Basic Electricity/Electronics

Related Training Instruction (RTI)Module 2 – Basic Electricity

Reading material associated with this module: None

► Items Covered in this section:1. Structure of Atoms2. Charge3. Concentrating Charge4. Current5. Ampere

Atoms

► All matter is made up of atoms.► Atoms are a collection of three kinds of

atomic particles.1) Protons2) Neutrons3) Electrons

Atomic Structure

►Protons and Neutrons are crowded together in the center of the atom forming the nucleus.

►Electrons orbit around the nucleus.►Intact atoms have the same number of

protons as electrons.►This number is referred to as the atomic

number which defines the kind of atom present.

Illustration of an Atom

Electric Charge

► Protons & Electrons are important topics in the study of electricity because they possess a quality called electric charge.

► Protons have a Positive Charge +.► Electrons have a Negative Charge -.► Neutrons have no Charge 0.► An intact atom has zero net charge because the

positive charge of the protons is balanced by the negative charge of the electrons.

►Like charges (+ to +, or – to -) tend to repel each other.

►Unlike charges (+ to -) tend to be attracted to each other.

►This is illustrated in the next slide.

Neutral Charge

Unlike Charge

Positive Like Charge

Negative Like Charge

Concentrating Charge

► This phenomenon causes the inner electrons that are close to the nucleus to be held tightly in place, but……

► In some atoms the outer electrons can become dislodged and move.

► At this point the atoms are no longer electrically neutral. ► If we rub a glass object with a smooth piece of cloth the

outer shell electrons in the glass move over to the cloth and crowd the cloth atoms outer shell.

► The two items are no longer electrically neutral they are said to be charged.

Current

Current Flow

► In the illustration on the previous slide electrons from atoms on the left side of the solid piece of metal/wire are strongly repelled by the negatively charged object.

► In those atoms on the right side of the solid piece of metal/wire the electrons are strongly attracted to the positively charged object.

► Inner shell electrons are tightly held in place, but outer shell electrons can break away from their parent atoms in reaction to these forces.

Current

► When a loosely held electron jumps away it lands on the outer shell of a neighboring atom.

► The neighboring atom now has a larger number of electrons than protons.

► This makes the hold on outer shell electrons even weaker making it more likely that one will migrate to an adjacent atom.

► There are an enormous number of electrons so this becomes a mass migration of electrons throughout the entire cross section of the metal/wire.

► This overall migration is called current.

Ampere

► It is necessary to have a method for describing the amount of current that is flowing.

► 1 coulomb of electrons is a huge number 6.24 X 1018 electrons.

► 1 Ampere is 1 coulomb of electrons passing a point in one second.

► The letter I is used to symbolize current in formulas, but the letter A is used to abbreviate it’s units (Ampere).

Example

► Suppose we measure 12.48 X 1018 electrons passing a point in a wire in an elapsed time of 10 seconds.

a) What is the amount of charge that has moved in coulombs?

b) What is the value of current flowing in the wire?

18

18

12.48 102

6.24 10Coulombs

20.2

10sec

Q CoulombsI Amperes

t onds

Test Your Understanding

►Which electrons actually jump from atom to atom when current flows; the inner shell electrons, or the outer shell electrons?

►What letter is used to symbolize current?►What is the basic measurement unit of

current?►What symbol is used for the basic

measurement unit of current?

Voltage & Voltage Sources

► This Section will outline1. The Voltage Idea2. Measurement Unit3. Differences between voltage & current4. Voltage Sources

The Voltage Idea

The Voltage Idea

► Look at the illustrations in the previous slide.► Note the difference in the net charge applied in

each illustration.► The force will be much greater in figure B

because the attraction and repulsion forces are greater if the net charge is more highly concentrated.

► The arrangement is figure B has a greater voltage than that of figure A since the electron moving force is greater.

► An electron moving force is often called electromotive force.

Water Tank Analogy

Water Tank Analogy► If we equate the electrical forces in figures A &

B to the Physical forces caused by the water in figures C & D.

► The height of the water in tank D is greater than tank C.

► The force tending to push water through the supply pipe depends on the total weight of water above the pipe.

► The greater amount of water in tank D provides more water moving force than the lesser amount in tank C.

► Just like the greater amount of net charge in figure B provides more electron moving force than the lesser amount of net charge in figure A.

Measurement Unit

►The basic unit of measurement for voltage is the Volt symbolized with the letter V.

►Voltage is an electron moving force.►A weaker force is specified by a smaller

number of volts.►A stronger force is specified by a larger

number of volts.

Difference between Voltage & Current

► Voltage is like a force, by itself it is not motion.► Current is motion-the motion of charge.► It is possible to have voltage without current

just as it is possible to have mechanical forces without motion.

► In our water tank examples the water can cause a force, but if the valve is closed no motion occurs.

► If we replace the solid metal/wire in our electron flow examples with a material that does not have a lot of free electrons we will have voltage without current.

► Voltage is the cause, current is the effect.

Voltage Sources

► Batteries-produce voltages by chemical reactions.► Generators-Create voltage using electromagnetic

induction.► Rectified power supplies-provides DC power from

AC wall outlets by allowing one voltage direction to appear, but blocking the other voltage direction.

► Solar Voltaic (Solar Cells)-Light Photons strike a system of “doped” silicon creating a voltage.

Test Your Understanding

►Which concept, voltage or current, conveys the idea of motion?

►Can voltage be present when current is not present?

►Is changing the voltage from 5 volts to 10 volts the equivalent to doubling the force exerted on the outer shell electrons?

Resistance

► Outline1. Meaning of Resistance.2. Unit of Resistance.3. Resistance in Series.4. Resistance in Parallel.5. Introduction to Ohms Law.6. Factors that determine Resistance.7. Resistance of Conductors.

Meaning of Resistance

►Electrical Resistance is the ability to oppose current flow.

►If a device allows very little current to pass when a voltage is applied the device has a high resistance.

►If it allows a large current to flow it has a low resistance

Resistance Units

► The basic unit is the ohm symbolized by the Greek letter omega V.

► In formulas we use the letter R to represent resistance.

► For the statement the resistance equals 10 ohms we write R=10V.

► The symbol for a resistor is:

Resistance in Series► Resistances connected in series have special

properties that are important to understand.1. The total resistance of a series circuit is the

sum total of the individual resistances. RT=R1+R2+R3…

2. The current flow in a series circuit remains constant. IT=I1=I2=I3…

3. The sum of the individual voltage drops across the resistances is equal to the source voltage. VS=VR1+VR2+VR3…

4. We will explore these in more detail during our introduction to Ohms Law.

Series Circuit Example

Resistance in Parallel► Resistances connected in parallel have their own special

properties that are important to understand.1. The reciprocal of the total resistance of a parallel circuit is

the sum total of the reciprocals of the individual resistances.

2. The voltage in a parallel circuit remains constant. VS=V1=V2=V3…

3. The sum of the current flow through the individual resistances is equal to the total current flow. IT=IR1+IR2+IR3…

4. We will explore these in more detail during our introduction to Ohms Law.

1 2 3

1 1 1 1...

TR R R R

Parallel Circuit Example

Test Your Understanding

►What is the basic unit of resistance?►What letter is used in formulas to symbolize

resistance?

Introduction to Ohms Law

►A voltage of 1 V (Volt), connected across a resistance of 1 V (Ohm), will allow 1 A (Ampere) of current to flow.

►V=I * R, Voltage is equal to the Current flow in the circuit times the resistance of the circuit.

►This is a very important law when analyzing DC circuits.

Going back to our series circuit

The Total Resistance

►Below is an example of the calculation of the total resistance of the series circuit shown in the previous slide.

1 2 3...

4 12 7 9 8 8

48

T

T

T

R R R R

R

R

The Current Flow

► The source voltage is given at 24 VDC in our example series circuit.

► We have calculated the total resistance of 48 V.

► We can now calculate the current flowing in the series circuit using ohms law. See the example calculation to the right

24

480.5

V I R

VI

RVDC

I

I Ampere

Back to our Rules for Series Circuits

1. The current flow in a series circuit remains constant. IT=I1=I2=I3…

► Since the current flow remains the same the current flow through each resistor is 0.5 Amperes as we have calculated in the example.

More of our Basic Rules

1. The sum of the individual voltage drops across the resistances is equal to the source voltage. VS=VR1+VR2+VR3…

2. To prove this we need to use ohms law. V=I*R

1 1 1

1

1

0.5 4

2

R R

R

R

V I R

V A

V Volts

2 2 2

2

2

0.5 12

6

R R

R

R

V I R

V A

V Volts

6 6 6

6

6

0.5 8

4

R R

R

R

V I R

V A

V Volts

5 5 5

5

5

0.5 8

4

R R

R

R

V I R

V A

V Volts

4 4 4

4

4

0.5 9

4.5

R R

R

R

V I R

V A

V Volts

3 3 3

3

3

0.5 7

3.5

R R

R

R

V I R

V A

V Volts

1 2 3.....

2 6 3.5 4.5 4 4

24

24

T R R R

T

T

S

V V V V

V V V V V V V

V V

V V

Now Lets Look at the Parallel Circuit

The Total Resistance ► Below is an example of the calculation of the total

resistance of the parallel circuit shown in the previous slide.

► Note that the total resistance is lower than the smallest resistance in the parallel circuit. This will always be true.

1 2 3

1 1 1 1...

1 1 1 1 1 1 1

4 12 7 9 8 8

1 126 42 72 56 63 63

504 504 504 504 504 504

1 422

504

504

1 4221.2

T

T

T

T

T

T

R R R R

R

R

R

R

R

The Current Flow

► The source voltage is given at 24 VDC in our example series circuit.

► We have calculated the total resistance of 1.2 V.

► We can now calculate the total current flowing in the parallel circuit using ohms law. See the example calculation to the right

24

1.220

V I R

VI

RVDC

I

I Ampere

Back to the Rules for Parallel Circuits

1. The voltage in a parallel circuit remains constant. VS=V1=V2=V3…

► Since the voltage in a parallel circuit remains constant, the voltage across each resistance in our example is the source voltage 24VDC. Looking at the schematic this makes sense because the source is directly connected across each resistance.

More of our Basic Rules

1. The sum of the current flow through the individual resistances is equal to the total current flow. IT=IR1+IR2+IR3…

2. To prove this we need to use Ohms Law I=V/R.

11

1

1

1

24

46

RR

R

R

VI

R

VI

I Amperes

66

6

6

6

24

83

RR

R

R

VI

R

VI

I Amperes

33

3

3

3

24

73.4

RR

R

R

VI

R

VI

I Amperes

44

4

4

4

24

92.6

RR

R

R

VI

R

VI

I Amperes

22

2

2

2

24

122

RR

R

R

VI

R

VI

I Amperes

55

5

5

5

24

83

RR

R

R

VI

R

VI

I Amperes

1 2 3.....

6 2 3.4 2.6 3 3

20

T R R R

T

T

I I I I

I A A A A A A

I A

Factors that determine Resistance

► The resistance of any piece of material depends on three factors:

1. The length-The longer the length the higher the resistance.2. The cross-sectional area-The larger the cross sectional area

the lower the resistance.3. The resistivity of the material-Greater resistivity causes

greater resistance.This gives us the relationship:

► Where R is resistance, r is resistivity, l is length, and A is cross-sectional area.

5. Understanding this relationship is imperative if we are to be able to complete voltage drop calculations for our Notification Appliance Circuits.

r lR

A

Wire has Resistance

► Since wire has resistance, Ohm’s Law applies!!► We have to insure that the Voltage Drop across our Wire

does not impair the circuits ability to operate.► This is especially important on Notification Appliance

Circuits.► We use Copper Wire which has a resistivity (r) at 75

degrees C of 13.09V-cm/ft. Note this figure is conservative because of the high temperature.

► From Table 8 of the National Electrical Code NFPA 70 we can obtain the cross-sectional areas of American Wire Gauge (AWG) sizes.

More on Wire

►Cross-sectional area for commonly used wire sizes.

►18AWG 1620cm►16AWG 2580cm►14AWG 4110cm►12AWG 6530cm

Resistance & Voltage Drop

►EXAMPLE PROBLEM►If we have a 200 foot length of 18AWG

cable and we intend to put an explosion proof horn strobe at the end of the cable that has a minimum operating voltage of 16VDC and a current draw of 1.7Amperes will the strobe operate properly when the circuit is activated?

Example Problem (Resistance)

► The first thing we need to do is calculate the resistance of the wire in the cable.

► We need to consider that there are two conductors in our cable so the length of the wire is two (2) times the cable length or 400 feet.

13.09 1400

1620

3.23

r lR

Acm

R ftft cm

R

Example Problem (Voltage Drop)

►Fire Alarms must operate between 85% and 110% of the rated name plate voltage.

►A typical fire alarm system operates at 24VDC nominal on battery power.

►We must consider a worst case scenario where the panel is operating at 85%

►24VDC * .85 = 20.4VDC►Our starting voltage is 20.4VDC

Example Problem (Voltage Drop)

► Since we know our current draw is 1.7 Amperes, and our wire resistance is 3.23 V.

► We can calculate the voltage that is dropped across the resistance of the cable using ohms law. V=I * R

1.7 3.23

5.49

V I R

V A

V V

Voltage at our Horn Strobe

► Since we had to consider our system operating at 20.4VDC and we have a voltage drop across the cable of 5.49VDC the voltage at our device is 20.4VDC-5.49VDC=14.91VDC.

► The minimum operating voltage of our device was 16VDC. The device would not operate if the panel was operating at 85% of its rated name plate voltage as required by code.

► To insure proper operation we would need increase the size of the conductors.

Practice on your own

►If we keep everything identical but increase the wire size to 16AWG complete the necessary calculations to determine if the device will operate properly or not.