MOECDC Class 8 Electricity and Magnetism
MOECDC Class 8 Science and Technology | Electricity and Magnetism | Nepal Class 8 | Exercise | Question Answer
MOECDC Class 8 English Medium Science Chapter 8 Electricity and Magnetism Exercise Question Answer Solution.
| Board | Government of Nepal. |
| Class | Eight (8). |
| Subject | Science and Technology. |
| Chapter | Electricity and Magnetism |
| Topic | Exercise Solve / Question Answer / Notes. |
(1) Choose the correct alternatives :
(a) Whichof the following is a natural magnet?
(i) limestone
(ii) nickel
(iii) loadstone
(iv) electromagnet
Answer :
(iii) Loadstone – is a natural magnet.
(b) To which wire in a home’s electrification system is the switch connected?
(i) neutral wire
(ii) earth wire
(iii) fuse wire
(iv) live wire
Answer :
(b) In a home’s electrification system the switch is connected to – (iv) live wire.
(c) Which one of the following figures gives evidence of geomagnetism?
(I)
(II)
(III)
(IV)
Answer :
(i) – gives evidence of geomagnetism.
(d) “Earthing prevents the electrical equipment from being damaged or destroye(d)” How is the earthing done in household wiring?
(i) A wire is buried in the earth.
(ii) A green-coloured wire is buried in the earth.
(iii) A wire is taken from the plug and connected to the earth.
(iv) Earth wire is taken from every electrical equipment and buried in the earth.
Answer :
The earthing done in household wiring – (iv) Earth wire is taken from every electrical equipment and buried in the earth.
(2) Differentiate between:
(a) natural magnet and artificial magnet
Answer :
Natural magnet and artificial magnet:
(i) Objects which contain magnetic properties naturally are called natural magnets. On the other hand, magnetic substances in which magnetic properties are developed as per need are called artificial magnets.
(ii) The magnetic strength possessed by the natural cannot be altered but the strength of the artificial magnets can be easily altered as required.
(b) plug and socket
Answer :
Plug and socket: Plug is movable, but socket is fixed on the wall.
(c) live wire and neutral wire
Answer :
Live wire and neutral wire:
(i) Live wire contains a higher potential current but an electric current of zero potential flows in a neutral wire.
(ii) Generally, red or dark-red colored wires are used for making live wire. On the other hand, blue or black colored wires are used for making neutral wire.
(3) Give reasons:
(a) The magnet gets demagnetized while hammering.
Answer :
The magnet gets demagnetized while hammering because the configuration of the molecular magnets and their parallel alignment gets disturbed and scattered in random directions.
(b) The angle of inclination or the angle of dip at the geomagnetic poles is 90º.
Answer :
The angle of inclination or the angle of dip at the geomagnetic poles is 90° because the needle of the dip circle rests perpendicular to magnetic equator.
(c) Different coloured wires are used in domestic wiring.
Answer :
Different colored wires are used in domestic wiring so that, the damaged wire and its location can be identified easily if there is any problem in the electric circuit.
(d) The fuse is always connected in phase or live wire in domestic wiring.
Answer :
The fuse is always connected in phase or live wire in domestic wiring because when more than the required current flows in the electric circuit due to some disturbance, the fuse melts and protects the other electrical equipment from being damaged.
(4) Answer the following questions:
(a) What are the conditions for the demagnetization of a magnet? Explain any one of them in brief.
Answer :
The conditions for the demagnetization of a magnet are, heating the magnet, hammering the magnet, passing alternating current in a magnet, keeping like poles together forcefully for a long time and natural loss.
Heating magnet – If the permanent magnet is heated, the magnetic molecules in the magnet vibrate and their pattern is disturbed. Magnet gets demagnetized when the parallel alignment of the molecular magnet in the magnet in the magnet is disturbed and its open chains of the molecular magnets are changed into closed chains.
(b) Explain, with a diagram, the molecular theory of magnetism.
Answer :
The molecular theory of magnetism –
(i) Magnet and magnetic substances are made up of magnetic molecules. Each magnetic molecule has North and South poles.
(ii) In magnets and magnetic materials, molecular magnets are organized as open or closed chains. As a result, magnet and magnetic materials exhibit magnetic behavior.
(iii) When the magnetic molecules are lined up or arranged in a certain pattern, magnetic material becomes magnetized.
(iv) The intensity of magnetization in magnetic materials is maximum when all the molecular magnet gets aligned or arranged or arranged in an open chain. This condition is called magnetic saturation. The north and south poles of all the molecular magnets are arranged at the extremities of the bar magnet in magnetic saturation.
(c) Write down any three applications of the magnet.
Answer :
Three applications of the magnet are,
(i) Magnets are used in ratio, television, fan, refrigerator, induction cooker, electric bells, generators, etc.
(ii) Magnets are also used in computers, sensors, headphones, hard disk, tape recorders, etc.
(iii) Magnets are used in medical imaging equipment like MRIs.
(d) What is terrestrial magnetism?
Answer :
Magnetic properties associated with the earth are called terrestrial magnetism.
(e) Write any three applications of terrestrial magnetism.
Answer :
Three applications of terrestrial magnetism are,
(i) Charged particles in the solar wind are reflected by the earth’s magnetic field towards the north and south poles to be discharged safely. In this way, terrestrial magnetism has saved the earth from the direct hit of harmful charged particles from the sun.
(ii) The age of the rocks is estimated by the magnetic materials found in the igneous rock. It helps to identify the new ores of metal.
(iii) A magnetic compass is based on terrestrial magnetism which is used to show the directions in submarines, ships and aero planes.
(f) What is household electrification? Write down the characteristics of neutral and phase wires used in household electrification.
Answer :
The process of bringing the electricity generated from the powerhouse to the people’s house through the transmission line is called household electrification.
Characteristics of neutral wires: An electric current of zero potential flows in a neutral wire. Generally, blue or black colored wires are used for making neutral wire.
Characteristics of phase wires: Phase wire contains a higher potential current. Generally, red or dark-red colored wires are used for making phase wire.
(g) “Magnetization of magnetic materials in nature is the effect of terrestrial magnetism.” Justify this statement with examples.
Answer :
“Magnetization of magnetic materials in nature is effect of terrestrial magnetism.” – 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.
(h) Write down the steps of wiring a plug.
Answer :
The steps of wiring a plug are,
(a) Open the cover of the plug or socket with a screwdriver.
(b) Using a knife or pliers, cut off about 1.5 inches of the insulation around the electrical cable and take out the three smaller cables inside it.
(c) Identify the live, neutral and earth wires and remove the insulation around the electrical cable and take out the three smaller cables inside it.
(d) Twist the naked wires inside each cable and bend the upper end of the twisted wire slightly.
(e) Loosen the screw on the right-hand pin with the screwdriver and insert the twisted bare tip of the live wire into the pinhole and tighten the screw.
(f) Unscrew the pin on the left and insert the twisted bare tip of the neutral into the hole and tighten the screw.
(g) Now unscrew the upper pin in the socket and put the twisted bare end of the earth wire in the hole and tighten the screw.
(h) Clamp the cord with several grippers and tighten the screws so that the power cord does not come out of the plug even with a slight pull.
(i) Finally, put the cover of the plug and tighten the screw.
(i) What is earthing? Why is earthing necessary in domestic wiring?
Answer :
Earthing is the process of instantaneous discharge of electrical energy by transferring charges directly to the earth through low resistance wire.
Earthing is necessary in domestic wiring because earthing protects the electrical equipment from leakage current by sending it to the ground and hence preventing electrical fires.
(j) “The orientation of magnetic molecules determines the condition of magnet and magnetic substance.” Are the magnetic and non-magnetic propertiesof a substance interchangeable according to the above statement? Justify.
Answer :
“The orientation of magnetic molecules determines the condition of magnet and magnetic substance.” – The magnetic and non-magnetic properties of a substance are interchangeable according to the statement because, a magnetic substance can lose its magnetic properties and become non –magnetic when molecular magnets aligned in a particular direction are oriented in a random direction or closed chain. Again, if all the molecular magnet gets aligned or arranged in an open chain then the substance will get back its magnetic properties.
(k) Is there a connection between magnets and electricity? Explain their connection with any activity.
Answer :
There is a connection between magnets and electricity. Artificial magnet formed by passing electricity through magnetic material which is called an electromagnet. In fact, if the alternating current is passed through a magnet it gets demagnetized.
(5) Solve the following numerical problems:
(a) The electric meter in Bhawana’s house records 15 units of consumption of electricity in 15 days. Calculate the average monthly electricity bill of Bhawana’s house if the cost of electricity is Rs. 9.5 per unit. (Ans: Rs 1425)
Answer :
Consumption of electricity in 15 days = 15 units
The cost of electricity = Rs. 9.5 per unit
Average monthly electricity bill of Bhawana’s house
= (15 * 9.5)
= Rs. 142.50
(b) If 8 bulbs each of power 25 W are installed in a house used for 4 hours 40 minutes daily, calculate the total electricity consumption, in units, in a day. (Ans: 0.0932 units)
Answer :
Power of bulb = 25 W = 0.025 kW
Number of bulb = 8
Time = 4 hrs. 40 minutes = 4.67 hrs.
Total electricity consumption, in unit, in a day
= (0.025 * 8 * 4.67)
= 0.934 unit
(c) The sub-meter in Sharmila’s room records 8.5 units of electricity in a month. If there is only one electric bulb of power 65 W in her room, calculate the average duration for which theelectric bulb is used in a day. (Ans: 4.353 hrs.)
Answer :
Power of bulb = 65 W = 0.065 kW
Number of bulb = 1
Total electricity consumption, in a month = 8.5 units
Total electricity consumption, in a day = 8.5/30 = 0.283 units
The average duration for which the electric bulb is used in a day
= (0.283 / 0.065)
= 4.353 hrs.
(d) In a certain place, the municipality installed 14 street lights each rated 1500 W. These street lights are used for 6.5 hours daily on average. If the municipality has to pay the electricity bill at the rate of Rs 12.50, calculate the monthly bill paid by the municipality to the electricity authority for using these street lights.
Answer :
Power of street light = 1500 W = 1.5 kW
Number of street light = 14
The average duration for which the street lights are used in a day = 6.5 hrs.
Rate of electricity bill = Rs. 12.50
Total electricity consumption, in a day = (1.5 * 6.5 * 14) = 136.5 units
Total electricity consumption, in a month = (136.5 * 30) = 4095 units
The monthly bill paid by the municipality to the electricity authority for using these street lights
= (4095 * 12.50)
= Rs. 51187.50
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