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VIII-EM-ELECTRICITY-EVALUATION-QUESTION & ANSWERS
Evaluation
I. Choose the best answer.
1. When an ebonite rod is rubbed with fur, the charge acquired by the fur is
a) negative
b) positive
c) partly positive and partly negative
d) None of these
Ans: b) positive
2. The electrification of two different bodies on rubbing is because of the transfer of
a) neutrons
b) protons
c) electrons
d) protons and neutrons
Ans: c) electrons
3. Which of the following a simple circuit must have?
a) Energy Source, Battery, Load
b) Energy Source, Wire, Load
c) Energy Source, Wire, Switch
d) Battery, Wire, Switch
Ans: d) Battery, Wire, Switch
4. An electroscope has been charged by induction with the help of charged glass rod. The charge on the electroscope is
a) negative
b) positive
c) both positive and negative
d) None of the above
Ans: b) positive
5. Fuse is
a) a switch
b) a wire with low resistance
c) a wire with high resistance
d) a protective device for breaking an electric circuit
Ans: d) a protective device for breaking an electric circuit
II. Fill in the blanks.
1. _____takes place by rubbing objects together.
Ans: charging
2. The body which has lost electrons becomes ________.
Ans: positively charged
3. ________ is a device that protects building from lightning strike.
Ans: Lightning arrester
4. ________ has a thin metallic filament that melts and breaks the connection when the circuit is overheated.
Ans: A fuse wire
5. Three bulbs are connected end to end from the battery. This connection is called ________.
Ans: series circuit
III. State True or False. If false, correct the statement.
1. The charge acquired by an ebonite rod rubbed with a piece of flannel is negative. Ans: True
2. A charged body induces an opposite charge on an uncharged body when they are brought near.
Ans: False
Correct Statement: An uncharged body acquires an opposite charge at the near end and similar charge at the farther end.
3. Electroscope is a device used to charge a body by induction.
Ans: True
4. Water can conduct electricity.
Ans: True
5. In parallel circuit, current remains the same in all components.
Ans: False.
Correct Statement: In parallel circuit, voltage across the resistors remains the same but the current flowing through the circuit gets divided across each resistor.
IV. Match the following.
Ans:
V. Give reason for the following.
1. When a glass rod is rubbed with silk cloth both get charged.
When a glass rod is rubbed with a silk cloth, the free electrons in the glass rod are transferred to silk cloth.
Because free electrons in the glass rod are less tightly bounded as compared to silk cloth.
Since the glass rod loses electrons, it has a deficiency of electrons and hence acquires a positive charge, but the silk cloth has excess electrons, so it becomes negatively charged.
2.When a comb is rubbed with dry hair it attracts small bits of paper.
Comb rubbed with hair gains electrons from the hair and becomes negatively charged. These electrons are accumulated on the surface of the comb.
When the bits of paper having positive and negative charges present at the edges of the bits come in contact with the comb, the negative charges in the comb attract positive charges in the bits. So the paper bits are moving towards the comb.
While combing hair, charges are transferred from the hair to comb due to friction.
3. When you touch the metal disc of an electroscope with a charged glass rod the metal leaves get diverged.
When you bring a charged object near the knob, electrons will either move out of it or into it. This will result in charges accumulating on the metal leaves inside the electroscope.
If a negatively charged object is brought near the top knob of the electroscope, it causes free electrons in the electroscope to move down into the leaves, leaving the top positive. Since both the leaves have negative charge, they repel each other and move apart.
If a positive object is brought near the top knob of the electroscope, the free electrons in the electroscope start to move up towards the knob. This means that the bottom has a net positive charge. The leaves will spread apart again now.
4. In an electroscope the connecting rod and the leaves are all metals.
Metals are good conductors of electricity.
The rod and leaves of the electroscope are made of metals, because they are good conductors.
5. One should not use an umbrella while crossing an open field during thunderstorms.
An umbrella can increase the chances of being hit by lightning as it makes the person the tallest object in the area.
The metal rod of an umbrella conducts electric current into the body, hence the person holding the umbrella will get a severe electric shock.
VI. Consider the statements given below and choose the correct option.
1. Assertion : People struck by lightning receive a severe electrical shock.
Reason : Lightning carries very high voltage.
2. Assertion : It is safer to stand under a tall tree during lightning.
Reason : It will make you the target for lightning.
a) Both assertion and reason are true and reason is the correct explanation of assertion.
b) Both assertion and reason are true and reason is not the correct explanation of
assertion.
c) Assertion is true but reason is false.
d) Assertion is false but reason is true.
Ans:
a) Both assertion and reason are true and reason is the correct explanation of assertion.
d) The assertion is false but reason is true.
VII. Answer briefly.
1. How charges are produced by friction?
Rubbing certain materials with one another can cause the build-up of electric charges on the surfaces. The material which gains electrons becomes negatively charged and the material which loses electrons becomes positively charged.
2. What is earthing?
A safety measure devised to prevent people from getting shocked if the insulation inside electrical devices fails is called earthing.
3. What is an electric circuit?
The path through which electrons flow from one terminal to another terminal of the source, is called an electric circuit.
A simple circuit consists of four elements: a source of electricity (battery), a path or conductor through which electricity flows (wire), a switch to control the circuit and an electrical resistor (lamp) which is any device that requires electricity to operate.
4. What is electroplating?
The process of depositing a layer of one metal over the surface of another metal by passing electric current is called electroplating.
In Electroplating, both anode and cathode are immersed in an electrolytic bath that is composed of a solution of salts, including the metal to be plated. A direct current (DC) of electricity is passed through the solution, to effect the transfer of metal ions onto the cathodic surface, plating the metal onto the item.
5. Give some uses of electroplating.
Electroplating is one of the most common applications of chemical effects of electric current.
Electro plating is applied in many fields.
We use iron in bridges and automobiles to provide strength.
The objects such as car parts, bath taps, kitchen gas burners, bicycle handlebars, wheel rims are made from a cheaper metal and only a coating of chromium is deposited over it.
Lot of components of medical devices are manufactured with nickel plating.
VIII. Answer in detail.
1. Explain three ways of charge transfer.
Charges can be transferred from one region to another region by any of the following three ways.
• Transfer by Friction
• Transfer by Conduction
• Transfer by Induction
Transfer by Friction:
Friction between objects results in transfer of electrons between them. When certain materials are rubbed with one another electrons get transferred from one material to the other, leaving them both with a net electric charge.
Transfer by Conduction:
The method of transferring charges from one body to other body is called
transfer by conduction. When a charged body touches another body, charges can be transferred from one body to another.
Transfer by Induction:
Induction is a process of charging an uncharged body by bringing a charged body near to it but not touching it is called induction. The uncharged body acquires an opposite charge at the near end and similar charge at the farther end.
2. What is electroscope? Explain how it works?
An electroscope is a scientific instrument used to detect the presence of electric charge on a body. In the year 1600, British physician William Gilbert invented the first electroscope. It is the first electrical instrument.
There are two types of electroscope:
pith-ball electroscope and
goldleaf electroscope.
An electroscope is made out of conducting materials, generally metal.
It works on the principle that like charges repel each other.
In a simple electroscope two metal sheets are hung in contact with each other.
They are connected to a metal rod that extends upwards, and ends in a knob at the end.
If you bring a charged object near the knob, electrons will either move out of it or into it.
This will result in charges accumulating on the metal leaves inside the electroscope.
If a negatively charged object is brought near the top knob of the electroscope, it causes free electrons in the electroscope to move down into the leaves, leaving the top positive.
Since both the leaves have negative charge, they repel each other and move apart.
If a positive object is brought near the top knob of the electroscope, the free electrons in the electroscope start to move up towards the knob.
This means that the bottom has a net positive charge. The leaves will spread apart.
3. Explain series and parallel circuit.
a) Series circuit:
In the series circuit battery, key and bulbs are connected in series. Here, the charges (electrons) from the battery have only one path to travel.
The current through the circuit remains the same throughout the circuit. But, the voltage gets divided across the bulbs in the circuit.
In the following series circuit two bulbs are used as resistors
Charges flow from the battery to each bulb, one at a time, in the order they are wired to the circuit. If one bulb in the circuit is unscrewed, the current flow to another bulb would be interrupted
Let us consider three bulbs connected in series. Let ‘I’ be the current through the circuit and V1, V2, V3 be the voltage across each bulb. The supply voltage V is the total of the individual voltage drops across the resistances (bulbs).
b) Parallel circuit:
In a parallel circuit, the bulbs are arranged on many paths. This means charges (electrons) can travel from one end of the cell through many branches to the other end of the cell.
Voltage across the resistors (bulbs) remains the same but the current flowing through the circuit gets divided across each resistor.
From the diagram it is clear that even when one resistor (bulb) burns out, the
other bulbs will work because the electricity is not flowing through only one path. All the light bulbs in our homes are connected in parallel circuit.
If one bulb burns out, the other bulbs in the rooms will still work. The bulbs in a parallel circuit do not dim out as in series circuits.
This is because the voltage across one branch is the same as the voltage across all other branches
Let us consider three bulbs connected in series. Let V be the voltage across the bulbs and I1, I2, I3 be the current across each bulb. The current I from the battery is the total of the individual current flowing through the resistances (bulbs).
I = I1 + I2 + I3
4. How lightning takes place?
Electric discharge takes place in a medium, mostly gases. Lightning is an example of discharge that takes place in clouds.
Lightning is produced by discharge of electricity from cloud to cloud or from cloud to ground.
During thunderstorms air moves upward rapidly and carries small ice crystals upward. At the same time, small water drops move downward.
When they collide, ice crystals become positively charged and move upward and the water drops become negatively charged and move downward.
When they come into contact, electrons in the water drops are attracted by the positive charges in the ice crystals. Thus, electricity is generated and lightning is seen.
Sometimes the lower part of the cloud which is negatively charged comes into contact with the positive charges accumulated near the mountains, trees and even people on the earth.
This discharge produces a lot of heat and sparks that results in lightning.
5. What is electroplating? Explain how it is done.
The process of depositing a layer of one metal over the surface of another metal by passing electric current is called electroplating.
In Electroplating, both anode and cathode are immersed in an electrolytic bath that is composed of a solution of salts, including the metal to be plated. A direct current (DC) of electricity is passed through the solution, to effect the transfer of metal ions onto the cathodic surface, plating the metal onto the item.
Take a glass beaker and fill it with copper sulphate solution.
Take a copper metal plate and connect it to the positive terminal of the battery. Connect an iron spoon to the negative terminal of the battery.
Now, dip them in the copper sulphate solution.
When electric current is passed through the copper sulphate solution, a thin layer of copper metal is deposited on the iron spoon and an equivalent amount of copper is lost by the copper plate.
