D.C circuit
Research findings in the modern electron theory reveal the matter, whether solid, liquid or gas consists of minute particles called molecules. These molecules are made up of minute particles called molecules. These molecules are made up of atoms which can be further split in to electron, protons and neutrons. The electrons revolve around the nucleus containing protons and neutrons. The electron present in the outermost orbit experience a very weak force of attraction for the obvious reason that according to coulomb’s law, the force between two charges varies inversely with the square of the distance. These electrons which are very loosely attached to the atom are called free electrons. They wander in random directions from one atom to another. If this random movement of electrons assumes steady flow, it causes electric current.
ELECTRIC CURRENT
The continuous flow of electrons constitutes electric current. It is denoted by ‘I’ and is measured in amperes.
‘I’ is also given by
I = dq/dt coulomb/second …………………………………………………….(i)
Milliampere and micro ampere are the smaller units of current, where
1 milliampere = 1 * 1/10 to the power 3 amperes
1 microampere = 1 *1/10 to the power 6 amperes
ELECTRIC POTENTIAL
The electric potential at any point in an electric field is defined as the work done (W) in bringing a unit positive charge (Q) from infinity to that point against the electric field.
V =W/Q Joules/coulomb ………………………………………………………….. (ii)
But, joules/ coulomb is also given by ‘volts’
Thus, the unit of electric potential is ‘Volts’. The smaller units of potential are millivolts and microvolts. The larger units of potential are Kilovolts and megavolts.
Thus 1 millivolt = 1/ 10 to the power 3 volts
1 microvolts = 1/ 10 to the power 6 volts
1 kilovolt = 10 to the power 3 volts
1 megevolt = 10 to the power 3 volts
RESISTANCE
It is the property of the conductor by which it opposes the flow of current. It is denoted by R and its unit is ohm. (Ω)
LAWS OF RESISTANCE
The resistance of a conductor
1. Varies directly with length (l)
2. Varies directly with its cross sectional area (A)
3. Depends on the nature of the material
4. Depends on the temperature
Thus, from the first two laws we have
R is directly proportional to l
And R is inversely proportional to A
From (i) and (ii) we have
Resistance(R) is inversely proportional to current/Area = I/A
=>R = specific resistance (ρ)* current (I) / Area (A)
Where ρ is called specific resistance
SPECIFIC RESISTANCE
It is defined as the resistance offered by unit cube of the material between its opposite faces. Its unit is ohm–meter.
Specific resistance = Resistance*Area/Current
TEMPERATURE EFFECT ON RESISTANCE
In the case of pure metals, the resistance increases with increases in temperature. In the case of alloys, the increase in resistance with increase in resistance is relatively small and irregular. The resistance of electrolytes and insulators decreases, with increase in temperature.
TEMPERATURE COEFFICIENT OF RESISTANCE
It is denoted as the change in resistance per ohm per degree change in temperature from 0°c. If a material has resistance of R0, R1 and R2 at temperature of 0 °C, t1 °C, and t2 °C respectively, then
R1 = R0 (1 + α0t1)
R2 = R0 (1 + α0t2)
=>R2 =[(1+α0t)/(1+α0t1)]R1
Where α0 is the temperature coefficient of resistance of the conductor at o° c.
The above expression can be further simplified using Binomial theorem and can be rewritten as
R2 = R1 (1 + α0 (t2-t2))
Knowledge the value of ‘α0’ we can find the value of ‘α’ at any temperature‘t’
Thus αt= α0/1+α0/α0*t
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