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61. Why is greater length of the potentiometer wire used?


As the potentiometer is based on the principle that the potential difference V across a portion of the wire is directly proportional to the length l of the portion of the wire i.e.


Potentiometer follows a null method. So, grater the length, the smaller is the potential gradient. The distance of the null position will thus increased which gives more accuracy in measuring emf. Hence, longer the wire, greater is the sensitivity of the potentiometer so greater length of the potentiometer is used.


62. Why should the cross-section of a potentiometer wire be uniform?
For the accuracy in the measurement using potentiometer, potential gradient throughout the length of potentiometer wire must be same. The uniformity of the potentiometer wire ensures a constant value of resistance per unit length which results its potential gradient throughout its length will be same and there will be accuracy in the measurement using that potentiometer. 
63. Is it true that Seebeck effect is the resultant of Peltier and Thompson effect? Discuss.
Yes, it is true that Seebeck effect is the resultant of Peltier and Thompson effect. Let us consider a Fe-Cu thermocouple in which if one junction is kept at higher temperature than other then it shows both Peltier and Thomson effect. For Fe, the direction of Thomson emf is from hot to cold junction and in Cu the direction of Thomson emf is from cold to hot junction. The electric field field due to Peltier effect at both the junctions are from Cu to Fe. The resultant of these two emf in the closed circuit is the Seebeck effect.

64. How does thermoelectric emf change in a thermocouple when the temperature of the hot junction is changed?


When the cold junction of a given thermocouple is kept constant at 00C and the hot junction temperature θ0C is varied, the thermoelectric emf E is found to vary as


where a & b are constants.


It give rise to a parabola shaped curve. At a certain temperature it becomes maximum and that temperature is called neutral temperature. On further increasing temperature it decreases and becomes again zero. After this temperature emf is negative, which is called inversion temperature.




65. What do you mean by thermoelectric series? Write down the series.


Seebeck arranged a large number of metals in the form of following series such that if any two of them form a thermocouple, the current will flow from the metal earlier in series to the metal later in series through cold junction.

     Sb, Fe, Zn, Ag, Au, Mo, Cr, Sn, Pb, Hg, Mn, Cu, Co, Ni & Bi

This series of arrangement of the above metals is known as the thermoelectric series.



66. Why is Sb-Bi thermocouple preferred to Fe-Cu thermocouple?

For a given temperature difference between the two junctions, the magnitude of thermo-emf produced by thermocouple will be maximum, if two metals forming thermocouple are spaced more apart in thermoelectric series. The thermoelectric series is given as:

            Sb, Fe, Zn, Ag, Au, Mo, Cr, Sn, Pb, Hg, Mn, Cu, Co, Ni & Bi

Here, Sb-Bi thermocouple are spaced more than Fe-Cu thermocouple. So, maximum thermo-emf can be produced by Sb-Bi thermocouple. Hence, Sb-Bi thermocouple is preferred to Fe-Cu thermocouple.


67. Is electrolysis possible with alternating current? Why?

No, electrolysis is not possible with alternating current since the direction of current changes periodically which prevents the ions reaching towing electrodes (positive ions flow towards cathode whereas negative ions flow towards anode). So, electrolysis is not possible with alternating current.



68. Is Ohmís law obeyed in copper and water voltameters?

Yes, Ohm's law is obeyed in copper voltameter because in copper voltameter, there is no back emf to oppose & the relation between the potential difference and current gives the straight line which follows Ohm's law.

In case of water voltameter, no current flows through it when back emf is greater than the emf of the source. But when the emf of the source is greater than back emf then the net emf is obtained which when relates with the current gives the straight line. Hence, water voltameter obeys Ohm's law only when emf of source exceeds the back emf.


69. Why is the conductivity of an electrolyte low as compared to that of a metal?
The conductivity of an electrolyte is low as compared to that of a metal because of the following reasons:

1. As the electric current is conducted by ions in the electrolyte so the conductivity of ions depends on the density of free ions. The density of free ions in an electrolyte is much less than that of free electrons in metals.

2. The drift velocity of ions is much less than that of the free electrons.

3. Resistance offered by solution of ions is much less than that of free electrons.


70. Why does the light of a torch glow dim when put on for a long time and why does it begin to burn brightly when it is switched off for a short time and then switched on?

A torch glows due to the chemical energy of dry cell converted into light energy according to law of conservation of energy. If current from a dry cell is continuously taken, polarization phenomena occurs & the current drawn by torch will be less. Hence, the light of a torch glow dim when put on for long time.

If it is switched off for a short time then the hydrogen gas resulted due to polarization is oxidized by the use of MnO2. Hence, a torch burn brightly when it is switched for a short time and then switched on.


71. Why do oppositely charged ions not combine in the electrolyte to form neutral molecules?

Ions are formed when the electrolyte is dissolved in water. So, due to the ionization process, the electrolytes dissociates into its component ions in the solution. The total number of positive ions is always equal to the number of negative ions in the solution. As a result of which the whole solution remains neutral. But, when electrolyte is dissolved in water then water also dissociates into its component ions i.e. hydrogen ion ( positive ion) and hydroxyl ion ( negative ion) into some extent. Positive ions and negative ions are attacked by hydroxyl ions and hydrogen ions of water which results in the erode of the electrolytes. So, oppositely charged ions do not combine in the electrolyte to form neutral molecules.


72. Why the lines of force in an electric field start at the +ve charge and terminate at the -ve charge?

It's purely by definition. It was originally thought that the 'body' that carried electric charge in a current conducting wire was positive, but of course we all now know that that body is the electron which carries a negative charge. So in fact, if we say a current is flowing from left to right, what we actually mean is that negatively charged electrons are moving from right to left.
Lines of force don't REALLY start at the +ve charge and end on the -ve charge! In fact, we don't know what way they go - they might be going the opposite way. So, to make sure that everyone agrees with them travelling in a certain direction, we define what an electric field line is.
We define it as 'The line along which an isolated positive charge would move if it were free to do so'. Now, if you consider the situation with like charges repelling and unlike charges attracting, that must mean that if you start with a positive charge and a negative charge separated by a distance, and then place another positive charge beside the first one, it will be repelled by this -ve charge and attracted by the negative charge. So, the direction in which this single positive charge will move is from positive to negative.
But this is only a convention adopted by scientists so that everyone is talking about the same thing. If you plot the magnetic field of a magnet using Iron Filings, you see 'lines' going from one pole of the magnet around the other. But there are no arrows showing you the actual direction. So, scientists agreed that to keep everyone talking about the same thing, that magnetic lines of force should travel from North to South. It is exactly the same for electric field lines and the definition of a magnetic field line is almost exactly the same as that of an electric field line, except it refers to isolated North Poles instead of isolated +ve charges.


73. Why do clouds darken to a very deep grey just before it is about to rain or prior to a heavy thunderstorm?

Clouds darken from a pleasant fluffy white just before rain begins to fall because they absorb more light.

Clouds normally appear white when the light which strikes them is scattered by the small ice or water particles from which they are composed. However, as the size of these ice and water particles increases--as it does just before clouds begin to deposit rain--this scattering of light is increasingly replaced by absorption.

As a result, much less light reaches the observer on the ground below and the clouds look darker.

74. Why is it that birds are able to stand/sit on electrical power lines whereas if we were to try this, we would be electrocuted?

Actually, if we were to sit on a power line and not touch anything else like the birds do, we would not get electrocuted! The reason for this has to do with the fact that current, a flow of electrons, flows along a path of least resistance. The electrons want to get to where they are going in the easiest possible way; much like a person might walk on the sidewalk instead of the grass because it is easier.

When a bird sits on a wire and the electrons reach the part of the wire where the bird is sitting, the electrons have two options. They could go through the bird's feet and encounter a large amount of resistance or they could go through the metal. All metals are conductors which mean that electrons flow through them easily. Because it is easier to travel through the metal instead, the electrons don't go through the bird's feet, so the bird stays safe.

The same thing would happen if a person sat on a wire and the electrons would go through the wire instead of through the person. If, however, the person reached out to a tree or anything else connected with the ground, there is a new path of lower resistance and the electrons would go through the person to the ground, electrocuting him.


75. Where do electrons get the energy to travel at such high speeds?

Electrons are charged particles. That is, they carry an electric charge. For this reason, they are influenced by electric fields. More precisely, they are accelerated in an electric field.

Since the mass of an electron is so very small compared with objects of ordinary experience, electrons are accelerated to very high velocities even by electric fields of only a few volts [per meter].

For example, the electrons in orbit in an atom have been accelerated through an electric field of only a few volts created by the positively charged nucleus as they 'fall' into the atom and are captured in orbit. Just these few volts are enough, due to the extremely small mass of the electron [mass of the electron is = 9.10938 ◊ 10-31 kg], to result in the electron attaining orbital speeds that, in some cases, may be an appreciable fraction of the speed of light.

Put another way, the fact that electrons usually seem to travel at very high speeds is not, as one might otherwise think, an indication of great energy.

It is because the mass of the electron is so small that its speed will be very great even when the electron has absorbed only a very small amount of energy.

To be more precise, for a non-relativistic particle, the speed v is given by
v =
where E is the kinetic energy and m the mass of the particle. As you can see, if m is very small, v may be very large for a modest energy E.


76. A comb run through oneís dry hair attracts small bits of paper, why?

When a comb runs through one's hair, it gets charged or electrified due to friction. It bears negative charge and when that comb is brought near a piece of paper, the electron on each atom of paper are repelled and positive charge being near to the comb there is a force of attraction developed between the comb carrying negative charge and paper.


77. It is difficult to produce charge on nylon rod if it is rubbed between your fingers, why?

As the human body is comparatively a good conductor with respect to many other substances, if it is tried to produce charge on nylon rod by rubbing it between the fingers, it won't work. It is because the charge produced in the nylon rod is conducted away through our body to the earth (ground). Hence, it is  difficult to produce charge on nylon rod if it is rubbed between your fingers.


78. An iron chain is suspended from a vehicle carrying inflammable materials. Why?

Vehicles carrying inflammable material have large metallic tank. The tank gets charge due to friction with air. As the charge cannot be transfer to the ground because rubber tyres are insulator. Due to huge charge and inflammable material, there is chance of explosion. If an iron is suspended from a vehicle then all the charges flow to the ground which reduced the risk of the explosion. Hence, an iron chain is suspended from a vehicle carrying inflammable materials.


79. Why are the tyres of aircraft made slightly conducting?

As the aircraft also contains large metallic tank. The tank of the aircraft gets charge due to friction with air. But dragging of a chain like in vehicles is not possible in the aircrafts. So, the tyres of aircrafts are made slightly conducting in order to make flow of the charge to the ground which reduces the risk of the explosion of that aircraft. Hence, the tyres of aircraft are made slightly conducting.


80. If a person intends to produce high voltage on his body without getting a shock, how can he do so?

If a person intends to produce high voltage on his body without getting a shock. This is only possible when he is not in contact with the ground. That means he should have wear shoe which behaves as an insulator so there is blockage of the flow of the electrons from his body to the ground i.e. earth.


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