ELECTRICITY AND CIRCUITS
                                               Electricity and Circuits


1.        Power station : Electricity we use at homes, in our factories, is supplied from a power station.

2.       
Electric cell :Electric cell is a source of electricity.

3.       
Production of electricity in a cell :An electric cell produces a small amount of electricity from chemicals stored inside it. When the chemicals in the electric cells are used up, the electric cells stop producing electricity.

4.       
Terminal : All types of electric cells have two terminals, a positive terminal and a negative terminal. In a dry cell used in your homes, the central carbon rod is the positive terminal (+) and the zinc contained is the negative (-) terminal.

 

5.       
Battery : When two or more cells are joined together, the combination is called a battery.
 


6.       
 Bulb : We get a light from a thin tiny wire inside the glass cover. This is called filament. It is supported by two thicker wires. One of these thick wires is connected to the metal casing around the base of the bulb. The other is connected to the metal tip of the base. The base of the bulb and the metal tip of the base are the two terminals of the bulb. These two terminals are fixed in such a way that they do not touch each other.

The inside portion of the bulb is filled up by inert gases, like argon.

 

7.       
Circuit : The complete path, from one terminal of the electric cell through the bulb and back to the other terminal of the electric cell, is called a circuit.


8.       
Open circuit : If there is any gap in the path of a circuit the bulb does not light up. Such a circuit is called an open circuit.

 
 
9.        Closed circuit :The bulb lights up only when a bulb and wire form a complete path, which starts at one terminal of electric cell and ends at the other terminal. Such a circuit is called a closed circuit.
 
 

 
10.     Flow of current in a circuit : As soon as the path from one terminal of electric cell to the other is completed, an electric current starts flowing through the circuit and the bulb lights up. The electric current flows from the positive terminal of the electric cell to its negative terminal.

                                             

 
11.     In the bulb, current enters through one of its terminals, flows through the filament inside the bulb and comes out through the other terminal of the bulb. When the current flows through the filament, it starts glowing.

12.    
Fused bulb : If the filament of the bulb is broken, the circuit is not completed and hence the current can not flow. The bulb with broken filament is called a fused bulb. When a bulb gets fused, it does not light up.

13.    
Electric switch :Electric switch is a simple device that either breaks the circuit or completes it to stop or start the flow of current.
(i)            When the switch completes the circuit, it is called closed switch.
(ii)          When the switch breaks the circuit, it is called open switch.

 

14.    
Conductors

(i)            Materials through which electric current can flow are called conductors.
(ii)          Most metals are conductors.
(iii)         Our body is also a good conductor.

15.    
Insulators
(i)            Materials, through which the electric current cannot pass, are called insulators.
(ii)          Rubber and wood are insulator.

16.    
Conduction of tester : It is a simple device to test whether a material is a conductor or insulator.

 
17.     Filament :The thin wire that give off light is called the filament of the bulb.

18.    
Drycell :Dry cell is a source of electricity. It generates direct current (DC) due to chemical reaction that take place inside it.
 


Q.1. Fill in the blanks :


(a)    A device that is used to break an electric circuit is called ……….. .
(b)    An electric cell has ………… terminals.

Ans.
(a) switch
         (b) two

Q.2. Mark ‘True’ or ‘False’ for following statements :

(a) Electric current can flow through metals.
(b) Instead of metal wires, a jute string can be used to make a circuit.
(c) Electric current can pass through a sheet of thermo Col.

Ans.
(a) True           (b) False            (c) False.


Q.3. Explain why the bulb would not glow in the arrangement shown in Fig.
 

Ans.
We know that a bulb lights up only when bulb and wire form a complete path, which starts at one terminal of the electric cell and ends at the other terminal. Here in this arrangement the circuit is not completed. The handle of the screw driver is made of some material which is an insulator. This is why the bulb will not glow.

 
Q.4. What is the purpose of using an electric switch? Name some electrical gadgets that have switches built into them.


Ans.
A switch is a simple device that either breaks the circuit or completes it. Hence we use electric switches in order to either – (i) stop an electrical gadget in function when required, and (ii) start an electrical gadget  when needed.
Some electrical gadgets that have switches built into them are – electric iron, table fan, television set, radio, etc.

Q.5. Would the bulb glow after completing the circuit , if instead of safely pin we use an eraser?


Ans.
No, the bulb will not light up. This is because an electric bulb lights up only when circuit is complete and eraser is a non-conducting material.


Q.6. Would the bulb glow in the circuit a wool is used to connect the battery and bulb?


Ans. In this circuit the bulb is connected through a woolen thread with the negative terminal of electric cell. Since woolen thread is not a conductor of electricity, it will not allow the electricity to flow through it that is from bulb to electrical cell. Thus the circuit remains incomplete and hence the bulb will not light up.


Q.7. Using the “conduction tester” on an object it was found that the bulb begins to glow. Is that object a conductor or an insulator? Explain.


Ans.
The material of that object is a conductor. Because with this object bulb lights up showing that electricity is flowing in the circuit that is the circuit is complete.This is possible only if the material of the object allows the electricity to pass through it and we know only conductors allow the electricity to pass, thus the material of the object must be a conductor.

Q.8. Why should an electrician use rubber gloves while repairing an electric switch at your home? Explain.


Ans.
Yes, while carrying out electrical repairs one must wear rubber gloves. This is because, our body is a good conductor of electricity, and if there is any contact of naked wire with our naked hand it will be very dangerous. And if we are with rubber gloves on our hands, the rubber being an insulator will check the contact with naked wire.

Q.9. The handles of the tools like screwdrivers and pliers used by electricians for repair work usually have plastic or rubber covers on them. Can you explain? Why?


Ans.
With screwdrivers, the electrician has to touch naked wires and screws in which electricity is flowing. If the screwdriver will have no handle made of plastic or wood, the electricity will flow through the screwdriver to his body, which is itself a good conductor of electricity and he has to face an electric shock. If handles are coated with wood or plastic, which are insulator, they will check this flow of electricity thus making him safe from any such electric shock.
 

ADDITIONAL IMPORTANT QUESTIONS

Q.1. What is an electric cell?


Ans.
Electric cell is a source of energy. It produces a small amount of electricity from chemicals stored inside it.

Q.2. What is meant by a battery?


Ans.
When two or more cells are joined together, it is called a battery.

Q.3. Give any difference between a cell and a battery.


Ans.
A cell has only one plate as a positive and only one plate as a negative electrode, while a battery which is a combination of cells in a series, can have many plates.

Q.4. What is meant by electric circuit?


Ans.
It is the path of electric current.

Q.5. Why a cell stops producing electricity after sometimes?


Ans.
An electric cell produces electricity from chemicals stored inside it. When the chemicals inside the cell are used up, the cell stops producing electricity.

Q.6. Why we should not join the wires connected to the two terminals of the electric cell



Ans.
We should never join the wires connected to the two terminals of the electric cell. If we do so, the chemicals in the electric cell get used up very fast and the cell stops working.

Q.7. Name some devices in which we use an electric cell.


Ans.
We use electric cell in an alarm clock, wrist watch, transistor, camera, torch etc.

Q.8. Two cells can be joined in two ways in series and in parallel. What do you mean by these two ways? In which way the cells are joined in a torch?


Ans. In series :
When (+) terminal of one cell is joined to (-) terminal of the other, the arrangement is called in series.
Parallel : When (+) terminal of a cell is joined to (+) terminal and (-) terminal to (-) terminal of other cell, the arrangement is called parallel.


Q.9. When a bulb gets fused, it does not light up. Can you explain why?


Ans.
A bulb with a broken filament is called a fused bulb. If the filament is broken, the circuit is not completed and hence the current can not flow and consequently the bulb will not light up.

Q.10. What is a conductor? Give examples.


Ans.
A conductor is a material that allow an electric current to pass through it. For example copper, aluminium, zinc, iron, etc.

Q.11. What is an insulator? Give few examples.


Ans.
Insulator is the material that does not allow an electric current to pass through it. For examples, wood, mica, asbestos, rubber etc.

Q.12. Why does one remove the plastic coating of connecting wires before making circuits?


Ans.
Plastic is bad conductor or insulator of electricity. It prevents us from electric shocks. To complete the circuit, we have to remove the plastic coating to connect wires so that the current may flow through the circuit.

Q.13. Why do you have to clean the ends of wires used for making a circuit with sand paper to make the bulbs shine?


Ans.
A layer of bad conductor polish is coated on the wire. The current may flow properly after removing this coat. Therefore, we have to remove the coating with sand paper.

Q.14. Is air a good or a bad conductor of electricity? You can answer this question on the basis of experiments you have done in this chapter.


Ans.
Air is a bad conductor of electricity. If air were the good conductor of electricity, electricity could reach to the bulb without wiring and the bulb could light without wires. Moreover, the electricity running through wire could scatter all around the wire and then it might be dangerous.

Q.15. How are cells arranged in a torch-in parallel or in series?


Ans.
In torch cells are connected in series.

Q.16. If in a circuit you have arranged cells in series, will the bulb continue to shine even if the connection of one of the cells is reversed?


Ans.
If cells are connected in a series in a circuit, change of ends of any one of the cell will stop the flow of electricity and the bulb might not illuminate as circuit will not complete.

Q.17. What would happen if air were a good conductor of electricity?


Ans.
If air were good conductor of electricity, the electricity could scatter all around in atmosphere nearby the power stations. The men at work could die with electric shocks. The electricity could not be transmitted to homes, factories from power stations.

Q.18. Shashi made a circuit consisting of two bulbs and a cell. While both bulbs were glowing, she removed one from the circuit. The other bulb also stopped glowing. What was the arrangements of the bulbs in the circuit?


Ans.
Shashi connected the bulbs in series. The electricity was flowing through these two bulbs. The circuit was completed. Removing one bulb from the circuit, made disconnection of electricity. It meant there was no flow of electricity and the bulb would not glow.

Q.19. You might have seen this sign at electric poles, substations and other places. What does it indicate?


Ans.
It indicates that the electricity can be dangerous. If handled carelessly, it can cause severe injuries or even death. Hence we should never attempt to experiment with the electricity in the wires and sockets found in homes, schools etc.


Q.20. Out of aluminium foil, paper, dry cloth and rubber sheet, which is the good conductor? Tell by using a torch.


Ans.
Aluminium is a good conductor of heat and electricity. For this explanation take a torch with cells. Switch on to see that it lights up. Now in the place between bulb and the cell put all things like, aluminium foil, dry cloth and rubber sheet one by one and check whether the torch lights up. It lights up with aluminium foil only. So, we can say that aluminium is a good conductor.


Q.21. Give some uses of Conductors and Insulators.


Ans.
Conductors and Insulators are equally important for us. Switches, electrical plugs and sockets are made of conductors. On the other hand, rubber and plastic are used for covering the electrical wires, plug tops, switches and other parts of electrical appliances, which people might touch.

Q.22. What is “conductions tester”? How it can be used to test whether a material is a conductor or insulator?


Ans.
It is a simple device to test whether a given object is a conductor or an insulator its set-up.

To test whether a given object is a conductor or an insulator we connect the two ends of that object to A and B. If after connecting the object between AB, the bulb lights up, the object is a conductor and if bulb does not light up, the object is an insulator.

Q.23. Why is the handle of electrician’s screw driver made of plastic?


Ans.
A screw driver is made of stell. Stell is a good conductor of electricity. So electricity can easily flow through it. Plastic is a bad conductor and does not allow electricity to pass through it. So, the electrician has a plastic handle to protect him from any shock.


Q.24. Using a conduction tester select conductors and insulators from the following. Note your observations in a table as follow :

Pin, notebook, chalk, duster, pen, comb, key, rubber, nail, paper.


S.NO.ThingBulb glows or notCircuit SituationObject is Conductor/ Insulator
1.
 
    
2.
 
    
 
 
S.NO.ThingBulb glows or notCircuit SituationObject is Conductor/ Insulator
1.
 
PinBulb glowsCircuit is completeConductor
2.
 
NotebookBulb doesn’t glowCircuit is not completeInsulator
 
3.
 
ChalkBulb doesn’t glowCircuit is not completeInsulator
 
4.
 
DusterBulb doesn’t glowCircuit is not completeInsulator
 
5.
 
PenBulb doesn’t glowCircuit is not completeInsulator
 
6.
 
CombBulb doesn’t glowCircuit is not completeInsulator
 
7.
 
KeyBulb glowsCircuit is completeConductor
8.
 
RubberBulb doesn’t glowCircuit is not completeInsulator
 
9.
 
NailBulb glowsCircuit is completeConductor
10.
 
PaperBulb doesn’t glowCircuit is not completeInsulator
 
 
 

OBJECTIVE TYPE QUESTIONS
Q.1. Match the following items given in Column A with that in Column B.
Column A
 
Column B
(a)  Cell
 
(b)  Battery
 
(c)  Conductor
 
(d)  Insulator
 
(e)  Switch
 
(f)   Bulb
 
(g)  Filament
 
(h)  Circuit
(i)    Allows electricity to pass through it
 
(ii)  Either breaks or completes a circuit
 
(iii) Converts electricity into light
 
(iv) Glows when electricity passes in it
 
(v) A device which produces electricity
 
(vi) Is a path of electricity
 
(vii) Does not allow electricity to pass
 
(viii) A combination of cells
 
Q.2. Fill in the blanks with appropriate words :
(i)        An Electrical cell has two terminals, a positive and a ……….. .
(ii)      Materials through which current can pass are called ………. .
(iii)     Materials through which current can not pass are called ………… .
(iv)     Our body is a …………. Conductor of electricity.
(v)      When a switch is off, the circuit becomes ………… .
(vi)     We use …………. to test conductivity of a material.
(vii)   A switch either breaks or …………. A circuit.
(viii)  Electricity we use in our homes is supplied from a ………… .
Q.3. State whether the statements given below are True or False :
(i)        A bulb has two terminals.
(ii)      An electric cell converts chemical energy into electrical energy.
(iii)     Rubber and wood are good conductors of electricity.
(iv)     A switch is made of an insulator.
(v)      Current flows from positive terminal of cell to its negative terminal.
(vi)     Current flows only through a complete path.
(vii)   Handles of electrical appliances are made up of insulators.
(viii)  In a torch, cells are joined in parallel arrangement.
(ix)     Air is a good conductor of electricity.
(x)      We should turn off main switch when there is a short circuit in our home.
(xi)     We should never join the wires connected to two terminals of a cell.
Q.4. Choose the correct option in the following questions:
(i)        Cell is a device which
(a)  Converts chemical energy into electrical energy
(b)  electrical energy into light energy
(c)  electrical energy into magnetic energy
(d)  none of these
(ii)      A bulb has
(a)  two terminals and a filament
(b)  two terminals and two filament
(c)  multiple terminals and single filament
(d)  single terminal and a filament
(iii)     Filament of a bulb is made up of
(a)  aluminium                                   (b) chromium
(c) platinum                                          (d) tungsten
(iv)     Bulb glows only in
(a)  closed circuit                                  (b) open circuit
(c) in both circuits                               (d) open circuit if bulb is not fused
(v)      A battery is
(a)  a single cell
(b)  a combination of cells in which cells are joined (+) to (-)
(c)  a combination of cells in which cells are joined (+) to (+)
(d)  none of these
(vi)     A substance which allows electricity to pass through it is called
(a)  a conductor                                    (b) an insulator
(c) semiconductor                               (d) super conductor
(vii)   Which is an example of an insulator
(a)  bakelite                                           (b) aluminium
(c) tap water                                         (d) all of these
(viii)  An example of a conductor is
(a)  tap water                                         (b) salt solution
(c) metal wire                                       (d) all of these
(ix)     How many terminal are there is a dry cell?
(a)  One                                                 (b) Two
(c) Three                                               (d) Four
(x)      To prevent electric shocks, the metallic electrical wires are covered with
(a)  Paper                                              (b) cotton
(c) aluminium                                      (d) plastic
 

ANSWERS TO OBJECTIVE TYPE QUESTIONS
Q.1. Match the items under Column A with Column B.
 
 
Column A
 
Column B
(a)  Cell
 
(b)  Battery
 
(c)  Conductor
 
(d)  Insulator
 
(e)  Switch
 
(f)   Bulb
 
(g)  Filament
 
(h)  Circuit
(v)  A device which produces electricity
 
(vii) A combination of cells
 
(i)  Allows electricity to pass through it
 
(viii) Does not allow electricity to pass
 
(ii) Either breaks or completes a circuit
 
(iii) Converts electricity into light
 
(iv) Glows when electricity passes in it
 
(vi) Is a path of electricity
 
 
Q.2. Fill in the blanks :
        (i) Negative                        (ii) Conductors                 (iii) Insulators
        (iv) Good                            (v) Open                           (vi) Conduction tester
        (vii) Completes                   (viii) Power house.
Q.3. True/False :
        (i) True              (ii) True             (iii) False           (iv) False            (v) True
        (vi) True            (vii) True           (viii) False          (ix) False           (x) True
        (xi) True.
Q.4. Choose the correct option:
(i)        (a) In a cell chemicals are used to produce potential difference.
(ii)      (a) A bulb has a filament which glows and two terminals which are joined to two terminals of the battery.
(iii)   (d) Tungsten has high melting point and high resistance.
(iv)   (a) Only a closed circuit is a complete circuit.
(v)      (b) In a battery two or more cells are combined is such a way that (+) terminal of one cell is joined to (-) terminal of other.
(vi)     (a) Substances which allow electric current to pass through them.
(vii)    (a) Aluminium and tap water are conductors of heat.
(viii)   (d) Tap water, salt solution and metal wire are the example of conduction.
(ix)     (b) Dry cell have two terminal (+ve) and (-ve).
(x)      Plastic is a insulator of electricity.