Domestic electric circuit

Q. 1     Name the types of wires or cables provided by state electricity board for houses & factories in electric power supply.

Ans.    The wires or cables used in the electric power supply provided by state Electricity Board are of three types 
         (i) Phase wire (or live wire which carries electric current)

         (ii) Neutral wire (iii) The earth wire.

Q. 2     In a domestic electric supply in India, what is the potential difference between the live wire & the neutral wire.

Ans.   In a domestic electric supply in India, the potential difference between the live wire & the neutral wire is 220V-250V.

Q. 3     Name the type of wire to which the main fuse is connected.

Ans.     The main fuse is connected to the live wire (phase wire).

Q. 4     What does the electricity meter measures ?

Ans.   The electricity meter measures electric energy consumption. It is expressed in ‘Units’, where 1 unit means 1 kilowatt hour (= 3.6 x 10⁶ Joules)

Q. 5     Name the types of wires across which an electric appliance is connected.

Ans.     An electric appliance is connected across the live wire (phase wire) & the neutral wire.

Q. 6     Write short notes :

I.          Earthing OR What is Earthing? Explain its importance.

Ans.     The system of connecting the metal casing of an electrical appliance to a copper plate. Buried   deep inside the earth is called earthing.

           Many appliances such as electric heater, a toaster, an oven, a refrigerator etc. have a metallic body (casing). If by chance the insulation of the wire or the cable melts, the live wire may touch the metal casing. In case, if we touch the casing carrying a current, we may receive a severe shock which may cause death. To avoid this metal casing of all electrical appliances are earthed. One end of a thick copper wire is connected to the metal casing of the appliance & the other end is connected to a copper plate buried deep inside the earth. The current through the metal caring then flows to the earth & the person touching the casing is saved.

II.        Safety measures in using electricity.

Ans. i) Electric switchboards, sockets & appliances should not be within the reach of very small children.

ii)     Never unplug by pulling the cord or the wire connected to the plug. Always pull it by properly holding the plug.

iii)    Before cleaning electric devices such as refrigerators, fluorescent tubes, bulbs, etc. switch off the current & unplug the plugs of the devices so that there is no chance of getting electric shocks.

iv)    Use only dry hands to switch on or switch off electrical devices.

v)    Never open an electrical or electronic device when you do not know how it works.

vi)   Never play with electricity or electronic devices. This can prove to be dangerous.

vii)  Replace all defective wires & devices.

viii)  Use fuse wires of the appropriate rating.

ix)   Electrical appliances must be switched off when there is lightning, to prevent a fire.

x)    Electrical appliances must be properly earthed.

xi)   Many electrical appliances of high power rating should not be connected in the same circuit.

III.       Short circuiting.

i)      Often the insulation of wires gets worn out & it gets removed.

ii)    When this happens, there are chances of the live wire & the neutral wire coming very close to each other or touching each other.

iii)    When this happen known as short circuiting.

vi)   During a short circuit, the resistance in the circuit becomes very small & a large current flows through it.

v)     This produces a large amount of heat & a very high temperature.

vi)   This causing a fire in the circuit. This fire may spread very fast & engulf even a whole building.

IV.       Overloading Or What is overloading ? When does it occur ?

            What does it cause ? How can overloading be avoided ?

Ans. i) A flow of large amount of current in a circuit, beyond the permissible value of current, is called overloading.

ii)     It occurs when many electrical appliances of high power rating, such as geyser, a heater, an oven, a motor, etc. are switched on simultaneously. This causes fire.

iii)     Over loading can be avoided by not connecting many electrical appliances of high power rating in the same circuit.

Q.7      Give scientific reasons.

«1.     Wires carrying electricity should not be touched when base-footed.

Ans. i) Our body contains different fluids which contains various ions. The cells in our body also contain many ions.

ii)         Therefore, our body is a good conductor of electricity.

iii)        When we are bare- footed, our body is directly in contact with the earth.

iv)        When we touch wires carrying electricity, we may come in contact with a live wire. The current passes through our body to the earth & we get a nasty shock. Hence we should wear shoes with rubber soles (insulator) & not handle wires barefooted.

«2.     We must not use many electrical appliances simultaneously.

 Ans. i) Many of the electrical appliances that we use at home have a high rating e.g. geyser, oven, grinder etc.

ii)         When many such appliances are switched on simultaneously, a large amount of current flows through the main circuit & the current may exceed the permissible amount.

iii)        This causes overloading, which may cause a fire.

iv)        This can be avoided by not using many appliances simultaneously.

3.         Most electrical appliances require earthing.

Ans. i) Many of the electrical appliances such as electric iron, toaster, oven, mixture, geyser, etc. have metal bodies which may come into contact with our body.

ii)         Due to damaged insulation or a defect in the appliances the live wire may come in contact with the metal body, or due to some other problems, some current may leak to the metal body.

iii)        In such cases, the person making a contact with the metal body of the appliance may get an electric shock as our body is a fairly good conductor of electric current.

iv)        If the appliance is properly earthed, there is no danger of an electrical shock as the earthing wire conducts the leakage current to the earth & then the current passing through the person’s body is extremely small. Hence most electrical appliances need earthing.

4.         Water must not be used to extinguish fire caused by short circuit.

Ans. i) Generally, water contains a number of salts & other impurities & therefore, it conducts electricity.

ii)         Hence, if water is used to extinguish the fire caused by a short circuit, electricity is conducted through it & the fire spread. Therefore water must not be used to extinguish fire caused by a short circuit.

5. Electric circuit should be switched off when there is lightning.

Ans. i) When there is a lightning, the cloud is at a very high potential & the earth is at zero potential.

ii)         Due to the low resistance, a very high current passes between the cloud & the earth.

iii)        This causes an enormous amount of heat & a very high temperature.

iv)        Due to this high temperature, the wires in the appliances can get burnt & the appliances can catch fire. This can be avoided by not switching on electrical appliances where there is lightning.

6.         Household wiring must be inspected periodically, & replaced, if found to be defective.

Ans. i) Due to the exposure of insulated wires to heat, light, chemical fumes, variation in temperature, etc. the insulation gets damaged over a long period of time.

ii)         Sometimes rats, mice, cockroaches & fungal growth may damage the insulations.

iii)        Damaged insulation, exposed or bare parts of the wires, can cause short circuits, fire & give electric shocks. Electric consumption can also increase due to leakage of current.

iv)        By inspecting the insulation of all the wires once a year & replacing the damaged ones, is the best way to maintain electrical safety in the house.

7.         In rainy season, we should not touch a wall of a building.

Ans. i) If a bare wire (live wire) comes in contact with a wet wall, an electric current begins to flow through the wall.

ii)         A person touching such a wall gets a severe shock. Hence, in rainy season, we should not touch a wall of a building.

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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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