Chapter 10:Electrical Resistance

IntroductionToday you will learn what resistance is, and how it can be useful in electrical appliances, some factors affecting resistance and how to work out the resistance of a component by measuring the current and voltage across it.you will also get to know some special resistors, and their uses

Resistance

Resistance is the opposition that an electrical device has to the flow of electrical current.

All devices have some resistance.

A resistor is a device that has a particular resistance.

Resistance EquationResistance=Voltage/Current R=V/I• Resistance is measured in

ohms• Voltage in volts(V),Current in

amps(I)• You can rearrange the equation

to find voltage & current

R=V/I V=R*I I=V/R

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Types Of ResistorsFixed Resistors, Variable Resistors & Special Resistors

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Fixed ResistorsThese resistors have a fixed resistance in order to control the size of currents and voltages.Resistors are included in circuits so that the correct voltage is applied across the bulb and the correct current flows through it.

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Variable ResistorsThese resistors have different resistance and it's possible to alter their resistance.when you alter the volume of television or speed of a fan you use a variable resistor to do so

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Special Resistors Diodes, Thermistor, LDR

Diodes

Diodes are resistors that direct the flow of current in one direction.

Used in rectifier circuit to convert AC to DC.

Thermistors A resistor whose resistance changes with the temperature

Resistance of a thermistor⬇ as the temperature⬆

Eg:Temperature sensors in fire alarms

High Temperature

Low Temperature

I

V

Light Dependent Resistors (LDR)

A resistor whose resistance depends on light intensityResistance of a LDR⬇ as light intensity⬆

Eg:Photographic equipment, automatic lighting control, some garage, gates

I

V

Bright light

Low light

Ohm's LawThe current that flows through a conductor is directly proportional to the potential difference across its ends,provided its temperature remains constant.

Obey's Ohm's Law

Doesn't Obey Ohm's Law

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Measuring resistance