• State ohm’s law
• Explain ohm’s law
• Draw the graph of current versus voltage
• Draw the graph of current versus resistance
2.4 OHM’S LAW
In 1826 George Simon Ohm found that the current, voltage, and resistance are
related in a specific way. Ohm expressed this relationship with a formula that is known today as Ohm‘s law. In this chapter, you will learn Ohm‘s law and how to used it in solving circuit problems.
Electric circuit can be of two basic forms: series and parallel. In this chapter, series, and parallel circuits are studied. you will also see how Ohm‘s law is used in series, and parallel circuits
Ohm‘s law is the most important mathematical relationship between voltage, current and resistance in electricity. Ohm’s Law is used in three forms depending on which quantity voltage, current or resistance you need to determine. In this section, you will learn each of these forms
2.4.1 Explanation of Ohm‘s Law
In the electric circuit, if the voltage across constant resistor value is increase, the current through the resistor will also increase; and, if the voltage is decrease, the current will decrease. For example, if voltage is double, the current will doubled. If the voltage is halved, the current will also be halved. This relationship is illustrated in the figure (4.1), with used voltage and current meter.
Ohm‘s law also stated that if the voltage is kept constant, less resistance results in more current, and, also, more resistance results in less current. For example, if resistance is halved, the current will doubled. If the resistance is doubled, the current is halved.
This relationship is illustrated by the meter indications in the figure (4.2). Where the resistance is increase and the voltage is constant.
From previous illustrated,
Ohm‘s law states that, current and voltage are linearly proportional at constant resistance; current and resistance are inversely related at constant voltage.
According this law the following three equivalent formula is derived:
This form of Ohm‘s low is used to determine current if voltage and resistance values are known
This form of Ohm‘s low is used to determine voltage if current and resistance values are known
This form of Ohm‘s low is used to determine resistance if voltage and current values are known
2.4.2 Ohm‘s Law triangle
There is an easy way to remember which formula to use. By arranging current, voltage and resistance in a triangle, one can quickly determine the correct formula. Ohm‘s Law triangle is shown in figure (4.3)
To use the triangle, cover the value you want to calculate, the remaining letters make up the formula as shown in figure (4.4).
Remember the following three rules:
Ohm‘s Law can only give the correct answer when the correct values are used.
Current is always expressed in Amperes or Amps (A)
Voltage is always expressed in Volts (V)
Resistance is always expressed in Ohms ( )
2.4.3The Relationship of Current, Voltage and Resistance
Ohm‘s law describes how current is related to voltage and resistance. current and voltage are linearly proportional at constant resistance; current and resistance are inversely related at constant voltage.
2.4.3.1 The Linear Relationship of Current and Voltage
Current and voltage linearly proportional; that is, if one is increased by a certain percentage, the other will increase or decrease by the same percentage, assuming that the resistance is constant value.
To draw a graph of current verses voltage, let‘s take a constant value of resistance. For example, R=10 , and calculate the current for several value of voltage ranged from 0 to 100 V by used the formula I=V/10 where R=10 . the current values obtained are shown in the table (4.1):
The graph of the current values versus the voltage values is shown in the Fig.
(4.5). This graph tells us that a change in voltage results in a linearly proportional change in current. From graph the change in voltage from 20 to 30 is increased by 50% by calculation the current must be increase by the same percentages 50% I = 2 + 2 X 50/100 = 3A.
2.4.3.2 Current and Resistance are Inversely Related
Current varies inversely with resistance as expressed ohms law, I=V/R. when the resistance is decreased, the current goes up; when the resistance is increased, the current goes down. For example, let‘s take a constant value of voltage V=10volts, and calculate the current for several value of resistance ranged from 10 to 100 by used the formula I=10/R. the current values obtained are shown in the next table (3-2), The graph of the current values versus the voltage values is shown in the figure(3-6).
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