Chapter – 13
Q.1: Electrons leave a dry cell and flow through a lamp back to the cell. Which terminal the positive or negative is the one from which electrons leave the cell? In which direction is the conventional current?
Ans: Electrons leave the negative terminal of the cell and move towards the positive terminal. However, as a convention, the conventional current is assumed to be the consisted of positive electric charges moving from a positive terminal to the negative terminal change flowing through the area per unit time (I = q/t).
Q.2: Both p.d. and e.m.f. are measured in volts. What is the difference between these concepts?
Ans: P.d. is the work done per unit charge across resistor in a closed circuit. But e.m.f. the total p.d. across the external and internal resistance, it refers to a source of current and is greater than the potential drop in an external circuit. (e.m.f. p.d. + internal resistance drop)
Q.3: Can you construct two wires of the same length, one of copper and one of iron, that would have the same resistance at the same temperature?
Ans: Yes, since resistibility is proportional to cross-sectional area. The resistivity of iron is about 7 times higher, than that of copper. Hence the iron wire must be 7 times thicker than a copper of the same Length to have the same resistance at the same temperature.
Q.4: Why does the resistance of a conductor rise with the rise in temperature?
Ans: As the temperature of a conductor rises, the amplitude of the vibration of the atoms in the lattice increases. This, in turn, increases the probity of their collision with free-electron. This impedes the drift of the electron. Hence the resistance of the conductor increases.
Q.5: Why is heat produced in a conductor due to the flow of electric current?
Ans: As electric charge flows due to certain p.d. through a conductor, it suffers loss of electrical potential energy. The energy is delivered to the lattice atoms. This energy is utilized in increasing their vibration kinetic energy which appears as heat. Consequently, the temperature of the conductor rises.
Q.6: When a metallic object is heated both its dimensions and its resistivity increase. Is the increase in resistivity likely to be a consequence of the increase in length?
Ans: The receptivity is given by . The increase in receptivity of a conductor due to heat is a consequence of the increase in resistance, and not a consequence of the increase in length.
Q.7: It is sometimes said that an electrical appliance “uses up” electricity. What does such an appliance actually use in its operation?
Ans: An electrical appliance, in its operation, uses the kinetic energy carried by the moving electrons, and not their quantity of charge.
Q.8: Do bends in a wire affect its resistance?
Ans: No, bends in a wire do not affect its resistance. However, it depends upon length, cross-sectional area, temperature, and nature of the material.
Q.9: Resistances of 10Ω, 30Ω and 40Ω are connected in series. If the current in 10Ω resistance is 0.1A, what is the current through the other?
Ans: When resistors are connected in series, then the same current flows through each (as there is only one path). Hence the current in this case will be 0.1A through all three resistors in series.
Q.10: The resistances of different values are connected in parallel. If the p.d. across one of them is 5V, what is the p.d. across the remaining at ne resistors?
Ans: When resistors are connected in parallel, then the same p.d. exists across each of them as they all are connected to two common points. Hence the p.d. in this case will be 5V across all the nine resistors in parallel.
Q.11: For a given potential difference V, how will the heat developed in a resistor depend on its resistance R? Will the beat be developed at a higher rate in a larger or smaller R.
Ans: The heat developed .
For a constant p.d. the resistance R should be small to develop heat at a higher rate.
Q.12: Is there any electric field inside a conductor carrying an electric current? Explain motion of charges here?
Ans: When a p.d. is applied across a conductor by connecting it to a battery and electric field E is established inside a conductor (parallel to the conductor, directed from the positive toward the negative terminal). The field exists here because the battery keeps the charges moving and prevents them coming to equilibrium on the outer surface of the conductor (in contrast to the situation in electrostatic), where they would cause the net electric field on the interior to be zero.
Q.13: Can the terminal voltage of battery be zero?
Ans: When a battery is short-circuited, the existence of negligibly small resistance in the circuit makes terminal voltage zero but current to a maximum value.
Q.14: Why is internal resistance of a cell not constant?
Ans: The internal resistance of a cell depends upon the resistance of electrolyte, terminals and electrons (and also on their area and separation) of the cell. Due to chemical changes (e.g. absorption of hydrogen and sulphate ions) in the electrolyte during the process of discharging the resistance of the electrolyte increase. Thus the internal resistance of cell does not remain constant.
Q.15: What is resistance? What is its mechanical analogue?
Ans: Resistance is a property of a given conductor which limits the current flow. It is due to the collisions of the drifted electrons with the crystal lattice which causes frequent scattering of the electrons under an electric field.
This property is analogous to mechanical friction or moving bodies.
Q.16: Often, you might have noticed crows sitting safely on high tension wires. Why are they not electrocuted, even when sitting on a part of the wire where the insulation has worn off?
Ans: For electrocution, the current should pass through the bodies of crows. When a crow sits on a signal wire, a p.d. is not developed for the flow of current because his both claws are the same potential. Hence they are not electrocuted.
Q.17: Why is it dangerous to touch a live wire standing on the earth barefooted?
Ans: We may be electrocuted if one touches a live wire while standing on barefooted; because we provide lower potential of the earth through our barefoot (conductor). [The effective resistance of the body is 50 kΩ /cm³ which reduces to 0.7 kΩ/cm when it is wet].
Q.18: A heavy duty battery of a truck maintains a current for 3.0A for 24 hours. How charge flows from the battery during this time.
Ans: The charge, .
Q.19: What a short circuit and open circuit mean to you?
Ans: A short circuit is a closed when no load is present i.e. external resistance (R) is zero. But an open circuit implies an infinite resistance (or gap) along its conduction path (i.e. wires).
Q.20: Is it possible to have a situation in which the terminal voltage will be greater than the e.m.f. of a battery?
Ans: In general, in the case when a battery is being charged.
Q.21: Why is resistance of a conductor inversely proportional to the area of cross sectional a conductor?
Ans: The larger the area of cross section of a conductor, the wider path is provided by it for the flow of charges through it . Hence, the resistance decreases.
