Transcript Electrical Principles Chapter 3

Electrical Principles
Chapter 3
Resistance, Power, Coulombs, Ohm’s
Law, Power Formula, Power – Current
Relationships, Impedance, Multimeters,
Calculating-Measuring Energy and Power
Resistance and Ohm’s
Resistance and Ohm’s
Resistance is the opposition to current
flow by the dissipation of heat.
The Resistor is a device that is included
within electrical and electronic circuits to
oppose current flow by introducing a
certain value of circuit Resistance.
Resistance is measured in Ohms and is
represented by the Geek letter Omega,
the symbol for Omega is Ώ.
Resistance and Ohm’s
The larger the resistance, the larger the
value of Ohms and the more the resistor
will oppose current flow.
As the resistance in the circuit is increased
the current will decrease and, conversely,
as the resistance of the resistor is
decreased the circuit current will increase.
Ohm’s Law
In 1827 George Ohm proved there was a
direct relationship between Voltage (E),
Current (I), and Resistance (R) in an
electrical circuit. This relationship is
known as Ohm’s Law.
Ohm’s Law states that current in a circuit
is proportional to the voltage and inversely
proportional to the resistance.
Ohm’s Law
E
I
E = Voltage - Volts
I = Current - Amps
R = Resistance or Reactance
(Impedence) - Ohms
R
Direction: Cover what you want and
perform the mathematical process
with what’s left over.
Example: Cover E = I X R
Ohm’s Law
Voltage/Current Relationship. Ohm’s Law
states that if the resistance in a circuit
remains constant, a change in current is
directly proportional to a change in
voltage. Example: A heat shrink sealing
gun connected to a variable power supply.
Ohm’s Law
Current/Resistance Relationship. Ohm’s
Law states that if the voltage in a circuit
remains constant, a change in resistance
produces an inversely proportional change
in current. The current in a circuit
decreases with an increase in resistance,
and the current in the circuit increases with
a decrease in resistance. Example: The
dimmer/brightness switch for lights inside
a car.
Ohm’s Law
In engineering applications, Ohm’s Law is
used to solve for the proper values of
voltage, current, or resistance during
circuit design and to predict circuit
characteristics before power is applied to a
circuit when two of the three electrical
values are know.
Ohm’s Law
In troubleshooting applications, Ohm’s
Law is used to determine how a circuit
should operate and how it is operating
under power.
Resistance measurements can not be
taken when a circuit is under load (power).
Current and Voltage can be taken when a
circuit is under load (power).
Power
Power
Transforming energy from one form to
another is called work. The greater the
energy transformed, the more work that is
done.
There are six basic forms of energy and
they are light, heat, magnetic, chemical,
electrical, and mechanical energy.
The unit for measuring work is called the
Joule (J).
Power
Power (P) is the rate at which work is
performed and is measured by the unit
called Watt (W). Watts = Joules per
second.
The output Power, or power ratings of
electrical, electronic or mechanical devices
can be expressed in Watts (W) and
describes the number of Joules of energy
converted every second.
Power
The output of Power of rotating machines
is given in the unit horsepower (hp).
The output of Power of heaters is given in
the unit British Thermal Units per hour
(BTU/h)
The output of Power of cooling units is
given in the unit Ton of Refrigeration.
Power
Despite the different names, all can be
expressed in the unit of Watts (W) by
using the following conversions:
1 hp = 746 W
1 BTU/h = 0.293 W
1 ton of refrigeration = 3520 W
The amount of energy stored (W) is
dependent on the coulombs of charge
stored (Q) and the voltage (V) and can be
expressed mathematically W = Q x V.
Power
Power is the rate at which electric energy
(W) is converted to some other form and
can be expressed mathematically as
P = I x V.
This formula states that the amount of
power delivered to a device is dependent
on the electrical pressure (or voltage
applied across the device) and the current
flowing through the device.
Power Formula
The Power Formula is the relationship
between Power (P), Voltage (E), and
Current (I).
P
P = Power -Watts
E = Voltage - Volts
I = Current - Amps
Direction: Cover what you want and perform the mathematical process
with what’s left over. Example: Cover E = I X R
E
I
Power Formula
The Power Formula states that if the
voltage in a circuit changes, the current in
the circuit also changes. The power
required from a circuit changes any time
loads are added (power increases) or
removed (power decreases).
The Power Formula is used when
troubleshooting and to predict circuit
characteristics before power is applied.
Combining Ohm’s Law and
Power Formula
Ohm’s Law and the Power Formula may be
combined mathematically and written as any
combination of Voltage (E), Current (I),
Resistance (R), or Power (P).
Ohm’s Law and the Power Formula are limited
to circuits in which electrical resistance is the
only significant opposition to the flow of current.
This limitation includes all DC circuits and AC
circuits that do no contain a significant amount of
inductance and/or capacitance – which we will
learn about later.
Combining Ohm’s Law and
Power Formula
Review
Ohm’s Law states that current in a circuit
is proportional to the voltage and inversely
proportional to the resistance.
The Power Formula states that if the
voltage in a circuit changes, the current in
the circuit also changes. The power
required from a circuit changes any time
loads are added (power increases) or
removed (power decreases).
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