Electric Generator

Q. 1    Define

Ans. a) Electric generator – An electric device which converts mechanical energy into electric energy is called an electric generator.

b)    A.C. generator – A generator which converts mechanical energy into electric energy in the form of an alternating current (A.C.) is called an A.C. generator.

c)    D.C. generator – A generator which converts mechanical energy into electric energy in the form of a direct current (D.C.) is called a D.C. generator.

Q. 2     On which principle the working of an electric generator is based?

Ans.     Principle of an electric generator :- Electric generator works on the principle of electromagnetic induction. When the coil of electric generator rotates in a magnetic field the magnetic field induces a current in this coil. This induced current then flows into the circuit connected to the coil.

«Q. 3   Explain the construction & working of an electric generator.

Ans.     An electric generator is a device used to convert mechanical energy into electrical energy. It works mainly on the principle of electromagnetic induction. There are two types of electric generators. 1. A.C. generator & 2. D.C. generator.

I.          Electric A.C. generator :

A]        Construction :- The main parts of an A.C. generator are –

            i. Armature coil (ABCD)                    ii. Strong magnets (N & S)

            iii. Split rings (Brass ring R₁ & R₂)     iv. Carbon Brushes (B₁ & B₂)

                                             Fig. Electric AC generator 

          A,B,C,D     – Armature coils

N & S        – Poles of magnet

R₁ & R₂      – Rings

B₁ & B₂      – Carbon brushes

G             – Galvanometer

B]        Working of A.C. generator : When the armature coil rotates in the magnetic field of strong magnets, it cuts magnetic lines of force. Thus, a potential difference is set up & an induced current is set up in the coil. The direction of the induced current is according to  Flemings right hand rule. In half a rotation of the coil, the current flows through the brush B₁ in one direction. In the other half rotation, the current flows through the brush B₂ in the opposite direction. This process is repeated & the induced current is an alternating current.

II.        Electric D.C. generator or Dynamo.

A]        Construction – The main parts of D.C. generator are

            i) Armature coil (ABCD)        ii) Strong magnets (N & S)

            iii) Split rings (R₁ & R₂)          iv) Carbon Brushes (B₁ & B₂)  v) Bulb.

                          Fig. Electric D.C. generator

ABCD    – Armature coils

N & S    – Poles of magnet

R₁ & R₂  – Rings

B₁ & B₂  – Carbon brushes

B           – Bulb

B]        Working of  a D.C. generator : The D.C. generator is provided with a bulb connected across the carbon brushes. The glowing of the bulb shows the output. When the coil of a D.C. generator rotates in a magnetic field, a potential difference is set up. This causes the flow of a current. The current is in the same direction because one brush is always in contact with the arm of the armature moving downwards in the magnetic field. Thus the current generated is flowing in the same direction & is a direct current.

«Q. 4             Distinguish between Electric motor & electric generator.

Electric motor

Electric generator

i.     Electric motor converts electrical energy into mechanical energy. ii.    The principle of a motor is that a conductor carrying a current placed in a magnetic field experience a force. iii.   A source of current supplies an electric current to the armature coil. iv.  Electric motor is used in many common appliances such as electric fans, hairdryers, grinders, blenders, etc.

i.        Electric generator converts mechanical energy into electrical energy. ii.    The principle of a generator is that when the coil of the generator rotates in a magnetic field, an induced current flows in the circuit. iii.   There is no source of current, but a bulb is introduced between the carbon brushes to check the output, when the generator rotates. iv.  Electric generator is mainly used in buildings, hospitals, etc. to generate power especially when there is a power failure.

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Electromagnetic induction

Q.1      What is electromagnetic induction? Who discovered it?

Ans.    The process by which a changing  magnetic field in a conductor induces a current in another conductor is called electromagnetic induction. A current can be induced in a conductor either by moving it in a magnetic field or by changing the magnetic field around the conductor.

           The phenomenon of electromagnetic induction was discovered by Michael Faraday (in 1832) & independently by Joseph Henry (in 1830).

Q. 2     What is galvanometer ?

Ans. i. A galvanometer is an instrument used to detect the presence of current in a  circuit.

ii.     A galvanometer is also used to determine the direction of the current. The deflection of its pointer is proportional to the current. The deflection is on either side of the zero mark (at the centre of the scale) depending on the direction of the current.

«Q. 3  State Fleming’s right hand rule.

Ans.   Fleming’s right hand rule : Stretch the thumb, forefinger & middle finger of the right hand so that they are perpendicular to each other. If the forefinger indicates the direction of the magnetic field & the thumb shows the direction of the motion of conductor, the middle finger will show the direction of induced current.

                                                 Fig. showing Fleming’s right hand rule.

Q. 4     What is a direct current (D.C.) ?

Ans.   A current that flows only in one direction is called a direct current (D.C.). The frequency of D.C. is zero.

Q. 5     What is an alternating current (A.C.) ?

Ans.   A current whose direction reverses periodically with time is called an alternating current. In India the frequency of A.C. is 50 Hz. (50 cycles per second).

Q. 6     State an advantage of A.C. Over D.C.

Ans.    An advantage of A.C. over D.C. is that electric power can be transmitted over long distances without much loss of energy.

Q. 7     Name the appliances in which D.C. is used.

Ans. i)   A direct current is used in a portable electric torch, radios, electric bell, a wall clock, etc.

ii)         A direct current is also used in the preparation of electromagnet & electrolysis process.

Q. 8     Name the appliances in which A.C. is used.

Ans.   An alternating current is used in an electric heater, a refrigerator, electric iron, a washing machine, an electric mixture, a food processor, an air-conditioner, an electric fan, etc.

Q. 9     Give difference between Direct current & alternating current.

Direct current	Alternating current
i.          The magnitude & direction of the current is constant. ii.     This type of current cannot be used on large scale of electricity for household purpose. iii.        The frequency of D.C. is zero. iv.   When carried over a long distances, the energy loss is considerable.	i.          The magnitude & direction of the current reverses periodically. ii.     This type of current is used in electrical household appliances such as heater, iron, refrigerator etc. iii.         The frequency of A.C. current in India is about 50 Hz. iv.  It can be carried over long distances with only a small loss of energy.

Electric Motor

«Q.1 Define electric motor.

Ans.   A device which converts electrical energy into mechanical energy is called an electric motor.

Q.2      On which principle the working of an electric motor is based ?

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

Q. 3   With a neat diagram explain the construction & working of an electric current.

Ans.    Principle: An electric motor converts electrical energy into mechanical energy.

Construction of an electric motor :

Fig. Electric motor.

i)      Armature coil ABCD – It consist of a large number of turns of insulated copper wire wound around a rectangular iron core ABCD.

ii)      Strong magnet : The armature coil is placed between the poles of a strong magnet (N & S) to create a strong magnetic field.

iii)     Split ring commutator : A metallic ring divided into two halves (R₁ & R₂). The ends of the armature are connected to the two rings. This commutator helps to reverse the direction of the current in the armature coils.

iv)     Brushes : Two carbon brushes B₁ & B₂ are used to press upon the commutator.

v)      Battery : It supplies the current (D.C) to the armature coil.

Working of the electric motor:

i.     When a current is switched on, the armature coil ABCD, due to the action of the magnetic, the arm AB experiences a force in the downward direction & arm CD in the upward direction. These forces are equal in magnitude & opposite in direction. This force rotates the coil in clockwise direction until it becomes vertical.

ii.     In this position, the contact between the commutator & the brushes  breaks & the supply of current to the coil is cut off. Hence, no force acts on the coil. But the coil continues to rotate due to the principle of inertia.

iii.    The commulator again comes in contact with the brushes. The current once again passes through the coil. This sets the arm AB in rotation. As the arm AB completes one rotation from 90⁰  to 360⁰, downwards & the arm CD moves upwards. This force again moves the coil in a clockwise direction.

iv.     Thus, the electrical energy helps the coil to rotate in the same direction. Hence, the coil of the motor continues to move in the same direction & the electrical energy of DC is converted into mechanical energy.

Q.4      Give the uses of DC motor Or  Explain the applications of a DC motor.

Ans.     Uses/ applications of a DC motor.

1.     These motors are used in domestic appliances like mixtures, blenders, refrigerators & washing machines.

2.      These motors are used in electric fans, hair dryers, record players, tape recorders & blowers.

3.      These motors are used in electric cars, rolling mills, electric cranes, electric lifts & electric trains, etc.

Q. 5     What is the characteristic feature of a magnetic levitation train?

Ans.  In the case of a magnetic levitation train, the currents passing through the electromagnets in the railway track & on the train produce magnetic fields that make the train ‘float’ above the rails & propel is forward.

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Force on a current carrying conductor in a magnetic field

Q.1    Give the statement for Fleming’s left hand rule.

Ans.   Fleming’s left hand rule : Stretch the forefinger, the central finger & the thumb of   your left hand mutually perpendicular to each other. It the forefinger shows the direction of the field & the central finger shows the direction of the current, then the thumb will point towards the direction of the motion of the conductor.

Fig. Showing Fleming’s left hand rule.

Q. 2     How magnetism is used in medical sciences ? Explain in brief.

Ans. i)   Electric current produces magnetic field, even if the current is very weak.

ii)       An ion current travels along the nerve cells in our body produces magnetic fields.

iii)      Thus current is very weak, hence magnetic field produced is also weak.

iv)      When we touch an object, the nerves in our body carry an electrical impulse to the muscles to be used.

v)      This impulse produces a very weak magnetic field for a short duration. This field is only about 10^-9  times the earth’s magnetic field.

vi)       Heart & brain are the two main organs where significant fields are produced.

vii)    This forms the basis of obtaining images of heart & brain or images of different body parts.

viii)    This is done by using magnetic Resonance Imaging (MRI) technique. Analysis of these images useful in medical diagnosis.

Q. 3     What is ‘MRI’ ? Where it is used ?

Ans.     ‘MRI’ stands for magnetic Resonance Imaging. An ion current is a weak current travelling along the nerve cells in our body. The electrical impulses carried by our nerves produces a weak magnetic field, especially in the heart & brain. This helps in obtaining images of heart, brain & other organs. Analysis of these images helps in the diagnosis of disorders.

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Magnetic field due to current carrying conductor

Q.1      State the factors on which the magnetic field due to a current carrying conductor depends.

Ans.     The magnetic field at a point due to a current carrying conductor depends on the current through the conductor & the distance of the point from conductor.

i)    The magnitude of the magnetic field produced at a given point is directly  proportional to the magnitude of the current passing through the conductor.

ii)         The magnetic field produced by a given current in the conductor decreases as the distance from the conductor increases.

«Q. 2 State the right hand thumb rule.

Ans.    Imagine you are holding a current carrying straight conductor in your right hand such that the thumb points towards the direction of current, then the curled fingers around the conductor will give the direction of the magnetic field.

Fig. Right hand thumb rule

Q.3      Describe with a neat labelled diagram, the pattern of magnetic field lines (magnetic lines of force) due to a current through a circular loop (coil). Also explain how the magnetic field depends on the number of turns (n) in the loop.
Ans:

.

Fig – Showing Magnetic field lines (magnetic lines of force) due to a current through a circular loop (coil) of ‘n’ turns.

1.                  It is seen that every point of the loop forms a centre of a large number of concentric lines forming a series. The circles are small near the wire & becomes large as we move away from the wire. At the centre of the loop, the arcs of these circles appear as straight lines because of very large radius of the circle.

2.                  The magnetic field produced by a current carrying wire at a given point is directly proportional to the current through the wire. The loop (coil) has ‘n’ turns, the field produced is n timer that produced by a single turn, because the current in each turn has the same direction & the field due to each turn contributes equally to the total field.

«Q.4   Define solenoid.

Ans.     A coil of many turns of insulated copper wire wrapped (wound) in the shape of cylinder is called as solenoid.

Q.5      What is a solenoid ? How will you use a solenoid ?

Ans.     A solenoid is a coil of many turns of insulated copper wire wound around a cardboard cylinder, to form a cylindrical coil. When a current is passed through the wire, the solenoid acts like a magnet. The magnetic lines of force are very similar to the lines of force of a bar magnet. The magnetic field in the hollow of the solenoid is very strong. When a carbon steel rod is placed in the hollow of the solenoid, & current is switched on, the rod gets magnetised. Rods of materials like carbon-steel, tungsten steel, cobalt steel & chromium steel are used in the preparation of permanent magnets, if the magnetic field of solenoid is strong enough. These magnets are prepared by using the alloys like alnico & nipermag also.

Q.6      Draw a neat labelled diagram showing the magnetic lines of force (magnetic field lines) due to a current carrying solenoid.
Ans.

Fig. – showing magnetic lines of force (magnetic field lines) due to a current carrying solenoid.

B – Battery     K – Plug key    I – Current    N – North pole    S- South pole

Q. 6.    Distinguish between

a) Bar magnet & Solenoid

Bar Magnet	Solenoid
i.  Bar magnet is solid, consist of soft iron or suitable alloy. ii. Its magnetic field is weaker      that that of solenoid. iii. It does not require electric current to function as a magnet. iv. It is a permanent magnet. v.   Its field has a fixed strength.	i.  Solenoid is a hollow coil of an insulated wire of copper or aluminium. ii.  A solenoid can be designed to produce a field much stronger or much weaker than that of a bar magnet. iii. It require electric current, without which it   cannot function. iv. It function as a temporary magnet. v.  It field can be varied by varying the current in it.

b) Bar magnet & Electromagnet

Bar magnet

Electro magnet

i. Bar magnet is a permanent magnet. ii. It does not require electric current to act as a magnet. iii. Its north pole & south poles are permanently fixed. iv. The strength of its magnetic fields is fixed. v.   It does not produce any heat, on its own. vi. It consists of a piece of iron or a suitable alloy in the shape of a bar.

i.      Electromagnet is a temporary magnet. In the absence of current it does not work as a magnet. ii.  It requires electric current to act as a magnet. iii.  The position of its poles can be reversed by reversing the direction of current passing through it. iv. The strength of its magnetic field can be varied by varying the current passing through it. v.  Electromagnet provides heat, as a part of electric energy is transformed into heat. vi.    It consists of an insulated wire wound in the form of a coil, usually wound on a piece of iron or a suitable alloys of various shapes bar, horse shoe, disc, etc.

Q. 7     Name the materials used to prepare permanent magnets.

Ans.     Hard steel, carbon steel, chromium, steel, cobalt & tungsten steel, Alnico (Alloy of aluminium-nickel-cobalt), nipermag (alloy of iron, nickel, aluminium & titanium) are the material used for the preparation of permanent magnets.

Q. 8     Where permanent magnets are used ?

Ans.  Permanent magnets are used in microphones, loudspeakers, electric clocks, ammeters, voltmeters & speedometers, etc.

Q. 9     Give scientific reasons.

«1.      Alloys like alnico & nipermag are used in industries.

Ans. i) Many instruments like loudspeakers, a telephone earpiece, measuring instruments    like an ammeter, powerful voltmeter, etc. require the use of permanent magnets.

ii)      Originally carbon steel was used to prepare permanent magnets.

iii)   Later research proved that addition of certain elements like nickel, chromium, cobalt, tungsten, etc improved the magnetic properties of permanent magnets.

iv)    Alnico is an alloy of iron containing aluminium, nickel & cobalt. Nipermag is an alloy of iron containing nickel,aluminium & titanium. Therefore, permanent magnets prepared from these alloys are most suitable for use in industries.

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