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# Chapter 11: Electric circuits

In grade 10 learners learnt about current, voltage and resistance. In this chapter they will learn about Ohm's law and power and energy. They will see how to apply the concepts learnt in grade 10 and series and parallel circuits to more complex circuit problems. The following list provides a summary of the concepts covered in this chapter.

• Ohm's law

In grade 10 learners learnt about parallel and series circuits, as well as the concepts of voltage, current and resistance. These concepts are now brought together in Ohm's law. Ohm's law relates voltage, current and resistance. Ohm's law only applies to Ohmic resistors (such as most resistors) and does not apply to non Ohmic resistors (such as light bulbs). Ohm's law is introduced and then learners get to see it in action in various circuits.

The first circuits covered are simple series or simple parallel circuits. Once learners are comfortable handling calculations for these circuits, series and parallel networks are introduced. In these circuits learners need to carefully work their way through the circuit calculating the equivalent resistances for each separate part of the circuit. It is advised for learners to circle the different parts of the circuit to help them in calculations. In the first example that they do they can redraw the circuit after each simplification replacing the part of the circuit with a resistor of the calculated equivalent resistance as the part of the circuit.

• Electrical power

The concept of electrical power is introduced. A source of energy is required to drive current round a complete circuit. This is provided by batteries in the circuits you have been looking at. The batteries convert chemical potential energy into electrical energy. The energy is used to do work on the electrons in the circuit.

Power is a measure of how rapidly work is done. Power is the rate at which the work is done, work done per unit time. Work is measured in joules (J) and time in seconds (s) so power will be $$\frac{\text{\text{J}}}{\text{s}}$$ which we call a watt (W).

• Electrical energy

The final part of this chapter deals with electrical energy. This has real world applications in teaching learners about how much electricity various appliances use around their home. Learners should be encouraged to make a list of as many appliances as possible that they use and find the power rating for the appliance. This is usually given on the back of the appliances. This part of the chapter helps learners understand some of the power saving tips that they are always told and helps to rationalise these tips. For example, learners can calculate how much it costs to leave a $$\text{100}$$ $$\text{W}$$ bulb burning all night compared to leaving it on for just an hour.

## 11.1 Introduction (ESBQ5)

The study of electrical circuits is essential to understand the technology that uses electricity in the real-world. We depend on electricity and electrical appliances to make many things possible in our daily lives. This becomes very clear when there is a power failure and we can't use the kettle to boil water for tea or coffee, can't use the stove or oven to cook dinner, can't charge our cellphone batteries, watch TV, or use electric lights.

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