Key Stage 4 – Ohm’s law and Ohmic conductors

Notes for teachers

At a glance

The digital age is built on an electrical component called a semiconductor. These are really important in electrical devices, as their electrical properties can be changed. Silicon is the most popular material for a

semiconductor, hence the naming of “Silicon Valley” where most of the world’s prominent technology companies operate in, in California.

Researchers studying semiconductors are looking to create new semiconductors that could revolutionize our technology.

Learning Outcomes

1. Recognise I-V graphs and the limitations of Ohm’s Law

2. Apply Ohm’s law to calculate resistance of a component and circuit

3. Identify and analyse uses and limitations of semiconductors

Each student will need

· Key Stage 4 – Ohm’s law and Ohmic conductors activity sheet.

· Calculators.

http://www.oxfordsparks.ox.ac.uk/content/soluble-semiconductors-revolution-printing-21st-century

· Prior knowledge: Students are expected to already have a basic understanding of current, charge, potential difference and resistance.

· OPTIONAL: Bulb, motor, buzzer, thermistor variable voltage supply and multi-meter.

Possible Lesson Activities

1. Starter activity: Recognise I-V graphs and the limitations of Ohm’s Law

· Show students the I-V curve of a wire, without the axes labelled. Tell them it’s a wire and ask if they know what the axes on the graph should be. Follow this up by showing the I-V curve for a filament lamp, and see if they recognise it. This leads into a curve-component sorting exercise on the activity sheet. Warn students that you expect them to be able to justify their answers.

▪At the end of the exercise, cold call students to justify their answers from the activity sheet. (Prompt for keywords such as temperature, resistance, electrons …). Depending on your class, you may want to pre-warn students who you will be cold-calling.

· OPTIONAL: This could also be done as group work as a think, pair, share exercise (students compare answers), depending on size of class.

· This exercise is important to get students into the mindset of analysing a situation and creating their own opinion. Later on the in the lesson students will need to use their knowledge of electricity and technology to decide if semiconductor research is important. (You may want to explain this to the students.) Adopting a ‘thinking pose’ will be very useful when receiving students answers in the cold call, and encouraging students to expand on each other’s answers.

· To lead onto the main activity, mention how filament bulbs are not as popular they used to be, due to the development of LEDs, light emitting diodes which are a type of semiconductor.

· MISCONCEPTIONS: “Heat will increase speed of electrons, making them move faster through the circuit and lowering resistance.” Kinetic energy of the electrons will increase with temperature and so electrons will move about faster, however, this will not make the flow of electrons faster. A simple analogy is a person jumping up and down on an escalator – it doesn’t matter how fast they jump, the escalator will not go any faster.

http://www.oxfordsparks.ox.ac.uk/content/soluble-semiconductors-revolution-printing-21st-century

2. Main activity: Apply Ohm’s law to calculate resistance of a component and circuit

· Discuss Ohm’s Law with the class, identifying what each term means. (It was made by Georg Ohm in 1826). If the graph from the starter is linear, then a component obeys Ohm’s law and it is called Ohmic.

· Use the apparatus to demonstrate how measurements can be obtained to calculate resistance and how it varies for the three different components, for example: two different filament bulbs and a motor (not a thermistor). Measurements can be recorded in the table on the student activity sheet. Vary voltage, component, and circuit (series only).

▪ Students must calculate the resistance of each measurement.

▪EXTENSION: Use values of resistance (to be recorded with a multimeter) to calculate current.

▪EXTENSION: Explain how adding of more components in series increases total resistance of a circuit, hence why bulbs in series get dimmer.

· To lead into the next activity, explain that a semiconductor does not follow Ohm’s law, as it is a mixture of an insulator and a conductor. A semiconductor conducts more when temperature increases.

· MISCONCEPTIONS: Some students may have heard of a superconductor and think it is the same as a semiconductor. The key difference is that a superconductor has zero resistance.

4. Plenary: Identify and analyse uses and limitations of semiconductors

· Silicon is the most popular material for a semi-conductor, hence the naming of “Silicon Valley” where most of the world prominent technology companies operate in, in California.

· Introduce and watch the Oxford Sparks animation.

· The research group at Oxford looks into creating a different kind of semi-conductor called a soluble semiconductors. As stated in animation, these are cheaper, more efficient and more technologically usable.

· Using the animation, get the students to work in pairs to judge the research and create a short speech to persuade others of their judgement. You can prompt the students and assess their learning during the process by asking:

▪ “Using the word insulator and conductor, what is a semiconductor?”

▪ “What is an example of a semiconductor and what are they usually made of?”

▪ “Why are semiconductors important?”

http://www.oxfordsparks.ox.ac.uk/content/soluble-semiconductors-revolution-printing-21st-century

▪ “When did you last use a semiconductor?”

▪ “What new technologies could be achieved with better semiconductors?”

▪ “Do you think the researchers will achieve their goal?” (As teacher, you should say your opinions on this.)

· Once the students have made a judgement, point out that the research group leader, Donal Bradley, has been recognised by numerous awards and accolades – Ask the students if this changes their opinion.

▪ (EU Descartes Prize, the Royal Society Bakerian Medal and the IOP and IET Faraday Medals. Also elected a Fellow of the Royal Society and awarded a CBE for services to science.

▪ Finish the lesson by asking two pairs of opposite opinion to share their speech.

Web links

· Oxford Sparks animation on Semiconductors: http://www.oxfordsparks.ox.ac.uk/content/soluble-semiconductors-revolution-printing-21st-century

· The semiconductor research group at Oxford: https://www2.physics.ox.ac.uk/research/physics-and-application-of-soluble-semiconductors

· Donal Bradley’s profile on the Royal Society website: https://royalsociety.org/people/donal-bradley-11123/

http://www.oxfordsparks.ox.ac.uk/content/soluble-semiconductors-revolution-printing-21st-century