Transcript Slide 1

2.2 Resistance
G482 Electricity, Waves & Photons
2.2.2 EMF
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2.1.1 Prior Learning
I
I
+
-
+
V
+
-
+
wire
-
lamp
-
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V
2.2.3 Resistance
Students can carry out practical work to investigate the I–V characteristics of a resistor,
lamp and different coloured LEDs. Students can discuss low-energy lighting, eg LED
torches.
(a) define resistance;
(b) select and use the equation for resistance V = IR
(c) define the ohm;
(d) state and use Ohm’s law;
(e) describe the I–V characteristics of a resistor at constant temperature, filament lamp
and light-emitting diode (LED);
(f) describe an experiment to obtain the I–V characteristics of a resistor at constant
temperature, filament lamp and light-emitting diode (LED);
(g) describe the uses and benefits of using light emitting diodes (LEDs). (Homework
Literacy Task – research and present on 1 A4 page)
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(a) define resistance;
(b) select and use the equation for resistance V = IR
(c) define the ohm;
(d) state and use Ohm’s law;
The electrical resistance of an electrical element is the opposition to the
passage of an electric current through that element. Electrical resistance
shares some conceptual parallels with the mechanical notion of friction.
The SI unit of electrical resistance is the ohm (Ω).
All materials show some resistance, except for superconductors, which
have a resistance of zero.
The resistance (R) of an object is defined as the ratio of voltage across it
(V) to current through it (I)
For a wide variety of materials and conditions, V and I are directly
proportional to each other, and therefore R is constant .
This proportionality is called Ohm's law, and materials that satisfy it are
called "Ohmic" materials.
http://en.wikipedia.org/wiki/Electrical_resistance_and_conductance
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d) 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
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e) Potential Divider or Variable Resistor
Pot Div
Variable
Resistor
Voltage: Pot div circuit can provide the full range of voltage from V -> 0V, while a
variable resistor circuit will not reach 0V.
Current: In a pot div circuit the load resistance (bulb) is in parallel with the variable
resistor which means that the overall resistance is less and more current flows in the
circuit.
Energy: In a pot divider circuit the current flow is more as there are two pathways for
current to flow. Hence energy flow is more!
Index
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e)
I
TASK: Draw out the axes and on an
A4 sheet leaving space copy traces
and add any explanations…
+
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V
+
+
-
+
wire
lamp
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I
V
I
High temp
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Amps
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low temp
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+
V
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0.6 V
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Milli-amps
thermistor
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diode
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V
e) Components and their characteristics
For metals:
I
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


resistance increases with temperature
positive ions vibrate more
conduction electrons movement is impeded
positive temperature coefficient
+
low temp
High temp
+ V
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For intrinsic semiconductors:
-
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


resistance decreases with temperature
more charge carriers become available
negative temperature coefficient
much larger change in resistance than with
metals
 thermistors used as temperature sensitive
component in a sensor
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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
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Mini Resistance Question….
The filament lamp is one example of a
non-ohmic device.
(i) State what is meant by the term
non-ohmic.
(ii) Name one other example of a nonohmic device.
2 a) The characteristic shown below is that
of a filament lamp. Explain why, as the
voltage is increased either positively or
negatively from zero, the characteristic has
the form shown in the figure.
b) At a certain point on the characteristic,
the power developed in the lamp is 20 W
and the current is 90 mA. Calculate the
resistance of the filament at this point on
the characteristic.
I
(i) V is not directly
proportional to I [or
resistance is constant] (1)
0
V
(ii) e.g. semiconductor diode
(1)
Basic
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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?
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Diode – example...
mA Scale
Low
Resistance
PN junction is
destroyed and current
flows
High
Resistance
About
30V
A Scale
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Semiconductors (Extension)
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, metallic-looking
substance. Which is a near perfect insulator.
Well on its own that is not amazing but a semi-conductor 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.
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Doping p or n? (Extension)
p-type
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.
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. P-type silicon is a good conductor.
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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. or Band Gap
Where free electrons have filled positive “holes” to form an area where there
are no free charge carriers.
P
N
P
N
P
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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
N
P
V> 0.7V
P
N
N
If we reverse the flow then the gap gets larger and does not conduct (up to 30V)
V< 0V
P
N
P
V<< 0V
N
P
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(Extension)
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Mini Diode Question….
Explain what happens to the diode in
terms of the band gap to explain this
shape.. (4 marks)
If a diode is placed in a circuit in the
forward bias direction the resistance of the
diode changes. Use the graph to explain
the difference for each part of the graph
and link this to the band gap in each case
(4). There are four areas to explain…
When voltage is in forward bias the
resistance is large below 0.7V as band gap is
large. Current is in mA.
When voltage is above 0.7 the band gap is
closed and the diode conducts freely and
resistance is low.
When voltage is in forward bias the
resistance is large below 0.7V as band
gap is large.
When in reverse bias direction the current
flow is very small A as band gap increases.
When voltage is above 0.7 the band
gap is closed and the diode conducts
freely and resistance is low.
Above the breakdown voltage Vb the band
gap is very large the PN junction breaks
down permanently.
Basic
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Connection
•
•
•
Connect your learning to the
content of the lesson
Share the process by which the
learning will actually take place
Explore the outcomes of the
learning, emphasising why this will
be beneficial for the learner
Demonstration
• Use formative feedback – Assessment for
Learning
• Vary the groupings within the classroom
for the purpose of learning – individual;
pair; group/team; friendship; teacher
selected; single sex; mixed sex
• Offer different ways for the students to
demonstrate their understanding
• Allow the students to “show off” their
learning
Activation
Consolidation
• Construct problem-solving
challenges for the students
• Use a multi-sensory approach – VAK
• Promote a language of learning to
enable the students to talk about
their progress or obstacles to it
• Learning as an active process, so the
students aren’t passive receptors
• Structure active reflection on the lesson
content and the process of learning
• Seek transfer between “subjects”
• Review the learning from this lesson and
preview the learning for the next
• Promote ways in which the students will
remember
• A “news broadcast” approach to learning
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