1.A current is passed through a steel wire, heating it to red hot. The half of the wire is immersed in cold water. Which half of the wire will heat up more and why?
When current flows through the wire, heat is produced. The heat produced in the portion of the wire immersed in cold water is immediately transferred to the water. So the rise of temperature of immersed portion is less than the outside portion of the wire. Due to the high temperature of the outside portion of wire, its resistance increases considerably and hence more heat will be produced in it. Thus the portion of the wire outside the water will heat up more.
2.Is there any connection of solar winds or cosmic rays with electric current?
Yes, solar winds are strong electric currents having electrons, protons and ions as the current carriers. Similarly cosmic rays are also high energy electric currents mainly due to protons as current carriers.
3.Is Ohm's law applicable to all conductors of electricity?
No, Ohm's law is applicable to only to those conductors whose current - voltage graphs are linear that is a straight line hence its resistance R is independent of V and I. Metals obey Ohm's law because they have V - I graphs in the form of straight line. For vacuum tubes, electrolytes etc, V-I graph is non linear and so they do not obey Ohm's law.
4.Why alloys are used for preparing standard resistance coils?
In the case of most alloys, the variation of resistance with change of temperature is very small as compared to the variation in the case of metals. That is the temperature coefficient of resistance is very small in the case of alloys. Certain alloys such as manganin, constantan and nichorme are least affected by temperature changes. In addition to having low value of temperature coefficient of resistance alloys have high resistance. Due to both these reasons alloys are used in preparing standard resistance coils.
5.At room temperature (27.0°C) the resistance of a heating element is 100 Ω. What is the temperature of the element if the resistance is found to be 117 Ω given that the temperature coefficient of the material of the resistor is 1.70 × 10-4 °C-1?
R2 = R1 [1 + (T2 - T1)]
117 = 100 [1 + 1.7 × 10-4 (T2 - 27)]
T2 = 1027 °C
6.Is electrical conductivity linked with thermal conductivity?If so how they are linked, Why?
The ratio of thermal conductivity and electrical conductivity is the same for all metals at a particular temperature and is proportional to the absolute temperature of the metal. This law is known as Wiedemann–Franz law. Electrical conductivity is related to thermal conductivity because the free electrons are responsible for the both types of conductivity.
7.A negligibly small current is passed through a wire of length 15 m and uniform cross-section 6.0 × 10-7m2 and its resistance is measured to be 5.0 Ω. What is the resistivity of the material at the temperature of the experiment?
I = 15m, A = 6.0 × 10-7 m2 ,
R = 5.0 Ω; ρ = ?
We know that
8.A heating element using nichrome connected to a 230 V supply draws an initial current of 3.2 A which settles after a few seconds to a steady value of 2.8 A. What is the steady temperature of the heating element if the room temperature is 27.0°C? Temperature coefficient of resistance of nichrome averaged over the temperature range involved is 1.70 × 10-4 °C-1 .
9.There is an impression among many people that a person touching a high power line gets struck with the line. Is that true? Explain.
This is a misleading notion. There is no special attractive force that keeps a person struck with a high power line. What happens is that a current of the order of 0.05 ampere or even much less is enough to disorganise our nervous system. The result is that the affected person may lose temporarily his ability to exercise nervous control to get himself free from the high voltage point.
10.In a potentiometer arrangement, a cell of emf 1.25 V gives a balance point at 35.0 cm length of the wire. If the cell is replaced by another cell and the balance point shifts to 63.0 cm, what is the emf of the second cell?
Given E1 = 1.25 V ; I1 = 35.0 cm
E2 = ? ; I2 = 63.0 cm
11.(a) Six lead-acid type of secondary cells each of emf 2.0 V and internal resistance 0.015 Ω are joined in series to provide a supply to a resistance of 8.5 Ω. What are the current drawn from the supply and its terminal voltage?
(b) A secondary cell after long use has an emf of 1.9 V and a large internal resistance of 380 Ω. What maximum current can be drawn from the cell? Could the cell drive the starting motor of a car?
(a) The total potential difference generated by the six cells is
V = 2 × 6 = 12 volt
The total internal resistance of six cells is
r = 6 × 0.015 = 0.09 Ω
R = 8.5Ω, I = ?, terminal voltage = ?
Applying Ohm's law V = I(R + r)
Therefore, 12 = I (8.5 + 0.09)
Terminal voltage = Voltage drop across R
= 1.4 × 8.5
= 11.9 V
The cell cannot drive the starting motor of a car as it requires a large current of around 100 A for a few seconds.
12.Describe super conductivity.
With the decrease in temperature, the resistivity of conductors decreases and conductivity increases. Near absolute zero resistivity becomes almost zero and conductivity becomes very high. This large conductivity near absolute zero is called, super conductivity.
At low temperature, free electrons becomes mutually coherent, and hence not deflected by ionic vibrations.
In recent days more and more substances have been discovered which exhibit superconductivity even at higher temperature. Days are not far off, when we may hear about substances super conducting at room temperature. Their use will set aside losses due to heating of substances.
13.What is meant by Avometer or Multimeter?
This is a commercial instrument combining ammeter, voltmeter and ohm - meter in one instrument, hence the name avometer. This contains a moving coil galvanometer, having several scales reading in amperes, volts and ohms. For several ranges, suitable resistance are provided inside, which get into the circuit by inserting the plug in the desired socket and the instrument begins to read either as an ammeter or as a voltmeter or as an ohm-meter of that range. This instrument is thus very compact and useful for the laboratory for detecting faults in radiosets and for other testing purposes.
14.What should be the properties of a good standard resistance?
The properties of a good standard resistances are (i) permanance (ii) negligible variation with temperature (iii) robust construction and convenient size (iv) very low inductance (v) capacity for carrying current without over-heating.
15.What are ohmic and non-ohmic resistances?
The conductors which obey Ohm's law are called ohmic conductors or linear resistor. For such conductors the graph between current and potential difference is a straight line. Resistances of these conductors are independent of potential difference and current.That is the resistance of a conductor remains constant when current is increased or decreased.Example Ag, Cu, AI etc.
The conductors which does not obey Ohm's law are called non-ohmic conductors or non-ohmic resistances. Here the graph between current and potential difference is not a straight line. When the current is varied, the resistance of non-ohmic resistors will vary. So they are also known as dynamic resistors.
16.A voltmeter, an ammeter and a resistance are connected in series with a lead accumulator. The voltmeter gives some deflection but the deflection of ammeter is negligible. Explain why this happens.
Since the voltmeter is connected in series the circuit resistance becomes very high and hence the current is considerably lowered. This current tends to deflect the voltmeter needle slightly but a larger portion of this current goes through the shunt of the ammeter. Thus current in the ammeter coil is too weak to deflect it.
17.What is the importance of fuse wire in a circuit?
A fuse is used in the electric circuit to protect it from high current due to overloading, short circuiting etc. A fuse wire has low melting point and high resistance. Hence if a high current is passed through it, heat is developed and immediately the wire melts before the circuit gets damaged.
Practice in Related Chapters
|Electromagnetic Induction and Alternating Current|
|Electromagnetic Waves And Wave Optics|
|Dual Nature of Radiation and Matter - Relativity|
|Semiconductor Devices and their Applications|
|Effects of Electric Current|