Electrical Engineering ⇒ Topic : Ohms Law
In the nineteenth century, a German philosopher, George Simon Ohm discovered that the current through a conductor, under constant temperature conditions, was proportional to the potential difference across the conductor.Expressed as an equation, Ohm's law become
where, / is the current in amperes, V is the potential difference in volts and R is the resistance in ohms.
This fact can be demonstrated by connecting a voltage source (V), an ammeter (A) and a load as shown in Fig. (a). Different currents can be obtained by varying the load resistance. For example, if the voltage across the load is 12 V and the load resistance is 3 Ω, the current through the circuit is 12/3 = 4 A. If the load resistance remains constant at 3 Ω, in accordance with Ohm's law, the current will double if the voltage doubles or will reduce to half if the voltage is reduced by half. Thus, if the voltage across the load reduces to zero, the current also becomes zero.
FIGURE (a) A simple circuit to demonstrate Ohm's law
The relationship between V and I of this example is shown in Fig. (b), in which V and I are plotted on the x and y axes, respectively. The graph is a straight line, the equation of which is I = V/3 A. The constant value, 3 represents the circuit resistance in ohms and is assumed to remain constant. The graph illustrates an important characteristic of the Ohm's lawnamely, "the current varies directly with the applied voltage if the resistance of the circuit is kept constant".
FIGURE (b) Current vs voltage in a circuit having constant resistance
If the voltage across the load in Fig. (a) is kept constant at 12 V, the current through the load will depend only on the effective resistance of the load. For example, if the resistance is 12 Ω, the current I = 12/12 = 1 A. If the resistance is halved, the current will be doubled; and if the resistance is doubled, the current reduces to half. In other words, if the voltage across the load is kept constant. The current in the circuit will vary inversely with resistance (I = V/R). The relationship between current and resistance of this example is shown in Fig. (c). It can be seen from this figure that as R becomes small, the current becomes very large.
FIGURE (c) Relation between resistance and current when the voltage is kept constant
On the other hand, if current through the load is maintained constant at 4 A,the voltage across the load will depend on the load resistance (V = 4R). The relationship between V and R is shown in Fig. (d). Voltage across the load varies directly with load resistance (and the graph is a straight line) if the current through the circuit is maintained constant.
FIGURE (d) Relation between resistance and voltage when the current is kept constant
Thus, Ohm's law can be stated as follows. "If a circuit having a resistance of R ohms is carrying a current of / amperes, the voltage across the circuit, V volts is given by V= IR".
The relationship between voltage (I), the current y) and resistance (R) in a d.c. circuit was first discovered by German scientist George Simon *Ohm. This relationship is called Ohm's law and may be stated as under
The ratio of potential difference (r) between the ends of a conductor to the current (I) flowing between them is constant, provided the physical conditions (e.g. temperature etc.) do not change i.e.
where R is the resistance of the conductor between the two points considered.
For example, if in Fig.(a), the voltage between points A and B is r 'volts and current flowing is I amperes, then I'd will be constant and equal to R, the resistance between points A and B. If the voltage is doubled up, the current will also be doubled up so that the ratio V/I remains constant.
If we draw a graph between rand I, it will be a straight line passing through the origin as shown in Fig.(b). The resistance R between points A and B is given by slope of the graph i.e.
R = tan θ = V/I= Constant
Ohm's law can be expressed in three forms viz.
I = V/R ; V= IR; R= VI
These formulae can be applied to any part of a d.c. circuit or to a complete circuit. It may be noted that if voltage is measured in volts and current in amperes, then resistance will be in ohms
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