Post on 05-Jan-2016
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Current-Voltage Characteristics of a bulbIn this experiment you are going to investigate how the current through a bulb changes according to the voltage across it.
Current (amp) Voltage (Volt)
0.00 0.00
A
V
To get negative currents and voltages, use reverse polarity on the battery pack
A graph of the results should look like this
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
-6 -4 -2 0 2 4 6
voltage (V)
Cur
rent
(A)
Cold filament
*constant resistance
*current is directly proportional to voltage.
*The metal filament inside the bulb follows ohms law
Hot filament
*resistance increases
*current does not increase in proportion to voltage
*filament is non- ohmic
BULB
Voltage / V
Current /A
0
0.5
1
1.5
2
2.5
3
-4 -3 -2 -1 0 1 2 3 4voltage (V)
Cur
rent
(A
)
DIODE In the forward direction the line curves towards the current axis.
(What is happening to the resistance?)
Diodes are non- ohmic conductors
In the reverse direction, current flow is very, very low.
Is the resistance of the diode high or low?
What is the resistance of the diode when the voltage is 2V?
1.2
R = V / I
R = 2 V / 1.2 A
R = 1.7 Ω
Thermistor
A thermistor’s resistance decreases as the temperature rises. USE: temperature sensor, fire alarm
Light dependent resistor (ldr)
An LDR’s resistance decreases as the amount of light falling on it increases. USE: street lights
What do the current-voltage graphs look like for a thermistor and an LDR?
Cur
rent
(A
)
Voltage (V)
High temperature
Low temperature
thermistor
Cur
rent
(A
)
Voltage (V)
Bright light
Dim light
LDR
The line is straight so the resistance is constant for a set temperature.
But at lower temperatures the resistance is higher (so the current is lower)
But at lower light levels the resistance is higher (so the current is lower)
The line is straight so the resistance is constant for a set light level.