Electric Current
The directed flow of free electrons (or charge) is called electric current. The flow of electric current can be beautifully explained by referring to. The copper strip has a large number of free electrons. When electric pressure or voltage is applied, then free electrons, being negatively charged, will start moving towards the positive terminal around the circuit as . This directed flow of electrons is called electric current. The reader may note the following points :
(i) Current is flow of electrons and electrons are the constituents of matter. Therefore, electric current is matter (i.e. free electrons) in motion.
(ii) The actual direction of current (i.e. flow of electrons) is from negative terminal to the positive terminal through that part of the circuit external to the cell. However, prior to Electron theory, it was assumed that current flowed from positive terminal to the negative terminal of the cell
via the circuit. This convention is so firmly established that it is still in use. This assumed direction(ii) The actual direction of current (i.e. flow of electrons) is from negative terminal to the positive terminal through that part of the circuit external to the cell. However, prior to Electron theory, it was assumed that current flowed from positive terminal to the negative terminal of the cell
of current is now called conventional current.
Unit of Current.
The strength of electric current I is the rate of flow of electrons i.e. charge flowing per second.
Current, I = Q/t
The charge Q is measured in coulombs and time t in seconds. Therefore, the unit of electric
current will be coulombs/sec or ampere. If Q = 1 coulomb, t = 1 sec, then I = 1/1 = 1 ampere.
One ampere of current is said to flow through a wire if at any cross-section one coulomb of
charge flows in one second. Thus, if 5 amperes current is flowing through a wire, it means that 5 coulombs per second flow
past any cross-section of the wire. Note. 1 C = charge on 625 × 1016 electrons. Thus when we say that current through a wire is 1 A, it means that 625 × 1016 electrons per second flow past any cross-section of the wire.
I = Q /t = ne/t where e = – 1.6 × 10–19 C ; n = number of electrons
Electric Current is a Scalar Quantity
(i) Electric current, I =Q/t As both charge and time are scalars, electric current is a scalar quantity.
(ii) We show electric current in a wire by an arrow to indicate the direction of flow of positive charge. But such arrows are not vectors because they do not obey the laws of vector algebra. This point can be explained by referring to The wires OA and OB carry currents of 3 A and 4 A
respectively. The total current in the wire CO is 3 + 4 = 7 A irrespective of the angle between the wires OA and OB. This is not surprising because the charge is conserved so that the magnitudes of currents in wires OA and OB must add to give the magnitude of current in the wire CO.
Types of Electric Current
The electric current may be classified into three main classes: (i) steady current (ii) varying
current and (iii) alternating current.
(i) Steady current. When the magnitude of current does not change with time, it is called
a steady current. shows the graph between steady current and time. Note that value of
current remains the same as the time changes. The current provided by a battery is almost a steady
current (d.c.).
(ii) Varying current. When the magnitude of current changes with time, it is called a varying
current.shows the graph between varying current and time. Note that value of current
varies with time.
(iii) Alternating current. An alternating current is one whose magnitude changes continuously
with time and direction changes periodically. Due to technical and economical reasons, we
produce alternating currents that have sine waveform (or cosine waveform) as shown in (iii).
It is called alternating current because current flows in alternate directions in the circuit, i.e., from
0 to T/2 second (T is the time period of the wave) in one direction and from T/2 to T second in the
opposite direction. The current provided by an a.c. generator is alternating current that has sine (or
cosine) waveform
The strength of electric current I is the rate of flow of electrons i.e. charge flowing per second.
Current, I = Q/t
The charge Q is measured in coulombs and time t in seconds. Therefore, the unit of electric
current will be coulombs/sec or ampere. If Q = 1 coulomb, t = 1 sec, then I = 1/1 = 1 ampere.
One ampere of current is said to flow through a wire if at any cross-section one coulomb of
charge flows in one second. Thus, if 5 amperes current is flowing through a wire, it means that 5 coulombs per second flow
past any cross-section of the wire. Note. 1 C = charge on 625 × 1016 electrons. Thus when we say that current through a wire is 1 A, it means that 625 × 1016 electrons per second flow past any cross-section of the wire.
I = Q /t = ne/t where e = – 1.6 × 10–19 C ; n = number of electrons
Electric Current is a Scalar Quantity
(i) Electric current, I =Q/t As both charge and time are scalars, electric current is a scalar quantity.
(ii) We show electric current in a wire by an arrow to indicate the direction of flow of positive charge. But such arrows are not vectors because they do not obey the laws of vector algebra. This point can be explained by referring to The wires OA and OB carry currents of 3 A and 4 A
respectively. The total current in the wire CO is 3 + 4 = 7 A irrespective of the angle between the wires OA and OB. This is not surprising because the charge is conserved so that the magnitudes of currents in wires OA and OB must add to give the magnitude of current in the wire CO.
Types of Electric Current
The electric current may be classified into three main classes: (i) steady current (ii) varying
current and (iii) alternating current.
(i) Steady current. When the magnitude of current does not change with time, it is called
a steady current. shows the graph between steady current and time. Note that value of
current remains the same as the time changes. The current provided by a battery is almost a steady
current (d.c.).
(ii) Varying current. When the magnitude of current changes with time, it is called a varying
current.shows the graph between varying current and time. Note that value of current
varies with time.
(iii) Alternating current. An alternating current is one whose magnitude changes continuously
with time and direction changes periodically. Due to technical and economical reasons, we
produce alternating currents that have sine waveform (or cosine waveform) as shown in (iii).
It is called alternating current because current flows in alternate directions in the circuit, i.e., from
0 to T/2 second (T is the time period of the wave) in one direction and from T/2 to T second in the
opposite direction. The current provided by an a.c. generator is alternating current that has sine (or
cosine) waveform
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