The document introduces electric current as the flow of charge, primarily involving electrons in metals, and describes its measurement in amperes. It explains conventional current direction, resistance, and provides calculations related to current, charge, and the number of electrons. Additionally, it defines resistance and its relationship with potential difference and current.

1. Introduction to
Electrical Elements

2. Electric Current
 An electric current is a flow of charge.
 In metals, current is the movement of negative charge,
i.e. electrons
 In gases and electrolytes (NaCl solution), both positive
and negative charges may be involved.

3. Electric Current
 Current is the rate at which charge is flowing in a circuit.
 It is the number of charges that pass through any point of the circuit per unit time.
 i.e. I = Q / t
 Current is measured in ampere, A, where 1 A = 1 C/s.
 One coulomb is equal to the charge on 6.241 x 1018
protons.
 Measuring instrument = Ammeter (Ampere meter)

4. Conventional current
 Scientist first thought that positive charges flow from
the positive terminal of a source to the negative
terminal. This is called the conventional current
direction.
 However, it was found that a current in a metal wire
is in fact a flow of negatively-charged electrons in
the opposite direction. Nevertheless, the
conventional current is still used.
+ –
electron
flow
convention
current

5. Problem 1
A hair dryer draws a current of 3 A. If it is switched on
for 5 minutes,
(a)how many charges, and
(b) how many electrons have passed through it?
 Solution:
(a) by equation
I = Q / t
3 = Q / (5 x 60)
Q = 900 C
(b) charge of 1 electron = 1.6 x 10-19
Coulombs
no. of electron = number of charges / charge per electron
no. of electron = 900 / (1.6 x 10-19
) = 5.625 x 1021

6. Practice Problem:
A hair dryer draws a current of 2A. If it is switched on for
7mins,
(a) how many charges, and
(b) how many electrons have passed through it?
 Solution:
(a) by equation
I = Q / t
2 = Q / (7 x 60)
Q = 840 C
(b) charge of 1 electron = 1.6 x 10-19
Coulombs
no. of electron = number of charges / charge per electron
no. of electron = 840 / (1.6 x 10-19
) = 5.25 x 1021

7. H.W. practice:
a laptop charger plugged-in for 10 minuets and 1900C of
charges have passed through it. Calculate:
(a) Current consumed by charger, also
(b) electrons passed through it during given time.
 Solution:
(a) by equation
I = Q / t
I = 1900 / (10 x 60)
I = 3.1 A
(b) charge of 1 electron = 1.6 x 10-19
Coulombs
no. of electron = number of charges / charge per electron
no. of electron = 1900 / (1.6 x 10-19
) = 1.187 x 1022

8. Resistance
 It may be defined as “the property of a substance
due to which it opposes (or restricts) the flow of
electricity (i.e. electrons) through it”.
 When the same p.d. is applied across different
conductors, different currents flow.
 Some conductors offer more opposition or
resistance to the passage of current than others.
Unit of Resistance
• The practical unit of resistance is ohm (Ω).
• A conductor is said to have a resistance of one ohm if it
permits one ampere current to flow through it when one
volt is impressed across its terminals.

10. Resistance
 The resistance R of a conductor is defined as the
ratio of the potential difference V across it to the
current I flowing through it.
conductor
through
current
conductor
across
.
d
.
p
resistance 
I
V
R 
+ –
A
V
• •