CH 12: CURRENT ELECTRICITY
Short Questions/Answers
12.1: why it is not possible to measure the drift speed for electron by timing their travel along a conductor ?
Ans: The speed is the distance covered per unit time. But drift speed is too small and cannot be measured with this method. It is because electrons collide with lattice atoms and follow the zigzag path with varying speed. So the electrons don’t cover appreciable distance in time t. that’s why we cannot measure the drift velocity for electrons by timing their travel along a conductor.
12.2. The relationship R= V/I tells us that the resistance of a conductor is directly proportional to the potential difference applied to it. What do you think of this proportion?
Ans: No. resistance of a conductor does not depend upon potential difference. It depends upon nature of material, temperature and dimension of conductor.
According to Ohm’s Law current is directly proportional to potential difference keeping the temperature of conductor remains constant.
V = IR where R is constant at constant temperature.
12.3: a heavy duty battery of a truck maintains a current of 3A for 24 hours. How much charge flows from the battery during this time?
Ans: Data: I =3A , t = 24 hour =24×3600 s
12.4: while analyzing a circuit the internal resistance of e.m.f . Sources are ignored why? Ans: As the relation b/w terminal potential and e.m.f is
E = Vt +Ir
Where ‘r’ is internal resistance of the cell offered by electrolyte of cell. Its value is very small so in general it can be ignored. So E ≈ Vt .
12.5: under what circumstances can the terminal P.D. of a battery exceed its e.m.f.?
Ans: In charging process the terminal potential difference is greater than emf of the cell. Because reverse current is flowing through the battery.
Vt = E – (-I)r = E + Ir
12.6: what is the difference between an e.m.f. and a P.D.?
Ans:
12.7: the loop rule is based on the conservation of energy principal and the junction rule on conservation of charge principal? Explain just how these are based on these principals?
Ans: Loop Rule (KVL): The algebraic sum of all the potential changes in a closed loop must be zero.
That is sum of the energy supplied per unit charge by source is equal to sum of energy losses per unit charge in a closed circuit, which is law of conservation of energy.
Junction Rule: Sum of currents flowing towards point must be equal to sum of currents flowing away from the point. That is charge flowing towards point is equal to charge flowing away from the point which is law of conservation of charge.
12.8: why rise in temperature of a conductor is accompanied by a rise in the resistance?
Ans: As the temperature is one of the factors upon which resistance depends. By increasing temperature resistance of the conductor increases because of
1. K.E of free electron increases
2. The amplitude of vibration of atoms in the lattice increases
3. Collision rate of the free electrons with lattice atoms increases.
12.9: does the direction of e.m.f. provided by a battery depend on the direction of current flow through the battery?
Ans: Yes direction of e.m.f. depends upon the direction of current flowing through the cell/battery.
Example: when current is flowing from negative to positive terminal within the cell/battery then emf is taken as positive and when current flowing from positive to negative terminal within cell/battery then e.m.f. is negative.
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