Basic Electrical Theory: Understanding Ohm’s Law Copyright © Texas Education Agency, 2011. All...

Basic Electrical Theory:Understanding Ohm’s Law

Copyright © Texas Education Agency, 2011. All rights reserved.

Copyright © Texas Education Agency, 2011. All rights reserved. 2

Introduction What is Ohm’s Law? Why is ohms law important? The Electrical Components of Ohm’s Law

1. Electromotive Force 2. Current 3. Resistance 4. Power

Ohm’s Law Formulas Using The Ohm’s Law Chart to calculate

electrical circuits.


Terms to Remember

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.


What is Ohm’s Law?

Ohm's Law states that, at constant temperature, the electriccurrent flowing in a conducting material is directly proportionalto the applied voltage, and inversely proportional to theResistance.

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) .


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.

Resistance

That characteristic of a medium

which opposes the flow of

electrical current through itself.

Resistance is measured in ohms.

Power The amount of current times the

voltage level at a given point

measured in wattage or watts.

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


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 relationshipbetween the current (I), the voltage (E),and the resistance (R) in a circuit.Ohm’s Law can be expressed in threedifferent ways, and can be applied tothe entire circuit or to any part of acircuit. When any two factors areknown, the third unknown factor can becalculated from Ohm’s Law.


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.


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


Finding Current

Ohm’s Law Formula for Finding Current

(I) (E) Current=

Voltage ————

(R) Resistance

To find Current Divide the Voltage by theResistance

E I = — R

Using the Magic Circle Cover the I to see the mathematical relationshipbetween Voltage and Resistance


Finding Resistance

Ohm’s Law Formula for FindingResistance

(R) (E) Resistance= Voltage

——— (I) Current

To find Resistance Divide the Voltageby the Current

E R = —

I

Using the Magic Circle Cover the R to see the mathematicalrelationship between Voltage and Current


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.


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?


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?


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?


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.


Ohm's Law is a very simple and useful tool for analyzing electric circuits. It is used extensively in the study ofelectricity 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

Basic Electrical Theory: Understanding Ohm’s Law Copyright © Texas Education Agency, 2011. All...

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