Electricity Tutorial 3 - Voltage
We can easily measure voltage and current, using the data to plot voltage current graphs. We use the following circuit, which you probably did in Year 10 (the 4th Year):
The variable resistor is there to change the voltage and the current. A variable power supply (like a lab-pack) will also do the job, and a variable resistor is not needed.
Remember that the voltmeter is connected in parallel across the component; the ammeter is connected in series.
The photograph shows a typical experiment:
The investigation of voltage-current characteristics lends itself well to data-logging techniques. The voltmeter and ammeter sensors are wired in exactly the same way as ordinary meters. They are then connected to the computer. If you see a question in the exam about data-logging, you should indicate clearly that the sensors are connected to the computer.
From this circuit we take readings of voltage and current plotting them as a graph called a VI characteristic.
normally put the voltage on the y-axis
and current on the x-axis.
This allows us to determine the resistance
from the gradient.
The straight line shows a constant ratio between voltage and current, for both positive and negative values. So when the voltage is negative, the current is negative, i.e. flowing in the opposite direction. Ohmís Law is obeyed.
For a filament lamp we see:
The resistance rises as the filament gets hotter, which is shown by the gradient getting steeper.
|Can you explain why the shape of this graph suggests that a light bulb does not obey Ohmís Law?|
A thermistor (a heat sensitive resistor) behaves in the opposite way. Its resistance goes down as it gets hotter. This is because the material releases more electrons to be able to conduct. Don't worry about why this happens; it's not on the syllabus.
Although it looks similar to the graph above, notice how the gradient is decreasing, indicating a lower resistance. There is, however, a health warning:
As the current goes up, the thermistor gets hotter.
As it gets hotter, it allows more current to flow;
Therefore it gets hotter and so on.
This is called thermal runaway, and is a feature of many semi-conductor components. At the extreme the component will glow red-hot, then split apart. Do NOT try it for yourself (unless you want an earful from your physics teacher, and, possibly, an interview with the vice-principal or deputy headmaster).
The thermistor is used wherever any electronic circuit detects temperature:
Here we see a thermistor protecting a power supply from too high a temperature.
|Why does a thermistor not obey Ohm's Law?|
You can investigate how temperature and resistance are related in a thermistor using equipment like this:
Diodes are semi-conductor devices that allow electric current to flow one way only.
The circuit to measure the characteristic of a diode is like this, based on a potentiometer.
The potentiometer allows a range of values from 0 volts to the battery voltage. We will look at the potentiometer in more detail in a later tutorial.
The diode characteristic graph looks like this:
Note the way it is presented. I have done it like this to be consistent with all the other characteristic graphs. However many text books show the graph with the voltage on the horizontal axis and current on the vertical. Watch out for this bear-trap in the exam.
The diode starts to conduct at a voltage of about +0.6 V. We call this forward bias. Then the current rises rapidly for a small rise in voltage. If the current is reversed (reverse bias) almost no current flows until the breakdown voltage is reached. This usually results in destruction of the diode.
|(Harder) Can you use the graph to explain why a diodes allows a current to flow one way only?|