8th Science Guide Unit 5 Electricity

8th Std Science Guide Lesson 5 Electricity  | English Medium

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8th Science Guide Unit 5 Electricity

I. Choose the correct answers:

1. When an ebonite rod is rubbed with fur, the charge acquired by the fur is

1. negative
2. positive
3. partly positive and partly negative
4. None of these

Ans : positive

2. The electrification of two different bodies on rubbing is because of the transfer of

1. neutrons
2. protons
3. electrons
4. protons and neutrons

Ans : electrons

3. Which of the following a simple circuit must have?

1. Energy Source, Battery, Load
2. Energy Source, Wire, Load
3. Energy Source, Wire, Switch
4. Battery, Wire, Switch

Ans : Battery, Wire, Switch

4. An electroscope has been charged by induction with the help of charged glassrod. Te charge on the electroscope is

1. negative
2. positive
3. both positive and negative
4. None of the above

Ans : positive

5. Fuse is

1. a switch
2. a wire with low resistance
3. a wire with high resistance
4. a protective device for breaking an electric circuit

Ans : a protective device for breaking an electric circuit

II. Fill in the blanks.

1. ________ takes place by rubbing objects together Ans : transfer of electron
2. The body which has lost electrons becomes ________. Ans : positive
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 : Electric fuse.
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 : True

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

Cu. St. : In in parallel circuit, voltage remains the same in all components.

IV. Match the following

1. Two similar charges – acquires a positive charge
2. Two dissimilar charges – prevents a circuit from overheating
3. When glass rod is rubbed with silk – repel each other
4. When ebonite rod is rubbed with fur – attract each other
5. Fuse – acquires a negative charge

Ans : 1 – c, 2 – d, 3 – a, 4 – e, 5 – b

V. Choose the correct answer from the following directions.

1. If both assertion and reason are true and the reason is the correct explanation of the assertion.
2. If both assertion and reason are true, but reason is not the correct explanation of the assertion.
3. If the assertion is true, but the reason is false.
4. If the assertion is false, but the reason is true.

1. Assertion : People struck by lightning receive a severe electrical shock.

Reason : Lightning carries very high voltage.

Ans : If both assertion and reason are true and the reason is the correct explanation of the assertion.

2. Assertion : It is safer to stand under a tall tree during lightning

Reason : It will make you the target for lightning.

Ans : If the assertion is false, but the reason is true.

VI. 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 piece, the free electrons in the glass rod are transferred to silk piece. Since the glass rod looses electrons, it has a deficiency of electrons and hence acquires a positive charge, but the silk piece has excess of 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 comb, the negative charges in the comb attract positive charges in the bits. So the paper bits are moving towards the comb.

3. When you touch the metal disc of an electroscope with a charged glass rod the metal leaves get diverged.

• When a positively charged glass rod is brought near the top knob of the electroscope, the free electrons in the electroscope start to move up towards the knob, which means the bottom has a net positive charge. Thus the leaves will spread apart again.

4. In an electroscope the connecting rod and the leaves are all metals.

• Since metals are good conductors of electricity, the connecting rod and the leaves of an electroscope are all made of metals.

5. One should not use an umbrella while crossing an open field during thunderstorm.

• Having an umbrella in the hands is the tallest in that area and the chances of getting hit by the lightning increases more. The rod of the umbrella is a metal and the person carrying it will get an electric shock as the current passes from the rod to the body of the person holding it.

VII. Answer briefly

1. How charges are produced by friction?

• Rubbing certain materials with one another can cause the build-up of electrical charges on the surfaces. It is also clear that charges are transferred by friction.

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 of the battery to the another terminal of the battery is called electric circuit.

4. What is electroplating?

• Electroplating is the process of depositing of layer of one metal over the surface of another metal by passing electric current.

5. Give some uses of electroplating.

Electroplating is used in many fields,

• Iron tends to corrode and rust. It is widely used to build bridges and in automobiles. So it is electroplated with zinc to protect it from corrosion and rust. This is called galvanisation.
• Steel is used for making objects like car parts, bath taps, gas burners bicycle handlebars, wheel rims. Steel is coated with chromium, to make the objects strong, durable and attractive. This electroplating is called chrom plating.

VIII. Answer in detail.

1. Explain three ways of charge transfer.

There are three methods of charging

1. charging by friction
2. charging by conduction
3. charging by induction

Charging by friction:

When an object is rubbed with another object, the atoms in the objects get rubbed and a transfer of electrons take place between the atom of the two objects, one object loses electrons and the other gains electrons, making both the objects charged.

Charging by conduction:

An object can by charged by simply touching it by an electrically charged object. This process of charging is called charging by conduction. When a charged body is placed in contact with another body, charges get transferred to the other body and both get charged.

Charging by induction:

When a charged body is brought close to the neutral body (uncharged), the uncharged body gets opposite charge on the nearer side close to charged body and similar charge at the farther end. Thus a neutral body is charged without touching the charged body. This is called as charging by induction.

2. What is electroscope? Explain how it works?

An electroscope is a device used to detect and measure electric charges.It works on the principle of transfer of charges by conduction or induction. (ie. like charges repel each other)

Structure of Electroscope:

• An electroscope is made up of metallic rod placed inside the jar.
• The upper end of the rod has a metallic knob and the lower end of the rod has thin metallic leaves hanging parallel to each other.
• The leaves are made up of gold or silver because they are good conductors of electricity.

Working:

• When a charged body touches the knob of the electroscope, the charge is transferred to the knob because of conduction.
• The charge is then transfered to the gold leaves through the metal rod.
• The leaves repel each other and separate out.
• From this effect, we can check if the body carries a charge.
• We can also find the nature of the charge by charging the gold leaf by induction.

3. Explain series and parallel circuit.

a) Series circuit:

• A series circuit has more than one resistor (bulb) which are connected end to end.
• The current through the circuit remains the same throughout the circuit ie. flow of current is unidirectional.
• The voltage gets divided across the bulbs in the circuit.
• In a series circuit, the 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 circuitElectricity Book Back Solution is unscrewed, the current flow to another bulb would be stopped.
• If more number of bulbs in a circuit with a battery increases, the light will be dimmer, because many bulbs are sharing the same power from the battery

1. b) Parallel circuit:

• In a parallel circuit, there are more than one resistor (bulb), which are connected separately to the battery.
• The flow of current has more paths to flow.
• The voltage across the resistors remains the same but the current gets divided across each resistor.
• When one bulbs burn out, the other bulbs will work because the electricity is not flowing through one path.
• If the number of bulbs increases, the bulbs do not dim out as in series circuit because the voltage across each path is the same as in all the other path.

4. How lightning takes place?

• Lightning is an electric discharge which takes place in clouds.
• Lightning is produced by the discharges of electricity from cloud to cloud or from cloud to ground.
• During the thunderstorms, air is moving upward rapidly.
• This air carries small ice crystals as it moves upward, the ice crystals becomes positively charged.
• At the same time, small water drops move downward, becomes negatively charged as it collide with the ice crystals.
• Thus the upper part of the cloud is positively charged and lower part of the cloud is negatively charged.
• When they come into contact, the electrons in the water drops are attracted by the positive charges in the ice crystals.
• As a result, electricity is generated and lightning in seen.
• Huge quantities of electricity are discharged in lightning flashes

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.

Procedure:

• Take copper sulphate solution in a beaker.
• Immerse two copper plates into the solution.
• One plate is connected to the positive end of the battery ie the anode, the other plate is connected to the negative end of the battery ie the cathode.
• When current passes through the solution, it splits up into positive Cu2+ ions and negative SO42– ions.
• The positive copper ions (Cu2+) move towards the cathode and gets deposited on the cathode.
• The negative sulphate ion (SO42–) move towards the anode and reacts with copper in the anode, gets converted into copper sulphate. This process is called electroplating.