Ohm's Law: When Does It Apply?

Ohm's law is a simple mathematical tool that helps us analyse electric circuits. It states that the electric current through a conductor between two points is directly proportional to the voltage across the two points. In other words, it defines the relationship between the three fundamental electrical quantities: current, voltage, and resistance. The formula for Ohm's law is V=IR, where V is the voltage, I is the current, and R is the resistance. This law is applicable to circuits containing only resistive elements and is used to determine voltage, current, impedance, or resistance in a linear electric circuit when the other two quantities are known.

Ohm's Law and its applications

Ohm's law is a fundamental principle in electrical engineering and physics that establishes a relationship between voltage, current, and resistance in an electrical circuit. It was formulated by German physicist Georg Simon Ohm in 1827. The law states that the electric current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, resistance, we arrive at the three mathematical equations used to describe this relationship:

V = IR

I = V/R

R = V/I

Where I is the current through the conductor, V is the voltage measured across the conductor, and R is the resistance of the conductor.

Applications of Ohm's Law

Ohm's law is widely used in circuit analysis and is applied in the design of resistors and electric heaters, among other equipment. It is also used to determine the desired circuit drop in circuit design and to maintain the required voltage drop between electrical components. Advanced laws such as Kirchhoff's Norton's law and Thevenin's law are based on Ohm's law.

Ohm's law is further used in ammeters, multimeters, and DC shunts to redirect current. It is also used to validate the static values of circuit components such as current levels, voltage supplies, and voltage drops.

Limitations of Ohm's Law

Ohm's law is an empirical law, and while it is found to be true for most experiments, it does have some limitations. It does not hold for unilateral networks, which allow current to flow in only one direction, such as those consisting of diodes or transistors. It is also not applicable to non-linear elements, where the resistance value changes with voltage and current, and the current is not exactly proportional to the applied voltage. Examples of non-linear elements include thyristors and semiconductors.

Additionally, Ohm's law only holds true for a conductor at a constant temperature, as resistivity changes with temperature. It is also not directly applicable to capacitor and inductor circuits.

Limitations of Ohm's Law

Ohm's Law is an important principle in the field of electronics, but it has its limitations and cannot be considered a fundamental law of nature.

Firstly, Ohm's Law is not applicable to unilateral networks, which allow current to flow in only one direction. Examples of such networks include diodes, transistors, and water voltameters. These are also known as non-Ohmic conductors, and the law is not applicable to them because the relationship between voltage and current is not linear. For instance, if the voltage is reversed while keeping its magnitude fixed, the resulting current will be of a different magnitude.

Secondly, Ohm's Law does not hold for non-linear elements, where the resistance value changes with different values of voltage and current. Examples of non-linear elements include thyristors, electric arcs, and PN junction diodes. In these cases, the relationship between voltage and current is not directly proportional, and Ohm's Law cannot be used to determine resistance or voltage and current values.

Additionally, Ohm's Law assumes that the physical conditions, such as temperature, of the conductor remain constant. However, in reality, when current passes through a material, it heats up, causing a change in resistance. This change in resistance affects the relationship between voltage and current, rendering Ohm's Law inapplicable in such cases.

Furthermore, Ohm's Law is not valid for certain materials and devices used in electric circuits. For example, semiconductors like Germanium and Silicon do not obey Ohm's Law and are known as Non-Ohmic conductors. Other examples include incandescent bulbs, where the filament's resistance changes with temperature increases, and LEDs, which are light-emitting diodes.

In summary, while Ohm's Law is a valuable tool for understanding the relationship between voltage, current, and resistance in many electrical circuits, it has limitations and does not apply universally.

Non-linear elements

Ohm's law applies to ohmic conductors such as iron and copper but not to non-ohmic conductors such as semiconductors. Non-linear elements do not support Ohm's law.

Incandescent Light Bulbs

The filament in an incandescent light bulb is heated when the voltage increases, changing the resistance of the wire. Therefore, Ohm's law cannot be applied to an incandescent light bulb.

Diodes

Diodes are unilateral electrical elements that only allow current to flow in one direction. They are non-linear elements and do not support Ohm's law.

Transistors

Transistors are another example of non-linear elements that do not support Ohm's law.

Vacuum Tubes

Vacuum tubes are non-ohmic conductors and therefore do not support Ohm's law.

Thyristors

Thyristors are non-linear elements and do not support Ohm's law.

Electric Arcs

Electric arcs are non-linear elements and do not support Ohm's law.

Semiconductors

Semiconductors are non-ohmic conductors and do not support Ohm's law. Examples of semiconductors include germanium, silicon, and gallium arsenide.

Current-voltage relationship

Ohm's law is a simple and powerful mathematical tool for helping us analyse electric circuits. It defines the relationship between the three fundamental electrical quantities: current, voltage, and resistance.

Ohm's law states that the electric current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, we arrive at the three mathematical equations used to describe this relationship:

• V = IR
• I = V/R
• R = V/I

Where I is the current through the conductor, V is the voltage measured across the conductor, and R is the resistance of the conductor.

Ohm's law is an empirical relation that accurately describes the conductivity of the vast majority of electrically conductive materials over many orders of magnitude of current. It is one of the most basic laws of electricity and is extremely useful for electrical/electronic engineering. It can be used to calculate the power, efficiency, current, voltage, and resistance of an element of an electrical circuit.

However, it is important to note that Ohm's law has limitations and is not applicable to all circuits. For example, it does not apply to unilateral networks or non-linear elements. Additionally, some materials do not obey Ohm's law and are called non-ohmic.

Calculating unknown resistance values

Ohm's law states that the electric current through a conductor between two points is directly proportional to the voltage across the two points. The law is represented by the equation:

V = IR

Where I is the current through the conductor, V is the voltage measured across the conductor, and R is the resistance of the conductor.

Ohm's law can be used to calculate unknown resistance values. This can be done by rearranging the equation above to isolate R, resulting in the following equation:

R = V/I

This equation can be used to calculate the resistance of a conductor when the voltage and current are known. For example, if a conductor has a voltage of 10 volts and a current of 2 amps, the resistance can be calculated as follows:

R = V/I = 10V / 2A = 5Ω

It is important to note that Ohm's law assumes that the resistance of the conductor remains constant and is not affected by changes in voltage or current. In some cases, the resistance of a conductor may change due to factors such as temperature or pressure. In such cases, Ohm's law may not be applicable.

Additionally, there are alternative methods to calculate resistance. For instance, if the electrical power (P) is known, resistance can be calculated using the following equation:

R = P/I^2

Furthermore, when dealing with parallel circuits, the total resistance is calculated differently. The total resistance in a parallel circuit is the reciprocal of the sum of the reciprocals of the individual resistances. This can be calculated using the following equation:

1/R_total = 1/R_1 + 1/R_2 + 1/R_3 + ... + 1/R_n

Ohm's law is a fundamental concept in electronics and is used to analyse electric circuits and validate the static values of circuit components.

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