Transcript Electrical Resistance and Ohm’s Law

Electrical Resistance
and Ohm’s Law
The Electric Current
 Electric current is a measure of the rate at which electric charges
move past a given point in a circuit.
 The SI Unit used to measure electric current is the ampere.
 The symbol for ampere is A.
 It is measured using an instrument
called an ammeter that would be
connected to the circuit in the series.
Analogy : Water wheel run by water
- if only one scoop of water each second – wheel would turn
relatively slowly – not much work could be done
-if have 20 scoops of water each second – wheel will turn more
quickly – more work could be done
Electrical Potential
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As the electric charges are released into the circuit from the power source they
carry a certain amount of energy.
The amount of energy each one has will determine how much work they can do
for us. The amount if energy it has is the electrical potential.
The SI Unit used to measure electrical potential is called the volt.
The symbol for the volt is V.
It is measured using an instrument called a voltmeter that would be connected to
the circuit in parallel.
Analogy
-if each scoop of water is raised only 6 cm and then released it will hit the water
wheel with very little force – moves the wheel very little and results in very little
work being done.
-if each scoop of water is raised 60 cm and then released it will
hit the water wheel with much more force – moves the wheel
much more resulting in much more work being done.
Electrical Resistance
 When electric charges move through various parts of a circuit they
can lose some of their energy (voltage).
 This is due to friction between the charges and the molecules that
make up the conductors in the circuit and loads.
 The ability of a conductor to slow down or impede the flow of
electricity is referred to as resistance.
 The loss of electric potential difference due to resistance is called
the potential difference.
 The SI Unit used to measure resistance is called the ohm (Ω).
 The symbol for resistance is R.
Analogy
-if one person is running on a flat empty road – relatively easy
then they will not be losing energy as quickly.
-if the same person is running on the same road which is crowded
with lots of people – makes it more difficult as the person has to
fight to get through crowds so they will use up more energy to
push through the crowds and therefore will get tired faster.
Ohm’s Law
 The potential difference between two points on
a conductor is proportional (directly related) to
the electric current flowing through the
conductor.
 This very simple law is used to calculate the
resistance of the load when designing many
different electrical devices.
 Although potential difference is the correct
term, the term voltage drop is commonly used
instead.
Ohm’s Law
 Potential Difference = Electric Current x
Electrical Resistance
V=IxR
Where potential difference (voltage drop) (V) is
measured in volts (V), electric current (I) is
measured in amperes (A), and resistance (R) is
measured in ohms (Ω).
Examples
 What is the voltage drop across the tungsten filament in a 100-W
light bulb? The resistance of the filament is 144 Ω and a current of
0.883 A is flowing through it.
 LIST THE VARIABLES
 I = 0.883 A
 R = 144 Ω
 V=?
EQUATION
V=IxR
SUBSTITUTE
V = (0.883 A) (144 Ω)
= 120 V
ANSWER
The voltage drop across a 100 W light bulb is 120 V.
You TRY!!!
 1. An electric toaster is connected to a
120-V outlet in the kitchen. If the heating
element in the toaster has a resistance of
14 Ω, calculate the current flowing through
it.
 2. The current required to operate an
electric can opener is 1.5 A. What is its
resistance of the supply voltage is 120 V?