OHM'S LAW

SUMMARY OF FORMULAS

WHAT IS OHM'S LAW?

**OHM'S LAW**

VOLTAGE IS . . .

**Group 11**

36 Princess Tipo

37 Joan Tolosa

38 Jill Topacio

36 Princess Tipo

37 Joan Tolosa

38 Jill Topacio

**OHM'S LAW IS . . .**

WHAT IS VOLTAGE?

push and pressure behind current flow through a circuit.

electric potential energy (work ) per unit charge.

measured in joules per coulomb ( = volts).

instrument used to measure voltage is called a VOLT METER.

WHAT IS CURRENT?

CURRENT IS . . .

defined as the rate at which charge flows through a surface.

the ratio of the quantity of charge and time.

1 ampere = 1 coulomb / 1 second

WHAT IS RESISTANCE?

RESISTANCE IS . . .

hindrance to the flow of charge.

R E M E M B E R :

VOLT - the practical unit for measuring electromotive force.

AMPERE - one ampere is equal to current flowing through a conductor.

OHM - an e.m.f of one volt will cause a current one ampere through a resistance of one Ohm.

the relationship between voltage (V or E), current (I) and resistance (R) in an ideal conductor.

used by electricians, automotive technicians and stereo installers.

1. Assuming the resistance does not change:

As voltage increases, current increases.

as voltage decreases, current decreases.

2. Assuming the voltage does not change:

As resistance increases, current decreases.

As resistance decreases, current increases.

GENERAL RULES

UNITS USED IN OHM'S LAW

BASICS TO SIMPLE CIRCUIT

A simple circuit contains the minimum things needed to have a functioning electric circuit.

1. source of voltage

e.g. battery, electrical outlet

2. conductive path

e.g. typically anything made of wire

3. resistor

e.g. light bulb, electric motor, heating element, speaker, etc.

E L E C T R I C F I E L D

- electric force per unit charge.

- direction of the field is taken to be the direction of the force it would exert on a positive test charge.

- radially outward from a positive charge, radially in toward a negative point charge.

Definition of Terms used in Voltage ( V ) :

a. ) ELECTROMOTIVE FORCE - also called emf (denoted and measured in volts), is the voltage developed by any source of electrical energy such as a battery or dynamo. It is generally defined as the potential for a source in a circuit.

b.) POTENTIAL DIFFERENCE - The difference in electric potential between two points, especially two points in an electric circuit.

DEFINITION OF TERMS :

+ BATTERY : series of dry cells

+ DRY CELL : a voltage-generating cell having an electrolyte in the form of moist paste.

+ CONDUCTIVE WIRE : a metal conductor or device that is designed to transmit energy, electricity or heat. it also carries electricity over a distance.

+ BULB : electric light which produces light with a wire filament heated to a high temperature by an electric current passing through it, until it glows.

FLOW OF SIMPLE CIRCUIT

voltage : ( v )

current : ampere ( a )

resistance : ohm (Ω)

SAMPLE PROBLEMS

If a circuit has a voltage of 500 V and a resistance of 250 Ω, what is the current?

question #1 :

ANSWER :

I=V/R

I=500/250

Ans: 2.0A

question #2 :

what is the voltage of the battery?

answer :

V=I*R

V=I*(R1+R2)

V=(0.5)*(10+10)

V=10V

question #3 :

If a circuit has a resistance of 100 Ω and a current of 0.05A, what is the voltage?

answer :

V=I*R

V=100*0.05

V=5V

If a circuit has a voltage of 45 V and a current of 5.0 A, what is the resistance?

question #4 :

answer :

R=V/I

R=45/5.0

R=9.0 Ω

An electrical device with a resistance of 3.0 Ω will allow a current of 4.0 amps to flow through it if a voltage drop of ________ Volts is impressed across the device.

question #5 :

answer :

12 volts

R E S I S T A N C E

What other words can you relate with resistance?

: Opposition offered by any object to the passage of electric current through it

: Anything that will RESIST the flow of electric current

: Regulates the amount of current flowing in a wire

The symbol used for resistance is R.

R E S I S T A N C E

There are four factors of resistance :

a) length

b) cross-sectional area

c) resistivity of material

d) temperature

FACTORS OF RESISTANCE

The BIGGER the cross-section area :

+ more CURRENT

+ less resistance

The amount of resistance is :

+ DIRECTLY PROPORTIONAL to its LENGTH.

+ INVERSELY PROPORTIONAL to its cross-sectional area.

Resistance = length

cross-sectional area

NO simple relationship between temperature (depends on the object)

R E L A T I O N S H I P S

Glass and synthetic polymers have great resistance because so many of their electrons are not free to move from atom to atom.

Silver and copper offer only slight resistance to an electric current because many of their electrons are free to move from atom to atom.

R E S I S T I V I T Y

The resistance of a material is given by its

resistivity

.

Allow comparison of the abilities of different materials to conduct current

The relationship of resistance (R) to length (l), cross-sectional area (A) and resistivity (ƿ) is given as

RESISTOR COLOR CODING

R E S I S T O R

an electrical component that implements electrical resistance as a circuit element.

HOW TO READ RESISTOR

3 400 000(0.05) = 170 000

3 4000 000 – 170 000 = 3 230 000

3 4000 000+ 170 000 = 3 570 000

= 3 230 000 to 3 570 000 Ω tolerance

240 000 (0.05) = 12 000

240 000 – 12 000 = 228 000

240 000 + 12 000 = 252 000

= 228 000 to 252 000 Ω tolerance

65 Ω ± 5%

65 (0.05) = 3.25

65 – 3.25 = 61.75

65 + 3.25 = 68. 25

61. 75 to 68.25 Ω tolerance

3 4 00,000

2 4 0,000

6 5

THANK YOU FOR LISTENING!

MOST RESISTORS HAVE 4 BANDS

SOME RESISTORS HAVE 5 BANDS

(52)(0.1) = 5.2 Ω ±10%

(5.2)(0.1) = 0.52

5.2 – 0.52 = 4.68

5.2 + 0.52= 5.72

= 4.68 to 5.72 Ω tolerance

5 2

SOME RESISTORS HAVE 3 BANDS, YOU MAY FIND THESE IN ANTIC RADIOS AND ANTIC TV.