Download Review of Current, Voltage and Resistance ppt

CURRENT, VOLTAGE, AND
RESISTANCE
Variable
Variable
Symbol
Unit
Unit Symbol
time
t
seconds
s
current
I
ampere
A
electric
charge
Q
coulomb
C
So what is electric charge in coulombs?
We count the amount of electric charge (Q) in
coulombs (C).
1 coulomb (C) of electric charge = the charge of
6.25 x 1018 electrons
i.e. 1 C = 6,250,000,000,000,000,000 electrons !
1 electron has a charge of 1.6 x 10-19 Coulombs
30 cars pass by a certain point in 3 hours
=
30 Coulombs of charge pass by a certain point in 3 s
=
WHAT IS VOLTAGE (OR
POTENTIAL DIFFERENCE)?
When the switch on a circuit is closed, the
energized electrons move from the negative
terminal, through a load and return to the
positive terminal with no remaining energy.
The chemical energy from the battery is
converted to light energy in a light bulb.
3. Electrons return
to positive terminal
with no energy left
1. Energized
electrons leave
the negative
terminal
2. Electrons give
up their energy to
the load
Potential difference:
The easiest way to think
about what batteries do is
to use a water analogy.
Batteries ‘lift’ electric
charges to a higher
potential. There is a
potential difference
between one end of a
battery to the other.
The electric charges with
higher potential energy
can now do work, by
flowing ‘downhill’ through
a load.
Potential difference is the difference in potential
energy per coulomb of charge between two
points in the circuit.
Potential difference is called voltage or electric
potential.
Potential difference is measured in volts by a
voltmeter.
Which of these
figures illustrates
higher potential
difference or voltage?
Variable
Variable
Symbol
Unit
Unit Symbol
Potential
difference or
voltage
V
volts
V
energy
E
joules
J
electric
charge
Q
coulomb
C
NOTE: Potential difference or voltage is the only variable
where the symbol for the units is the same as the
symbol for the variable.
WHAT IS RESISTANCE?
WHAT IS OHM’S LAW?
Electrical resistance is the ability of a conductor to
impede or oppose the flow of charge in a circuit.
The SI symbol for resistance is R. Resistance is
measured in units called ohms, and is represented
by the symbol Ω (the Greek letter “Omega”).
Ohm’s Law relates voltage, electrical current, and
resistance:
R=V/I
The resistance between 2 points on a circuit is
proportional to the potential difference and inversely
proportional to the current.
1 Ω is the resistance when 1 A of current flows due
to a potential difference of 1 V.
V=R×I
V
R = V/I
R
I
Variable
Potential difference
or voltage
I = V/R
Variable
Symbol
V
Unit
Volts
Unit
Symbol
V
Current
I
Amperes
A
Resistance
R
Ohms
Ω
Example: A motor has a current of 5 A running
through it and a resistance of 10 Ω. What is the
voltage across the motor?