Download ppt - Animated Science

VI Characteristics Ohms Law
12.1.2 Current/voltage characteristics
For an ohmic conductor, a semiconductor diode and a filament
lamp Candidates should have experience of the use of a current
sensor and a voltage sensor with a data logger to capture data
from which to determine VI curves
12.1.3 Ohm’s law understood as a special case where I  V
Mr Powell
Resistor
 As the voltage increases
the current also increases
at the same rate.
 This is what is called
“ohms law”
 True for resistor at a
constant temperature
 It is directly proportional
Filament Lamp / Bulb
 The resistance of a filament lamp increases as the temperature
of the filament increases.
 i.e. the resistance changes as the temperature of the wire
changes.
 The gradient of the graph represents the resistance
Semi Conducting Diode
The current through a diode flows in one direction only.
The diode has a very high resistance in the reverse
direction.
Often used in transformers to change A.C to D.C.
currents or computers!
Why does
this happen?
30V
Summary

Ohms law states that V I when T = const

If we plot a graph of a component of v on y-axis and I on the x-axis the gradient is
the resistance of the component.

Light bulb and diode are examples of non-ohmic components
Semiconductors?
Carbon, silicon and germanium (germanium, like
silicon, is also a semiconductor) have a unique
property in their electron structure - each has four
electrons in its outer orbital.
This allows them to form nice crystals. The four
electrons form perfect covalent bonds with four
neighbouring atoms, creating a lattice.
In carbon, we know the crystalline form as diamond.
In silicon, the crystalline form is a silvery, metalliclooking substance. Which is a near perfect insulator.
Well on its own that is not amazing but a semiconductor is made from a silicon lattice with an
impurity which will enable it to conduct. We often refer
to these types of materials as intrinsic semiconductors.
2D morphed view of lattice
Doping p or n?
In N-type doping, phosphorus or arsenic is
added to the silicon in small quantities.
Phosphorus and arsenic each have five outer
electrons, so they're out of place when they
get into the silicon lattice. The fifth electron
has nothing to bond to, so it's free to move
around. It takes only a very small quantity of
the impurity to create enough free electrons
to allow an electric current to flow through the
silicon. N-type silicon is a good conductor.
Electrons have a negative charge, hence the
name N-type.
p-type
n-type
In P-type doping, boron or gallium is the
doping agent. Boron and gallium each have
only three outer electrons. When mixed into
the silicon lattice, they form "holes" in the
lattice where a silicon electron has nothing to
bond to. The absence of an electron creates
the effect of a positive charge, hence the
name P-type. Holes can conduct current. A
hole happily accepts an electron from a
neighbour, moving the hole over a space. Ptype silicon is a good conductor.
2D morphed view of lattices
PN junctions?
N-type and P-type silicon are not that amazing by themselves; but when
you put them together, you get some very interesting behaviour at the
junction. That's what happens in a diode.
The place where they meet in the diagram forms what is called a
depletion zone.
Where free electrons have filled positive “holes” to form an area where
there are no free charge carriers.
P
N
P
N
P
N
PN junctions - behaviour to applied pd
This typically takes 0.7V to overcome and bridge the gap and make it conduct!
V< 0.7V
P
V> 0.7V
N
If we reverse the flow then the gap gets larger and does not conduct (up to 30V)
V< 0V
P
N
V<< 0V
PN junction
superimposed
Thermistor
The resistance of a thermistor decreases as the temperature increases
so if we look at it from the VI perspective it is the opposite of a bulb!
How do they work?
The exact conduction mechanisms are not fully understood but metal oxide NTC thermistors
behave like semiconductors, as shown in the decrease in resistance as temperature
increases. The physical models of electrical conduction in the major NTC thermistor
materials are generally based on this theory;
A model of conduction called "hopping" is relevant for some materials. It is a form of ionic
conductivity where ions (oxygen ions) "hop" between point defect sites in the crystal
structure.
The probability of point defects in the crystal lattice increases as temperature increases, hence
the "hopping" is more likely to occur and so material resistivity decreases as temperature
increases.
Extension
Temperature Sensors?

They are inexpensive, rugged and reliable. They
respond quickly to changes and are easy to
manufacture in different shapes.

An example could be made from a combination of
Fe3O4 + MgCr2O4 (metallic oxides)

A NTC thermistor is one in which the resistance
decreases with an increase in temperature.

The circuit shows how you can use the thermistor as a
potential divider. As the temperature changes the
division of voltage or energy will change. You need the
5k resistor or the voltage would be that of the cell a
constant 3V.

A common use is the glass heat sensor in a car or the
temperature sensor in a conventional oven.
Extension Devices
As Physics
Mr Powell
LDR
The resistance of a light-dependent resistor
(LDR) decreases as light intensity increases.
This is a similar process to a thermistor