Electricity Tutorial 3 - Voltage Current Characteristics

 Contents

Measuring Voltage against Current

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.

Voltage Current Characteristics

From this circuit we take readings of voltage and current plotting them as a graph called a VI characteristic.

We normally put the voltage on the y-axis and current on the x-axis.  This allows us to determine the resistance from the gradient. This is a voltage current graph for an ohmic conductor: The straight line shows a constant ratio between voltage and current, for both positive and negative values.  We say that the voltage is directly proportional to the current.  This means that the graph is a straight line of positive gradient going through the origin.  So when the voltage is negative, the current is negative, i.e. flowing in the opposite direction.  Ohm’s Law is obeyed. In the exam, you will, most likely, see that the current is plotted against the voltage.  Therefore the gradient is the conductance, and you have to find out the reciprocal to get the resistance.  The higher the resistance, in this case, the less steep the gradient. It is entirely possible that the graphs are plotted with the voltage against the current.  Read the question carefully

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 Ohm’s Law? Thermistor

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 the 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

Diodes are semi-conductor devices that allow electric current to flow one way only.

• If the positive of the diode is connected to the positive terminal of the battery, the diode will conduct.
• The voltage across the diode is about 0.6 V.
• This is called forward bias.
• The negative terminal, sometimes called the cathode is shown by the silver band. 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: Most text-books show it like this, as do electronic engineers.  However you may see the graph plotted as voltage against current.

Here it is the other way round: 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.

 Question 3 (Harder) Can you use the graph to explain why a diodes allows a current to flow one way only? 