class 10 numerical with solutions of electricity
Answers
Answer:
Question: A torch bulb has a resistance of 1 Ω 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 R = V/I = 2/.2 =10 Ω
(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.
3. Question: Calculate the resistance of 1 km long copper wire of radius 1 mm. (Resistivity of copper = 1.72 x 1 0-8
Answer: L = 1 km = 1000 m
R = 1 mm = 1 x 1 0-3
p = 1.72 x 1 0-8 W m
Area of cross section = p r2 = 3.14 x 1 0-3 x 1 0-3 = 3.14 x 1 0-6
R = pl/A = (1.72 x 1 0-8 x 1000 ) / 3.14 x 1 0-6 = 5.5 W
4. Question: When a potential difference of 2 V is applied across the ends of a wire of 5 m length, a current of 1 A 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.
Answer: (i) V = I R ----- Ohm's law R=V/I=2/1= 2 Ohm
Resistance per unit length: 2/5= 0.4 Ohm/m
(ii) Resistance of 2 m length of the wire = 0.4 x 2=0.8 ohm
(iii) When the wire is doubled on itself:
(a) the area of cross-section is doubled. If A is the original C.S. area, now it is 2 A.
(b) The length becomes half i.e.L/2
Resistance of this wire =R' = p (l/2)/(2A) = 1/4(p(L/A)
But p(L/A) = 2 ohm
R' = 1/4 x 2=0.5 Ohm
Question 1.
Define electric potential.
Answer:
Electric potential at a point in an electric field is defined as the work done in moving a unit positive charge from infinity to that point in the electric field.
Question 2.
State the relation between work (W), change (q) and electric potential (V).
Answer:
V = W/q.
Question 3.
What is the S.I. unit of electrical potential ? (CBSE 2005, 2012)
Answer:
Volt
Question 4.
Define 1 volt electric potential.
Answer:
Electric potential is said to be 1 volt if 1 Joule of work is done in moving 1 coulomb charge from infinity to a point in the electric field.
Question 5.
Is electric potential a scalar or a vector physical quantity ?
Answer:
Electric potential is a scalar physical quantity.
Question 6.
What is meant by potential difference between two points ? (CBSE 2011)
Answer:
Work done per unit charge in moving a unit positive charge from one point to another point in an electric field is called potential difference between two points.
Question 7.
Name the instrument used to measure the electric potential difference. (CBSE 2011, 2012)
Answer:
Voltmeter.
Question 8
State the relation between the current I passing in a conductor, number of electrons (n) flowing through any cross-section of the conductor, magnitude of charge on an electron (e) and time t.
Answer:
I = ne/t.
Question 9.
Mention the factor that maintains the flow of charge through a conductor. (CBSE 2013, 2014)
Answer:
Potential difference across the ends of the conductor.
Question 10.
Define electric current.
Answer:
Electric current is defined as the amount of electric charge flowing through any cross-section of a conductor per unit time
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