Basic Electrical Theory:
Understanding Ohm’s Law
Copyright © Texas Education Agency, 2011. All rights reserved.
Introduction
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What is Ohm’s Law?
Why is ohms law important?
The Electrical Components of Ohm’s Law
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1. Electromotive Force
2. Current
3. Resistance
4. Power
Ohm’s Law Formulas
Using The Ohm’s Law Chart to calculate
electrical circuits.
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Terms to Remember
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The Source can be any source of electrical energy. In practice, there are
three general possibilities: it can be a battery, an electrical generator, or some
sort of electronic power supply.
The Load is any device or circuit powered by electricity. It can be as simple as
a light bulb or as complex as a modern high-speed computer.
(Path) a wire or pathway which will allow electron to flow throughout a circuit.
Electricity can be described as the flow of charged particles. If the
particles accumulate on an object, we term this static electricity.
(Direct Current) An electrical current that travels in one direction and used
within the computer's electronic circuits.
(Alternating Current) The common form of electricity from power plant to
home/office. Its direction is reversed 60 times per second.
Circuit is a conducting path for electrons.
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What is Ohm’s Law?
Ohm's Law states that, at constant temperature, the electric
current flowing in a conducting material is directly proportional
to the applied voltage, and inversely proportional to the
Resistance.
Why is Ohms Law important?
Ohm’s Law is the relationship between power, voltage, current
and resistance. These are the very basic electrical units we
work with. The principles apply to alternating current (ac),
direct current (dc), or radio frequency (rf) .
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The Electrical Components of Ohm’s Law
Voltage
The electrical "pressure" that
causes free electrons to travel
through an electrical circuit. Also
known as electromotive force (emf).
It is measured in volts.
Power
The amount of current times the
voltage level at a given point
measured in wattage or watts.
Resistance
That characteristic of a medium
which opposes the flow of
electrical current through itself.
Resistance is measured in ohms.
Current
The amount of electrical charge
(the number of free electrons)
moving past a given point in an
electrical circuit per unit of time.
Current is measured in amperes
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Ohm’s Law Formulas
This is Ohm's law, where E = Volts;
I = current in amperes and R =
resistance in ohms.
Ohm’s Law expresses the relationship
between the current (I), the voltage (E),
and the resistance (R) in a circuit.
Ohm’s Law can be expressed in three
different ways, and can be applied to
the entire circuit or to any part of a
circuit. When any two factors are
known, the third unknown factor can be
calculated from Ohm’s Law.
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Characteristics Of Ohm’s Law
Voltage: Difference of potential, electromotive force, ability to do work.
Unit of measure Volt Symbol E or V (This symbol will be used in the Ohms
Law Formula)
Compares to Pressure in a gas or liquid system.
Current: Flow of electrons Unit of measure Ampere Symbol I
(This symbol will be used in the Ohms Law Formula)
Compares to The flow of gas or liquid in a piping system.
Resistance: Opposition to current flow Unit of measure Ohm often seen as the
Greek letter Omega Symbol R (This symbol will be used in the Ohms Law
Formula) Compares to Orifice effect or the size restriction of inside pipe
Diameter.
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Finding Voltage
Ohm’s Law Formula for Finding Voltage
(E)
(I)
(R)
Voltage = Current x Resistance
To find Voltage Multiply the Current times the
Resistance.
E=I x R
Using the Magic Circle
Cover the E to see the mathematical
relationship between Voltage and Current
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Finding Current
Ohm’s Law Formula for Finding Current
(I)
Current=
(E)
Voltage
————
(R)
Resistance
To find Current Divide the Voltage by the
Resistance
E
I= —
R
Using the Magic Circle
Cover the I to see the mathematical relationship
between Voltage and Resistance
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Finding Resistance
Ohm’s Law Formula for Finding
Resistance
(R)
(E)
Resistance= Voltage
———
(I)
Current
To find Resistance Divide the Voltage
by the Current
E
R=
—
I
Using the Magic Circle
Cover the R to see the mathematical
relationship between Voltage and Current
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In this algebraic expression, voltage (E) is equal to current (I) multiplied by
resistance (R). Using algebra techniques, we can manipulate this equation
into two variations, solving for I and for R, respectively:
Let's see how these equations might work to help us analyze simple circuits:
In the above circuit, there is only one source of voltage (the battery, on the
left) and only one source of resistance to current (the lamp, on the right).
This makes it very easy to apply Ohm's Law. If we know the values of any
two of the three quantities (voltage, current, and resistance) in this circuit,
we can use Ohm's Law to determine the third.
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In this first example, we will calculate the amount of current (I) in
a circuit, given values of voltage (E) and resistance (R):
What is the amount of current (I) in this circuit?
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In this second example, we will calculate the amount of resistance (R) in a
circuit, given values of voltage (E) and current (I):
What is the amount of resistance (R) offered by the lamp?
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In the last example, we will calculate the amount of voltage supplied by a
battery, given values of current (I) and resistance (R):
What is the amount of voltage provided by the battery?
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Ohm's Law is a very simple and useful tool for
analyzing electric circuits. It is used extensively in the
study of electricity and electronics. Remember When
any two factors are known, the third unknown factor
can be calculated from Ohm’s Law.
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Ohm's Law is a very simple and useful tool for analyzing
electric circuits. It is used extensively in the study of
electricity and electronics. Remember When any two factors
are known, the third unknown factor can be calculated from
Ohm’s Law.
REVIEW:
 Voltage measured in volts,
symbolized by the letters
"E" or "V".
 Current measured in
amps, symbolized by the
letter "I".
 Resistance measured in
ohms, symbolized by the
letter "R".
 Ohm's Law: E = IR ; I =
E/R ; R = E/I
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