Page 18 - Physics - XII

Q4. A voltmeter is always connected in parallel and an ammeter in series with the circuit. Why?
Will they record the respective parameters if connected in opposite manner? (NCERT)
Ans. An ammeter is connected in series with the circuit because the purpose of the ammeter is to measure
the current through the circuit. If it is connected in parallel across any load then all current in the

circuit will choose a lower resistive path (i.e. ammeter) to cause its circuit to be damaged. Hence, it
is used in series.
A voltmeter has very high resistance to ensure that its connection does not alter the fl ow of current
in the circuit. Now, if it is connected in series then no current will be there in the circuit due to its
high resistance. Hence, it is connected in parallel to the load across which the potential diff erence is
to be measured.
Q5. Defi ne Ohm’s Law.
Ans. Ohm’s Law states that the electric current I fl owing through the conductor is directly proportional to
the potential diff erence V across the two ends of the conductor, provided that the physical conditions
of the conductor remains same.

Q6. How do you know if a circuit obeys Ohm’s Law?
Ans. If the graph plotted between V and I is a straight line, its obeys Ohm’s Law.

Q7. Why are copper wires normally used for connecting diff erent components in an electric circuit?

(NCERT)
Ans. The resistance of connecting wires should be very low. Copper wires off er low resistance. Copper
is an excellent electrical conductor; no other metal compares in terms of electrical conductivity. It is
inexpensive. So, due to the above-mentioned reasons, copper wires are used as connecting wires.

Q8. What happens if the current is allowed to fl ow through the circuit continuously for a long
time? Why? (NCERT)
Ans. When a current passes through the wire for a long time, the heating eff ect occurs, i.e. temperature of
the wire gets increased due to which the resistance of the wire gets increased.

Q9. If the metre bridge wire is not of uniform cross-section, then how will it aff ect the observations?
(NCERT)
Ans. If the metre bridge wire is of non-uniform cross-section, then the resistance per unit length of bridge
wire will be diff erent over diff erent lengths of metre bridge.

Q10. What is a galvanometer?
Ans. It is an instrument which is used to detect the presence of a feeble current in a circuit.

Q11. Is it necessary to keep the length of metre bridge wire 100 cm i.e. 1 m? What, if not? (NCERT)
Ans. No, the metre bridge works on the principle of Wheatstone’s Bridge. If the bridge is balanced, the
product of resistances in opposite arms is same. So, we can use wire of any length in the metre
bridge. Finally, we should fi nd the resistance per unit length of wire and then the balancing length.

Q12. If the same experiment is performed with AC = 50 cm instead of 1 m, what changes would be
there in the result? (NCERT)
Ans. There will not be any change in the result because the resistance depends on the length ratio instead

of entire length of bridge wire.

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