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Electrical Resistance

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 Sub Topics Resistance is a property of all electrical components. Sometimes the effect of resistance is undesirable, other times it is constructive. Every material offers some resistance or opposition to the flow of current. Some conductors such as silver, copper and aluminum offer very little resistance to current flow. Insulators such as glass, wood and paper offer high resistance to current flow. Resistors are components manufactured to possess a specific value of resistance to the flow of current. A resistor is the most commonly used components in an electric circuit. Resistors are available with fixed or variable resistance values.

Definition

The opposition to current flow in electrical circuits is called resistance. Resistance is not the same for all materials. The number of free electrons in a material determines the amount of opposition to current flow. With constant voltage, current flow is increased by decreasing resistance. Decreased current results from more resistance. By increasing or decreasing the amount of resistance in a circuit, the amount of current flow can be changed. Resistance can be determined using Ohm's law. The mathematical expression of Ohm's law is

R = $\frac{V}{I}$
R is the resistance, V is the voltage and I is the current

The resistance of any material depends on four factors:
• The material of which it is made
• The length of the material
• The cross sectional are of the material
• The temperature of the material

Unit

Resistance has the SI unit volt per ampere, called an Ohm (Ω). Resistance can be calculated using the expression Ohm's law which is given above. One Ohm can be defined as the resistance offered by a material if 1V is applied to a circuit and it produces a current of 1A.

How does Temperature affect Resistance

Among these four factors, temperature is an important one which will affect directly to the resistance. For most materials, the higher the temperature, the more resistance it offers to the flow of electric current. This effect is produced because a change in the temperature of a material changes the ease with which a material releases its outer electrons. A few materials, such as carbon, have lower resistance as the temperature increases. The effect of temperature on resistance is the least important of the factors that affect resistance.

Resistance in Series

A series circuit contains two or more resistors and provides one path for current to flow. The current flows from the negative side of the voltage source through each resistor to the positive side of the voltage source. If there is only one path for current to flow between two points in a circuit, the circuit is a series circuit. The more resistors connected in series, the more opposition there is to current flow. The more opposite there is current flow, the higher the resistance in the circuit. In other words, when a resistor is added in series to a circuit, the total resistance in the circuit increases. The total resistance in a series circuit is the sum of the individual resistances in the circuit. This can be expressed as,

R = $R_{1}$ + $R_{2}$ + $R_{3}$ + .........+ $R_{n}$

Resistance in Parallel

A parallel circuit contains two or more resistors and provides two or more paths for current to flow. Each current path in a parallel circuit is called a branch. The current flows from from the negative side of the voltage source, through each branch of the parallel circuit, to the positive side of the voltage source. If there is more than one path for current to flow between two points in a circuit, the circuit is termed as parallel circuit. The more resistors connected in parallel, the less opposition there is to current flow. So the resistance is small in the given circuit. In other words, when a resistor is added in parallel to a circuit, the total resistance in the circuit decreases. Total resistance for a parallel circuit is given by the formula:

$\frac{1}{R}$ = $\frac{1}{R_{1}}$ + $\frac{1}{R_{2}}$ + $\frac{1}{R_{3}}$ + .........+ $\frac{1}{R_{n}}$

Law of Resistance

Law of resistance is nothing but the Ohm's law. It states that, for a conductor which is kept at constant temperature, the current I flowing through this conductor is proportional to the potential difference V across its ends. The ratio of potential difference to current is called the resistance R of the conductor. The expression of Ohm's law is,

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

Examples

Some of the solved problems related to resistance is given in this section.

Solved Examples

Question 1: Calculate the resistance of a circuit in which a voltage of 10V produces a current of 2.5A?

Solution:

From the question, it is clear that,
V = 10V and I = 2.5A
The equation for electrical resistance is,

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

R = $\frac{10}{2.5}$ = 4Ω

Question 2: If an applied voltage of 15V produces a current of 3A, find out the resistance of the circuit?

Solution:

From the question, it is clear that,
V = 15V and I = 3A
The equation for electrical resistance is,

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

R = $\frac{15}{3}$ = 5Ω