Skip to 0 minutes and 1 second [Current-Voltage characteristics] In this video, I’m going to be talking through one possible way of teaching current voltage characteristics through practical work, whilst enabling formative assessment along the way. We are going to look at the current-voltage behaviour
Skip to 0 minutes and 23 seconds of three common components: the bulb —- resistor —– and diode Our first step is for students to copy and draw this circuit on their mini white boards. Each group will have a tray with the circuit components they need set up the circuit. Our first opportunity for assessment is to check whether the students can place the components on the correct symbols. They should remember to connect all the components in a series loop, except for the voltmeter, which needs to be connected in parallel across the bulb. After connecting up their circuit, their first check is that the bulb turns on and that they are getting positive readings for both current and voltage.
Skip to 1 minute and 4 seconds When taking readings of current and voltage, students can immediately start to plot a graph on a mini-white board. When taking this approach, it should be made clear to the students that it is the shape of the graph that is important, rather than getting the graph precisely correct. This should also be repeated for negative values. Once groups have sketched their graphs, these can be compared in the class to see whether all the groups are getting the same shape. This is a good test to see if they have swapped the values on their axes. At this point, you can discuss with the students why they think the graph has that shape.
Skip to 1 minute and 40 seconds As the applied voltage increases in the circuit, so does the current. The current has a heating effect which acts to increase the resistance of the bulb. This would be a good opportunity for the students to calculate the resistance of their component, Voltage over current, for each reading – this should show them that the resistance of the bulb is increasing. An alternative to get a quick graph would be to set up a table and scatter-graph in a spreadsheet and to fill in the values as they go.
Skip to 2 minutes and 12 seconds We can now discuss the behaviour of two other components: the resistor and the diode. First the resistor. For low voltages the fixed resistor has the same resistance, whatever the voltage. With this knowledge, students can sketch a graph of their prediction of the current voltage characteristics for this component, and they can share this with the whole class. The diode only lets current through one way, and this is when it reaches a certain threshold voltage. There’s two types of diode. The light emitting diode with a threshold voltage of 2 volts and also a silicon diode, which has a threshold voltage of only 0.7 volts. Once the potential difference reaches this voltage the component has very little resistance.
Skip to 2 minutes and 55 seconds This is harder, but again, students can sketch their predictions. Can they explain their sketches in terms of voltage, current and resistance? It is now up to the teacher to decide whether the students could repeat their experiments using a fixed resistor and then with a diode, or whether to demonstrate these set-ups and sketch the graphs together as a plenary. This could be dependent on the availability of components and of digital multimeters in the milli-amp region. It should be noted that if you are using an LED as a diode, then a protective resistor should be used in series with the LED to prevent damage to the LED.
Skip to 3 minutes and 33 seconds Most silicon diodes will have a voltage rating that enables you to use them without a protective resistor in series. Once the experiment has been carried out, students and the teacher can then compare their predictions against their measured results. Do they match? If not, why not? Using this method of predict, test and evaluate, allows both the teacher and the students to test their understanding before and after testing the circuit in practise. Remember this is only one possible teaching sequence to look at current-voltage characteristics - we’d like you to share your ideas too.
I-V characteristics in the classroom
In this video, we show how students can set up a simple circuit to measure the current vs. voltage for a bulb, resistor and diode.
Before the practical is carried out, some discussion of the behaviour of the circuit components with increasing applied voltage would be advantageous. This theory is discussed in the previous step of this course. Part of this discussion could include students sketching expected I-V graphs based on the description of the circuit components’ behaviour (see subject knowledge primer in the previous step). After the practical, they can compare their predictions to their graphs from measured values and discuss any differences.
Setting up circuits can be a significant challenge for many students. By drawing the circuit components out on whiteboards first, this improves their knowledge of circuit symbols and diagrams and helps them link those diagrams to the real components. It also aids the teacher in assessing their understanding of the circuit before they set it up.
Mini-white boards, or “show me” boards, are a common way to quickly assess understanding across the class.
Can you identify a way you could use this kind of feedback in a topic that you are teaching this week? How could you use this method to assess progression in learning?
© STEM Learning