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Electric current
The flow of electric current or electricity, arises from a flow of negative charges (electrons) in the opposite direction of the conventional current.
Figure below shows the flow of electric current (I) from the positive terminal of a chemical cell (battery) to the negative terminal. It is a conventional current which is always from positive to the negative terminal of the battery.
The magnitude of the current can be measured by an ammeter placed some where along the electrical circuit. R is a resistance of a resistor.
S.I. unit of current is ampere (A).
About 6 million million million electrons (6.25 X 1018 electrons) flowing round a circuit every second would give a current of 1 A.
You know 1C of charge contains 6.25 X 1018 electrons.
One Ampere of electric current
The electric current is said to be one ampere when a charge of one coulomb passes in one second through any cross-section of a conductor. Its smaller units are milli amp and micro amp.
1 milli ampere = 10-3 A
1 micro ampere = 10-6 A
Direction of electric current
In the circuits arrow is shown to give the direction of electric current. They are following the conventional current direction, from positive to negative terminal of the battery round the circuit. But electrons flow in the opposite direction which is called electronic current, from negative to positive terminal of the battery round the circuit.
The direction of conventional current was defined before the discovery of electrons. However it is not wrong mathematically, because magnitude of the flow of charge in both direction (from positive to negative or from negative to positive) per unit time is the same.
Basic relation of electric current
The electric current and the charge passing through the circuit are related with each other. If 1C of charge passes through the circuit in 1s, we say 1A current is established. If 2C of charge passes through the circuit in 1s, we say 2A current is established. This relation can also be expressed as
Electric current = Charge / time
I = Q / t
Charge (C) = Current (A) X Time (s)
Q = I X t
Some typical values of electric current:
| Current through bulbs of headlights of the car | 4 A |
| Current through an electric kettle element | 10 A |
| Current through small torch bulb | 0.2 A |
Direct current
Direct current is that which flows only in one direction (unidirectional flow).
In order to explain it, consider a voltage V, applied across a resistor R. A constant current I, flows through the circuit as shown in the figure below (a).
If the voltage across the resistor is reversed , the current I, is also reversed as shown in the figure (b) .
While polarity of the cell is reversed, only the direction of current is reversed while its magnitude remains the same as before.
Alternating current
It is that current whose direction changes with time. In order to us reverses continuously. The end A, of the resistor is earthed i.e. at zero and the voltage of end B, varies with time in a way represented in the figure(b) the electric current in the resisor R, then flows as shown in the figure, the shape of the graph remains the same during the time interval 0, to T & T to 2T etc. The interval of time after which the value of electric current repeats itself is called time period T and the electric current is said to have completed one cycle. During the time interval 0 to T/2, the current increases from 0 to certain maximum value and then again falls to zero. Simmilarly during time interval T/2 to T, the electric current varies in the same manner but in opposite direction. Such a flow of current is called Alternating Current.
Ammeter
Ammeter is an instrument used to measure electric current. It is always connected in series to the component for which we want to measure the electric current in the circuit.
It’s basically a galvanometer which is modified to measure current. A galvanometer will give full scale deflection with very small current of few milliamperes or micro-amperes. So it can not be directly used to measure heavier currents.
To measure heavier currents a low resistance is connected parallel to galvanometer. Then it becomes ammeter. The low resistance so connected is called shunt. The shunt provides an alternate path to flow of current since its resistance is low most of the current passes through it and only a small friction of current passes through galvanometer. However the deflection in galvanometer will be proportional to the total current because if the total current is increased its fraction through galvanometer will also increase and there will be greater deflection.
By using shunt resistances of different suitable values , the ammeters of different ranges of current can be made by same galvanometer.