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Revision Question Bank
Magnetic Effects of Current
1. The flow of current in a circular wire creates a magnetic field at its centre. How can
existence of this field be detected? State the rule which helps to predict the direction of
this magnetic field.
Solution:
The existence of magnetic field can be detected by magnetic compass, which is as follows
Place a magnetic compass in between the circular coil carrying current. The needle of
magnetic compass will align itself in a definite direction, which indicate the presence of
magnetic field. Right hand thumb rule gives the direction of magnetic field. It states that
'imagine that you are holding a current-carrying conductor in your right hand, with the
thumb pointing in the direction of current, then the direction in which fingers encircle,
gives the direction of magnetic lines of force.
2. (a) Mention any two properties of the magnetic field lines.
(b) Draw the pattern of field lines due to a bar magnet
Solution:
(a) Characteristics of magnetic lines of force are
(i) Magnetic lines of force originate at the
North pole and end at the South pole,
(ii) Magnetic lines of force never intersect each other
(b) The pattern of magnetic lines of force due to a bar magnet is shown below
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3. The wire in figure below is being moved downwards through the magnetic field so as to
produce induced current.
What would be the effect of
(a) Moving the wire at a higher speed?
(b) Moving the wire upwards rather than downwards?
(c) Using a stronger magnet?
(d) Holding the wire still in the magnetic field?
Solution:
(a) The induced current increase at a higher speed.
(b) The induced current is reversed.
(c) The induced current increase.
(d) The induced current becomes zero.
4. Why is pure iron not used for making permanent magnets? Describe how permanent
magnets are made electrically. State two examples of electrical instruments made by using
permanent magnets.
Solution:
Pure iron is soft and cannot be converted into permanent magnet. The substance
commonly used for making permanent magnet is chromium steel, carbon steel or alnico.
A permanent magnet can be made by winding a coil of insulated copper wire around a
hard steel rod. When current is passed through the coil, it gets converted to a magnet.
Permanent magnets are used in microphone, voltmeter, etc
5. Draw the lines of force of magnetic field as through
(a) a single loop, of wire carrying current.
(b) Solenoid carrying a direct current.
Solution:
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(a) Magnetic lines of force due to a circular current-carrying coil are shown below
(b) Magnetic lines of force due to a circular current-carrying solenoid are shown below
6. Why does a compass needle get deflected when brought near a bar magnet?
Solution:
Due to the magnetic field around a bar magnet, a magnetic compass shows deflection
when placed near it.
7. Explain why two magnetic lines of force do not intersect.
Solution:
If the two magnetic lines offeree cross each other, it would mean that at the point of
intersection, the compass needle would point towards two directions corresponding to
the two tangents at the two lines. This is not possible and hence no two field lines
intersect.
8. State the principle of an electric generator.
Solution:
Electric generator works on the principle that ‘whenever there is a relative motion
between magnet and a coil, a current is induced in the coil.’
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Magnetism {Chapter Test}
M:M: 30
M: Time:
40Mins.
1.
(a) How is the direction of magnetic field at a point determined?
[3]
(b) What happens to the field lines pattern if the magnet is broken into two halves?
Solution:
(a) The direction of magnetic field at a point is determined by placing a small compass
needle.
(b) If magnet is broken into two halves, the field lines pattern remains the same.
2.
Under what conditions permanent electromagnet is obtained if a current carrying
solenoid is used? Support your answer with the help of a labelled circuit diagram.
[4]
Solution:
An electromagnet is a solenoid coil that attains magnetism due to flow of current. It
works on the principle of magnetic effect of current. It consists of a long insulated
copper wire wound around an iron core. The magnetic effect remains only till the
current is passing through the solenoid.
Permanent electromagnet is obtained if following conditions are satisfied
(i) The current through the solenoid should be direct current,
(ii) The rod inside is made of a magnetic material such as steel,
(iii) The magnetic material should be of high retentivity and should not loose it easily.
The labelled circuit diagram of obtained permanent etectromagnete is shown below
3.
Distinguish between alternating and direct current. Show both of them graphically. Give
one advantage of alternating current over direct current. What is the frequency of AC in
India?
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Solution:
The differences between alternating and direct current are as follows
S. No.
Alternating Current (AC)
(i)
The polarity of current
changes periodically.
(ii) The supply in our homes at
mains is AC.
Direct Current (DC)
The polarity of current does not change.
The DC is produced by batteries and electro
chemical cells.
One advantage of AC over DC is that alternating current is transferred to long distances
without much loss of energy through the use of transformers.
Frequency of AC in India is 50 Hz.
4. What is the commercial unit of energy? Derive its relation with SI unit of energy. A bulb
is marked 25 W, 220 V. It is used for 10 h daily. Calculate its resistance, while glowing
and energy consumed in kWh per day.
[4]
Solution:
The commercial unit of energy is kilowatt hour (kWh), commonly known as 'Unit'.
(I)
1 kWh = 1000 watt × 3600 second
= 3.6 ×106 watt second
= 3.6 ×106 joule (J)
Given, Power (P) = 25 W, Voltage (V) = 220 V
Time (t) = 10h
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V2 
V2 
Resistance (R)=  as P  
P
R 

220  220
 1936
25
Energy consumed in one day

25
kW  10h  0.25kWh
1000
5. Which device is used to measure electric current through a resistor in a circuit?
[2]
Solution:
Ammeter, connected in series is used to measure electric current through a resistor in a
circuit.
6. How is electric fuse made? How is it connected? What is the function of electric fuse?[4]
Solution:
Fuse wire is made up or an alloy of lead and tin. It is connected as a safety device in the
live wire in household circuits near the main supply.
When current exceeds a safe limit in the circuit, the heat produced melts the fuse wire
to break the circuit. This protects our appliances from heavy current and damage
caused by it.
7. (a) Define electromagnetic induction (EMI).
[5]
(a) Give one application of electromagnetic induction.
(b) Describe an activity to demonstrate EMI.
Solution:
(a) The phenomenon of inducing electric current in a coil of insulated conducting wire
under the effect of relatively changing magnetic field is called EMI.
(b) This principle is used to make electric generator.
(c) The following activity demonstrates EMI
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(i) Take a coil and connect its ends to galvanometer terminals. Take a strong bar
magnet and move it towards the coil. The galvanometer needle deflects, showing
induced current.
(ii) Change the direction of motion of magnet. The current is induced in opposite
direction.
(iii) The induced current is not produced when magnet and coil are stationary with
respect to each other.
(iv) The magnitude of induced current increases with number of turns in the coil, rate
of movement of magnet and strength of magnet.
8. (a) What is an electromagnet?
[5]
(b) Give uses of electromagnets.
(c) How can the strength of electromagnet be increased?
Solution:
(a) An electromagnet is a coil of insulated conducting wire wound around a core of
suitable material. When electric current passes through it, it behaves like a magnet,
while magnetic properties vanish when current is switched off.
(b) Uses In electric bell, motor, fan, telephones, etc. It is also used in scrapyard to
separate magnetic junk material.
(c) The strength of an electromagnet can be increased by increasing the number of
turns in the coil, increasing magnitude of current and using soft iron core.
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Sources of Energy
1.
Which form of energy is provided by wind to propel sail boat?
Solution:
Kinetic energy is provided by wind to proper sail boat.
2.
Name the type of reaction by which sun produces its energy. List two conditions which
are present at the centre of the sun responsible for this reaction.
Solution:
Nuclear fusion reaction takes place at the sun. High temperature and high pressure are
present at the centre of the Sun, which are responsible for the nuclear reaction.
3.
Write four characteristics used for selecting a suitable fuel.
Solution:
The characteristics of a good fuel are as follow
(i) It should have high calorific value,
(ii) It should not cause pollution.
(iii) It should have proper ignition temperature.
(iv) It should not leave much ash behind after burning.
4.
Name three main varieties of coal. Which one of these has the highest carbon content?
Solution:
Three main varieties of coal are anthracite (best), bituminous and lignite ( also known
as peat which is the worst variety). Anthracite has maximum carbon content (90%) and
so has maximum-calorific value.
[Note Calorific value of coal increases with increase in carbon content in coal].
(b) Magnetic lines offeree due to a current-carrying solenoid are shown below
5.
(a) State one limitation of solar energy available from solar cells.
(b) What is the minimum wind velocity required to obtain useful energy with a wind
mill?
(c) Define the term nuclear fission
Solution:
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(a) Solar cells cannot be used at night for the production of electricity.
(b) 15 km/h is the minimum wind velocity required to obtain useful energy with a wind
mill.
(c) A resaction in which a heavy nucleus splits into two smaller nuclei with the evolution
of large amount of energy is called nuclear fission,
i.e.,
6.
92
U235 0 n1 56 Ba139 36 Kr94  30 n1  Energy
(a) Describe the steps involved in obtaining biogas and explain what is meant by
anaerobic decomposition?
(b) Which isotope of uranium can undergo fission readily?
Solution:
(a) Biogas is obtained by the anaerobic decomposition of organic matter like cow dung,
human waste, etc. The process is as follows
(i) The cow dung is first mixed with some water to prepare the slurry. This slurry is then
fed in the inlet chamber.
(ii) From the inlet chamber slurry goes into the digestor tank, where it is acted upon by
anaerobic bacteria to produce biogas.
(iii) This biogas is collected above and filled in cylinders or directly supplied. The
decomposition of organic matter in the absence of air by anaerobic bacteria is called
anaerobic decomposition.
(b) U235 is the isotope of uranium which can undergo fission readily.
7.
‘Hydro-energy is an indirect source of solar energy.' Justify this statement.
Solution:
The hydro energy is said to be indirect source of solar energy. It is due to, the enormous
amount of water in rivers and oceans not only acts as a big collector of solar energy but
it has also great storage capacity for solar heat energy. Water evaporates from oceans,
rivers, etc. Ultimately the water comes back to surface in the form of snow and rain.
During evaporation, a part of solar energy gets converted to potential energy (PE) of
water, which rise up in the air to form clouds. The PE of molecules gets converted into
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KE during rain and snowfall.
The water, due to melting of ice and rains in higher altitude, flows through the river as it
has a higher gravitational potential energy. In this process, potential energy is converted
to kinetic energy of flowing water. Energy of flowing water is utilised to produce
electricity (hydroelectricity). Hence, hydroenergy is an indirect source of solar energy.
8.
A student has set up a model of box type solar cooker. He used a black translucent
plastic sheet to cover the open face of the box. He found that this cooker does not
function well. What modification should he make to enhance its efficiency? Give reason.
Mention the maximum temperature that can be reached in a box type solar cooker.
Solution:
The student should replace the back translucent plastic sheet with a simple transparent
glass sheet. He/she should also use a proper mirror as reflector, to focus large amount of
heat radiations in the box.
The reason is that a translucent glass sheet does not let most of the solar radiations to
enter into the box. However, the simple glass sheet not only allows large amount of solar
radiations to enter inside the box but also traps the solar radiations due to greenhouse
effect. The temperature attained, inside a box type solar cooker is of the order of 100°C
to 140°C.
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Sources of energy {Chapter Test}
M:M: 30
M: Time:
40Mins.
1.
What problems are associated with construction of dams?
[3]
Solution:
The problems associated with construction of dams are as follows
(i) There are limited sites suitable for dam construction.
(ii) A large number of people and animals in downstream regions have to be displaced.
(iii) A large area of land and vegetation get submerged and ecosystem is affected.
(iv) Submerged plants perform anaerobic respiration, which produce harmful polluting
gas, methane.
2.
What are the environment consequence of using fossil fuels? Suggest the steps to
minimise the pollution caused by various sources of energy including non-conventional
sources of energy.
[3]
Solution:
The environmental consequences of using fossil fuels are as follows
(i) The air pollution caused by burning of coal or petroleum products.
(ii) The oxides of carbon, nitrogen and sulphur that are released on burning fossil fuels
are acidic oxides. These lead to acid rain which affects our water and soil resources.
(iii) The generation of greenhouse effect of gases like carbon dioxide leading to global
warming.
Steps to minimise the pollution are as follows
(a) The pollution caused by burning fossil fuels can be reduced by increasing the
efficiency of the combustion process and using various techniques to produce the
smokeless appliances.
(b) The air pollution caused by burning of coal or petroleum products can be
reduced by afforestation.
(c) The planned and judicious use of energy can minimise the pollution.
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3.
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What are (a) solar concentrators
[4]
(b) solar cell panels?
(c) How are they an improvement on simple devices? Why are solar cell panels costly?
Solution:
(a) Solar concentrators are the devices which concentrate solar heat in a small area to
raise its temperature to high degrees. This heat can be used to cook food, heat water
and generate electricity.
(b) Solar cell panels are the assembly of solar cells fabricated together to produce
electricity when sunlight falls on them.
(c) They are an improvement over simple devices because they efficiently use the
abundantly available solar energy to do work. They have also raised the possibility to
minimise the use of fossil fuels and other conventional sources to obtain energy.
Solar cell panels are costly because the connections of solar cells in a panel are made
using silver wire which is an expensive metal.
4.
Expand OTEC. How does it work?
[3]
Solution:
OTEC means Ocean Thermal Energy
Conversion. (T)
Power plant works on temperature gradient between surface water of oceans and
depths up to 2 km below it.
The warm surface water is used to boil a volatile liquid like ammonia. The vapours of
the liquid are then used to run the turbine of the generator. The cold water from the
depths of the oceans is used to condense vapour again to liquid.
5.
Discuss the drawbacks of nuclear energy.
[2]
Solution:
The drawbacks of nuclear energy are given below
(i) The main problem with nuclear energy is disposal of radioactive waste, which
contaminates earth's environment.
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(ii) Nuclear power plants are very expensive,
(iii) Leakage of nuclear radiation may have disastrous effects on life.
6.
What is biogas?
[1]
Solution:
Biogas is the fuel gas produced from biomass with the help of bacteria.
7.
(a) What type of mixture are separated by fractional distillation? Name the fraction of
crude oil whose boiling point is more than 350°C. Where is it mostly used as fuel?
(b) A student has set up a solar cooker in a box by using a black painted aluminium
sheet, a black cooking vessel, some glass wool, a glass sheet and a mirror plate. What is
the role of each item used in the solar cooker?
[5]
Solution:
(a) If the various components of a mixture have different boiling points then these are
separated by fractional distillation.
Fuel oil is the fraction of crude oil i.e., petroleum whose boiling point is above 350oC.
It is used as a fuel in various industries to heat boilers because it does not leave any
residue afterburning.
(b) The role of each item used in solar cooker is as follows
(i) Black painted aluminium sheet and black cooking vessel absorb heat radiation, (ii)
Glass wool is a bad conductor of heat which prevents the loss of heat,
(iii) Glass sheet traps the heat radiation in the box.
(iv) Mirror plate reflects the heat radiation onto the glass sheet.
8.
(a) If energy in the universe is constant, why is the world yelling out for energy
crisis? What does judicious use of energy imply?
(b) You have wood, kerosene and LPG. Which one source of energy would you
use for cooking your food and why? Give the reason.
[4]
Solution:
(a) The energy in the universe is constant. However, when energy is converted
into
non-usable forms and cannot be converted back into usable forms, it gets wasted. If all
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the energy available is in non-usable form, we suffer the crisis for usable form of
energy.
Judicious use of energy implies minimisation of wastage of energy by controlled use of
sources and enabling reuse of energy wherever possible.
(b) I will prefer LPG instead of wood and kerosene for cooking food because
(i) It has high calorific value.
(ii) It is easy to use.
(iii) No residue is formed after its burning.
(iv) No existence of pollution.
9.
Give one advantage and one limitation of solar cell.
[2]
Solution:
Advantage Non-polluting
10. (a) What are the energy conversions in a wind energy farm?
[3]
(b) What condition is necessary for wind farm to produce energy?
(c) Why are wind farm expensive to maintain?
Solution:
(a) In wind energy farm, kinetic energy of wind is converted into electrical energy.
(b) To produce electric energy, wind should have minimum speed of 15 km/h.
(c) Wind energy farm are expensive to maintain, because they require large area of
land, the initial cost of establishment of the farm is quite high.
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