Q.1. Distinguish between static electricity and current electricity.Electric Current and Resistance Question and Answer
Ans. Static electricity deals with the study of charges at rest and current electricity deals with charges in motion.
Q.2. A wire is carrying a current. Is it charged?
Ans. No. The current in a wire is due to flow of free electrons in a definite direction. But the number of protons in the wire at any instant is equal to number of electrons and charge on electron is equal and opposite to that of proton. Hence net charge on the wire is zero.
Q.3. A large number of free electrons are present in metals. Why is there no current in the absence of electric field across it, but there is a current in the presence of electric field?
Ans. The current is due to flow of charge in a definite direction. In a metal, the free electrons move in all directions haphazardly, in the absence of electric field; the average thermal velocity of electrons is zero. Therefore, there is no current in metal in the absence of electric field.
In presence of electric field, each free electron experiences a force opposite to the direction of field. Due to which the electrons acquire a drift velocity and there is a net flow of charge in a definite direction which constitutes the current.
Q.4. Of metals and alloys, which has greater value of temperature co-efficient of resistance?
Ans. The value of temperature co-efficient of resistance is more for metals than for alloys. For example, in case of copper, and for manganin, .
Q.5. The standard resistance coils are made of manganin. Why?
Ans. For manganin, the temperature coefficient of resistance is very low and its resistivity is quite large. Due to which the resistance of manganin wire remains almost unchanged with change in temperature. It is due to this fact, the wire of manganin is used for making standard resistance coils.
Q.6. The connecting wires are of copper. Why?
Ans. The electrical conductivity of copper is high. Therefore, it conducts the current without offering much resistance. The copper being diamagnetic material does not get magnetized due to current through it and hence does not disturb the current in the circuit.
Q.7. A potential difference V is applied to a conductor of length L, diameter D. How are the electric field E, the drift velocity vd and the resistance R affected when (i) V is doubled (ii) L is doubled (iii) D is doubled?
Ans. We know,
(i.). When V is doubled, E becomes double, vd becomes double and R remains unchanged.
(ii). When L is doubled, E becomes half, vd becomes half and R becomes double.
(iii) When D is doubled, E remains unchanged, vd is also unchanged and R becomes one-fourth.
Q.8. Explain, why bending a wire does not affect electrical resistance?
Ans. Free electrons in a wire have small value of drift velocity and hence low value of inertia of motion. Due to it, they are able to go around the bends easily.
Q.9. Give relation between drift velocity and electric field.
Ans. We know, or
Q.10. A steady current is flowing in a cylindrical conductor. Is there any electric field within the conductor?
Ans. Yes, the current flows through a conductor only when the electric field established within the conductor exerts force on the free electrons, due to which they move in a definite direction, accounting for a current in a conductor. The electric field E inside the conductor is given by
Q.11. When we switch on the lights one after the other, what is the effect on the resistance of the electric circuit of the house? On the current flowing in the main circuit?
Ans. In a house all lights are in parallel circuits, to a fixed supply voltage 220 V. When we switch on lights one after the other, the effective resistance of the circuit goes on decreasing. As a result of it, the main current goes on increasing.
Q.12. What do you understand by electromotive force? State its S.I. unit.
Ans. E.M.F. of a cell is the maximum potential difference across the two electrodes of the cell when no current is drawn from the cell or cell in the open circuit. S.I. unit of emf of a cell is volt.
Q.13. Bends in pipe slow down the flow of water through it. Do bends in a wire increase its electrical resistance 2 Explain.
Ans. Bends in the wire do not increase the electrical resistance of a wire. It is so because the free electrons in a wire have small value of drift velocity and hence low value of inertia of motion. Due to it, the electrons are able to go around the bends easily.
Q.14. Is electric current a vector or scalar quantity? Explain.
Ans. Electric current is a scalar quantity, for its addition, the laws of scalar addition are applicable and not the laws of vector addition, i.e., the angle between the wires carrying the current does not affect the total current in the circuit.
Q.15. Why a current is not possible, without the source of e.m.f.?
Ans. The current in a circuit is possible only, when current carriers move in a particular direction through a conducting material. The same is possible by supplying the energy in some form, which in turn is possible by the use of source of e.m.f.
Q.16. When does the current flow in an electric circuit?
Ans. Current flows in an electric circuit only when there is a source of emf and circuit is closed.
Q.17. How can the current be kept continuous?
Ans. By maintaining a constant potential difference between two points.
Q.18. Why copper wires are used as connecting wires?
Ans. Copper wires are used as connecting wires because electrical conductivity of copper is high and its electrical resistivity is low.
Q.19. What should be the properties of the standard resistances?
Ans. The properties of standard resistances should be:
(i). Its value should not change with time.
(ii). It should show negligible variation with temperature.
(iii). It should have proper capacity for carrying current without overheating.
(iv). It should have low inductance and self-capacitance.
(v). It should be of robust construction and of convenient size.
Q.20. Why constantan or manganin are chosen for the construction of standard resistances?
Ans. (i) It is so because they possess a high value of resistivity. Therefore, for making a particular resistance, the wire chosen is not either abnormally long or extremely thin.
(ii) Their temperature coefficient of resistance is extremely small. Therefore, the value of the resistance remains practically unchanged when the temperature of the surrounding changes.
(iii) They are least effected by atmospheric conditions, i.e., they are neither easily oxidized, nor they are effected by moisture.
(iv) Their contact potential with copper is small. Therefore, when these resistances are connected in a circuit with copper connecting wires, there is a least disturbance in the circuit due to them.
Q.21. When we switch on an electric bulb, it lights almost instantaneously though drift velocity of electron in copper wires is very small. Explain.
Ans. When we switch on the bulb, the electric circuit gets closed, the electric field is established in the circuit instantly with the speed of electro-magnetic waves (= 3 x 108 ms-1) causing at every point a local electron drift. The establishment of current does not wait for the electron to flow from one end of circuit to another end. Hence the bulb lights almost instantaneously.
Q.22. What is the order of the number of free electrons in metals? What is their function?
Ans. The order of number of free electrons in a metal is 1029 m-3, their movement in a definite direction will constitute the electric current in the metal.
Q.23. Is Ohm’s law true for all conductors? Name two types of commercially available resistors.
Ans. No, it is true only for metallic conductors. Commercially available resistors are: carbon resistors and wire bound resistors.
Q.24. What is the effect of temperature on the relaxation time of electrons in a metal
Ans. Relaxation time decreases with increase in temperature.
Q.25. What do you mean by a linear resistor?
Ans. A linear resistor is one which obeys Ohm’s law, i.e., for which voltage current graph is a straight line.
Q.26. What is the effect of rise in temperature on the electric resistivity of semiconductor?
Ans. Electrical resistivity of semi-conductor decreases with rise of temperature.
Q.27. How is the current conducted in metals? Explain.
Ans. Every metal conductor has large number of free electrons which move at random at room temperature. Their average thermal velocity at any instant is zero. When a pot. diff is applied across the ends of a conductor; an electric field is set up. Due to it, the free electrons of the conductor experience force due to electric field and drift towards the positive end of the conductor, causing the electric current (i.e. conduction current) in the conductor whose direction is opposite to the direction of motion of the free electrons in the conductor.
Q.28. Define ampere, volt and ohm.
Ans. If 1 coulomb charge is flowing per second through a cross-section of a conductor, then current through the conductor is 1 ampere.
If 1 joule work is done in bringing 1 coulomb of charge from one point to another against the electric forces, then pot. diff, between those two points is 1 volt.
If 1 ampere current flows through a conductor when 1 volt pot, diff, is applied across its two ends, then the resistance of conductor is 1 ohm.
Q.29. What is terminal potential difference of a cell? Can its value be greater than the e.m.f. of a cell? Explain.
Ans. Terminal potential difference of a cell is defined as the potential difference between the two electrodes of a cell in a closed circuit. The value of terminal potential difference of a cell is less than the e.m.f. of a cell, when current is drawn from the cell (i.e. during discharging of cell). The value of terminal potential difference of a cell becomes greater than the e.m.f. of the cell during charging of the cell, i.e., the positive electrode of the cell is connected to positive terminal of battery charger and negative electrode of the cell is connected to negative terminal of battery charger.
Q.30. While making a standard resistance the coil is made of manganin. The coil is double folded and is wound over non-conducting frame. Why?
Ans. For manganin, the temperature coefficient of resistance is very low and its resistivity is quite high. Due to it, the resistance of manganin wire remains almost unchanged with change in temperature. It is due to this fact, the wire of manganin is used for making standard resistance coil. The coil is double folded and is wound over the non-conducting frame in order to avoid the inductive effect and the leakage of current.
Q.31. Lights of a car are dimmed when the starter is operated, why?
Ans. When the motor starter of a car is operated, it draws more current from the battery for the operation of car. Due to which the voltage across the lightbulb is lowered, hence the light of a car is dimmed.
Q.32. To reduce the brightness of a lightbulb, should an auxiliary resistance be connected in series with it or in parallel?
Ans. To reduce the brightness of a lightbulb, we should decrease the current flowing through the bulb, which is possible when an auxiliary resistance is connected in series with the bulb.
Q.33. What is the emf of a cell? On what factors does it depend?
Ans. E.M.F. of a cell is defined as the maximum value of potential difference between the two electrodes of a cell when the cell in the open circuit, i.e., no current is drawn from the cell. E.M.F. of a cell depends upon (i) nature of electrodes (ii) nature and concentration of electrolyte and (iii) temperature of electrolyte used in cell.
Q.34. If the current flowing in a copper wire be allowed to flow in another copper wire of double the radius, then what will be the effect on the drift velocity of the electrons. If the same current be allowed to flow in an iron wire of the same thickness, then?
Ans. We know that if two wires are in series, there will be same current in each wire. The current I through the wire is related with drift velocity vd by the relation
Since in the second wire, the radius of wire becomes 2 times hence the drift velocity will become one fourth. In case of iron wire, the number density of free electrons will be less in comparison to copper wire and hence the drift velocity will be more in comparison to that in copper wire.
Q.35. What is super-conductivity? Write its two applications.
Ans. Superconductivity is a phenomenon of disappearance of all signs of resistance in a conductor at a critical temperature or transition temperature which is usually low. Superconductivity can be used (i) to produce very high speed computers and (ii) to transmit electrical power from generating station to consumers over superconducting cables without any loss of power.
Q.36. What are super-conductors? Write its two applications.
Ans. As the temperature of certain metals and alloys decreases, their resistance also decreases. When the temperature reaches, a certain critical value called critical temperature, the resistance of material completely disappears, i.e., it becomes zero. Then the material behaves as a super-conductor. Thus, super-conductors are those material conductors whose resistance disappear at critical temperature. The critical temperature is different for different materials. Super conductors are used (i) in power transmission (ii) to produce very high speed computers.