4.2.3: I-V characteristics

I-V characteristics show the relationship between the electric current in a component and the potential difference across it. This information is commonly collected using a potentiometer/variable resistor. One main point to look at is whether the component behaves the same way if the current is reversed (goes through it in the opposite direction). We need to know the shape of I-V graphs for a few components...

Resistors

• Fixed resistors are designed to keep constant resistance regardless of temperature fluctuations
• The potential difference across a resistance is proportional to current so the resistor can be said to obey Ohm's law.
• This means it is an ohmic conductor (just a conductor that obeys Ohm's law)
• The resistors behave in the same way regardless of polarity
• The shallower the gradient the greater the resistance

Filament lamps

• The potential difference across a filament lamp is not proportional to the current through the resistor so a filament lamp does not obey Ohm's law (it is a non-ohmic conductor)
• The resistance of a filament lamp is not constant, it increases as the p.d. increases (remember: shallower gradient = greater resistance). The increase in resistance is caused by the wire getting hot as it glows (this point here).
• The filament lamp behaves the same way regardless of polarity

Light emitting diodes

• Diodes only allow current in one direction - its behaviour depends on the polarity. With a negative p.d there is no current - resistance is infinite and the diode will not conduct. As p.d. increases resistance gradually starts to drop. This is known as the threshold p.d.. Above threshold p.d., an increase in p.d. leads to a larger decrease in resistance and the diode begins to have very little resistance. The value for threshold p.d. varies depending on the colour (wavelength) of light they emit.
• The potential difference across a diode is not directly proportional to the current through it therefore it does not obey Ohm's law (it is a non-ohmic component)
• Resistance is not constant (seen by a fluctuating gradient)

Thermistor

Thermistors are temperature sensing components with a negative temperature coefficient. This means that their resistance DROPS when temperature increases (this is the opposite of a metal wire). This effect can be explained in terms of number density. In certain semiconductors (the ones with negative temperature coefficients) the number density of the charge carriers increases as the temperature increases. Often the resistance drop is large meaning a small change in temperature can be detected by monitoring the resistance of the thermistor. Thermistors are used in thermostats (controlling heating/air-conditioning units), to measure the temperature of engines and electrical devices to ensure they do not overheat, and in simple thermometers.

Like LEDs and filament lamps, thermometers are non-ohmic. With a thermistor, as the current increases temperature also increases (like a filament lamp) but this leads to a DROP in resistance because the number density of the charge carriers increases. The graph is pretty similar to a filament lamp (don't get them confused!) but the gradient increases for a thermistor (= decreased resistance) and the gradient decreases for a filament lamp (= increased resistance)...

We need to know how to investigate the relationship between resistance and temperature. This can be done using an ohmmeter and a water bath, or also an ammeter and voltmeter at different temperatures then use V=IR to calculate resistance. Results for this experiment will vary slightly with different thermistors. This can ensure that the best thermistor is selected for the right application.

LDR

Light dependant resistors are little electrical components which change their resistance depending on the light intensity. Some uses are in sports/street lamps/brightness metres in phones and laptops. An LDR is made from a semiconductor in which the number density of charge carriers changes depending on the incident light intensity. Dark conditions = low number density = high resistance, light conditions = high number density = low resistance.

We need to know about the relationship between the resistance and light intensity or an LDR and how to experiment it...

Image credit: Kerboodle OCR A Physics textbook p160

Vary the distance from the LDR to the constant light source (the lamp). This will change the intensity of light recieved by the LDR. Putting a small black cardboard tube around the LRD will redice background light interfering. The results will give a calibration curve that you can use to read unknown readings off...

Image credit: Kerboodle OCR A Physics textbook p160

Image sources: Kerboodle OCR A Physics textbook p.149-159