Lesson 2, Topic 1
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Ohm’s Law

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A German scientist George Ohm, investigated the relationship between voltage or potential difference and current flowing along a metallic conductor. His finding is known as Ohm’s law which is stated as follows;

The current flowing along a metallic conductor at constant temperature is directly proportional to the potential difference across its end provided other physical conditions remain constant.

If V represents the potential difference in volts, I represent the current flowing in amperes, then

$$\scriptsize V \propto I$$

V = IR

R = $$\frac {V}{I} = \frac {Potential\; difference}{current}$$

R is a constant of proportionality known as resistance of a conductor. It depends on the nature of the conductor

∴ V = IR

Limitations of Ohms Law

Ohm’s law holds for metals and some certain materials but some materials do not obey Ohm’s law. Such materials include, diode, transistors, rectifiers, semiconductors, rare gas, acids.

The unit of resistance is Ohm (Ω).

Example 1

Calculate the current produced when 250 volt cell is connected across 12 ohms resistor.

Solution

Values in the question:

Voltage V = 350 volt,  Resistance R = 5 ohms, current I = ?

Formula:

Voltage V = IR

Substitute the values given into the equation

350 = I x 5

Make current the subject of the formula:

current I = $$\frac{350}{5}$$  .

Current I = 70 Amperes 