The most important law applicable for study of electricity is Ohm’s Law. This law outlines the relationship between voltage, current and resistance in an electrical circuit. Ohm’s experiments show that whenever electric current is flowing through a conductor the following three factors are present:
(a) The pressure or potential difference (V) across the conductor (measured in volts) causing the current to flow.
(b) The resistance (R) of the conductor (measured in Ohm) which must be overcome.
(c) The current strength (I), which is maintained in the conductor (measured in Amp) as a result of potential difference overcoming the resistance.
There exists a definite and exact relationship among the three quantities involved. This is known as Ohm’s Law.
Definition. In a DC electrical circuit, the ratio between the applied potential difference (V) and the current (I) flowing is constant at a constant temperature. That is V/ I is constant, and this constant is the resistance of the conductor. The relationship between EMF, current and resistance is expressed by “Ohm’s Law” which states that the current in a circuit is directly proportional to the potential difference and inversely proportional to the resistance of the circuit. Thus, other factors remaining constant, if the potential difference is doubled, the current is doubled, if the resistance is doubled, the current is halved; Ohm’s law can be expressed as an equation:
e.m.f in volts
Current in amperes= ———————————-
Resistance in ohms
In symbols I = V/R
R = V/I or V = IR
Application. The applications of Ohm’s law are as follows:
(a) This law is applicable to all DC circuits.
(b) In a modified form it is applicable to AC circuits also provided account is taken of the induced EMF resulting from the self-inductance of the circuit and distribution of the current in the cross section.
A German physicist GR Kirchoff elaborated on Ohm’s Law and developed two statements that are known as Kirchoff’s Laws for current and Voltage. An understanding of these the laws enable the aircraft technician to gain a better understanding of the behaviour of electricity. Using Kirchoff’s Law, it is possible to find the following:
(a) The current in each branch of a network circuit, when both the resistance and EMF in each branch are known.
(b) The EMF in each branch when both the resistance and the current in each branch are known.
These laws are stated as follows:
(a) Current Law or Point Law. In an electrical network, the algebraic sum of the current meeting at a point (or junction) is zero. In simple means that the amount of current leaving a junction is equal to the total current entering to that junction. i.e., S I = 0. Consider the case of a few conductors meeting at a point as shown in Fig1.12, let I1, I4current of a conductor entering at the junction and I2, I3, I5 current of conductor leaving at the junction. Assuming the incoming current positive and outgoing current negative. Next applying current law, algebraic sum of current meeting at a junction is zero.
I1 + (-I2) + (-I3) + I4 + (-I5) = 0
Or I1 + I4 -I2 -I3 -I5 = 0
Or I1+ I4 = I2 + I3 +I
Or incoming currents = outgoing currents
Or S I = 0
(b) Mesh Law or Voltage Law. It states that the algebraic sum of the product of current and resistance in each of the conductors in any closed mesh (or path) in a network plus the algebraic sum of the EMF in that path is zero. In other words:
S IR + SEMF= 0