ELECTRIC CURRENT
CONDUCTOR
In some materials, the outer Electrons of each atoms or molecules are only weakly bound to it. These electrons are almost free to move throughout the body of the material and are called free electrons. They are also known as conduction electrons. When such a material is placed in an electric field, the free electrons move in a direction opposite to the field. Such materials are called conductors.
INSULATOR
Another class of materials is called insulators in which all the electros are tightly bound to their respective atoms or molecules. Effectively, there are no free electrons. When such a material is placed in an electric field, the electrons may slightly shift opposite to the field but they can’t leave their parent atoms or molecules and hence can’t move through long distances. Such materials are also called dielectrics.
SEMICONDUCTOR
In semiconductors, the behaviour is like an insulator at low levels of temperature. But at higher temperatures, a small number of electrons are able to free themselves and they respond to the applied electric field. As the number of free electrons in a semiconductor is much smaller that in a conductor, its bahaviour is in between a conductor and an insulator and hence, the name semiconductor. A freed electron in a semiconductor leaves a vacancy in its normal bound position. These vacancies also help in conduction.
ELECTRIC CURRENT, VELOCITY AND CURRENT DENSITY
| n → number of free charge particles per unit volume
q → charge of each free particle i → charge flow per unit time V → drift velocity i = nqvA |
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Current density – A vector, at a point have magnitude equal to current per unit normal area at that point and direction is along the direction of the current at that point.
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Current is flux of current density. Due to principle of conservation of charge:
Charge entering at one end of a conductor = charge leaving at the other end, so current does not change with change in cross section and conductor remains unchanged when current flows through it.
MOVEMENT OF ELECTRONS INSIDE CONDUCTOR
| All the free electrons are in random motion due to the thermal energy and relationship is given by
At room temperature its speed is around 106 m/sec or 103 km/sec but the average velocity is zero so current in any direction is zero. |
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When a conductor is placed in an electric field. Then for a small duration electrons do have an average velocity but its average velocity becomes zero within short interval of time |
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When by some means a constant potential difference is applied across the conductor. Then the electrons start moving with an acceleration and due to collision with other atoms, electrons, its average velocity becomes nearly constant and is called as drift velocity. The electric field between the plate = E =v÷l |
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Vd = drift velocity = average velocity along the wire
hence i = nAeVd
Vd is of the order 10-3 m/s.
Illustration–1
Find the number of free electrons per unit volume in a metallic wire of density 104 kg/m3, atomic mass number 100 and number of free electron per atom is one.
Solution
Illustration–2
Find the approximate total distance traveled by an electron in the time–interval in which its displacement is one meter along the wire (speed of electron is 106 m/s).
Solution
RELATION BETWEEN I & V IN A CONDUCTOR
In absence of potential difference across a conductor no net current flows through a cross section. When a potential difference is applied across a conductor the charge carriers (electrons in case of metallic conductors) start drifting in a direction i.e., opposite to electric field with average drift velocity. If electrons are moving with velocity vd, A is area of cross section and n is number of free electrons per unit volume then,
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