Questions from CBSE Board Paper 2014 for 2015
Q. A wire of resistivity ρ is stretched to double its length. How does it affect the (a) resistance (b) resistivity?
Solution:
(a) wire is stretched to double its length
So, New length of wire l'= 2l
New Area of cross section or thickness of wire A'= A/2
New resistance = r =( ρ x 2l) /(A/ 2) = 4ρl/ A = 4 R
Hence, new resistance = 4r
(b) the resistivity is a property of a material and thus is a constant.
So, in this case, by increasing the length of the wire we are not altering its resistivity. It will remain the same, ρ
Q. A wire of resistance 10 ohm is bent in the form of a closed circle. What is the effective resistance between the two points at the end of any diameter of the circle?
Solution: A wire of resistance 10 ohm is bent in the form of a closed circle
So, R1 = R2 = 5 Ohm form a parallel loop
R' = (R1 R2)/(R1+ R2) = (5x5 )/(5+5) = 2.5 Ohm
Q. List two safety measures commonly used in electric circuits. Explain the main function of each. [CBSE 2014]
Ans: Two safety measures commonly used in electric circuits and appliances are
(i) Electric Fuse: An electric fuse is connected in series it protects the circuit from overloading and prevents it from short circuiting.
(ii) Proper earthing of all electric circuit in which any leakage of current in an electric appliance is transferred to the ground and people using the appliance do not get the shock.
Q. With the help of a circuit diagram prove that when a number of resistors are connected in parallel the reciprocal of the equivalent resistance of the combination is equal to the sum of the reciprocals of the individual resistances of the resistors.
Find the resistance between A and B in the following network [CBSE 2014]
Solution: The resistance between a to and B to C = R1= 2 + 2 = 4 ohm
R2 = 4 Ohm
The resistance between A and B =R' = (R1 R2)/(R1+ R2) = 4x4/(4+4) = 2 Ohm
Q. (a) How does the resistance of a wire change when
(i) its length is tripled ?
(ii) its radius is tripled ?
(iii) its material is changed to one whose resistivity is three times ?
(b) List two reasons why nichrome is used for making heating element of electrical appliances. [CBSE 2014]
Solution: (a)
(i) We know that resistance of wire is directly proportional to length of wire
So,if its length is tripled the resistance increases 3 times
(ii) We know that resistance of wire is inversely proportional to area of cross section of wire
New radius = 3r so,New area increases 9 times
Thus the resistance of a wire decreases 9 times
(ii) R = Pl/A
new resistivity p' = 3p
New Resistance R' = 3Pl/A = 3R
Thus, the resistance of wire increases 3 times
(b) Nichrome is used for making heating element of electrical appliances because:
(i) Nichrome does not oxidize, i. e. burn easily at high temperatures as it has higher melting and boiling point than metals.
(ii) Nichrome has higher resisistivity and consequently a higher resistance. Therefore, it will resist the flow of charges more, and lead to development of heat faster
Q. Mention one reason why tungsten is used for making filament of electric lamp. [CBSE 2014]
Ans: tungsten does not oxidize, i. e. burn easily at high temperatures as it has higher melting and boiling point than metals
Q. List two characteristics of the material to be used in fuse wire. Name the material it is made of. A fuse is always connected in series in an electric circuit ? Justify this statement giving reason [CBSE 2014]
Answer:
two characteristics of the material to be used in fuse wire
(a) Low melting and boiling point (b) high electric resistance
Fuse is generally made up of an alloy of tin and lead
In series connection the current for the entire house pass through the fuse. So, when fuse melts , it breaks down the entire home circuit and no current flows to the household circuitry. Thus, a fuse is always connected in series
Q. A circuit has a line of 5 A. How many lamps of rating 40 W; 220 V can si
Q. A wire of resistivity ρ is stretched to double its length. How does it affect the (a) resistance (b) resistivity?
Solution:
(a) wire is stretched to double its length
So, New length of wire l'= 2l
New Area of cross section or thickness of wire A'= A/2
New resistance = r =( ρ x 2l) /(A/ 2) = 4ρl/ A = 4 R
Hence, new resistance = 4r
(b) the resistivity is a property of a material and thus is a constant.
So, in this case, by increasing the length of the wire we are not altering its resistivity. It will remain the same, ρ
Q. A wire of resistance 10 ohm is bent in the form of a closed circle. What is the effective resistance between the two points at the end of any diameter of the circle?
Solution: A wire of resistance 10 ohm is bent in the form of a closed circle
So, R1 = R2 = 5 Ohm form a parallel loop
R' = (R1 R2)/(R1+ R2) = (5x5 )/(5+5) = 2.5 Ohm
Q. List two safety measures commonly used in electric circuits. Explain the main function of each. [CBSE 2014]
Ans: Two safety measures commonly used in electric circuits and appliances are
(i) Electric Fuse: An electric fuse is connected in series it protects the circuit from overloading and prevents it from short circuiting.
(ii) Proper earthing of all electric circuit in which any leakage of current in an electric appliance is transferred to the ground and people using the appliance do not get the shock.
Q. With the help of a circuit diagram prove that when a number of resistors are connected in parallel the reciprocal of the equivalent resistance of the combination is equal to the sum of the reciprocals of the individual resistances of the resistors.
Find the resistance between A and B in the following network [CBSE 2014]
Solution: The resistance between a to and B to C = R1= 2 + 2 = 4 ohm
R2 = 4 Ohm
The resistance between A and B =R' = (R1 R2)/(R1+ R2) = 4x4/(4+4) = 2 Ohm
Q. (a) How does the resistance of a wire change when
(i) its length is tripled ?
(ii) its radius is tripled ?
(iii) its material is changed to one whose resistivity is three times ?
(b) List two reasons why nichrome is used for making heating element of electrical appliances. [CBSE 2014]
Solution: (a)
(i) We know that resistance of wire is directly proportional to length of wire
So,if its length is tripled the resistance increases 3 times
(ii) We know that resistance of wire is inversely proportional to area of cross section of wire
New radius = 3r so,New area increases 9 times
Thus the resistance of a wire decreases 9 times
(ii) R = Pl/A
new resistivity p' = 3p
New Resistance R' = 3Pl/A = 3R
Thus, the resistance of wire increases 3 times
(b) Nichrome is used for making heating element of electrical appliances because:
(i) Nichrome does not oxidize, i. e. burn easily at high temperatures as it has higher melting and boiling point than metals.
(ii) Nichrome has higher resisistivity and consequently a higher resistance. Therefore, it will resist the flow of charges more, and lead to development of heat faster
Q. Mention one reason why tungsten is used for making filament of electric lamp. [CBSE 2014]
Ans: tungsten does not oxidize, i. e. burn easily at high temperatures as it has higher melting and boiling point than metals
Q. List two characteristics of the material to be used in fuse wire. Name the material it is made of. A fuse is always connected in series in an electric circuit ? Justify this statement giving reason [CBSE 2014]
Answer:
two characteristics of the material to be used in fuse wire
(a) Low melting and boiling point (b) high electric resistance
Fuse is generally made up of an alloy of tin and lead
In series connection the current for the entire house pass through the fuse. So, when fuse melts , it breaks down the entire home circuit and no current flows to the household circuitry. Thus, a fuse is always connected in series
Q. A circuit has a line of 5 A. How many lamps of rating 40 W; 220 V can si
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