# RESIstance

Document Sample

WELCOME
Conducting materials
by the end of this lesson you will be able to:
1. State and explain the factors which
affect the resistance of a resistor.
2. define semi-conducting materials and
explain how they conduct electricity.
3. explain how the diode operates.
4. explain the function of the diode in half-
wave rectification.
resistance
RESISTANCE IS THE OPPOSITION TO CURRENT
FLOW.
ALL MATERIALS SHOW SOME RESISTANCE TO
CURRENT FLOW.
GOOD CONDUCTORS SHOW LITTLE RESISTANCE
TO CURRENT FLOW.
POOR CONDUCTORS(INSULATORS) SHOW HIGH
RESISTANCE TO CURRENT FLOW.
FACTORS WHICH AFFECT RESISTANCE:

1. Length of resistor.
2. Cross sectional area.
3. Type of material.
4. Temperature.
FACTORS AFFECTING RESISTANCE
Length of resistor.

The longer the resistor
Greater the resistance
Lesser the current flow
Cross sectional area.
Thinner the resistor
Higher the resistance
Thicker the resistor
Lower the resistance
Type of material
Silver allows electrons to flow through most rapidly.
Copper is next best and is widely used since it is relatively
cheap.
When high resistance is needed, alloys are used such as
constantan, manganin or nichrome. These have about 50-60
times the resistance of copper.
Some materials allow current to flow based on its
temperature.
Temperature.
The resistance of pure metals increases with temperature.
The resistance of certain other conducting materials decrease with
temperature. E.g. carbon.
Certain other materials- germanium, silicon and selenium- show a
decrease in resistance as the temperature increases.
These materials are called semi-conductors.
The resistance can be further decreased if these materials are
doped with certain impurities.
SEMI-CONDUCTORS
Semi-conducting materials can conduct current
because of
1. Intrinsic conduction which is due to the natural
material.
2. Extrinsic conduction which is artificial-due to
INTRINSIC CONDUCTION

In pure silicon, all the outer electrons are occupied in bonding so no
current will flow.
INTRINSIC CONDUCTION
When the temperature is high, few electrons become loose.
As these electrons move, they leave ‘holes’, which are positive.
INTRINSIC CONDUCTION
INTRINSIC CONDUCTION

The net effect is that of ‘holes’ moving in the direction of
conventional current, while electrons move in the direction
of real flow of current.
Semi-conductors are better conductors at higher
temperatures.
Conduction in a semi-conductor by means of positive holes
and thermal electrons is called intrinsic conduction.
EXTRINSIC CONDUCTION
Silicon has four electrons in its outermost shell. If it is doped with an atom which
has five electrons in its outermost shell, then there will be extra electrons to carry
current.
EXTRINSIC CONDUCTION

When the doping provides extra electrons, the semi-conducting material is
called a n-type (negative) material.
EXTRINSIC CONDUCTION
Silicon, when doped with atoms which contains less
electrons in the outermost shell such as Aluminium-
(3 electrons) will have less electrons to carry current
EXTRINSIC CONDUCTION

When doping provides less electrons- i.e. more positive holes, for the conduction of
current, the material is said to be p-type (positive). Conduction is by the movement
of ‘holes’.
THE JUNCTION DIODE
p-n junction diodes can be used to control the direction of current flow in a
circuit. The junction diode conducts electricity in one direction only.
THE JUNCTION DIODE
Whether or not the current will flow depends on how the diode is connected in the
circuit. The connection can be connected in forward bias – current flow or reverse
bias – no current flow.

Forward bias - Current flows.                 Reverse bias - No current flows.
Bulb lights.                                  Bulb does not light.
FORWARD BIAS
Electrons flow from the
negative end of the battery
and enters the n-type side of
the diode. These electrons are
attracted across the junction to
the p-type end (positive). As
electrons jump over, holes are
left on the n-type side. The
holes appear to move from the
p-type to the n-type side.
FORWARD BIAS
Current will flow in the
circuit and the diode is
said to be forward bias.

The negative end of the
battery is connected to
the n-type end of the
diode.
FORWARD BIAS
REVERSE BIAS
Electrons flow from the
negative end of the battery to
the p-type end of the diode.

The electrons simply fill the
‘holes’ and do not jump across
the junction.

There is no attraction for the
electrons to move across the
junction.
REVERSE BIAS
No current will flow in
the circuit and the diode
is said to be in reverse
bias.

The negative end of the
battery is connected to p-
type end of the diode.
REVERSE BIAS
HALF-WAVE RECTIFICATION
Diodes are used to convert alternating current into direct current.
The current is said to be rectified.
HALF-WAVE RECTIFICATION
The diode only allows current to flow in a certain direction as
shown.
HALF-WAVE RECTIFICATION
If the diode is reversed, then it will only allow the current to flow
in the opposite direction.
Thank you much
NOW STUDY IT!                      TYSON
NATHANIEL,
FORGET CHARMED,PASSIONS AND OTHER GARBAGE!

MYRIAM                           DEMERRIS
TERRAN                           DANFORD!
JOHNNY                           OR ELSE WE
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