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Question (1):
What is the SI unit of charge? |
| Answer:
The SI unit of charge is coulomb. |
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Question (2):
How many electrons constitute a charge equal to 1 C? |
| Answer:
1 Coulomb of charge has 6.25 x 1018electrons. |
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Question (3):
State ohm's law. |
| Answer:
When temperature and other physical parameters remain constant, the current flowing through a conductor is directly proportional to the potential difference across its ends. |
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Question (4):
Which effect of current is used in the following appliance?
a) electric bulb
b) immersion rod
c) electric iron
d) galvanometer
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| Answer:
For electric bulb, immersion rod and electric iron, heating effect of current is used. For galvanometer magnetic effect of current is used. |
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Question (5):
Define watt and watt hour. |
| Answer:
Watt is the unit of power. If 1 joule of energy is consumed in 1 second, we say the power of the device is 1 watt.
Watt hour is the unit of energy. If a device with a power 1 watt works for 1 hour 1 watt hour amount of energy is consumed.
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Question (6):
Which of the following graphs depict ohm's law.
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Answer:
Graph (a) represents ohm's law since a straight line which shows that 'I' is directly proportional to 'V' (i.e., ohm's law).
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Question (7):
Two coils have a combined resistance of 25
when connected in series and a resistance of 4
when connected in parallel. What is the resistance of each coil?
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Answer:
Let the resistance be R
1
and R
2
, and R
s
represents resistances in series and R
p
represents resistance in parallel.
According to the given data
R
1
+R
2
=R
S
=25
_______(
1)
+
=
=
 _______(
2)
R
1
+ R
2
= 25
R
1
R
2
= 100
(R
1
- R
2
)
2
= (R
1
+ R
2
)
2
- 4 R
1
R2
= 25
2
- 4 (100)
= 625 - 400
= 225
R
1
- R
2
= 15
R
1
+ R
2
= 25
Adding 2R
1
= 40
R
1
= 20
R
2
= 5
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Question (8):
On what factor's does the heating effect of current depend on. |
Answer:
The heating effect of current depends on
- The square of the amount of current flowing
- The resistance of the wire and
- The time of flow of current
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Question (9):
Name the instrument that measures the potential difference across the ends of a current carrying conductor. How is the instrument connected to the circuit? |
| Answer:
Voltmeter measures the potential difference across the ends of a conductor. It is connected in parallel across the element through which current flows due to a certain potential difference. |
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Question (10):
When are several resistors in a circuit said to be connected in parallel? |
| Answer:
Several resistors are said to be connected in parallel when the potential difference across the resistors remain the same. |
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Question (11):
Two bulbs have ratings 100 W, 220 V and 60 W, 220 V respectively. Which one has a greater resistance? |
Answer:
P=VI=
For the same V, R is inversely proportional to P. Therefore, the bulb 60 W, 220 V has a greater resistance.
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Question (12):
A torch bulb has a resistance of
 when cold. It draws a current of 0.2 A from a source of 2 V and glows. Calculate
(i) the resistance of the bulb when glowing and
(ii) explain the reason for the difference in resistance.
|
Answer:
(i) When the bulb glows:
V = I R ---- Ohm's law
(ii) Resistance of the filament of the bulb increases with increase in temperature. Hence when it glows its resistances is greater than when it is cold.
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Question (13):
Calculate the resistance of 1 km long copper wire of radius 1 mm. (Resistivity of copper = 1.72 x 10-8
 m) |
Answer:
L = 1 km = 1000 m
R = 1 mm = 1 x 10-3 m
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Question (14):
When a potential difference of 2 V is applied across the ends of a wire of 5 m length, a current of 1A is found to flow through it. Calculate:
(i) the resistance per unit length of the wire
(ii) the resistance of 2 m length of this wire
(iii) the resistance across the ends of the wire if it is doubled on itself.
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Answer:
(i) V = I R ----- Ohm's law
(ii) Resistance of 2 m length of the wire = 0.4 x 2
(iii) When the wire is doubled on itself:
(1) the area of cross-section is doubled. If A is the original C.S. area, now it is 2 A.
(2) The length becomes half i.e. 
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