Ohm's Law -Elecetricity.

 

Class X (Physics)

 CHAPTER: Electricity

 TOPIC: Ohm’s Law

I hope so all of you must know these questions' answers.

Ø  What do you mean by term Electricity?

Ø  Define the terms electrostatic force and electrodynamics force.

Ø  Define the terms electric current and voltage.

Ø  What is the difference between e.m.f and terminal voltage in an electric circuit?

 

CONTENT DESCRIPTION: In 1828, George Simon Ohm, a German physicist, derived a relationship between electric current and potential difference. This relationship is known as Ohm’s law.

Ohm was awarded the Royal Society’s highest award, the Copley Medal. Gradually, the term

Ohm was adopted as the unit of electrical resistance in 1872.

 

What is Ohm’s Law?

Ohm’s Law states that the current flowing through a conductor is directly proportional to the potential difference applied across its ends, provided the temperature and other physical conditions remain unchanged. Mathematically it can be represented as,

   

                                

Where, 

·        V is Voltage in volts (V)

·        R is Resistance in ohm (Ω)

·        I is Current in Ampere (A)

 

The SI unit of resistance is Ohm (Ω).

                                                                

One Ohm: If the potential difference across the ends of a conductor is 1 volt and the current flowing through it is 1 ampere, then the resistance of the conductor R is 1 ohm.

How does Ohm’s Law work? 

                Ohm’s Law describes the current flow through a resistance when different electric potentials (voltage) are applied at each end of the resistance. Since we can’t see electrons, the model or an analogy of electric circuits used to help us understand circuits better is the water-pipe analogy. Water flowing through pipes is a good mechanical system that is analogous to an electrical circuit.

            Here, the voltage is analogous to water pressure, the current is the amount of water flowing through the pipe, and the resistance is the size of the pipe. More water will flow through the pipe (current) when more pressure is applied (voltage) and the bigger the pipe, (lower the resistance). 

 

In a circuit ammeter is always connected in series and voltmeter is connected in parallel across the points between which potential difference is to be measured.

                                 A straight line graph obtained between V and I verifies the Ohm’s law.

 

Least Count: It is very important to find the least count of ammeter and voltmeter before using them.

If in the ammeter, there are 10 divisions from 0 to 0.1 A then each division indicates 0.01 A.

A.    To calculate the least count of ammeter. Range of ammeter = AR…………………….

Number of divisions in ammeter = AN…………………………..

.’. Least count of ammeter = AR    / AN    =................ ampere.

B.  To calculate the least count of voltmeter. Range of voltmeter = VR……………………

Number of divisions in voltmeter = VN

.’. Least count of voltmeter =VR    / VN =.................. volt.

Different Applications of Ohm’s Law

The main applications of Ohm’s law are:

 

1.     To determine the voltage, resistance or current of an electric circuit. 

2.     Ohm’s law is used to maintain the desired voltage drop across the electronic components.

3.     Ohm’s law is also used in dc ammeter and other dc shunts to divert the current.

 Limitations of Ohm’s Law

Following are the limitations of Ohm’s law:

 

1.                   Ohm’s law is not applicable for unilateral electrical elements like diodes and transistors as they allow the current to flow through in one direction only.

2.                   For non-linear electrical elements with parameters like capacitance, resistance etc the voltage and current won’t be constant with respect to time making it difficult to use Ohm’s law.

Calculating Different Parameters Using Ohm’s Law

As an equation, the Ohm’s Law serves as an algebraic recipe for calculating the current when the resistance and the potential difference are known. Likewise, if any two parameters in the equation are known, then the unknown third parameter can be easily calculated as follows:

 

               

Analyzing Simple Circuits Using Ohm’s Law

Resistive Circuits are analyzed using the Ohm’s Law. The law is often used to find the voltage, current or resistances of the circuit. The law relates these three quantities by the equation V = IR. Ohm’s Law Triangle helps us remember this equation while solving problems. Here, the three quantities V, I and R are superimposed into a triangle with voltage at the top and current and resistance below. The magic V I R triangle can be used to calculate all formulations of Ohm’s law. 

 

 

 

 

 

The Ohms law Triangle

 

·        If the value of voltage is asked and the values of the current and resistance are given, then  to  calculate  voltage  simply  cover V at   the   top.   So,   we   are   left   with  the I and R orI X R. So, the equation for Voltage is Current multiplied by Resistance. Examples of how the magic triangle is employed to determine the voltage using Ohm’s law is given below.

 

Solving Problems Using Ohm’s Law Triangle

Example 1: If the resistance of an electric iron is 50Ω and 3.2A Current flows through the

resistance. Find the voltage between two points. Ans.

If the value of Resistance is asked and the values of the current and voltage are given, then to calculate resistance simply cover the R. Now, we are left with the V at the top and I to the bottom left or V ÷ I.

Given, Resistance (R) = 50Ω Current (I) = 3.2A Therefore,

Voltage (V) = I X R = 3.2A x 50 Ω =160V

Example 2: An EMF source of 8.0 V is connected to a purely resistive electrical appliance (a light bulb). An electric current of 2.0 A flows through it. Consider the conducting wires to be resistance-free. Calculate the resistance offered by the electrical appliance.

Ans.

If the value of current is asked and the values of the resistance and voltage are given, then to calculate current simply cover the I. We are left with Voltage over Resistance or V ÷ R. So the equation for Current is Voltage divided by Resistance.

Given,

Voltage (V) = 8.0 V Current (I) = 2.0 A

Therefore,

Resistance (R) = V ÷ I = 4 Ω

Frequently Asked Questions

 

What is Ohm’s law simplified?

Ohm’s law states that the current passing through a conductor is proportional to the voltage over the resistance..

How do you calculate current draw?

To calculate the current drawn, divide the power by the operating voltage.

 

Do all metals obey Ohm’s law?

When the temperature of a metal increases, resistance decreases. Good conductors possess non-zero electrical resistances.

 

Why is Ohm’s law important?

Ohm’s law formula is used to calculate electrical values so that we can design circuits and

use electricity in a useful manner.