Unit 8: Electricity and Magnetism

 

Unit 8: Electricity and Magnetism

Multiple Choice Questions

Circle the correct option to the following questions.

1.       Which of the following is a natural magnet?

¢ a) Loadstone                        b) bar magnet  

c) U shaped magnet                d) electromagnet

2.       Which of the following is not the cause of demagnetization?

a) Heating the magnet.

¢ b) Keeping the magnet by facing north-south direction.

c) Hammering the magnet.

d) Passing electricity in magnet.

3.       What is the value of angle of declination in geomagnetic poles?

a) 0⁰               b) 45⁰                    ¢ c) 90⁰                                d) 42⁰

4.       What is the angle made by the needle of freely suspended magnet with horizon at any place?

a) declination                             b) meridian

c) declinometer                        ¢ d) inclination

5.       What is the value of inclination(angle of dip) at the geometric equator?

¢ a) 0⁰          b) 45⁰                    c) 90⁰                     d) 42⁰

6.       Which wire return the electric current after the consumption of electricity by electric equipment?

a) Live wire                                 ¢ b) Neutral wire

c) Earthing wire                         d) Electric wire

7.       Which is the unit to measure electricity consumption?

a) watt hour                               b) kilowatt

¢c) kilowatt hour                    d) hour

8.       What is the safety device used in domestic wiring?

a) switch                      b) socket             c) bulb                  ¢d) fuse

9.       Electric light bulbs are connected in parallel combination in household electrification, why?

¢a) Each bulb can be operated by separate switch.

b) All the bulbs can be operated by same switch.

c) Due to increase in brightness of bulb.

d) Due to flow of electric current into electric device through live wire.

10.   What is called for a device used to increase or decrease the voltage of alternating current?

a) main switch           b) fuse                  ¢c) transformer              d) socket

1.       What is magnet?

A magnet is a substance that always points north and south direction when hanged freely and attracts magnetic substance.

2.       Mention the cause of neutral point?

Equal but opposite magnetic force of terrestrial magnet and magnet is the cause of neutral point.

3.       Write any one application of magnetic declination.

Angle of declination is used in aeroplane, submarine, ships for navigation.

4.       What is the value of magnetic inclination in the capital of Nepal (Kathmandu)?

The value of magnetic inclination in the capital of Nepal (Kathmandu is 42⁰.

5.       Write the formula to calculate electricity consumption.

The formula to calculate electricity consumption is :

E=p*n*t

6.       Write the full form of MCB.

The full form of MCB is Miniature Circuit Breaker.

7.       Which metals form fuse wire?

Alloy of Tin and Lead are used to make fuse wire.

8.       What is the colour of live wire?

The colour of live wire is red or brown.

9.       Why is earthing wire connected in domestic wiring?

Earthing wire is connected in wiring to protect the electric equipment when there is any leakage in electric current by sending it to ground.

10.       What is meant by geomagnetic meridian?

An imaginary line that connects two poles of terrestrial magnet is called geomagnetic meridian.

11.       What is electricity?

Electricity is a form of energy produced due to the motion of charge particles.

12.       What is terrestrial magnetism?

The earth is composed of huge magnetic material and it behaves like a magnet. Such a property of earth is called terrestrial magnetism.

Short answer type question:

1.       Draw a diagram showing molecular magnet in magnetic substance and magnet.

 2.       Write two differences between fuse and MCB.

Two differences between fuse and MCB are as follows:

Fuse	MCB
It consist of thin wire made of alloy of tin and lead.	It consist of bimetallic automatic switch.
It needs to be replaced once it melts.	It does not melts but opens the circuit by bending of metallic strip.

 

3.       What changes occur in the molecular magnet when a magnet is demagnetized? Write with diagram.

When a magnet is demagnetized, the molecular alignment in a particular direction are oriented in random direction.

4.       What is fuse? Write it’s one use.

A fuse is a thin wire made of tin and lead having low melting point and high resistance.

5.       Fuse should be connected in phase line, why?

A fuse is connected in a phase wire and not in a neutral wire because the phase wire carries the current from the electrical supply to the load. In the event of an overload or short circuit, the fuse in the phase wire will interrupt the current flow, protecting the circuit from damage and preventing the risk of fire or electrical hazards.

6.       Magnetic poles cannot be separated even the magnet is made many pieces, why?

According to the theory of magnetism, magnetic pole of a magnet lies in pair and it is called molecular magnet. These molecular magnets cannot be broken even a magnet is made into smaller pieces. So, magnetic poles cannot be separated even the magnet is made many pieces.

7.       Demagnetization occurs when the magnet is heated or dropped, why?

When a magnet is heated or dropped then then the pattern of alignment of molecular magnet in a linear position get disrupt and they get distributed in random position. So,

8.       Write to differences between magnet and magnetic substances.

Two differences between magnet and magnetic substances are:

Magnet	Magnetic substance
A magnet always points north-south direction when hanged freely.	A magnetic substance may not point north-south direction when hanged freely.
Its molecular magnets are aligned in linear direction.	Its molecular magnets are aligned randomly.

 

9.       A freely suspended magnet faces North-South at rest, why?

A freely suspended magnet faces North-South at rest because north pole of a freely suspended magnet is attracted by south pole of terrestrial magnet of the earth which lies towards the geographical north pole.

10.   Write two advantages of using MCB in a house.

Two advantages of using MCB in a house are:

a)       It is easy to reset without replacing any part.

b)      It prevents wire during short circuiting and overloading better than fuse.

11.   Write anything methods to conserve the magnetic energy.

Two methods to conserve magnetic energy are:

a)       Magnet should be kept in magnetic keepers.

b)      It should not be dropped or hammered.

12.   Magnetic poles cannot be separated, why?

Same as QN 6.

13.   Write any two differences between natural magnet and artificial magnet.

Natural magnet	Artificial magnet
It is found in the earth crust.	It is prepared artificially by magnetization.
It is not available in different shapes.	It is available in different shapes like U-shape, horse-shoe shape, etc.

 

14.   Write any two differences between plug and socket.

Two differences between plug and socket are:

Plug	Socket
It is attached with electrical appliances.	It is connected with wiring of house.
It is removable.	It is fixed with the wall or device.

 

15.   Write any two differences between live wire and neutral wire.

Two differences between live wire and neutral wire.

Live wire	Neutral wire
Live wire carries electric current from the source.	Neutral wire completes the electric circuit by returning the electric current to the source.
It is usually brown or red in colour.	It is usually blue or black in colour.

 

16.   A magnet get demagnetize by while hammering, why?

While hammering, molecular magnets get dislocated from their position of open chain into closed chain.

17.   The angle of inclination or the angle of dip at geomagnetic pole is 90. Why?

18.   The angle of inclination or angle of dip at geomagnetic pole is 90⁰ because the dip needle lie vertical with the horizontal surface of the earth.

19.   Different colored wires are used in domestic wiring. Why?

Different colored wires are used in domestic wiring because different colours of the wire makes easy to detect live, neutral and earth wire and makes easy to repair.

20.   Write any two application of magnet.

Two application of magnet are:

a)       Magnets are used in loudspeakers, radios, generators, dynamos, electric motors, etc.

b)      They are used in tape recorders and video recorders.

21.   Write down the steps wiring a plug.

The steps of wiring a plug are:

a)       The little screws in each of the plug pins are unscrewed.

b)      Twisted copper wires are inserted into the holes in the pins.

c)       Green wire is inserted at the top. It is earthing wire.

d)      Blue wire is inserted into the left pin. It is neutral wire.

e)      Red wire is inserted into the right pin. It is live wire.

f)        The screw is then made tight by covering the wire and plug pins.

22.   “The orientation of magnetic molecules determines the condition  of a substance  being magnetic and non magnetic.” Are the magnetic and non magnetic properties of a substance interchangeable in the world according to the above statement? Justify.

The orientation of magnetic molecules determines the condition of a substance being magnetic and non magnetic. Yes, the magnetic and non magnetic properties of as substances are interchangeable in the world by the orientation of molecular magnet. In a magnetic substance, molecular magnets are in closed chain. They can be changed into open chain using magnetization while the open chain of magnet can be changed into closed chain by demagnetization. 

Long answer questions.

1.       Write any four properties of magnet.

Four properties of magnet are:

a)       A magnet attracts magnetic materials.

b)      A magnet always points North-South direction when hanged freely.

c)       The magnetic force is more at the end of a magnet.

d)      Unlike poles of magnet attract each other and like poles repel each other.

2.       What is the demagnetization? Write any three conditions due to which demagnetization occur?

The process by which a magnet loses its properties is called demagnetization. Three condition due to which demagnetization occur are:

a)       By heating strongly.

b)      By continues dropping or hammering

c)       By rubbing the like poles.

3.       Molecular magnet is shown in the given figure. Answer the following questions on the basis of the figure.

i) Which figure is magnet?

Figure B is magnet.

ii) What is called the position of molecules in figure B?

The position of molecules of molecular magnet in figure B is called open chain.

iii) Which principle is shown by above diagram?

Principle of magnetism is shown by the above diagram.

iv) What is the cause of not showing molecular property by every magnetic substance?

In every magnetic substance, the molecular magnets are not aligned in open chains. This is the cause of not showing molecular property by every magnetic substance.

4.       The use of MCB is better than fuse, why? Write any two properties of fuse.

The fuse melts during overloading or short circuit. It must be replaced to use circuit again. But MCB trips when there is any fault in the circuit. It can be easily reset after repairing faults. So, the use of MCB is better than fuse. Two properties of fuse are:

a)       It is made up of tin and lead.

b)      It has high resistance and low melting point.

5.       What type of combination of load is used in domestic wiring? Name any two advantages of this type of combination. In which wire should we connect switch, why?

Parallel combination of load is used in domestic wiring. Two advantages of this type of combination are:

a)       Different switches can be used for different appliances or load.

b)      There is equal distribution of potential differences (voltage)

6.       Fuse should not be connected with neutral wire, why? Write any two electric devices and their functions.

Fuse should not be connected with neutral wire because if the fuse is connected with the neutral wire, then the fuse burns when there is excessive flow of electric current. Even when current stops flowing in the circuit, the appliance remains connected to the high potential point of the supply through the live wire. Now if a person touches the appliance, he may get a shock.

Two electric devices are and their functions are:

a)       MCB: It prevents the electrical appliance during short circuiting and overloading.

b)      Socket: It is used to supply electric current to electrical devices. 

7.       How is MCB different from fuse? Write any three precautions that should be taken while using fuse in domestic electrification?

MCB trips and shut down when overloading or short circuiting but fuse melts when overloading or short circuiting. Three precautions that should be taken while using fuse in domestic electrification are:

a)       A fuse must have high resistance and low melting point.

b)      A fuse should always be connected with live wire.

c)       The capacity of the fuse must be a little bit higher than the limit value.

8.       Explain the molecular theory of magnetism.

The molecular theory of magnetism are:

a. Magnet and magnetic substances are made up of magnetic molecules. Each magnetic molecule has North and South poles.

b. In magnets and magnetic materials, molecular magnets are organised as open or closed chains. As a result, magnet and magnetic materials exhibit magnetic behaviour.

c. When the magnetic molecules are lined up or arranged in a certain pattern, magnetic material becomes magnetised.

d. The intensity of magnetization in magnetic materials is maxin1um when all the molecular magnet gets aligned or arranged in an open chain.

 

9.       Write any four evidences for the existence of terrestrial magnetism on Earth.

Four evidences for the existence of terrestrial magnetism on the earth are:

a)       If the iron rod is buried under the earth's crust in the north-south direction, it shows magnetic properties due to the influence of terrestrial magnetism.

b)      A freely suspended niagnet always rests in a geographical north[1]south direction.

c)       The existence of a neutral point is the combined effects of terrestrial and other magnetic fields.

d)      The occurrence of natural magnets on Earth is due to terrestrial magnetism.

10.   In a house five electric bulbs of power 100 W each are used for 5 hours in a day. What is the electricity bill for a month if the cost of a unit of electricity is Rs. 12?

Solution,

Power of bulb (p)=100 W = 100/1000 KW = 0.1 KW

Time(t) = 5 hours

Number of bulbs (n) =5

Energy consumption for a day(E) = p*n*t

= 0.1*5*5

=2.5 KWh

Total energy consumbed in one month = 2.5KWh*30 = 75KWh  = 75 units

Cost of 1 unit = Rs. 12

Therefore cost of 75 units = Rs. 12*75 = Rs. 900

 

11.   What are the conditions for the demagnetization of a magnet? Explain any one of them in brief.

The conditions for the demagnetization of a magnet are:

a)       By heating

b)      By dropping or hammering

c)       By rubbing

d)      Haphazard storing

e)      By passing electricity

Here is the explanation of condition (a) by heating

When a magnet is heated, it’s molecular magnet gain heat energy and increase their kinetic energy. The open chain of molecular magnet get disoriented and hence demagnetized.

12.   Is there any connection between electricity and magnets? Explain any one connection with any one activity.

Yes there is a connection between electricity and magnets. An artificial magnet is made by using electricity. Such a magnet is called electromagnet. Activity to show connection between electricity and magnets is shown below:

Objective: To make an electromagnet.

Required materials: 1.5-volt cell, enamel-coated insulated copper wire, switch, pliers, pins and small iron nails.

Procedure: A long insulated copper wire is cut with the help of Iron Nail. A spiral formation of insulated copper wire around the iron nail is made as shown in the figure. The spiral formation of wire is called a solenoid. Two ends of the wire is connected to the opposite terminals of the cell. The switch is kept in the path of electrical circuits. Observation and discussion:  Some pins or iron nails near the electromagnet and it is observe that the iron nails are attracted by the electromagnet when a switch is on.

13. In a certain place, the municipality installed 14 street light each rated 1500 W. These street lights are used 6.5 hours daily on average. If the municipality has to pay the electricity bill at the rate of Rs. 12.50 per unit, calculate the monthly bill paid by the municipality to the electricity authority for using these street light.

Solution,

Power of bulb (p)=1500 W = 1500/1000 KW = 1.5 KW

Time(t) = 6.5 hours

Number of bulbs (n) =14

Energy consumption for a day(E) = p*n*t

=1.5*14*6.5

=136.5 KWh

Total energy consumbed in one month = 136.5 KWh*30 = 4095KWh  = 4095 units

Cost of 1 unit = Rs. 12.5

Therefore cost of 4095 units = Rs. 12.5*4095 = Rs. 51,187.5

14. There are 8 electric bulbs each of power 60 W in a house used for 7 hours in a day. What is electricity bill for the month if the cost of a unit of electricity is Rs. 12? 

(Try Yourself)