upward. Why?
Ans: When current in a solenoid (with an iron core) increases, an induced current is produced in a copper ring (held above it) in the opposite direction. This is analogous to opposite currents in two parallel wires. Thus they develop similar poles and repel each other. Consequently, the ring moves up.
Q.24: A very long copper pipe is held vertically. Describe the motion of a bar magnet dropped lengthwise down the pipe?
Ans: Suppose a bar magnet falls through a very big copper pipe (under gravity). When the magnet is well inside the pipe, the configuration of the magnetic field remains the same. So it will fall freely with acceleration of gravity only.
Q.25: A solenoid is viewed in such a way that the solenoid current appears clockwise to the viewer. What is the direction of the field within the solenoid?
Ans: The end viewed will develop south polarity. So the direction of the magnetic field will be away from the viewer inside the solenoid.
Q.26: A hollow copper tube carries. Why is inside the tube? Is non-zero outside the tube?
Ans: The charges always reside or move on the outer of a conductor. Since inside the tube, current is zero, hence (according to Ampere’s law). The outer surface of the tube behaves like a set of parallel wires carrying current down their length. The magnetic field outside the tube exists, and its direction is given by the right-hand grip rule.
Q.27: Can a current-carrying coil be used as a compass?
Ans: A current-carrying coil behaves like a bar magnet (magnet dipole). Thus when it is suspended freely, it can be used as a compass.
Q.28: When a charged particle enters a magnetic field, it is deflected by the magnetic force? Can the magnetic force do work on the moving charged particle?
Ans: No, magnetic force can do no work on a moving charged particle because it is always perpendicular to the velocity of the particle.
Q.29: If both electric field (E) and magnetic field (B) act on a charged particle, what is the total force on it?
Ans: The total force is . This force is called the Lorenz force.
Q.30: Can an isolated magnetic pole (monopole) exist? What is the source of the magnetic fields?
Ans: No, magnetic monopoles cannot exist. The only known source of magnetic fields are magnetic dipoles (current loops), even in magnetic materials.
