# Current and EMF

Students will have met basic electrical ideas at GCSE, particularly current, potential difference and energy. However it is useful to revisit all the basics again at A level as students often have developed misunderstandings, for example, in how current flows round a circuit and what affects it.

- electric current as rate of flow of charge, I = Δq/Δt
- It is useful to discuss and think about electron flow vs convential current here. Calculation of drift velocity is a useful challenging introduction to electricity. Also get students to work out number of charge carriers that are required for one coulomb of charge.

- EMF and potential difference
- potential difference in terms of energy transfer
- The definitions are important and this is a difficult part of the topic to get right. Emphasis should be made on the energy transferred or work done by the power supply or battery and the components in the circuit.

- definition of EMF and concept of internal resistance
- The trickiest part of the electricity topic. It sometimes benefits students to leave this until they have a strong understanding of the potential divider, as this essentially is the same mathematical problem. Emphasis on the different forms of the formula and relating it back to V = I R is important. Students find this topic very challenging. It's worth breaking up the topic to revisit the ideas as students gain confidence with circuit calculations.

Whilst this list provides a source of information and ideas for experimental work, it is important to note that recommendations can date very quickly. Do NOT follow suggestions which conflict with current advice from CLEAPSS or recent safety guides. eLibrary users are responsible for ensuring that any activity, including practical work, which they carry out is consistent with current regulations related to Health and Safety and that they carry an appropriate risk assessment. Further information is provided in our Health and Safety guidance.

### Episode 100: Preparation for Electric Current Topic *suitable for home teaching*

This introduction to the topic gives some useful tips on language used and using bulbs in place of resistors. It is worth reading before teaching this topic.

The Teaching Advanced Physics resources are an excellent set of resources to follow when teaching this topic. The practicals are clearly explained and there are plenty of questions to test and challenge the students.

### Voltage, Current, Resistance, and Ohm's Law

This tutorial from the spark fun website is a good overview of the relationship between voltage current and resistance. You can use this as a reminder for students and to check what they learned about electricity at GCSE.

### PhET Ohm's Law

An interactive simulation that helps the user visualise how changing either voltage or resistance affects current.

### Episode 101: Introduction to Circuits

Students struggle to bring their knowledge of circuits from GCSE into A Level. It is worth taking the time to reintroduce all the aspects. The activities in this resource are well suited for students to use to build initial confidence with circuit practicals. Spending time on using multimeters for example is definitely worth doing.

### Episode 102: Current as a Flow of Charge

The shuttling ball demonstration is not really necessary. It is to provide an analogy of current, and as a visual representation it does it's job, but not without confusing some students if not explained carefully.

The questions are fairly basic but the spooning charge demonstration are good introductions to the idea of charge and current.

### Episode 103: Currents and Charge Carriers

Lots of ideas for demonstrations here, as well as a derivation of drift velocity. A nice discussion about safety and conduction of electricity through the human body is useful again for teasing out ideas and differences between potential difference and current. Take note of the safety advice at the end of this resource.

### Episode 106: Electrical Power

This resource has lots of questions and grids for students to use to practice their calculations as well as demonstrations and questions on power used by devices.

### Episode 120: Energy Transfer in Electric Circuits

This resource introduces the basic principle that electrical sources, for example batteries or power supplies, do work on the charges that flow round the circuit, but also transfer energy themselves in the form of heat. The basic idea that there is energy lost across a battery or power supply is investigated through a number of experiments - the most useful being loading a battery with lamps in parallel and recording the terminal voltage.

### Episode 121: Electromotive Force and Internal Resistance

This final resource has a comprehensive treatment of internal resistance with a number of experiments including finding the internal resistance of a potato (which would certainly have been useful for the 2015 AQA Unit One paper - the potato question caused quite a reaction). The questions and explanations of the equation manipulation are very useful especially the graph interpretation of potential difference on page four.