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Miscellaneous

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15.8Miscellaneous


In these revision notes for Miscellaneous, we cover the following key points:

  • How do we measure the current in a circuit?
  • How do we measure the potential difference in a circuit?
  • What is superconductivity?
  • What is the condition for a material to become a superconductor?
  • How is the current produced in liquids?
  • What is electrolysis?
  • What does the Faraday's Law of Electrolysis say?
  • How to calculate the amount of substance produced through electrolysis?
  • How the current is produced in gases?

Miscellaneous Revision Notes

Ammeter is used to measure the current in the circuit; it can be placed at any position of the conductor since the current is the same at every point of a circuit branch. An ammeter has a very low internal resistance.

Voltmeter is used to measure the potential difference across the terminals of a circuit element; it must be connected in parallel to it that is one wire of voltmeter is connected before the appliance and the other wire after the appliance. This is because the voltmeter has a very high resistance in order to affect as less as possible the value of current flowing in the circuit.

A superconductor is a material whose resistance is very low, close to zero. It is known that the resistance of material largely depends on its temperature, i.e. higher the temperature of material, higher its resistance. Therefore, a material will behave as a superconductor only at very low temperatures (close to the absolute zero).

If scientists are able to produce a strong superconductor with a higher transition temperature than the actual materials, this will bring a revolution in technology.

Electric current can flow in liquids as well. Pure water does not conduct electricity. However, we can add some acidic element in water to turn it into a conductor. The cheapest and most accessible material for this purpose is the table salt. The liquid in which two solid electrodes are immersed is known as electrolyte.

Michael Faraday introduced an equation which shows the relationship between the amount (mass) m of substance decomposed to the anode (deposited in the cathode) and the current I flowing through the electrolyte. This equation, known as the Faraday's Law for Electrolysis is

m = k ∙ Q = k ∙ I ∙ t

where k is a constant known as the electrochemical constant of electrolysis. It has the unit of [kg/A ∙ s]. I shows the current and t the time during which the electrolysis takes place.

The acid added in water is not the only material that is decomposed. Water also decomposes in two separate elements: hydrogen and oxygen. The salty water simply increases the conductive ability of water. The chemical reaction that takes place during the water electrolysis is

H2 O ⟶ H2 + 1/2 O2

The process of electrolysis is used to obtain pure hydrogen or oxygen from water decomposition.

In normal conditions, gases are insulators. When a gas is heated, the kinetic energy of its molecules increases. When this kinetic energy exceeds the binding energy (i.e. the energy that keep electrons bound with the atoms), electrons escape from atoms and as a result, atoms begin to ionise. If the temperature of gas is high enough, the gas becomes a mixture of positive ions and free electrons. Such a state is known as plasma and it represents the fourth state of matter besides solid, liquid and gas. A material in plasma state is a good conductor of electricity as it contains a lot of free charges.

Heating the gas is not the only method to obtain free charges. Electromagnetic radiation is another factor that produces electric current in metals due to ionization of gas particles, as shown in the figure below.

A third factor that can cause gas ionization (and therefore an electric current) is a high potential difference produced due to the existence of an electric field inside the gas. For this, a power source such as a battery is needed. Electrons accelerate due to the electric field and gain a high kinetic energy over a small distance. When electrons strike the gas atoms, they pull out new electrons from atoms turning them into positive ions. This kind of ionization is called discharge by self-ionisers.

The phenomenon of discharge by self-ionisation is used to construct light bulbs and self-illuminating structures such as billboards, light tubes etc.

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