CHAPTER TWO: CURRENT  
ELECTRICITY  
Meaning  
2.1 Introduction  
Electric current is the rate of flow of charge  
through the conductor  
A conductor such as an electric wire  
contains charges (negative or positive).  
When these charges are compelled to move,  
they constitute an electric current. To initiate  
this movement, we must establish a potential  
difference across the conductor ends.  
charge  
I =  
time  
Q
I =  
. . . . . . . . . . . (i)  
t
But 푄 = 푛푒 . . . . . . . . . . . (푖푖)  
Whereby n = number of electrons and e =  
charge of an electron  
ne  
I =  
. . . . . . . . . . . (iii)  
t
Methods used to create potential  
difference  
Current is the measure of amount of charge  
that passes a given point in a circuit per unit  
time  
Various methods have been utilized to create  
a potential difference, such are;  
The SI - unit of current is Ampere.  
(i) By using a battery  
(ii) By using a charged capacitor  
Points to note  
Ampere: is the amount of current when the  
charge of one coulomb flows through the  
conductor in one second  
Note  
To ensure a steady flow of current, we must  
use a battery, as the capacitor discharges  
immediately once the current flow  
(i) The charge is carried by particles such as  
electrons and ions  
(ii) A higher current indicates a greater flow  
of charge  
(iii) The direction in which the positive  
charge will flow, gives the direction of  
convectional current. However, in metallic  
conductors, free electrons are carriers of  
electricity and hence the electrons constitute  
the electric current  
1.25 × 1019 × (1.6 × 10−19 C)  
1푠  
Example 01  
n =  
Given that the charge of an electron is  
1.6 × 10−19퐶, find the number of electrons  
that pass in one second through any cross -  
section of a conductor with steady current of  
1 Ampere.  
n = 2.25 × 1020  
The amount of current flowing is 2A  
Solution  
Example 04  
ne  
I =  
t
An electric current of 0.12 A passes a  
certain point along a conducting wire. How  
much electric charge is flowing past this  
point in a minute?  
It  
n =  
e
1A × 1s  
n =  
Solution  
1.6 × 10−19  
퐶
Q = It  
n = 6.26 × 1019  
The number of electrons = 6.26 × 1019  
Example 02  
Q = 0.12 A × 60 s  
Q = 7.2 퐶  
Therefore, the electric charge flowing past  
the point = 7.2 C  
How many electrons pass through a lamp in  
2 minutes if the current is 300 mA? Given  
charge on electron is 1.6 × 10− 19  
퐶
Solution  
ne  
I =  
t
Example 05  
It  
n =  
e
How long would it take for a charge of 1.2 C  
to flow when a current of 0.01 A is flowing  
in a circuit?  
300 × 10−3 × (2 × 60 s)  
n =  
1.6 × 10−19  
퐶
Solution  
n = 2.25 × 1020  
Q
t =  
I
The number of electrons = 2.25 × 1020  
1.2 C  
t =  
0.01 퐴  
Example 03  
푡 = 120 푠푒푐표푛푑푠  
푡 = 2 푚푖푛푢푡푒푠  
If 1.25 × 1019 electrons pass a given point  
in one second. Calculate the current flowing  
in the circuit given that charge of an electron  
is 1.6 × 10−19  
퐶
Therefore, the time taken by the charge = 2  
min  
Solution  
ne  
t
I =  
Convectional current  
This is the current that flows from  
positive pole to the negative pole of a  
cell in an external circuit.  
(iii) Solar cells  
(iv) Electric generators  
Potential difference (p.d)  
Significant of convectional current  
The direction of convectional current is  
The electric potential difference is the work  
done in moving a unit charge in a circuit  
from one point to another.  
opposite  
in  
direction of  
flow  
of  
electrons.  
It is also called voltage, The SI – Unit is J/C  
or Volt  
Electromotive force (e.m.f)  
(
)
푊표푟푘 푑표푛푒 푊. 푑  
The electromotive force (e.m.f) is the  
potential difference across the terminals of a  
source when no current is flowing  
( )  
푝. 푑 푉 =  
퐶ℎ푎푟푔푒 (푄)  
푊. 푑 = 퐶ℎ푎푟푔푒 × 푝. 푑  
Function of electromotive force  
푊. 푑 = 푄 × 푉  
It provides the energy required to move the  
electrons through a conductor, leading to an  
electric current.  
Example 01  
The energy spent by a car battery is 48 J in  
moving 4 C of charge from one point to  
another. Calculate the potential difference  
between the terminals of the battery  
With higher e.m.f, more electrons flow  
through the conductor. The electric pressure  
is measured in volts.  
Solution  
Volt: is the energy given to each coulomb  
of charge in order to move in a circuit.  
(
)
푊표푟푘 푑표푛푒 푊. 푑  
( )  
푝. 푑 푉 =  
퐶ℎ푎푟푔푒 (푄)  
The e.m.f of a source is always labeled on  
the device as shown on the figure below.  
48 퐽  
( )  
푝. 푑 푉 =  
4 퐶  
( )  
푝. 푑 푉 = 12 푉  
Note: When an electric device such as a  
bulb is connected to a cell, electric current  
flows through the device, and some  
electrical energy is converted to heat and  
light.  
Note: A dry cell has an e.m.f of 1.5 V while  
the car battery has an e.m.f of 12 V  
The amount of energy converted per unit  
charge equal to the potential difference (p.d)  
across the device.  
Source of e.m.f  
(i) Electrochemical cells such as dry cells  
and car batteries  
(ii) Thermal electric devices such as thermal  
couples  
It is always connected in parallel with the  
cell to measure the e.m.f or parallel with the  
load (resistor) to measure the p.d because  
the e.m.f of a cell is compared to the p.d  
across the voltimeter terminals  
Polarity (connection) of the voltimeter  
To measure the e.m.f and the p.d, the  
positive terminal of the voltimeter is  
connected to the positive terminal of the  
cell, and the negative terminal of the  
voltimeter is connected to the negative  
terminal of the cell  
Differences between e.m.f and p.d  
e.m.f  
p.d  
This is the potential  
difference across the  
cell terminals when  
there is no current  
flows or no load is  
connected  
This is the work  
done per unit  
charge in moving  
electric charge from  
one point to another  
Is measured when no Is measured when  
current is flowing to  
the external circuit  
current flows  
through the circuit or  
when the load is  
connected  
Is greater than the  
potential difference  
Is less than e.m.f due  
to internal resistance  
of the cell  
Note: Measuring the e.m.f by a voltimeter  
only provides an estimated value, because a  
small current is drawn by the voltimeter.  
It maintains its  
It causes current to  
flow in the circuit  
hence the p.d  
In order to obtain the correct value of e.m.f,  
no current is drawn.  
potential difference  
decreases  
Consider the diagram below which shows a  
circuit used to measure the e.m.f  
andthep.dof a conductor  
Measurement of e.m.f and the p.d across  
the conductor  
The e.m.f and p.d are measured by a high  
resistance device called voltimeter.  
A voltimeter measures the potential  
difference between two points.