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Thursday, 15 December 2011

Electromagnetism questions for SSC Science


ALL ABOUT ELECTROMAGNETISM
Questions/Answers
Q.1
Why does a compass needle get deflected when brought near a bar magnet?



Compass needle is a small magnet which experiences a force in the magnetic field of a bar magnet. Due to this force, it gets deflected.

Q.2
Draw magnetic field lines around a bar magnet.





Q.3
List the properties of magnetic lines of force.



(i) Magnetic lines of force are closed continuous curves.
(ii) The tangent at any point on the magnetic line of force gives the direction of the magnetic field at that point.
(iii) No two magnetic lines of force can cross each other.

Q.4
Why don’t two magnetic lines of force intersect each other?



The tangent at any point on a magnetic field line gives the direction of magnetic field at that point. If two magnetic field lines cross each other, then at the point of intersection, there will be two tangents. Hence, there will be two directions of the magnetic field at the point of intersection. This is not possible. Hence, no two magnetic field lines can cross each other.


Q.5
Consider a circular loop of wire lying in the plane of the table. Let the current pass through the loop clockwise. Apply the right-hand rule to find out the direction of the magnetic field inside and outside the loop.



Magnetic field inside the loop is perpendicular to the plane of table and in the downward direction. However, outside the loop, magnetic field is perpendicular to the plane of the table and in the upward direction.

Q.6
Choose the correct option.
The magnetic field inside a long straight solenoid-carrying current
(a) Is zero
(b) Decreases as we move towards its end
(c) Increases as we move towards its end
(d) Is the same at all points



(b) Decreases as we move towards its end

Q.7
Which of the following property of a proton can change while it moves freely in a magnetic field? (There may be more than one correct answer.)
(a) Mass
(b) Speed
(c) Velocity
(d) Momentum



A force acts on a proton when it moves freely in a magnetic field. Hence its velocity and momentum can change.

Q.8
A positively-charged particle (alpha-particle) projected towards west is deflected towards north by a magnetic field. The direction of magnetic field is
(a) Towards south   
(b) Towards east
(c) Downward
(d) Upward



(d) Upward

Q.9
What is the principle of an electric motor?



Electric motor works on the principle that a current carrying conductor placed perpendicular to a magnetic field experiences a force.

Q.10
Explain different ways to induce current in a coil.



(i) By moving a bar magnet towards or away from the coil.
(ii) By placing a coil near another coil connected across a battery.
Q.11
State the principle of an electric generator.



It is based on the principle of electromagnetic induction. That is, the changing magnetic field induces current in the coil.

Q.12
Name two safety measures commonly used in electric circuits and appliances.



(i) Electric fuse and (ii) earthing.

Q.13
What precaution should be taken to avoid the overloading of domestic electric circuits?



(i) We should not connect many appliances in the same socket.
(ii) Electrical appliances of high power rating should not be switched on simultaneously.

Q.14
Which of the following correctly describes the magnetic field near a long straight wire?
(a) The field consists of straight lines perpendicular to the wire
(b) The field consists of straight lines parallel to the wire
(c) The field consists of radial lines originating from the wire
(d) The field consists of concentric circles centred on the wire



(d) The field consists of concentric circles centred on the wire

Q.15
The phenomenon of electromagnetic induction is
(a) The process of charging a body
(b) The process of generating magnetic field due to a current passing through a coil
(c) Producing induced current in a coil due to relative motion between a magnet and the coil
(d) The process of rotating a coil of an electric motor



(c) Producing induced current in a coil due to relative motion between a magnet and the coil

Q.16
The device used for producing electric current is called a
(a) Generator
(b) Galvanometer
(c) Ammeter
(d) Motor



(a) Generator

Q.17
At the time of short circuit, the current in the circuit
(a) Reduces substantially
(b) Does not change
(c) Increases heavily
(d) Vary continuously



(c) Increases heavily

Q.18
State whether the following statements are true or false.
(a) An electric motor converts mechanical energy into electrical energy.
(b) An electric generator works on the principle of electromagnetic induction.
(c) The field at the centre of a long circular coil carrying current will be parallel straight lines.
(d) A wire with a green insulation is usually the live wire of an electric supply.



(a) False. It converts electrical energy into mechanical energy.
(b) True
(c) True
(d) False. Live wire has red insulation cover.

Q.19
List three sources of magnetic fields.



(i)a permanent magnet
(ii) a current carrying conductor
(iii) a current carrying solenoid

Q.20
How does a solenoid behave like a magnet? Can you determine the north and south poles of a current-carrying solenoid with the help of a bar magnet? Explain.



When electric current flows through a solenoid, magnetic field is set up around the solenoid. The pattern of the magnetic field is same as that of the magnetic field of a bar magnet. One end of the solenoid behaves as north pole and the other end of the solenoid behaves as south pole. To determine the north and south poles of a current carrying solenoid with the help of a bar magnet, suspend it with a strong thread. Now bring the north pole of a bar magnet towards one end of the solenoid. If the solenoid attracts towards the magnet, then that face of the solenoid is south pole. If the solenoid moves away from the bar magnet, then that face of the solenoid is the north pole.

Q.21
When is the force experienced by a current-carrying conductor placed in a magnetic field largest?



When current carrying conductor is placed perpendicular to the magnetic field.

Q.22
Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of magnetic field?



Movement from electron beam from back wall to the front wall is equivalent to the flow of electric current from front wall to the black wall. The deflection of the beam means, the force is acting towards our right side. according to Fleming''''s Left Hand Rule, the direction of magnetic field is vertically downward. That is, the magnetic field is perpendicular to the plane of the paper and directed inward. Such magnetic field is shown by http://www.extraminds.com/Uploads/EditorImages/chap13_phy_92.gif


Q.23
Name some devices in which electric motors are used?



Electric cars, rolling mills, electric fans, hair dryers, mixers, blenders etc.

Q.24
A coil of insulated copper wire is connected to a galvanometer. What will happen if a bar magnet is (i) pushed into the coil, (ii) withdrawn from inside the coil, (iii) held stationary inside the coil?




(i) When a bar magnet is pushed into the coil, induced current flows through the coil due to the phenomenon of electromagnetic induction. This induced current is indicated by the deflection of the needle of the galvanometer as shown in figure (a)
(ii) When a bar magnet is withdrawn from inside the coil, again induced current flows through the coil due to the phenomenon of electromagnetic induction. In this case, the  direction of induced current is opposite to the direction of the current in case (i) as shown in figure (b)
(iii) When the bar magnet is held stationary inside the coil, there is no change in magnetic field around the coil. Hence, no induced current flows through the coil. Therefore, galvanometer shows no deflection as shown in figure (c).

Q.25
Two circular coils A and B are placed closed to each other. If the current in the coil A is changed, will some current be induced in the coil B? Give reason.



When current in coil A is changed, a changing magnetic field is set up around it. This changing magnetic field also links with coil B and hence some current will be induced in coil B due to electromagnetic induction.

Q.26
State the rule to determine the direction of a (i) magnetic field produced around a straight conductor-carrying current, (ii) force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it, and (iii) current induced in a coil due to its rotation in a magnetic field.



(i) Right hand thumb rule.
(ii) Fleming''s left hand rule.
(iii) Fleming''s right hand rule.

Q.27
When does an electric short circuit occur?



When live wire and neutral wire touch each other (i.e. come in direct contact.).

Q.28
What is the function of an earth wire? Why is it necessary to earth metallic appliances?



Earth wire acts as a safety measure. When the live wire touches the metallic casing of an electric appliance, the electric current flows from the casing of the appliance to the earth through the copper wire. As the earth offers very low or almost no resistance to the floe of current, so large current passes through the copper wire instead of human body. This large current heats the circuit and hence the fuse in the circuit melts. As a result of this, the circuit is switched off automatically and hence the electric appliance is saved from burning and the human body suffers no electric shock.

Q.29
The essential difference between an AC generator and a DC generator is that
(a) AC generator has an electromagnet while a DC generator has permanent magnet.
(b) DC generator will generate a higher voltage.
(c) AC generator will generate a higher voltage.
(d) AC generator has slip rings while the DC generator has a commutator.



(d) AC generator has slip rings while the DC generator has a commutator.

Q.30
What is the role of the split ring in an electric motor ?



The split-ring in an electric motor reverses the direction of current in the armature coil of the motor. Therefore, the direction of the force acting on the two arms of the coil is also reversed. As a result of this, the coil of d.c. motor continues to rotate in the same direction.
Q.31
Which source produces alternating current ?



AC generator (which converts mechanical energy into alternating current or electricity) and an oscillator (a device which converts D.C. into A.C.) are the sources which produce alternating current.

Q.32
Draw a labeled diagram of an electric motor. Explain its principle and working. What is the function of a split ring in an electric motor?




Electric motor converts electrical energy into mechanical energy.

Principle. Electric Motor is based on the fact that a current carrying conductor placed perpendicular to the magnetic field experiences a force.       

Working:

1. When current flows through the coil as shown in figure (a) , arms AB and CD experience magnetic force. According to Fleming''''s left hand rule, arm AB of the coil, experiences a force in the downward direction. Similarly, arm CD of the coil experiences a force in the upward direction. Both these forces are equal but opposite direction.
            Two equal and opposite forces acting at different position of the armature constitute a couple. This couple rotates the coil in clockwise direction until the coil is in the vertical position. At this position, the contacts of commutator and brushes break. So the supply of current to the coil is cut off. Hence, no force acts on the arms of the coil. But the coil does not come to rest. It goes on rotating due to the inertia of motion of the coil until commutator again comes in contact with the brushes B1 and B2
2. When the commutator comes in contact with brushes B1 and B2 after rotation, the direction of the current in arms AB and CD is reversed as shown in figure (c). Split-ring type commutator. It consists of two halves (R1 and R2 ) of a metallic ring. The two ends of the armature coil are connected to these two halves of the ring. Commutator reverses the direction of current in the armature coil.

Q.33
Explain the underlying principle and working of an electric generator by drawing a labeled diagram. What is the function of brushes?



An electric device used to convert mechanical energy (kinetic energy) into electric energy (electricity) is called an electric generator.

Principle. Electric generator works on the principle of electromagnetic induction. When the coil of electric generator rotates in a magnetic field, induced current flows in the circuit connected with the coil.

Working : When the armature coil ABCD rotates in the magnetic field provided by the strong field magnet, it cuts the magnetic lines of force. Thus, the changing magnetic field produces induced current in the coil. The direction of the induced current in the coil is determined by the Fleming''s right hand rule.The current flows out through the brush B1 in one direction in the first half of the revolution and through the brush B2 in the next half revolution in the reverse direction.
           This process is repeated. Therefore, induced current produced is of alternating nature. Such a current is called alternating current.

Function of brushes: Two carbons brushes (B1 and B1), are pressed against the slip rings. The brushes are fixed while slip rings rotate along with the armature. These brushes are connected to the external circuit across which the output is obtained

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