Chapter 13: Magnetic Effects of Electric Current


1(a) State Fleming’s left hand rule. or  Why does a current carrying conductor kept in a magnetic field experience force? On what factors does the direction of this force depend? Name and state the rule used for determination of direction of this force. Or  What does the direction of thumb indicate in the right hand thumb rule? In what way this rule is different from Fleming’s left hand rule?
(b)Write the principle of working of an electric motor.
(c)Explain the function of the following parts of an electric motor.
(i)Armature (ii) Brushes (iii) Split ring

2.(a) Explain any three properties of magnetic field lines.
(b) Give two uses of magnetic compass.

Ans (a)Properties of magnetic field lines:
1.Magnetic field lines emerge from N  pole  and merge at S pole outside a bar magnet and travel from S pole to N pole inside the magnet.
2.These are continuous and closed curves.
3.Two field lines never intersect each other.
(b)Uses of magnetic compass :
1.In navigation it is used to find direction.
2.It is used to detect the magnetic field.
3.It can be used to test whether a substance is magnetic or not.
3.State one main difference between ac and dc. Why   ac is preferred over dc for long range transmission of electric power? Name one source each of dc and ac.
Ans : The magnitude and direction of ac remains same whereas a.c. changes its magnitude and direction periodically. Low AC voltage can be increase to high voltage to prevent loss in electric energy during its long distance transmission.
AC generator and DC generator/or cell.
4.Under what condition does a current carrying conductor kept in a magnetic field experience maximum force? On what other factors does the magnitude of this force depend? Name and state the rule used for determination of direction of this force.
Ans :Force on a current carrying conductor in a magnetic field depends upon
a.length of the conductor
b.strength of the magnetic field
strength of the current
c.angle between direction of magnetic field and current.


Fleming’s Left Hand Rule:


Fleming’s left hand rule gives the direction of force experienced by a current carrying straight conductor placed in a magnetic field which is perpendicular to it. According to Fleming’s left hand rule if we stretch our left hand thumb, forefinger  and  middle  finger in such a way that forefinger points the direction of magnetic field, middle finger points the direction of current then thumb will give the direction of force on the conductor.

5.What is meant by overloading of an electrical circuit? Explain two possible causes due to which overloading may occur in household circuit. Explain one precaution that should be taken to avoid the overloading of domestic electric circuit.
Ans :Connecting large number of electric appliances in  one socket whose load is more than the maximum permitted limit. The two possible cause are
1.all of a sudden supply of high voltage and
2.too many devices connected in a single socket.

Precautions:1.use of voltage regulator,

2.load of a socket must be greater than its permitted rating.

4.Explain the meanings of the words “electromagnetic” and “induction” in the term electromagnetic induction. List three factors on which the value of induced current produced in a circuit depends. Name and state the rule used to determine the direction of induced current. State one practical application of this phenomenon in everyday life.
Ans The word electromagnetic means that an electric potential dipole is being produced in a coil due to change in magnetic field. The word induction means that the current has been induced. The value of induced current produced in a circuit depends on the following factors:
         number of turns in given coil
         area of each turn in coil
         rate of change of magnetic field.
The rule is Fleming’s right hand rule.  Stretch  the thumb, forefinger and middle finger of right- hand perpendicular to each other that forefinger indicates the direction of magnetic field, thumb gives the direction of motion (or force) of the conductor, then middle finger will point the direction of induced current.
Application: AC generator or DC generator.

6.Name two safety measures commonly used in electric circuits and appliances, what precautions should be taken to avoid the over loading of domestic electric circuits?
Ans :  Proper earthing and using a fuse load in the electric circuit must be as per rating of the fuse and do not connect to many plugs in a single socket.
7. What is a solenoid? Sketch magnetic field lines produced around a current carrying solenoid. Mark the region where field is uniform. Compare its field with that of a bar magnet.
AnsA solenoid is a large number of insulated turns of the copper wire in the shape of helix (or cylinder).
The patterns of the magnetic field lines around a current carrying solenoid is as given in figure. At the centre of the solenoid the magnetic field is uniform and magnetic field lines are parallel.
Similarities:In both the cases of a current carrying solenoid and bar magnet the magnetic lines of forces inside the body is strong and uniform. In both the cases there exists stronger magnetic field at the poles compared to the middle parts.
Dissimilarities:
(i)The poles in a bar magnet do not exist at the extreme ends of the magnet whereas in solenoid the poles can be considered to be lying at the edge.
(ii)In a bar magnet, magnetism is retained naturally, but in solenoid magnetism is there so long current flows through it.

8.(a) What is meant by a ‘magnetic field’ ?
(b)How is the direction of magnetic field at a point determined?
(c)Describe an activity to demonstrate the direction of the magnetic field generated around a current carrying conductor.
(d)What is the direction of magnetic field at the centre of a current carrying circular loop?
Ans :a.Magnetic field is the space around a magnet or a current carrying conductor in which its magnetic force can be experienced.
b.A magnetic compass is used to demonstrate the direction of the magnetic field generated around a current carrying conductor.
c.Fix a cardboard and insert a wire to pass through its centre normal to the plane of the card board. Sprinkle iron filings on card board uniformly. Pass the current in the wire. Tap the cardboard gently. You will find that iron  filings  align  themselves in the concentric circles around the wire. These circles represents magnetic field lines around the conductor.

d.At the centre of circular loop, the magnetic field lines are straight

9.Two coils C1 and C2 are wrapped around a non- conducting cylinder. Coil C1 is connected to a battery and key and C2 with galvanometer G. On pressing the key (K), current starts flowing in the coil C1  State your observation in the galvanometer.

a.What key K is pressed on.
b.When current in the coil C1 is switched off.
c.When the current is passed continuously through coil C1.
d.Name and state the phenomenon responsible for the above observation. Write the name of the rule that is used to determine the direction of current produced in the phenomena.
Ans:a.Induced current in coil C2 is produced so galvanometer shows a deflection.
b..Again galvanometer shows a deflection but in opposite direction to the previous one.
c.There will be no deflection in galvanometer.
d.This phenomenon is called electromagnetic induction. The phenomenon in which a changing magnetic field in a coil induces a current in another coil kept near it. Fleming’s right hand rule is used to find the direction of induced current.
10.What is the advantage of the third wire of earth connection in domestic appliances?
Ans :  In case of any electric fault in domestic appliances, current may comes in appliance body. The third wire called earth wire transfer this current to the earth and user remains safe from any such electric shock.
11.How can you show that the magnetic field produced by a given electric current in the wire decreases as the distance from the wire decreases?
Ans : If we bring a magnetic compass from a distance to near a current carrying conductor its deflection goes on increasing and when magnetic compass is brought away from the current carrying wire its deflection goes on decreasing which shows that magnetic field near current carrying wire is maximum and decreasing on increasing the separation.
12When is the force experienced by a current carrying conductor placed in a magnetic field the maximum?
 Ans : A current carrying conductor experience maximum force in a magnetic field when the direction of current is perpendicular to the magnetic field.
13.How is the induced current in a secondary coil related to current in a primary coil?
Ans :  Induced current in a secondary coil may be more or lesser than the current in primary coil depending upon the number of turns in secondary.

14. What is the role of fuse, used in series with any electrical appliance? Why should a fuse with defined rating not be replaced by one with a larger rating?
Ans : Fuse wire is safety device to prevent electrical devices due to short circuiting or overloading. The fuse wire is rated for a maximum current which has high resistance and low melting point. When there is short circuiting large current is passed in the circuit. Due to large current in fuse wire heat is produced and     by melting fuse wire breaks the circuit to keep other appliances safe.
If a fuse wire is replaced by an ordinary copper wire which has low resistance and high melting point it will not melt and domestic appliance may get damaged due to excessive heat due to short circuiting or overloading.

15. Differentiate overloading and short- circuiting.
Ans :  Overloading means to draw current more than the permitted maximum current in the circuit which may be due to connecting many appliances in one socket.
In short circuiting, when live wire and neutral wire come in contact with each other then resistance of the circuit becomes minimum consequently the current in the circuit increases abruptly. It may be due to damage of insulation of wire.

16.What are permanent magnet and electromagnet? Give two uses of each.
Ans :  Permanent Magnet: It has constant magnetic field around it. e.g. generator, loud speaker.
Electromagnet: When a soft iron is placed in a solenoid it gets magnetised till there is current in solenoid, e.g. electric bells, cranes.
17.Explain briefly different methods of producing induced emf.
Ans : a.By mutual induction: If we change current in one coil, a current is induced in the neighbouring coil.
b..By giving a relative motion to a magnet and coil in its neighbourhood.

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