19 Resistance

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RESISTANCETOPIC 19 : CURRENT ELECTRICITY

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WHAT IS RESISTANCE?

The measure of the degree to which an

electrical componentopposes the flow

of an electric current .

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Higher Resistance,

Small Current

Lower Resistance,

Large Current

An electrical conductor that has:

flowing through it.


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ELECTRICAL CONDUCTOR ?

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Any components that allows current

to flow through it.


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RESISTANCEThe resistance of an electrical component

is given by the equation

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Resistance Potential Difference

Current=

S.I unit of resistance is the Ohm ( )


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MEASURING RESISTANCE

The resistance of

conductor R can be

found by measuring

the current in the

circuit and the

potential difference

across R.PreparedbyCg


Ammeter-Voltmeter

Method

Rheostat

Voltmeter

Ammeter

Power Source

R

R


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MEASURING RESISTANCE

A graph of potential

difference (V)

against current (I) isplotted.

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Gradient of V-I graphgives the resistance

of the component.


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OHMS LAW

The current through a metallic

conductor is directly

proportional to the voltageacross its ends if the

temperature and other

conditions are constant.PreparedbyCg


Georg Simon OhmI V


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I-V GRAPHS

GraphsofCurr

entagainstVoltageisusedtoillustratewhetheracomponentobeysOhmsLaw.

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Current

Voltage


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I-V GRAPH OF

METALLIC CONDUCTORS

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Obey Ohms Law

Current is directly

proportional to Voltage

I V

Ohmic Conductors

Constant Resistance


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I-V GRAPH OF

FILAMENT BULBS

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Doesnt obey Ohms Law

Resistance as

Current

Non-Ohmic Conductors


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I-V GRAPH OF

SEMICONDUCTOR DIODES

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Doesnt obey Ohms Law Non-Ohmic Conductors

Resistance if Voltage +Resistance if Voltage -


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I-V GRAPH OF

THERMISTORS

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Resistance if

Temperature



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I-V GRAPH OF

LIGHT DEPENDENT RESISTOR

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Resistance ifLight Intensity



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Milliammeter

Voltmeter

Fixed Resistor

LDR


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LIGHT DEPENDENT RESISTOR

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When an LDR and fixed resistor are connected inseries, it forms part of a simple burglar alarm system.

FixedResistor

LDR


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FACTORS AFFECTING RESISTANCE

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Resistance =

ifTemperature =

Metallic Conductors Semi-conductors

Resistance if

Temperature Resistance

if

Temperature


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For a uniform wire:

Resistance , Length

Shorter wire, Lower Resistance

Longer wire, Higher Resistance

Length

Length

Longerwiresprovideslongerpathforthecurrenttopassthrough.


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For a uniform wire:

Resistance , Temperature

Cooler wire, Lower Resistance

Temperature

Temperature

Hot wire, Higher Resistance

Inhotterwire,the

particlesarevigorouslyvibrating,providingmoreobstacleforthecurrenttopass

through.


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For a uniform wire:

Resistance , Cross-Sectional Area

Thickerwiresprovidesmoreareaforthecurrenttopassthrough.

Cross-sectional Area

Thinner wire, Higher Resistance

Cross-sectional Area

Thicker wire, Lower Resistance


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For a uniform wire:

Resistance alsodepends on the

materials of the wire

Resistance , Resistivity


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RESISTANCE

The resistance of a wire can also be calculated by

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S.I unit of resistance is the Ohm ( )

ResistanceResistivity

Cross-sectional Area=

LengthX


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RESISTORS & RESISTANCEResistors are components in an electrical circuit

which provides a known value of resistance.

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Fixed ResistorsLDR

VariableResistors

Thermistors


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RESISTORSSome examples of resistors

Fixed Resistors

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Variable Resistors / Rheostats

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Light Dependant Resistor (LDR)PreparedbyCg




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Thermistor

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FUNCTION OF RESISTORS

Resistors in a circuitPreparedbyCg



Mainfunctionistocontroltheamountofcurrentflowinginacircuit.


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RESISTORS & RESISTANCE

There are two types of resistors:

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Fixed Resistors Variable Resistors


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FIXED RESISTORSFixed resistors are usually made from wires of special

alloys or from carbon and have fixed values.

Fixed Resistors & Its Circuit SymbolPreparedbyCgEdnaMariaHjOsman.E-m


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FIXED RESISTORS

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Each band and its placement on the resistor

represent its resistance value and tolerance.

Fixed resistors have colour coded bands.


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FIXED RESISTORS



il.comEach band and its placement on the resistor represent

its resistance value and tolerance.

4 7 0 10%

470

10%6 8 5%

20%

10%

1 0 000

2 2 00

68 5%

10000 10%

2200 10%

Resistor Value


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VARIABLE RESISTORS

Variable resistors have varying resistances that canbe altered by moving a contact / turning a dial.

Variable Resistors / Rheostats



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VARIABLE RESISTORS

Using variable resistor as a rheostat.



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BulbVariable Resistor

Switch Battery

Changingtheresistanceofthevariableresistor

changestheamountofcurrentflowinginthe

circuit.

Circuit diagram showing the arrangement of a variable resistor used as a rheostat.


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VARIABLE RESISTORS

Using variable resistor as a potential divider.



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Device X

Fixed

ResistorBattery

Switch

Variable

Resistor

Changingtheresistanceof

thevariableresistorchangesthepotentialdifferenceacrossDeviceX.

Circuit diagram showing the arrangement of a variable resistor used as a potential divider.


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RESISTORS & RESISTANCESeries & Parallel Arrangements



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Resistors can be arranged either in series or in parallel to

obtain the required total resistance needed in the circuit.

Resistors In Series

Resistors In Parallel


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RESISTORS & RESISTANCESeries Arrangements



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Total resistance of resistors arranged in series can be

determined by the equation:

R1 R2 R3

Rseries R 1 R2 R3= + +


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RESISTORS & RESISTANCEParallel Arrangements



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Total resistance of resistors arranged in parallel can be

determined by the equation:

R1

R2

R3

= + +R1 R2 R31 1 1 1

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19 Resistance