Current-Voltage Characteristics of a bulb
In this experiment you are going to investigate how the current
through a bulb changes according to the voltage across it.
Current (amp)
V
0.00
Voltage (Volt)
0.00
A
To get negative currents and voltages,
use reverse polarity on the battery pack
A graph of the results should look like this
Current(A)
BULB
-4
*resistance increases
0.6
Voltage / V
-6
Hot filament
0.8 Current /A
0.4
*current does not increase in
proportion to voltage
0.2
*filament is non- ohmic
0
-2
0
2
4
6
-0.2
Cold filament
-0.4
*constant resistance
-0.6
*current is directly
proportional to voltage.
-0.8
voltage (V)
*The metal filament inside
the bulb follows ohms law
In the forward direction the
line curves towards the
current axis.
3
DIODE
2.5
Current (A)
2
(What is happening to the
resistance?)
1.5
1.2
Diodes are non- ohmic
conductors
1
0.5
0
-4
-3
-2
-1
0
1
voltage (V)
2
In the reverse direction,
current flow is very, very low.
Is the resistance of the diode
high or low?
3
4
What is the resistance of the
diode when the voltage is 2V?
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?
thermistor
Current (A)
High temperature
Low temperature
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)
Voltage (V)
Current (A)
LDR
Bright light
The line is straight so the
resistance is constant for
a set light level.
Dim light
But at lower light levels
the resistance is higher
(so the current is lower)
Voltage (V)