OHMS LAW.ppt

8/13/2019 OHMS LAW.ppt

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Ohms Law explains therelationship between voltage (V

or E), current (I) and resistance(R)

Used by electricians, automotivetechnicians, stereo installers


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Every conversion of energy from one formto another can be related to this equation.

In electric circuits the effect we are trying toestablish is the flow of charge, or current.Thepotential difference, or voltagebetweentwo points is the cause (pressure), andresistanceis the opposition encountered.

Opposition

CauseEffect


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Simple analogy: Water in a hose

Electrons in a copper wire are analogous to water in ahose.

Consider the pressure valve as the applied voltage andthe size of the hose as the source of resistance.

The absence of pressure in the hose, or voltage across thewire will result in a system without motion or reaction.

A small diameter hose will limit the rate at which waterwill flow, just as a small diameter copper wire limits theflow of electrons.


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Developed in 1827 by Georg Simon Ohm

For a fixed resistance, the greater the voltage (orpressure) across a resistor, the more the current.

The more the resistance for the same voltage, theless the current.

Current is proportional to the applied voltage andinversely proportional to the resistance.


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It is the push or pressure behindcurrent flow through a circuit,

and is measured in (V) volts.


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Current refers to thequantity/volume of electrical

flow. Measured in Amps (A)


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Resistance to the flow of thecurrent. Measured in Ohms


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1. Assuming the resistance does notchange:

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.


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What is Voltage?

What is Current?

What is Resistance?


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V (E) = I x R

I = VR

R = V

I


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Battery voltage is 12V

Current is Amp ?

Resistance 2 Ohm


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Voltage is 12V

Current is 4 Amps

Resistance Ohms ?


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Multi meters/ DVOM


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Why do we need resistors

To decrease the amount of voltageapplied to a component

The value of the resistor is marked on thebody using coloured rings


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What is Voltage?

What is Current?

What is Resistance?

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What is a load

It is a any electric load on a circuit that doeswork.

Example: Power windows, light bulbs, motors.


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Series circuit : One pathway for current to flow.

Example: Old Christmas lights


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More then one path way for current toflow.

Used in most electrical vehicle circuits.


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What is Voltage?

What is Current?

What is Resistance?

What are wiring diagrams?

What are the different circuitsand there characteristics?


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Power is an indication of how much work (theconversion of energy from one form to another)can be done in a specific amount of time; that

is, a rateof doing work.


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Energy (W) lost or gained by any system isdetermined by:

W =Pt

Since power is measured in watts (or joulesper second) and time in seconds, the unit of

energy is the wattsecond(Ws) orjoule(J)


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The watt-second is too small a quantity for mostpractical purposes, so the watt-hour(Wh) andkilowatt-hour(kWh) are defined as follows:

The killowatt-hour meteris an instrument usedfor measuring the energy supplied to a residentialor commercial user of electricity.

1000

(h)time(W)power(kWh)Energy

(h)time(W)power(Wh)Energy


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Efficiency () of a system is determined by thefollowing equation:

=Po/ Pi

Where: = efficiency (decimal number)

Po= power output

Pi= power input


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The basic components of a generating (voltage)

system are depicted below, each component has anassociated efficiency, resulting in a loss of powerthrough each stage.

Insert Fig 4.19


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Insert Table 4.1


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Power coming into any facility or item must be limitedto ensure that the current through the lines or electricalequipment is not above the rated value.

Fuses or circuit breakers are installed where the power

enters the installation.Fuses have an internal metallic conductor which begins to meltif the current exceeds the fuse rated value on the case.

In recent years fuses have been replaced with circuit breakers.Circuit breakers have an electromagnet that, when the currentexceeds the rated value, has sufficient strength to draw the

connecting metallic link out of the circuit and open the path.


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National Electrical Code requires that outlets inthe bathroom and other sensitive areas be of theGround Fault Circuit Interrupt (GFCI) variety.

GFCIs are designed to trip more quickly than thestandard circuit breaker.

GFCI senses differences in input and outputcurrents to the outlet, and trips if they are not the

same.


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Microwave ovens

Most microwaves are rated at 500 W to 1200 W at afrequency of 2.45 GHz.

Heating occurs because the water molecules in thefood vibrate at such a high frequency that thefriction with neighboring molecules causes theheating effect.

Most microwaves are between 50% and 60%efficient.


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Household wiring

Most older homes, without electric heating, have a100 A service.

Power is broken down into different circuitsutilizing 15 A, 20 A, 30 A and 40 A protectivebreakers.

Maximum load on each breaker should not

exceed 80% of its rating (12 A of a 15 A circuitbreaker).


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The correct gauge of wire must be used withthe right circuit breaker #14 wire up to a 15 Abreaker, #12 wire up to 20 A, #10 wire up to 30

A. Grounding is a very important part of safety.

The National Electric Code requires that the neutralwire of a system be grounded to an earth-driven rod,

a metallic water piping system of 10 ft or more, or aburied metal plate.

OHMS LAW.ppt

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