The charged particles whose flow in a definite direction constitutes the electric current are called current barriers.
Current carriers in solid conductors. In solid conductors like metals the valence electrons of the atoms do not remain attached to individual atoms but are free to move throughout the volume of the conductor. Under the effect of an external electric field, the valence electron moves in a definite direction causing electric current in the conductors. Thus valence electrons are the current carriers in solid conducts.
Current carriers in liquids. In an electrolyte like CuSO4, NaCI etc, there are positively and negatively charged ions (like Cu ++ SO4 ---, Na+, CI-) there are forced to move in definite directions under the effect of an external electric field causing electric current. Thus in liquids, the current carriers are positively and negatively charged ions.
Current carriers in gases. Ordinarily the gases are insulators of electricity. But they can be ionized by applying a high potential difference at low pressures or by their exposure to X rays etc. the ionized gas contains positive ions and electrons. Thus positive ions and electrons are the current carriers in gases.
Electric current is possible in a closed circuit if there is a source of external force (in fact a source of energy) which compels the current carriers (electrons or ions) to move in a definite direction (from lower potential energy to higher potential energy) this external force which makes the current carriers to move in a definite direction is called electromotive force
The electromotive force is associated with an arrangement or mechanism which can supply energy or does work to move the electric charge from lower potential energy to higher potential energy. Such an arrangement is called a source of which may be a cell, a battery, a generator or dynamo. It is a seat of power.
An electrolytic cell consists of two electrodes called positive electrode (p) and negative electrode (N) and an electrolyte. When the electrodes are immersed in electrolyte they exchange charges with the electrolyte. The positive electrode acquires positive potential V+ w.r.I electrode solution adjacent to it. The negative electrode acquire is negative potential – v relative to the electrolyte solution adjacent to it. When no current is drawn from the cell, then the maximum potential difference between the two electrodes of the cell (called emf of the cell) is
ε = v+ - (- v-) v + + u- > 0
E.M.F. of a cell is defined as the maximum potential difference between the two electrodes of the cell when no current is drawn from the cell or cell is in the open circuit. It may also be defined as the energy supplied by the cell to drive a unit charge once around the complete circuit and is given by
E= ?E . dl
Where E is the electric field (force per unit charge) and dl is the small path segment vector.
EMF of a cell is also equal to the sum of the potential drops across the various resistors in the closed circuit. Symbiotically a cell is represented with tow unequal parallel lines having a small separation. The taller line represents the positive electrode P and smaller line prep resents the negative electrode N.
The S.I unit of emf or a cell is volt (V) or joule per coulomb (JC-1) the emf of a cell is said to be one volt if l joule energy is supplied by the cell to drive one coulomb of charge once around the whole circuit (including the cell).
The emf of a cell depends upon the nature of electrodes nature and the concentration of electrolyte used in the cell and its temperature.
Mechanical analogy of emf
Consider iron balls be dropped at a steady rate thorough a vertical column of a ciscoes liquid
A man lifts each iron ball from the floor and transfers to the tip. The balls thus go through the closed path at a steady rate in the same manner as the electric charges flow through a closed circuit at a steady speed causing a steady current. The work done by the man on the balls while transferring the form bottom to top is at the cost of his own internal chemical energy. Here man is the source of emf.
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