Note: An ammeter must have low resistance  
so that:  
(i) It does not drop much voltage  
(ii) It does not affect the current flowing  
through the circuit.  
(iii) To ensure that the reading remains  
accurate  
Example 01  
VOLTIMETER  
With the aid of diagram, explain how the  
ammeter is connected to the cell?  
A voltimeter is a device used to measure the  
potential difference (voltage) between two  
points in an electrical circuit.  
Answers  
An ammeter is always connected in series  
with the component whose current is to be  
measured (cell).  
Characteristics of ammeter  
1. High internal resistance  
A voltimeter has a very high resistance so  
that only a small current pass through it.  
This prevents it from affecting (loading) the  
circuit being measure.  
2. Connected in parallel  
It is always connected across (parallel to)  
the component where the voltage is being  
measured, because the voltage is a potential  
difference between two points.  
Example 02  
Explain why an ammeter is connected in  
series with the component?  
Answers  
Because all the current to be measured must  
pass through the ammeter. The current is  
the same at every point in series path, by  
placing ammeter in series enables it to  
measure all the current in a circuit.  
Example 03  
How the voltimeter is protected in a  
circuit  
Why an ammeter made of low resistance?  
Answers  
(i) It has very high resistance, so that only  
tiny current can pass through it.  
So that it does not change the current it is  
supposed to measure  
(ii) It is always connected in parallel, so that  
the current divides to other parts of the  
circuit  
resistance  
resistance  
Always is placed in  
the path of current  
(i.e. in series)  
Always is connected  
in parallel with the  
conductor (resistor)  
whose potential  
difference is to be  
determined  
Example 01  
Why is the voltimeter made of high internal  
resistance?  
Answers  
(i) It does not change the current in the  
circuit  
2.2: RESISTANCE TO ELECTRIC AN  
CURRENT  
(ii) It does not affect the voltage being  
measured  
Resistance: - is the opposition of flow of  
electric charges offered by the conductor  
(iii) It gives accurate and reliable reading  
Voltage  
Resistance =  
Current  
Example 02  
The SI – Unit is Ohm (Ω)  
With the aid of diagram, explain how the  
voltimeter is connected in a circuit?  
Answers  
Resistance: - is the ratio of voltage across  
the conductor to the current flowing  
It is always connected across (parallel to)  
the component to measure the potential  
difference between two points in a circuit.  
Ohm: - is the resistance of a conductor  
which carriers a current of 1 ampere when  
the potential difference of 1 volt is applied  
across its terminals.  
Points to note:  
✓ As charges flow in a material, they  
encounter numerous collisions with  
atoms, resulting in a resistance to the  
flow of charges. This is known as  
electric resistance  
✓ The electrical current loses its energy  
as it flows through a conductor, due  
to resistance  
AMMETER AND VOLTMETER  
Ammeter  
Voltmeter  
This is a device  
This is a device used  
Conductor: - is any material that offers low  
resistance to the current flow.  
used to measure  
current  
to measure potential  
difference  
Electrical conductor: - is a material that  
permits current to flow free through it under  
an applied voltage  
It has a very low  
It has a very high  
Insulator: - is any material that does not  
allow the flow of electric current through it  
Semi conductor:- is any material that has  
resistance between that of a conductor and  
insulator  
퐼 ∝ 푉  
or 퐼 = 푘 푉  
Where 푘 is the constant of proportionality  
called conductance of the conductor denoted  
by G  
CHARGE CARRIERS  
퐼 = 퐺 푉  
Charges: - are moving particles that are  
responsible for carrying electric current  
in the given substance. Sometimes  
charge carriers are known as current  
carriers. The SI – Unit of charge is  
coulomb.  
퐼
퐺 =  
푉
1
푉
퐼
( )  
… … . 푖  
=
퐺
But  
1
Definition: - Coulomb is defined as the  
quantity of charge which passes any  
section of a conductor when a current of  
1 Ampere flows in a conductor in 1  
second.  
( )  
= 푅 … … … . . 푖푖  
퐺
Substitute equation (ii) into equation (i)  
푉
(
)
푅 =  
… … … … 푖푖푖  
퐼
푄 = 퐼푡  
Also, Ohm’s law can be expressed as  
푉 ∝ 퐼 or 푉 = 퐼푅  
1퐶 = 1퐴 × 1푠  
Example of charge carriers are.  
a. In metals / conductors: - the  
charge carriers are free electrons  
b. In electrolytes: - the carriers are  
free ions.  
Where, R is the constant of proportionality  
called resistance of a conductor.  
Ohms: - is the resistance of a conductor  
such that, when a potential difference of 1  
volt is applied between two points of a  
conductor, a current of 1 ampere flows  
through it.  
c. In semiconductors: - the charge  
carriers  
are  
holes  
(+)  
and  
electrons (-)  
Consider the circuit below used to verify  
the ohm’s law  
Relationship between potential difference  
across the conductor, current and  
resistance  
For a conductor such as copper wire, the  
current flowing through it is directly  
proportional to the potential difference  
across the ends of the conductor  
This is summarized by Ohm’s law which  
states that “The current passing through a  
conductor at constant temperature is  
proportional to the potential difference  
between its ends”  
The graph of V against I for a straight line  
passing through the origin  
That is:  
NON – OHMIC CONDUCTORS  
They are also called non – linear conductors  
Non – Ohmic conductors are conductors  
which does not obey ohm’s law.  
The graph of I against V passing through the  
origin is not a straight line. Examples are  
junction diode, diode valve and gases like  
neon gas.  
NB: The slope of the graph of V against I,  
represents the resistance of the conductor.  
∆푉  
푆푙표푝푒 =  
∆퐼  
푆푙표푝푒 = 푅  
Example 01  
A cell develops a potential difference of 2V  
across a resistor of 4Ω. Calculate the current  
flowing through the resistor and the  
conductance of a resistor  
Conductance: - is the reciprocal of  
resistance  
Solution  
(i) To find the current using Ohm’s law  
푉 = 퐼푅  
1
( )  
… … … … 푖푣  
퐺 =  
푅
OHMIC AND NON – OHMIC  
푉
퐼 =  
푅
COBNDUCTORS  
2푉  
퐼 =  
Ohmic conductors: - are the conductors  
which obey ohm’s law (i.e 퐼 ∝ 푉). the  
graph of V against I for Ohmic  
conductor is linear and passes through  
the origin.  
4Ω  
퐼 = 0.5 퐴  
(ii) To find the conductance of a conductor  
1
퐺 =  
푅
1
퐺 =  
4Ω  
퐺 = 0.25Ω−1  
Therefore, the current is 0.5A and  
conductance of a resistor is 0.25Ω−1