• SLC Science
  • Physics
    • Gravitational force
      • Law of gravitation
      • Acceleration(g)
      • Free fall
      • Numerical
    • Pressure
      • Def.Pressure
      • Principles
      • Upthrust
      • Numerical
    • Heat
      • Heat Energy
      • Anomalous expansion of water
      • Specific heat capacity
      • Numerical
    • Electricity
      • Electricity
      • Fuse
      • Faraday'sPrinciple
      • Transformer
      • Numerical
  • Chemistry
  • Biologi
• SLC Maths
• slc.Opt.Math
• +2Science
  • Wave Motion

Short Question Answers on Direct current :

Short Question Answers on Direct current

1. A large number of free electrons are present in metals. Why is there no current in the absence of electric field across it?
Ans In the absence of electric field, the free electrons in the metal have random motions. During motion, they collide with positive ions of the metal again and again and after each collision, their direction changes. So there is no net flow of charge carriers in a particular direction and hence no current flows.
2. In a resistance box, there is a resistance marked infinity. What is the length of the wire used for making this infinite resistance?
Ans No wire is used to produce an infinte resistance. The infinite resistance is produced by an air gap as air is an insulator.
3. The electron drift speed in metals is small in the order of few mm s ⁻ and the charge of the electron is also very small, 10 ^{− 19} C, but we can still obtain a large amount of current in a metal. Why?
Ans The current in metal depends not only on charge (e) and drift speed (V_b), but also on the number density (n) of free electrons. Although e and v_d are small, we can still obtain a large current because n is very large, -10²⁹ m^{− 3}
4. A steady current is flowing in a metallic conductor of non-uniform cross-section. Which of these quantities is constant along the conductor: current, current density, electric field, drift speed?
Ans Since the current is steady, it remains constant in the conductor. All other quantities vary inversely with area of cross-section.
5. Is Ohm's law universally applicable in all conducting materials? Explain with examples of material.
AnsNo. Examples of materials that are not applicable to Ohms law (i) semiconductor diodes, vaccum diodes, transistors, etc. The I-V graph is a curve but not a straight line.
6. There is animprression among many people that a person touching a high power line gets shock. Is it true? Expalin.
Ans This expression is not true. The current disorganizes our nervous system. It makes the person to lose temporarily his ability to exercise his nervous control to get himself free from high power line. It is dangerous only if we touch to ground with power line at the same time. At this condition, the current will pass through our body and produce harmful effect.
7. The electron drift in a conductor arises due to the force experienced by electrons in the electric field. This force should cause acceleration of electrons. But the electrons acquire a steady drift velocity. Why ?
Ans The reason is that electrons suffer a large number of collisions with the positive ions of the conductor. Although the electric field makes the electron to accelerate, the collision reduces the acceleration. So the gain in speed between collisions is lost in the next collision. Thus the net acceleration averages out to zero and the electron acquires a constant average drift speed.
8. Why are standard resistors made of alloys such as constantan and manganin ?
Ans The temperature coefficient of resistance for constantan and manganin is very small. This means that there is a negligible change in the resistance due to moderate changes in temperature. Having high value of α, the wire occupies less space in devices for high value of resistance.
9. Two wires, one of copper and other of manganin, have equal lengths and equal resistance. Which wire is thicker?
Ans
10. Why is an ammeter connected in series in a circuit ?
Ans An ammeter is a low resistance galvanometer. When it is connected in series in a circuit, the resistance of the circuit does not increase appreciably and consequently and consequently the current in the circuit remains unaffected.
11. Why voltmeter connected in parallel across a circuit element ?
Ans A voltmeter is a high resistance device. When it is connected in parallele across any component of a circuit, it draws a very small current from the main circuit. Most of the current passes through the element. Hence the p.d. across the component is not affected.
12. What happens when an ammeter is placed in parallel with circuit ?
Ans When an ammeter is connected in parallel in a circuit, the resistance is considerably reduced. A large current flows through the circuit, which can damage the ammeter. Moreover, the ammeter wil read the current passing through it only and not the current in the circuit.
13. What happen when voltmeter is connected in series in circuit ?
Ans When voltmeter is connected in series, the resistance of the circuit becomes high. The current decreases considerably. Voltmeter will not read actual potential difference.
14. Though the direction of electric current is given by the direction of flow of positive charges but even then it is regarded as a scalar quantitiy. Explain why ?
Ans : Current is regarded as scalar quantity due to the following facts:
          (i) Currents can be added or subtracted according to ordinary rules of algebra.
          (ii) It is dot product of two vectors i.e. I = J.A.
          (iii) Strength and direction of current always remains unchanged even if (a) wire carrying current has different cross-sections at different points along its length and (b) wire may be bent at different angles at its different points.
15. Though electrons are constantly in motion within metals, but there is not current until a potential difference is established across it. Expalin why?
Ans : Electrons in metals move randomly in all directions and there is no net motion in any direction in the absence of a potential difference. When a potential difference is applied across the metal an electric field is established within the metal, and electrons drift towards higher potential which generates an electric current.
16. Although the drift velocity of electron is very small, yet an electric bulb lights up almost instantly when switched on, whatever be the distance of the bulb from the switch. Why?
Ans : As soon as the switch is made on, an electric field is established in the circuit instantly with the speed of light, causing at every point a local electron drift. Hence, current in the circuit is established almost instanlty when the switch is made on the bulb also lights up instantly. This implies that information for the flow of current is transmitted through the propagation of electromagnetic waves (electric impulse) and not with drift velocity of the electrons.
17. A steady current is flowing in a cylindrical conductor. Is there any electric field within the conductor?
Ans : Yes, current flows in a conductor only when electric field established within the conductor exerts force on the free electrons.
18. Why don't we consider the drift velocity of positive ions ?
Ans : The positive ions in a conductor also experience a force in the presence of an electric field. Since the positive ions are heavy and tightly bound in the metal, they are hardly able to move making the drift velocity negligibly small.
19. A wire is stretched to double its length. What will be its new resistivity ?
Ans : The resistivity of the wire will remain the same as it only depends upon the nature of the material of the wire and is independent of the lenght or area of cross-section of the wire. 20. Is the value of temperature coefficient always positve?
Ans : No, the value of temperature coefficient of resistance is positive only for metals and alloys and is negative for semiconductors and insulators.
21. Why do we use connecting wires made of copper?
Ans : Copper has low resistivity and therefore it is a good conductor. Moreover, it is diamagnetic and so does not get magnetised due to current flowing through it. Thus it does not disturb the current flowing in the circuit.
22. What are the factors on which resistance of a conductor depend?
Ans : The resistance of the conductor depends upon length (l) of the conductor such that R α l, Area of cross section (A) of the conductor such that R α $\frac{l}{A}$, Temperature and nature of the materials of the conductor.
Therefore, at a given temperature,
R α $\frac{l}{A}$
i.e. R = $\rho \frac{l}{A}$
where ρ is the proportionality constant which is called the resistivity or specific resistance of the material of the conductor.
23. You are given n wires each of resistance r. what is the ratio of maximum to minimum resistance that can be obtained from these wires ?
Ans :Since in series combination effective resistance increases, therefore, when n wires each of resistance R connected in series the maximum resistance is obtained is given by
R_s = R + R + R + ...... = n R
or R_s = n R ............(i)
In parallel combination effective resistance decreases, so minimum resistance is given by
$\frac{1}{\mathrm{R_p}}=\frac{1}{R}+\frac{1}{R}+\frac{1}{R}+............=\frac{n}{R}$
or, R_p = $\frac{R}{n}$ ..................(ii)
where R_s and R_p be the effective resistance in series and parallel combination.
Dividing Eq. (i) by Eq. (ii), we get
$\frac{\mathrm{R_s}}{\mathrm{R_p}}=\frac{\mathrm{n\; R}}{\mathrm{R/n}}=n²$
The required ratio = n².
24. You are given 2 wires each of resistance R. What is the ratio of maximum to minimum resistance that can be obtained from these wires?
Ans : In series combination effective resistance increases and in parallel combination effective resistance decreases. Therefore, when 2 wires each of resistance R are connected in series, the maximum resistance is obtained and when they are connected in parallel minimum resistance is obtained.
R_s = R + R = 2R
i.e. R_s = 2R ................(i)
R_p = $\frac{\mathrm{R\; \times \; R}}{\mathrm{R\; +\; R}}=\frac{R}{2}$
i.e. R_p = $\frac{R}{2}$ ................(ii)
where, R_s and R_p be the effective resistance in series and parallel combination.
Dividing Eq. (i) by (ii), we get
$\frac{\mathrm{R_s}}{\mathrm{R_p}}=\frac{\mathrm{2R}}{\mathrm{R/2}}$ = 4
the required ratio of R_s and R_p is 4:1.
25. A wire is stretched to double its length. What happens to its resistance and resistivity?
Ans : If R be the resistance of the wire of rsistivity ρ whose length is l and area of cross section is A. Then resistance
R = $\rho \frac{l}{A}$.
Therefore, R = $\rho \frac{\mathrm{l²}}{\mathrm{A\; l}}$
or, R = $\rho \frac{\mathrm{l²}}{v}$, where A l = v is the volume of the conductor
If the wire is stretched to double its length, then the new length is l' = 2 l and the new resistance R' can be written as
R' = $\frac{\mathrm{\rho \text{'}\; l^\text{'}2}}{\mathrm{v\text{'}}}=\frac{\mathrm{\rho \; (2l)²}}{v}$, where, v =v' because volume remains constant.
Also ρ' = ρ, because the material of the conductor is same
Therefore R' = $\frac{\mathrm{4\; l²\rho }}{v}=\frac{\mathrm{4\; l²\rho }}{\mathrm{A\; l}}=4\; R$ [since R =$\frac{\mathrm{4\; l²\rho }}{\mathrm{A\; l}}$]
Thus, when a wire is stretched to double its length, its resistance becomes 4 times of the original resistance. Since the resistivity depends upon the material of the conductor, therefore the resistivity remains same for the same conductor.