current-voltage characteristics of a bulb

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.