  WWW http://sajitsir.tripod.com

 1 21. Two wires one of copper and the other of maganin have equal lengths and equal resistance. Which wire is thicker? If two wires one of copper and the other of maganin have equal lengths l and equal resistance R. Let resistivities & cross-sectional area copper & maganin wires are and respectively. The resistance is define as  Experimentally, it is found that which implies that . Hence, manganin wire is thicker.   22. Why are alloys constantan and manganin used to make standard resistors?       23. A wire carrying current stays electrically neutral. Why?   Electric current is the flow of free electrons in the conductor. The number of proton in wire is equal to the number of electrons at any instant so, the net charge is zero. A wire carrying current stays electrically neutral because many electrons enter one end of wire & leave it from other end.     24. The electron drift speed in metals is small and the charge of the electron is also very small, but we can still obtain a large amount of current in a metal. Why? The electron drift speed in metals is definitely a small, of the order of millimeters per second. Had there been less number of free electrons, the overall charge passing through a point per unit time would be less. However metals have very high number of free electrons. So even if they traveled with less velocity, their large number carries more charges every second and then according to the relation , the current will be high.     25. A wire is stretched to double its length. What happens to its resistance? The resistance R of the resistor of the length l whose resistivity is ρ is given by: As the resistivity of the resistor is the characteristics of the material of the wire which does not altered and remains constant for the same material. If a wire is stretched to double its length, then its length increases more than its cross-sectional area decreases so from above relation, resistance increases and becomes four times the initial resistance.   26. In a car, one terminal of the battery is 'said' to be connected to ground even if it is not really connected to the ground. Explain.   A large body has large capacitance and due to its large capacitance, if some charge is given to it then its potential does not change but remains constant. Hence, such a body is considered as at zero potential or ground. In a car, one terminal of a battery is connected to the metal body which is assumed to grounded even if it is not grounded. Hence, one terminal of the battery is said to be connected to ground even if it is not really connected to the ground in a car.       27. Why is an ammeter always connected in series with the load?   An ammeter is used to measure current flowing through the circuit. The current flowing through the circuit can be easily measured when the ammeter is connected in series. As the resistance of an ammeter is negligible or zero ( in practice, the resistance of ammeter is not zero but very low as compared to other resistance) so, that when it is connected in the circuit the equivalent resistance of the circuit remains same. So, the current in the circuit is unaltered. Hence, an ammeter is always connected in series with the load.       28. Why is a voltmeter always connected in parallel with the load (resistance)?   A voltmeter is an instrument used to measure the voltage across the load so when it is connected in the circuit it should not disturb the current flowing through the circuit. As a voltmeter contain high resistance so when it is connected in parallel with a load, resistance is not disturbed which doesn't disturbs the current. But, if a voltmeter is connected in series with the load then the equivalent resistance of the circuit increases which alters the current so, there is error in the reading of voltage. Hence, a voltmeter is always connected in parallel with the load.        29. Why do electrons acquire a steady average drift velocity?   When a battery is connected across the ends of a conductor, an electric field is set up. This electric field accelerates the electrons in the conductor. Electrons gain kinetic energy depending on the amount of the field. During their movement, electrons collide with atomic lattice and loses their energy. So, they cannot be accelerated continuously but they soon acquires drift velocity.       30. Why is there no current established in a conductor in the absence of electric field although a large number of free electrons are moving with high speed? The flow of current requires a flow of charges in a particular direction, which of course requires guiding the electrons somewhere.   There are large number of free electrons moving in a metallic conductor. But in the absence of any external force (i.e. the electric field), they are not guided anywhere. So, for every electron moving to the right, there are equal number of electrons moving to the left. For every of them moving upwards, equal number of them would be moving downwards and so on. Therefore there will not be any overall movement in any direction, which makes the charge flow zero and so current also zero.     31. In which condition will the terminal p.d of a cell be more than its emf?   In case of charging of cell having internal resistance r, the terminal potential difference V of the cell is greater than its emf E i.e. V = E + Ir: where I is the current. Here, the terminal p.d. exceeds the emf of a cell by the term Ir.       32. The thermal speeds of the free electrons in a conductor are quite large. Then, why don’t they escape from the surface of the conductor?   For the propagation of free electrons, the medium of good conductor is required. However, the thermal speeds of the free electrons in a conductor are quite large, they do not escape from the surface of the conductor because there are no such medium through which free electrons can flow from the surface of the conductor. There is only the air medium which behaves as insulator through which  free electrons can not flow. However, the thermal speeds of the free electrons in a conductor are quite large they do not escape from the surface of the conductor.       33. Explain how do you distinguish whether two resistances in a circuit are connected in parallel or in series?   Firstly, I would like to measure whole resistance of a circuit. Then, I would separate two resistors and I would measure the value of resistances of each resistor. Then I would calculate the equivalent resistance of two resistors when they are connected in parallel as well as in series. After that, I would compare the value of whole resistance of a circuit with the equivalent resistance of two resistors when they are connected in parallel as well as in series. If one of the value of equivalent resistance of two resistors satisfies the value of resistance of whole circuit, then I would easily know whether two resistors are connected in parallel or series.       34. When electron drift in a metal from a lower to higher potential, all the free electrons of the metal move in the same direction. Is this statement true? Explain.   When electron drift in a metal from a lower to higher potential, all the free electrons of the metal move in the same direction. Yes, this statement is true because the drift velocity is superposed over the random thermal velocities of electrons.       35. How does the resistance of a material vary with the temperature? The resistance of a metal increases with increase in the temperature. Let R1 & R2 are the resistance of resistor at temperature t10C & t20C respectively then                R2 = {1 + α(t2 - t1)} where 'α' is the constant called the 'temperature coefficient' of the material. For metal, 'α' is positive but for insulator and semiconductor, 'α' is negative.   36.       37. In a metal, there are a large number of free electrons. Then, why does it offer resistance?   When an electric field is applied across a metal, the free electrons are accelerated. When accelerated, the electrons then move across the metal. During their motion, they have to make a large number of inelastic collisions with the positive ions of the metal. In each collision, electrons lose their energy and their speeds and directions changes during the motion. In addition, they also undergo collisions with the electrons present in various orbits of atoms, which are also responsible to cause loss of energy. Hence, a metal offers resistance to the free electrons.       38. Why is copper used as connecting wire?   Copper is the metal that has less resistance and it is cheap as well as it is easily available in its pure form. So, it can make flow of the electric current by least resisting or opposing it. Hence, copper is used as connecting wires.       39. How are lamps connected in a house? How is the fuse connected?   Generally, lamps are connected in a house in parallel combination. If the lamps are connected in the series then if the switch is turned off then all lamps will not glow but if they are connected in parallel then all lamps can be made to glow at a time and also individual lamp can also be made to glow. Hence, lamps are connected in a house in parallel combination.   As the fuse is the safety device which is made of the alloys of metals (tin and lead) having low melting point. It suddenly breaks when the high voltage flow through it resulting in the break up of the circuit of our houses that saves our expensive electronics device as well as it saves from firing. So, it must be connected in series with the circuit so that whole circuit is break off while it melts.     40. A conductor is carrying a current that is charge per unit time. Can you say that the conductor was charged?   We cannot say that the conductor was charged. The flow of current in a conductor is due to the drifting of free electrons in a direction. In a conductor, at any instant, the total number of electrons is equal to the total number of protons so the net charge is zero. Hence, we cannot say that the conductor was charged.           If you want certain questions answered, please write to
Electricity 2