AS LEVEL PHYSICS : ELECTRONS AND PHOTONS

1
AS LEVEL PHYSICS ELECTRONS AND PHOTONS

• By the end of this presentation, you should be
  able to
• Describe an experiment to verify Ohms law.
• Recall Ohms law and the equation V IR
• Describe the effects of changing temperature on
  the resistance of a metallic conductor.
• Describe the characteristics of thermistors, LDRs
  and diodes.

2
TO TEST OHMS LAW
You may have used this circuit.
Power pack
A
R RV
V
3
Set at a fixed value at the start of the
experiment
Power pack
A
CURRENT
Fixed resistor, unknown value.
R RV
V
Adjust this variable resistor and take readings.
VOLTAGE
4
THE RESULTS
I
What happened here?
x
x
One precaution in this experiment may be to
ensure that readings are taken quickly, then
switch off the power. Wait for 2 minutes before
taking the next reading.
x
x
x
x
x
V
5
I
Ohms law is verified if You get a straight
line graph which passes through the origin
x
x
This shows that the voltage across a conductor is
proportional to the current flowing through it. V
a I V / I constant
x
x
x
Providing the temperature is constant!
x
x
V / I R
V
6
V / I R
This equation, which is derived from Ohms law,
is used to define both resistance and the
Ohm. (check the summary on p. 95)
7
If the temperature is allowed to change
I
WHY? (see p. 93) If the temperature
increases, the ratio V/I will increase also, i.e.
the resistance increases.
V
This is the case with a filament lamp, and any
metallic conductor where the temperature changes.
R
T
8
If the conductor is not metallic, its behaviour
may be affected differently by changing
temperatures.
Write down (in point form) what you can remember
about thermistors, including a sketch of its
characteristics (R vs T).
Now use this circuit to obtain 6 pairs of
readings of R and T, plot the graph, and double
check your initial ideas!
I
T
R
R
9
Semi conducting materials are also used to make
light dependent resistors. The characteristics
are similar to that of a thermistor, but very
different to that of a filament lamp
A thermistor
An LDR
A filament lamp
R
R
R
T
T
Light intensity
10
Diodes are manufactured from layers of
semi-conducting materials.
Write down (in point form) what you can remember
about diodes, including a sketch of its
characteristics (I vs V).
Now use this circuit to obtain 6 pairs of
readings of V and I, plot the graph, and
double check your initial ideas!
-
A
V
11
The resulting graph