Ohm - Unit of Electrical Resistance
Ohm is the unit of electrical resistance in the SI system. It was named in the honor of the famous German physicist Georg Simon Ohm. It is mathematically equal to the resistance of a circuit in which a potential difference of one volt can produce a current of one ampere or, the resistance in which one watt of power is dissipated when one ampere of current starts flowing through it.
Ohm’s law provides a direct relationship between electric current and potential difference. The current which is flowing through any conductors is directly proportional to the voltage applied to it. In this topic we have discussed what is Ohm, let’s understand Ohm definition and laws and some numerical examples.
Ohm Definition
Ohm’s law states that the voltage across any conductor is directly proportional to the current flowing through it. Assuming all the physical conditions and temperature remain constant.
Ohm’s law is valid only if the temperature provided and other physical factors remain constant. Ohm’s SI unit is rho (Ω), In certain components, the current raises the temperature. Eg: The filament of a light bulb where the temperature rises as the currents are increased. In this case, Ohm’s law will fail. The lightbulb filament is violating Ohm’s Law.
Calculating Electrical Power Using Ohm’s Law
The rate at which one form of energy is converted from the electrical energy of the moving charges to some other form of energy Eg: mechanical energy, heat, magnetic fields, or energy which is stored in electric fields, is known as electric power. The electrical power can be calculated by using Ohm’s law and by substituting the values of voltage, current, and resistance.
When the values of current and voltage are given, the formula for finding power will be: P = V I
When the values of power and voltage are given, the formula for finding current will be: I = P / V
When the values of power and current are given, the formula for finding voltage will be: V = P / I
Experimental Verification of Ohm’s Law
Aim: To verify the Ohm's law.
Apparatus Required:
Resistor
Ammeter
Voltmeter
Battery
Plug Key
Rheostat
Theory: Ohm’s law states that the voltage across any conductor is directly proportional to the current flowing through it, Assuming all the physical conditions and temperature remain constant.
Procedure:
1. The key K is closed in the first step and the rheostat Rh is adjusted to get the minimum reading in the ammeter A and the voltmeter V.
2. The sliding terminal of the rheostat is then moved slowly to increase the current gradually and each time the value of current I flowing in the circuit and the value of potential difference V across the resistance wire is recorded. So, different sets of values of V and I are recorded.
Then for each set of values of Voltage and Current, the ratio V / I is calculated.
The ratio of V / I gives a constant value called R which is called the resistance of the conductor.
Plot a graph between current and the potential difference, it will be a straight line. This brings us to the conclusion that the current is proportional to the potential difference.
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The Main Applications of Ohm’s Law are:
The main applications of Ohm's law are:
It is used to determine the voltage, resistance, and current of an electric circuit.
Ohm’s law is majorly used in order to maintain the desired voltage drop across any electronic components.
Ohm’s law is to find its application in dc ammeter and other dc shunts to divert the current.
Following are the Limitations of Ohm’s Law:
Ohms law has a certain limitation:
Ohm’s law is not applicable for some electrical elements like diodes and transistors as they allow the current to flow through them in a single direction only.
For those electrical elements which don’t follow linear relation with parameters eg: capacitance, resistance, etc the voltage and current won’t be constant with respect to time making it difficult to use Ohm’s law.
Solved Example
Example 1: If the resistance of an electric iron is 60 Ω and a current of 3.2 A flows through the resistance. Find the voltage between two points.
Answer: It is asked to calculate the value of voltage provided current and resistance have given to us: T, we use the following formula to calculate the value of V:
V = I × R
Substituting the values in the equation, we get
V = 3.2 A × 60 ÷ = 192 V
V= 192 V
Example 2: Calculate the resistance of an electrical circuit having a voltage supply of 100 Volts and a current of 10 mA.
Answer: V = 100 V, I = 10 mA = 0.010 A
R = V / I
= 100 V / 0.010 A
= 10000 Ω = 10 kΩ
FAQs on Ohm
1. What is electrical resistance and what is its SI unit?
Electrical resistance is a fundamental property of a material that measures its opposition to the flow of electric current. When electrons flow through a conductor, they collide with the ions and atoms of the material, which obstructs their path. This obstruction is known as resistance. The SI unit of resistance is the ohm, represented by the Greek symbol omega (Ω).
2. How is one ohm (1 Ω) of resistance defined according to Ohm's law?
One ohm (1 Ω) is defined as the electrical resistance between two points of a conductor when a constant potential difference of one volt (1V), applied across these points, produces a current of one ampere (1A) through the conductor. In simple terms, it is the ratio of voltage to current, where 1 Ω = 1 V / 1 A.
3. What factors influence the resistance of a conducting wire?
The resistance (R) of a conducting wire is influenced by four primary factors:
- Length of the conductor (L): Resistance is directly proportional to the length. A longer wire has more resistance.
- Cross-sectional area (A): Resistance is inversely proportional to the cross-sectional area. A thicker wire has less resistance.
- Nature of the material (ρ): This is determined by the material's resistivity, an intrinsic property. Materials like copper have low resistivity, while materials like nichrome have high resistivity.
- Temperature: For most metallic conductors, resistance increases as the temperature rises.
4. What is the key difference between resistance and resistivity?
The key difference lies in their dependency on dimensions. Resistance (R) is an extrinsic property of a specific object that depends on its length and cross-sectional area. In contrast, resistivity (ρ) is an intrinsic property of the material itself, which defines how well that material resists current flow, irrespective of its shape or size. For example, two copper wires of different lengths will have different resistances but the same resistivity.
5. Does Ohm's law apply to all electrical components and materials?
No, Ohm's law does not apply universally. Materials that obey Ohm's law, where the V-I graph is a straight line, are called ohmic conductors (e.g., most metals at a constant temperature). Materials that do not obey this law are called non-ohmic conductors. For these, the resistance changes with voltage or current. Common examples of non-ohmic components include semiconductor diodes, transistors, and thermistors.
6. Why does the resistance of a metallic conductor typically increase with an increase in temperature?
When the temperature of a metallic conductor increases, its positive ions (atomic nuclei) gain thermal energy and begin to vibrate more vigorously about their fixed positions. This increased vibration raises the probability of collision with the free electrons that constitute the electric current. More frequent collisions result in a greater obstruction to the electron flow, which is observed as an increase in the conductor's overall electrical resistance.
7. What are some real-world examples where the concept of Ohm's law is important?
Ohm's law is crucial in many everyday applications. For instance, it is used to:
- Design heating elements in appliances like toasters, electric heaters, and kettles, where high resistance is used to generate heat (Joule heating).
- Select the correct resistor to protect sensitive components like LEDs from excessive current.
- Understand how fuses and circuit breakers work; they melt or trip when current exceeds a safe limit, a value determined by Ohm's law (I = V/R).
- Control the speed of a fan using a regulator, which essentially varies the resistance in the circuit to adjust the current flowing to the motor.
8. How can Ohm's law be represented graphically for an ohmic conductor?
For an ohmic conductor, the relationship between potential difference (V) and current (I) is linear. This is represented by a V-I graph, where voltage is plotted on the y-axis and current on the x-axis. The graph is a straight line passing through the origin. The slope of this line (ΔV/ΔI) gives the constant resistance (R) of the conductor, visually confirming that the resistance does not change with voltage or current.
