Topics of Interest CBSE Physics
Raj Kumar Parashari 22 Dec, 2019 Views: 347
1. What is conductance? Write its SI unit.
Ans. The reciprocal of resistance is called conductance. It is denoted by G.
is mho, or Siemen.
2. What is the resistance of an open key? Explain it.
Ans. An open key has infinite resistance because it makes current zero .
3. (a) Define resistivity or specific resistance of the material of conductor.
(b) State factor on which resistivity of a material depends.
Hence, resistivity is defined as the resistance of a conductor of unit length and unit cross-sectional area. The unit of resistivity is ohm-metre Resistivity depends on nature of material and temperature.
4. Define electrical conductivity. Write its SI unit.
Ans. It is reciprocal of resistivity. It is represented by the symbol . The unit of electrical conductivity is Siemen per metre .
5. What is the order of magnitude of resistivity of conductors?
Ans. Resistivity of the conductors is of the order of
6. Give unit of temperature coefficient of resistance.
Ans. The unit of temperature coefficient is .
7. What is the condition for a Wheatston’s bridge to become balanced?
Ans. In a balanced Wheatstone’s bridge, we have, A Wheatstone’s bridge is said to be balanced, when no current flows through the galvanometer and it gives zero deflection.
8. Will the interchange of positions of cell and galvanometer effect the balance condition?
Ans. No. The condition of balanced Wheatstone bridge remains satisfied.
9. When is a Wheatstone’s bridge most sensitive?
Ans. The bridge is most sensitive when all the four resistances P, Q, R and S are of same order of magnitude.
10. What are applied forms of a Wheatston’s bridge?
Ans. The applied forms of a Wheatstone’s bridge are: - (i) Metre Bridge or Slide Wire Bridge. (ii) Post Office Box.
11. Why is a metre bridge so called?
Ans. Since the bridge uses one metre long wire, it is called a metre bridge.
12. Why the jockey should not be pressed too hard on the wire when sliding over it?
Ans. Sliding the jockey with a hard press, will scratch the wire and make its thickness non-uniform. Then the resistance per unit length of the wire will not remain constant because resistance depend upon area of cross-section.
13. Why is it advised to keep null point between 35 cm and 65 cm?
Ans. It is done to minimise the effect of neglecting of end resistances in calculation and Wheatstone bridge is most sensitive when all four arms have same order of resistances.
14. What are end resistances?
Ans. The resistances of thick copper strips which keep the two ends of the wire pressed, are called end resistances.
15. Why copper strips, used to pressed the ends of wire, are thick?
Ans. Thick Cu strips have negligible resistance over the resistance of alloy metre bridge wire and minimise effect of end resistances.
16. Why the bridge method for resistance measurement is better than Ohm’s Law?
Ans. It is so because the bridge method is a null method (at null point, there is no current flowing in galvanometer) and more sensitive.
17. Why the metre bridge is suitable for measuring moderate resistances?
Ans. Because, Wheatstone bridge is suitable for moderate values of resistances. Therefore, meter bridge is more sensitive for moderate values.
18. How does position of null point affected if (a) value of x is increased (b) x and y are interchanged.
Questions based on Potentiometer
1. What do you understand by the e.m.f. of a cell?
Ans. Electromotive force i.e., e.m.f. of a cell is the potential difference across the terminals of the cell when the cell is in an open circuit i.e., when no current is drawn from the cell.
2. Why is it called a potentiometer?
Ans. Because it measures potential difference between any two points of electric circuits.
3. What is potential gradient? Give its SI unit.
Ans. It is the fall of potential per unit length of the potentiometer wire i.e. . Unit is volt per meter.
4. How does the potential gradient vary along the length of the wire from end P to end Q?
Ans. Potential gradient is same throughout if the wire has uniform cross-section and material density.
5. What kind of source of e.m.f. should be used as auxiliary battery?
Ans. The e.m.f. of the source must be steady. A freshly charged accumulator should be used for this purpose.
6. On what factors does the potential gradient depend?
Ans. Potential gradient depends directly on the strength of the current and resistance per cm of the wire.
7. What is the preferred material used for making potentiometer wires?
Ans. Manganin. It is characterised by a low temperature coefficient of resistance and a high resistivity.
8. Why don’t we use a copper wire as a potentiometer wire?
Ans. Because it has very less resistivity so extremely large wire is required.
9. Why is a ten-wire potentiometer more sensitive than a four- wire one?
Ans. The potential gradient, under same conditions, decreases with an increase in the length of the potentiometer wire, Hence, a 10-wire potentiometer (having a smaller potential gradient) is more sensitive than a 4-wire one.
10. What will you conclude if the deflection of the galvanometer is in same direction at both the ends?
Ans. The reasons may be
11. Why do we need a protective series resistance/ shunt along with a sensitive galvanometer?
Ans. To prevent it from damage from the flow of excessive currents that may exist when the jockey is far from the balance point.
12. On what factors does the internal resistance of a cell depend?
Ans. Internal resistance of a cell depends upon:
13. Does the internal resistance depend on the current drawn from the cell?
Ans. Yes, the internal resistance usually increases as more current is drawn from the cell.
14. Can we find the internal resistance of an accumulator or secondary cell?
Ans. No; the internal resistance of an accumulator is so small that this method cannot be used.
15. Can you measure e.m.f. by a voltmeter?
Ans. No. The voltmeter measure the terminal potential difference of a cell because it draw some current
16. Is the terminal potential difference (V) and e.m.f. (E) of a cell different? Explain.
17. Does the potentiometer is used to determine the internal resistance of (i) primary cell (ii) secondary cell?
Ans. The potentiometer is used to determine the internal resistance of primary cell only but not secondary cell because of very small resistance .
18. What are the factors on which the e.m.f. of a cell depends?
Ans. (i) Nature of electrodes, (ii) Nature of electrolyte, (iii) concentration of electrolyte, (iv) Temperature of electrolyte.
19. Why is a potentiometer preferred over a voltmeter for measuring the e.m.f. of cell?
Ans. A potentiometer draws no current from the cell whose e.m.f. is to be measured. On the other hand, the voltmeter always some current. Thus e.m.f. measured by voltmeter will be slightly less than the e.m.f. measured by potentiometer.
20. Can we consider the potentiometer as an ideal voltmeter?
Ans. Yes. At null point, the potentiometer does not draw any current. Hence it measure the emf. The potentiometer is equivalent to an ideal voltmeter.
21. In the potentiometer circuit shown in fig, the balance (null) point is at X. State with reason, where the balance point will be shifted when.
(i) Resistance R is increased, keeping all parameters unchanged.
(ii) Resistance S is increased, keeping R constant.
(iii) Cell P is replaced by another cell whose emf is lower than that of cell Q.
Ans. (i) Null point shift toward point B.
(ii) No change.
(iii) Balancing length will not obtain on wire AB.