# Ohm's Law

## What is Ohm's law?

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

According to Ohm’s law,

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,

Potential difference ∝ Current

V ∝ I   (So when the value of V increases then the value of I also increases simultaneously)

V = IR

Where,

V is Voltage in volts (V)

R is Resistance in ohm (Ω)

I is Current in Ampere (A)

It is very important to remember this formula, so that if any two values of the resistance, voltage or current quantities are given then we can use Ohm’s law to find out the third missing value.
• To find Voltage(V),

V = IR

• To find Current(I),

$$I=\frac{V}{R}$$

• To find Resistance(R),

$$R=\frac{V}{I}$$

Ohm’s law relationship can also be remembered using pictures. So, to easily remember this formula there is another way called Ohm’s magic triangle. The magic V I R  triangle can be used to calculate all formulations of Ohm’s law.

### The triangle for Ohm’s law:

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

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. Given,

Resistance (R) = 50Ω

Current (I) = 3.2A
Therefore, Voltage (V) = I X R
= 3.2A x 50 Ω

=160V

• 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, we are left with the V at the top and I to the bottom left or V ÷ I.

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. Given,

Voltage (V) = 8.0 V

Current (I) = 2.0 A

Therefore, Resistance (R) = V ÷ I = $$\frac{V}{I}$$

= $$\frac{8}{2} = 4$$

• 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.

Example 3: If the filament resistance of an electric bulb is 330 Ω and Potential difference of two points 110V. Find the current flowing through the filament.

Ans. Given,

Resistance (R) = 330 Ω

Voltage (V) = 110V

Therefore, Current (I) = V ÷ R = $$\frac{V}{R}$$

= $$\frac{330}{110} = 3A$$

The rate at which energy is converted from the electrical energy of the moving charges to some other form, e.g., mechanical energy, heat, or magnetic fields or energy stored in electric fields, is called as electric power. The unit of power is watt.

The electrical power can be calculated using the Ohm’s law and by substituting the values of voltage, current and resistance.

### Formulae to find power:

• When the values for voltage and current are given,

P = V X I

• When the values for voltage and resistance are given,

P = $$V^{2}\div R$$

• When the values for voltage and current are given,

P = $$I^{2}\times R$$

### Power Triangle:

In the power triangle, the power (P) will be on the top and current(I) and voltage (V) at the bottom.

• When the values of current and voltage will be given, the formula for finding power will be,

P = I X V

• When the values of power and voltage will be given, the formula for finding current will be,

I = $$\frac{P}{V}$$

• When the values of power and current will be given, the formula for finding voltage will be,

V = $$\frac{P}{I}$$

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#### Practise This Question

If the power of two lenses and P1 and P2; when they are combined; the effective power is: