Example 02

A cell develops a p.d of 1.5V and the current

of 0.1A flows through a conductor, find the

resistance of a conductor. Neglect the

internal resistance of a cell.

Solution

푉

푅 =

퐼

1.5푉

푅 =

Solution

0.1퐴

푅 = 15훺

Step 1: To find the slope of the graph

∆퐼

푆푙표푝푒 =

∆푉

Example 03

퐼₂− 퐼₁

푆푙표푝푒 =

A current of 0.75 a flows through a coil of

wire when the potential difference across its

ends is 60 V. Calculate the resistance of the

coil

푉 − 푉

2

1

(40 × 10 ⁻³− 0)퐴

푆푙표푝푒 =

Solution

(2 − 0) 푉

푉

푅 =

퐼

푆푙표푝푒 = 20 × 10 ⁻³퐴푉⁻¹

60 푉

푅 =

0.75퐴

푅 = 80 훺

Step 2: To find the resistance, R

1

푅 =

푆푙표푝푒

Example 04

1

The figure below shows a sketch graph of

current, I, through an Ohmic conductor and

the potential difference, V, across it.

푅 =

20 × 10 ⁻³퐴푉⁻¹

푅 = 50 훺

Calculate the resistance of the conductor.

Example 05

The values of current I flowing in a given

resistor for the corresponding values of

potential difference V across the resistor are

given below.

I(A)

0.5 1.0 2.0

3.0

4.0

Resistors are divided according to the

material used to make it and the value of

resistance.

V(V) 0.5 2.5 6.75 11.0 15.0

Plot the graph of V against I and calculate

the resistance of the resistor

Solution

Types of resistors due to material used

Wire wound resistor, metal film resistor,

carbon resistor and metal oxide film resistor

Types of resistors due to value offered

Fixed resistor and variable resistor

Note

(i) Fixed resistor is the one which has a

value of resistance which cannot

change. E.g. 5Ω resistor

(ii) Variable resistor is a resistor whose

value of resistance change. Example

potential metre, Thermistor, photo

resistor and rheostat.

Consider the points A (2.0, 6.75) and B (4.0,

15)

∆푉

푆푙표푝푒 =

∆퐼

• Fixed Value Resistor: Fixed value

resistors are those types of resistors

whose value are fixed already during

manufacturing and cannot be changed

during its usage.

푆푙표푝푒 = 푅

Then,

∆푉

푅 =

∆퐼

푉 − 푉

2

1

푅 =

퐼₂− 퐼₁

• Variable Resistor or Potentiometer:

Variable Resistors or Potentiometers are

those types of resistors whose value can

be changed during its usage.

(15 − 6.5)푉

푅 =

(4 − 2) 퐴

푅 = 4.25 훺

RESISTORS

Variable resistor (rheostat) is a resistor

whose resistance can be changed smoothly

in order to change the current flowing.

Resistor: - is a device specially designed to

offer resistance to flow of electric current. It

is used to control the magnitude of current

and voltage according to Ohm’s law.

Resistance box consists of a number of

resistors connect in series through thick

brass blocks (any of the resistors may be

TYPES OF RESISTORS

∆ꢀ

short-circuited by putting a plug into the

associated socket).

Temperature coefficient,∝ =

, since the

ꢀ ∆푇

ꢁ

resistance of a thermistor decreases with

temperature, the temperature coefficient is

negative.

A Thermistor is a variable resistor whose

resistance changes when the surrounding

temperature changes.

There are two types of Thermistor

❖ We can classify the resistor as

follows

(i) Positive

Thermistor: - is the one whose

resistance increases when

temperature increase.

temperature

coefficient

1. Linear Resistor: A linear resistor is the

type of resistor whose resistance remains

constant with increase in the potential

difference or voltage applied to it. The

V-I characteristics of such resistor is a

straight line.

(ii) Negative temperature coefficient: - is

the one whose resistance decreases

with increase in temperature.

2. Non-Linear

Resistor:

Non-Linear

Resistor are those types of resistors in

which the Current passed through it is

not exactly directly proportional to the

Potential difference applied to it. These

types of resistors have non-liner V-I

characteristics and does not strictly

follow ohm’s Law.

Uses of Thermistor

Thermistor are used in sensing circuits such

as the one used to control cooling fan’s

speed.

THERMISTOR

This i a heat sensitive resistor made from

semiconductor

which

has

negative

temperature coefficient.

MAGNETO – RESISTOR

This is a resistor in which the resistance

changes with the strength and direction of a

magnetic field.

Negative temperature coefficient: -means,

the resistance R – decreases with rise in

temperature as shown in the graph below.

The resistance of magnetic field increases

when the magnetic strength applied to a

magneto – resistor increases and decreases

when the strength of the magnetic field

decreases.

LIGHT DEPENDENT RESISTOR

A light – dependent resistor is a variable

resistor in which resistance changes when

light energy is applied to the resistor. The

resistance decreases when the intensity of

light increases.

The first and second bands together present

two digits integers. Example in the figure

above, yellow (4), Purple (7)

The third band is called Multiplier and

gives the number of zeros that must be

appended to the number obtained from the

first two bands to get the resistance in

Ohm’s.

RESISTANCE COLOUR CODES

Resistors which are used in electronic

devices always are painted different colour

texture called band. The band represent the

exactly value of resistance. It contains four

bands with different meaning (codes).

Example 푟푒푑 = 10²푎푛푑 표푟푎푛푔푒 = 10³

The next band gives the tolerance of the

resistor and is called tolerance band. The

tolerance band is typically either Gold or

silver in colour.

TABLE OF COLOUR CODES

If the resistor has one more band past the

tolerance band, it is a quality band. This

band is read as percentage failure rate per

1000 hours, assuming the maximum rated

power is being dissipated by the resistor

Example 01

What is the resistance of the resistor shown

below?

Solution

Note

푅 = 47 × 10²Ω ± 5%

푅 = (4700Ω ± 5%)

(i) The first three bands encode the value of

the resistance.

(ii) One band (the fourth band from left)

encodes the tolerance (uncertainty) as

percentage.

Example 02

Draw the resistor colour codes for the

resistor whose resistance is 4500kΩ ±1%

Consider the resistor colour codes below

Solution

First colour is yellow = 4

Second colour is green = 5

Third (multiplier) is green = 10⁵

Tolerance band is silver = ±10%