Current electricity
is a branch of physics and electrical engineering in which we study the behaviour
of charges in motion, i.e. moving charges. In current electricity, we consider
the flow of electric charges. It is
a unique characteristic of electric charge that it is mobile, i.e. it can be
transferred from one place to another.
When a conducting wire is connected to a source of electromotive force like a cell or
battery, the charges in atoms of the conductor wire are forced to move, where
positive charges move in one direction whereas negative charges move in the
opposite direction. This movement of electric charges creates an electric
current in the conductor wire.
It is conventional to take the flow of electric current as
the movement of positive charges which is opposite to the flow of negative
charges. This convention was first introduced by American scientist and inventor Benjamin Franklin.
But we now know that the flow of electric current in
metallic conductors is only due to negatively charged particles, i.e. electrons. Although, we still use the
universally accepted convention that the electric current is the net flow of
positive charges.
Electric Current
The net flow of free electrons in a conducting wire in a
particular direction is known as electric
current. Mathematically, the electric
current is defined as the time rate of flow of electric charges (or
electrons).
The electric current is denoted by symbols I or i.
Where, the symbol I is generally used
to denote a constant electric current, while the symbol i is used to denoting a time-varying current.
From the definition of electric current, we can write the expression of the electric current as
`\I=\frac{Q}{t}`
Where Q is the total charge in coulombs and t is time in seconds.
In differential form, the electric current
is given by,
Since, according to the quantization theory of electric charge, we have,
Q = n e
Where n = 0, 1, 2, 3,… and e is the charge on an electron.
`\e=1.6×10^{-19}" C"`
Therefore, the electric current can also be given by,
`\I=\frac{n e}{t}`
Direction of Electric Current
We can easily understand the direction of
the flow of electric current with the help of the above circuit. Here, we have
taken a copper conductor that has a large number of free electrons. When an
electric pressure or voltage is applied across the conductor, the free electrons
being negatively charged will start moving towards the positive terminal of the
source through the connected wires. This directed flow of electric charge or
electrons is termed electric current.
From this discussion, we understand
that the electric current is the flow of negatively charged electrons through
the conductor. Hence, the actual direction of electric current is from the source's negative terminal to the source's positive terminal through
the circuit. This current is known as actual
current or electron current.
However, before the knowledge of the
electron theory of matter, it was
considered that the electric current flows from the positive terminal to the
negative terminal of the source. This assumed direction of the electric current
is known as conventional current.
Unit of Electric Current
Since the electric current is expressed as the ratio of charge and time, i.e.
`\I=\frac{Q}{t}`
Therefore,
`\"Unit of current"=\frac{"Coulomb"}{"Second"}`
Where,
`\1" C⁄s"=1" Ampere (A)"`
Thus, the SI unit of electric current is Ampere, denoted by A. It is named in honour of French mathematician and physicist
Andre-Marie Ampere.
One
ampere of current is said to flow through a
conductor if at any cross-section one coulomb of charge flows in one second.
Types of Electric Current
Depending on the fashion in which the
flow of electric charge takes place, the electric current is classified into
two major types:
- Direct Current (DC)
- Alternating Current (AC)
Direct Current
The type of electric current which always
flows in the same direction, where it does not reverse (or change) the
direction is known as direct current or
DC. Since it always flows in the
same direction so it is also known as a unidirectional
current. The direct current (DC) can be constant or time-varying.
By convention, we use the symbol I to represent a constant direct current
and the symbol i to represent the
time-varying direct current. The following figure graphically illustrates the
constant direct current and time-varying direct current.
The constant DC is usually obtained from the cells and batteries, whereas the time-varying DC is obtained at the output of the rectifier.
Alternating Current
An electric current whose magnitude
changes continuously and direction changes periodically is known as alternating current or AC. In practice, due to some technical
and economic reasons, we produce alternating currents that have sinusoidal
waveform (i.e. sine wave or cosine wave). The following figure represents an alternating
current having a sine waveform.
The electric current produced by an AC generator (or alternator) is an alternating current that usually has a sinusoidal waveform.
Numerical
Example (1) – How much current is flowing in a
circuit where 2.35 ×
1014 electrons move past a given point in 80 ms?
Solution – Given data,
`\n=2.35×10^{14}" electrons"`
`\t=80" ms"=80×10^{-3}" s"`
The electric current is given by,
`\I=n e/t`
`\⟹I=((2.35×10^{14} )×(1.6×10^{-19} ))/((80×10^{-3} ) )`
`\∴I=4.7×10^{-4}" A"`
Numerical
Example (2) – How long does it take 100 µC of charge to pass a point in a
circuit if the current flow is 20 mA?
Solution – Given data,
`\Q=100" μC"=100×10^{-6}" C"`
`\I=20" mA"=20×10^{-3}" A"`
`\∵I=Q/t`
`\∴"Time",t=\frac{Q}{I}`
`\⟹t=\frac{100×10^{-6}}{20×10^{-3} }`
`\∴t=5×10^{-3}" s"=5" ms"`
Numerical Example (3) – The electric current in a certain
conductor is 50 mA. Find the total charge in coulombs that passes through the
conductor in 2 seconds.
Solution – Given data,
`\I=50" mA"=50×10^(-3)" A"`
`\t=2" sec"`
`\∵I=Q/t`
`\∴"Charge",Q=It`
`\⟹Q=50×10^(-3)×2`
`\∴Q=0.1" A"`
Conclusion
Now, we can conclude this article as we discussed that electric current is a directed flow of electrons. Although, it is a convention that the current is the flow of positive charges. The SI unit of electric current is Ampere. The electric current can flow through the conducting materials only. Electric current is broadly classified into two types namely direct current (DC) and alternating current (AC).
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