Electricity
Current is the rate of flow of charge.
Current
I = ΔQ
Δt
19
The potential difference between two points is the electrical energy transferred per unit of charge passing through the points.
Potential Difference
V = W
Q
Resistance is the opposition to the flow of charges.
Resistance
Charge is
measured in
Coulombs (C)
Current is measured
in Amps (A)
e

Current (A) = Charge (C)
Time (s)
A charge of 1.24 C flows in a period of 0.63 s.
What is the current?
Use Q = It
I = Q/t
= 1.24
0.63
= 1.97 A
 Discuss the definitions of Current, Voltage and Resistance
 Use Ohm's Law to calculate these quantities
Electricity
R = V
Example:
A Bulb of resistance 25Ω has a potential difference of 8V applied across it.
a)Calculate the current produced in the bulb
b)Calculate the charge delivered to the bulb in 1 minute
c)Calculate the number of electrons flowing through the bulb in 1 minute
d)Calculate the energy dissipated in the bulb during 1 minute
Ohm's Law
It is the
Work Done per unit of charge
.
It is measured in
Joules per Coulomb
or
Volts
.
It is measured in Ohms (Ω).
It can be defined by the equation.
A box contains 1C of charge.
How many electrons is this?
I
Activity: in the diagram below if 4C of charge passes through the bulb entering with a total energy of 12.6J and leaving with 3.4J then what is the potential difference?
In a metal, the charge carrier is an electron. One electron has 1.6 x 10
Coulombs
of charge.
The current in a metallic conductor is directly proportional to the potential difference between its ends provided that the temperature and other physical conditions are the same.
Ohm's Law
Conventional Current
At any junction in an electrical circuit,
the sum of currents flowing in is equal to the sum of currents flowing out:
Kirchoff's First Law
Kirchoff's First Law
4A
7A
3A
1A
?
4A
3A
4A
?
In a circuit the electrons actually flow towards the positive terminal of the cell. However for historical reasons we always assume that current flows from __________ to __________. This is known as conventional current flow.
In metallic conductors the charge carriers are electrons. Each electron has a negative charge of ____________.
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Electricity 1  Current, Voltage, Resistance
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