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NEET-XII-Physics

32: Electric Current in Conductors

with Solutions - page 2

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  • Qstn #14
    We often say, “A current is going through the wire.” What goes through the wire, the charge or the current?
    Ans : When there is a transfer of charge through a wire, current is said to be flowing through it. It is the electron/charge that drifts through the wire. The assertions "charge is going" and "current is going through the wire" are correct, as they signify the same thing, that is, flow of charge.
    Page No 196:
  • Qstn #15
    Would you prefer a voltmeter or a potentiometer to measure the emf of a battery?
    Ans : A potentiometer is preferred to measure the emf of a battery, as it gives a more accurate result. This is because a potentiometer uses the null method to measure emf and it hardly draws any current from the primary circuit.
    When a voltmeter is used in the circuit, its equivalent resistance is connected parallel to some element of the circuit. This changes the overall current in the circuit and, hence, the potential difference to be measured also changes. The error can be minimised if the equivalent resistance of the voltmeter is increased. However, we also need to keep in mind the heat dissipated due to high resistance while deciding the value of resistance of the voltmeter.
    Hence a potentiometer is preferred.
    Page No 196:
  • Qstn #16
    Does a conductor become charged when a current is passed through it?
    Ans : No, a conductor does not become charged when a current is passed through it. The free electrons present in the valence shell in a circuit drift from a lower potential to a higher potential and, thus, current is produced. A battery does not provide any extra electrons or charge to the circuit. It just provides a potential difference across two points, which helps in creating an electric field. This further helps in moving the electrons along the conductor.
    Page No 196:
  • Qstn #17
    Can the potential difference across a battery be greater than its emf?
    Ans : The potential difference across a battery cannot be greater than its emf. Basically, emf is the maximum potential difference between the terminals of a battery when the terminals are not connected externally to an electric circuit. When the same battery is connected to an electric circuit, current flows in the closed circuit. When current flows, the potential difference across the terminals of the battery is decreased as some potential drop due to its internal resistance.
    Due to the internal resistance in the battery, the potential difference across it is less than its emf. However, for an ideal battery, potential difference and emf are equal.
    Page No 196:
  • #
    Section : ii
  • Qstn #1
    A metallic resistor is connected across a battery. If the number of collisions of the free electrons with the lattice is somehow decreased in the resistor (for example, by cooling it), the current will
    (a) increase
    (b) decrease
    (c) remain constant
    (d) become zero
    digAnsr:   a
    Ans : (a) increase
    If the number of collisions of the free electrons with the lattice is decreased, then the drift velocity of the electrons increases.
    Current i is directly proportional to the drift velocity 'Vd' and is given by the following relation:
    i = neAVd , where 'n' is the number density of electrons and 'A' is the area of the cross-section of the conductor.
    So, we can easily see that current increases with increase in drift velocity.
    Page No 196:
  • Qstn #2
    Two resistors A and B have resistances RA and RB, respectively, and RA < RB. The resistivities of their materials are ρA and ρB.
    (a) ρA > ρB
    (b) ρA = ρB
    (c) ρA < ρB
    (d) The information is not sufficient to find the relation between ρA and ρB.
    digAnsr:   d
    Ans : (d) The information is not sufficient to find the relation between ρA and ρB.
    The resistance R of a conductor depends on its resistivity ρ, length l and cross-sectional area A. Thus,
    `` R=\rho \frac{l}{A}``
    From the given comparison of resistances, we cannot derive the correct relation between the resistances. We also need to know the cross-sectional areas and the lengths of both the conductors before concluding about their resistivities. Only then can the relation between the resistivities be found.
    The information is not sufficient to find the relation between ρA and ρB.
    Hence, the correct option is
    (d)
    Page No 196:
  • Qstn #3
    The product of resistivity and conductivity of a cylindrical conductor depends on
    (a) temperature
    (b) material
    (c) area of cross section
    (d) None of these
    digAnsr:   d
    Ans : (d) None of these
    The relation between the resistivity ρ and conductivity σ of a material is given by
    `` \rho =\frac{1}{\sigma }``
    `` \Rightarrow \rho \times \sigma =1``
    The product of conductivity and resistivity is unity. So, this product does not depend on any of the given quantities.
    Page No 196:
  • Qstn #4
    As the temperature of a metallic resistor is increased, the product of its resistivity and conductivity
    (a) increases
    (b) decreases
    (c) remains constant
    (d) may increase or decrease
    digAnsr:   d
    Ans : (d) may increase or decrease
    The product of resistivity and conductivity is independent of temperature. As the temperature of a metallic resistor is increased, the resistivity increases and conductivity decreases. Hence, both the conductivity and resistivity of the metallic resistor nullify the effect of the change in temperature.
    Page No 196:
  • Qstn #5
    In an electric circuit containing a battery, the charge (assumed positive) inside the battery
    (a) always goes from the positive terminal to the negative terminal
    (b) may go from the positive terminal to the negative terminal
    (c) always goes from the negative terminal to the positive terminal
    (d) does not move
    digAnsr:   b
    Ans : (b) may go from the positive terminal to the negative terminal
    In the study of electric current, the direction opposite the flow of electrons is regarded as the direction of flow of positive charge. In a battery, positive charge flows from the negative terminal to the positive terminal when it is discharging (connected to external circuit). But when the battery is charged, the positive charge flows from the positive terminal to the negative terminal.
    Page No 196:
  • Qstn #6
    A resistor of resistance R is connected to an ideal battery. If the value of R is decreased, the power dissipated in the resistor will
    (a) increase
    (b) decrease
    (c) remain unchanged
    digAnsr:   a
    Ans : (a) increase
    As the resistance is connected to an ideal battery, it provides a constant potential difference across the two terminals. The internal resistance of the battery is also zero.
    The power dissipated in the resistor,
    P = `` \frac{{V}^{2}}{R}``
    V is constant; hence V `` \propto \frac{1}{{R}^{}}``
    Thus, if the value of the resistance is decreased, the power dissipated in the resistor will increase.
    Page No 196:
  • Qstn #7
    A current passes through a resistor. Let K1 and K2 represent the average kinetic energy of the conduction electrons and the metal ions, respectively.
    (a) K2 < K2
    (b) K1 = K2
    (c) K1 > K2
    (d) Any of these three may occur
    digAnsr:   c
    Ans : (c) K1 > K2
    The metal ions are bound at their positions and vibrate due to collisions with electrons and due to thermal energy. The conduction electrons are free to move. They get energy from the electric field set inside the conductor by connection with a battery and due to thermal motion. The velocity of the electrons is high.
    Thus, the kinetic energy of the electrons is greater than the kinetic energy of the metal ions.
    Page No 196:
  • Qstn #8
    Two resistors R and 2R are connected in series in an electric circuit. The thermal energy developed in R and 2R are in the ratio
    (a) 1 : 2
    (b) 2 : 1
    (c) 1 : 4
    (d) 4 : 1
    digAnsr:   a
    Ans : (a) 1 : 2
    Thermal energy developed across a resistor,
    U = i2Rt ,
    where i is the current flowing through the resistor of resistance R for time t. Since the resistors are connected in series, the current flowing through both the resistors is same and the time for which the current flows is also same.
    Thus, the ratio of the thermal energy developed in R and 2 R is 1 : 2.
    Page No 196:
  • Qstn #9
    Two resistances R and 2R are connected in parallel in an electric circuit. The thermal energy developed in R and 2R are in the ratio
    (a) 1 : 2
    (b) 2 : 1
    (c) 1 : 4
    (d) 4 : 1
    digAnsr:   b
    Ans : (b) 2 : 1
    Thermal energy developed in the resistances,
    H =`` \frac{{V}^{2}}{R}t``
    Heat developed in the resistance R, H1=`` \frac{{V}^{2}}{R}t``
    Heat developed in the resistance 2R, H2 = `` \frac{{V}^{2}}{2R}t``
    Thus, Heat developed in the resistance R and 2R are in ratio 2:1.
    Page No 196:
  • Qstn #10
    A uniform wire of resistance 50 Ω is cut into 5 equal parts. These parts are now connected in parallel. The equivalent resistance of the combination is
    (a) 2 Ω
    (b) 10 Ω
    (c) 250 Ω
    (d) 6250 Ω
    digAnsr:   a
    Ans : (a) 2 Ω
    Resistance of a wire is directly proportional to its length.
    So, when we cut the wire into 5 equal parts, the resistance of each part will be 10 Ω.
    On connecting these wires in parallel, the net resistance will be
    `` \frac{1}{R}=\frac{1}{10}+\frac{1}{10}+\frac{1}{10}+\frac{1}{10}+\frac{1}{10}``
    `` \Rightarrow R=2\,\mathrm{\,\Omega \,}``
    Page No 197:
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