Simple Circuits

Introduction

The practical application of the relationships between voltage, current, and resistance is in the creation of electric circuits. A circuit provides a closed path for current to flow from the positive terminal of a voltage source through to the negative terminal. A circuit will contain different elements that control the current or use the energy of the moving charges.

To create a current you must have a voltage source. The voltage source is often referred to as a source of electromotive force or emf, for short. A battery is an example. The term force here is misleading. This is another historical reference that we continue to use. From now on we will refer to only sources of emf. In equations we will use the symbol to represent emf. When you see that your car requires a 12 V battery, that 12 V is the emf of the battery. In reality, when you use the battery to start your car you will get less than this 12 V emf. This is because the battery has an internal resistance, r, when current flows through it. You need to understand the difference between the emf, , and something called terminal voltage. Emf is the natural voltage of the battery when there is no current flowing in the circuit. Terminal voltage is what you measure across the terminals of a battery when it is connected in a circuit and current is flowing. To calculate terminal voltage:

So the terminal voltage is equal to the emf minus the voltage drop through the battery due to its internal resistance, r.

Simple Circuits

Simple Circuits (Series Parallel) Review

Watch this video to refresh your understanding of how resistors work in series and parallel.

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