# DC Circuits and Capacitors

Students find the circuit rules can be difficult to get the hang of. It is a good idea to keep revisiting Kirchoff's Laws as you teach each aspect of the circuit:

- conservation of charge and energy in circuits
- relationships between currents, voltages and resistances in series and parallel circuits
- power dissipated

The idea of ratio in a potential divider circuit can throw some students. There are a number of different ways to calculate the effect of a potential divider circuit. However if students don't have a good grasp of the mathematical relationship they will struggle. It is a good idea to introduce potential dividers, then go back to teach internal resistance, then revisit potential dividers when the students are more confident:

- potential divider circuits

Capacitance is a good topic to introduce exponential decay as the decay curves are easily produced using data logging software:

- definition of capacitance
- energy of a capacitor
- quantitative treatment of charge and discharge curves

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.

### PhET Circuit Construction Kit *suitable for home teaching*

The PhET circuit construction kit is a very useful simulator for building circuits on the computer and making measurements. You can use this to complement circuit construction in the lab. It is also useful to demonstrate circuits on the board for students to answer questions.

You can demonstrate internal resistance as well as energy losses in wires as all components (including the wires) can be assigned resistance.

There are many ways to measure the current and voltage.

The best way to use this tool is to build circuits and then save them for later use. These circuits can then be distributed for the students to load into the simulator.

### Episode 114: Components in Series and Parallel

This topic allows the students the chance to investigate series and parallel circuits and make measurements to see how the current and potential difference vary. They should have some idea about what happens when connecting components in series and parallel from GCSE, but they need to use the formula to calculate effective resistance. There are plenty of example questions.

### Episode 117: Kirchhoff’s Laws *suitable for home teaching*

Kirchhoff's Laws are fundamental to understanding circuit design. This resource covers both laws with experiment and questions. It is important that students know both the definition of the laws and the mathematical representation so that they can perform calculations of current and potential difference around the circuit.

### Episode 118: Potential Dividers

Potential dividers are often poorly understood, and students often tie themselves up in knots trying to calculate the answer to sometimes trivial problems. This resource leads you through a logical set of steps and experiments to make sure that the topic is well understood. The experiments are fairly straight forward and the mathematics is well explained. The students need plenty of practice with sensor circuit questions.

### Bin-bag Capacitor

This videos explains how to safely build a large scale capacitor using bin-bags, tin foil and an EHT power supply.

This is a dangerous demonstration. Students must not touch the capacitor when it is connected to the power supply and great care must be taken to discharge properly.

A very effective demonstration of the construction of the capacitor.

### Episode 125: Introducing Capacitors

This resource contains instructions for the previous demonstration ( the bin-bag capacitor ) and other basic circuits to allow students to investigate what happens when a capicitor is charged or discharged.

### Episode 126: Capacitance and the Equation C = Q/V

This resource describes the construction of capacitors as well as introducing the equation C=Q/V.

There is an experiment to charge a capacitor with a constant current which is hard for the students to do, but good for developing their practical skills. This experiment allows students to better understand how a capacitor charges and to show the relationship between Q and C.

### Episode 127: Capacitors in Series and Parallel

This resource has circuits to show the behaviour of capacitors in series and parallel and shows how to derive formulae for capacitors in series and parallel.

### Episode 129: Discharge of a Capacitor

This resource is very thorough and explains in detail the decay equation of a discharging capacitor. There are plenty of questions for students to do and graphs to analyse.

There is also an excel spreadsheet that can be used to model the exponential decay which is a useful method for students to try as they get more hands on with the mathematical relationships.

### Capacitors: delivery guide for AS and A level physics

There is a good description of common misconceptions at the beginning of this resource, as well as some practical resources.