ELECTRICITY Q&A CLASS 10

Chapter 12

 

 Electricity

Page 200

1.      What does an electric circuit mean?

An electric means a closed and continuous pathway for the passage of electric current.

 

2.      Define the unit of current.

The SI unit of current is ampere (A)

      1 ampere is defined as the amount of current that is constituted by the flow of 1 coulomb of charge through any point of a wire for every 1 second.

 

3.      Calculate the number of electrons constituting one coulomb of charge.

One coulomb of charge is constituted by a total of 6 x 10¹⁸ electrons.

 

Page 202

1.      Name a device that helps to maintain a potential difference across a conductor.

A cell or a battery is a device that helps to maintain a potential difference across as conductor.

 

2.      What is meant by saying that the potential difference between two points is 1 V?

‘The potential difference between two points is 1 V’ means that 1 joule of work has to be done if a charge of 1 coulomb is moved from one of the point to the other.

 

3.      How much of energy is given to each coulomb of charge passing through a 6 V battery?

     According to definition,

                V = W/Q

Therefore, W= 6 x 1  [V=6 and Q=1C]

                   = 6 J         [energy = work done]

Thus 6 J of energy is given to each coulomb of charge.

 

Page 209

 

1.      On what factors does the resistance of a conductor depend?

The resistance of a conductor depends on the following factors –

a)  Length of the conductor.

b) Cross-sectional area (thickness) of the conductor.

c)  Material of the conductor.

 

 

2.      Will current flow more easily through a thick wire or a thin wire of the same material, when connected to the same source? Why?

The flow of current through a wire depends on its resistance. The resistance in turn is inversely proportional to the thickness of the wire, which means that a thicker wire offers lesser resistance than a thinner wire of the same material. Thus current will flow more easily through the thick wire due to lesser resistance.

 

3.      Let the resistance of an electrical component remain constant while the potential difference across the two ends of the component decreases to half of its former value. What change will occur in the current through it?

According to Ohm’s law the amount of current is directly proportional to the potential difference. Again the amount of current also depends on the resistance of the wire. Now, as the resistance remains constant current will depend only on potential difference. Thus, if the potential difference decreases to half its former value the current also will reduce to half its initial value.

 

4.      Why are coils of electric toasters and electric irons made of an alloy rather than pure metal?  

     Electric toasters and irons are devices that utilize the heating effect of current and inorder to increase the heating effect materials of high resistivity is preferred. The resistivity of alloys is generally higher than that of pure metals. So the coils of electric toasters and irons are made of alloy rather than pure metals.

 

5.           Use the data in table 12.2 to answer the following –

a)  Which among iron and mercury is a better conductor?

  According to table 12.2, iron has a resistivity less than that of mercury and so it is a better conductor than mercury.

 

b) Which material is the best conductor?

  Silver has the least resistivity. So silver is the best conducting material.

 

Page 213

 

1    and 2 refer attachments (images)

 

Page 216

 

1.      Judge the equivalent resistance when the following are connected in parallel –

     (a) 1Ω and 10⁶Ω   

          In a parallel connection of resistors the resultant resistance is always less than the smallest resistor. So here the equivalent resistance will be less than 1.

 

(b)1Ω, 10³Ω and 10⁶Ω

          In a parallel connection of resistors the resultant resistance is always less than the smallest resistor. So here also the equivalent resistance will be less than 1.

 

2.      An electric lamp of 100 Ω, a toaster of resistance 50 Ω and a water filter of resistance 500 Ω are connected in parallel to a 220 V source. What is the resistance of an electric iron connected to the same source that takes as much current as all three appliances, and what is the current through it?

     Take r₁ = 100Ω,   r₂ = 50Ω and   r₃ = 500Ω

Add r₁, r₂, and r₃ as in parallel connection to get R. This R [about 31.25 Ω] will be the resistance of the electric iron. Again, use Ohm’s law equation [I=V/R] and find out I. This I will be the current through it.

 

3.      What are the advantages of connecting electrical devices in parallel with the battery instead of connecting them in series?

          The advantages of connecting electrical devices in parallel with the battery are –

a)  All the devices receive the same potential difference.

b) The effective resistance of the circuits remains low.

c)  Defect in any device does not hamper the supply of current to the other devices.

 

4.      Refer attachments (images)

 

5.      What is the (a) highest, (b) lowest total resistance that can be secured by combination of four coils of resistance 4Ω, 8Ω,12Ω and24Ω?

a)  For highest resistance, work out as series connection with the four resistances.

b) For lowest resistance, work out as parallel connection with the four resistances.

 

Page 218

 

1.      Why does the cord of an electric heater not glow while the heating element does?

     Copper or aluminium is used in the cord on an electric heater and such materials have low resistivity. On the other hand alloy or other such materials with high resistivity are used in the heating elements. So the cord does not glow as it is not heated up while the heating element starts to glow as it is heated updue to high

resistance.

 

 

2.      Compute the heat generated while transferring 96000 coulomb of charge in one hour through a potential difference of 50V.

     Take Q= 96000 C, t=60x60 sec and V= 50 V

 

First use    I=Q/t      then use P=VI

 

Then use  H=Pt     [H=I²Rt =Pt     since P=I²R]

 

3.      An electric iron of resistance 20 Ω takes a current of 5 A. Calculate the heat developed in 30 sec.

     Take     I=5A,   R=20Ω    and   t=30 sec

 

Then use     H=I²Rt

 

Page 220

1.      What determines the rate at which energy is delivered by a current?

     The energy delivered by a current is determined by the power of the electrical appliance.

 

2.      An electric motor takes 5 A from a 220 V line. Determine the power of the motor and the energy consumed in 2 hr.

     Take     I=5 A      V=220V      t=2hr

      Then use      P=VI

      Again use,       energy consumed=Pt (kWh   or   units)

 

 

 

                                       EXERCISE

1.      Option (d)

2.      Option (b)

3.      Option (d)

4.      Option (d)

5.      How is a voltmeter connected in the circuit to measure the potential difference between two points?

     A voltmeter is connected in parallel across the two points between which potential difference is to be measured.

 

18. Explain the following.

a)      Why is tungsten used almost exclusively for filament of electric lamp?

     The filament of an electric lamp produces light as it glows due to the heating effect of current. Tungsten increases the heating effect of current due to its high resistivity. At the same time it also has a high melting point. So it is used almost exclusively for filament of electric lamp.

 

b)     Why are the conductors of electric heating devices, such as bread toasters and electric iron, made of an alloy rather than a pure metal?

     Electric heating devices require materials of high resistivity in order to increase the efficiency of heating. In general, alloys have higher resistivity than that of pure metals. So conductors of electric heating devices, such as bread toasters and electric iron are made of an alloy rather than a pure metal.

 

c)      Why is the series arrangement not used for domestic circuits?

     The series arrangement is not used for domestic circuits for the following reasons –

i)       In a series arrangement all circuits do not receive the same voltage.

ii)     In a series arrangement circuits cannot be operated individually.

iii)    In a series arrangement if defect occurs even in one circuit, other circuits will also be hampered.

iv)   In a series arrangement the overall resistance increases.

 

 

d)     How does the resistance of a wire vary with its area of cross-section?

The resistance of a wire varies inversely to its area of cross-section. This means that a thicker wire will offer lesser resistance while a thinner wire will offer a greater resistance.

 

e)      Why are copper and aluminium wires usually employed for electricity transmission?

     Copper and aluminium are pure metals. Such materials are good conductors of electricity due to their low resistivity. So they are employed for electricity transmission.