Lesson D

Basic Electric Circuits

The Open Circuit

The open circuit is a very basic circuit that we should all be very familiar with. It is the circuit in which no current flows because there is an open in the circuit that does not allow current to flow. A good example is a light switch. When the light is turned off, the switch creates an opening in the circuit, and current can no longer flow.

You probably figured that since there are "open circuits" that there are probably also "closed circuits". Well, a closed circuit is when the switch is closed and current is allowed to flow through the circuit.

A fuse is a device that is used to create an open circuit when too much current is flowing. We will see how that can be caused in the following section...

The Short Circuit

A short circuit can be caused by incoming power wires (wires that are normally insulated and kept separate) coming in contact with each other. Since a circuit usually has resistance, and the power wires that "short out" have very little resistance, the current will tend to flow through the path of least resistance... the short. Less resistance at the same amount of voltage will result in more current to flow.

Therefore a short circuit will have too much current flowing through it. What's the best way to stop a short circuit from doing damage (because it is drawing too much power from the source)? By using a fuse. Fuses are designed to work up to a certain amount of current (e.g. 1 amp, 15 amps, ...). When that maximum current is exceeded, then the wire within the fuse burns up from the heat of the current flow. With the fuse burnt up, there is now an "open circuit" and no more current flows.


Every circuit uses a certain amount of power. Power describes how fast electrical energy is used. A good example is the light bulbs used in each circuit of your home. When you turn on a light bulb, light (and heat) are produced. This is because of the current flowing through a resistor built into the bulb. The resistance turns the electrical power into primarily heat, and secondarily light (assuming an incandescent bulb).

Each light bulb is rated at a certain power rating. This is how much power the bulb will use in a normal 110 Volt house circuit. Three of the most popular power values for inside light bulbs are 60, 75, and 100 Watts (Power is measured in Watts). Which of these light bulbs uses the most power? The 100 Watt bulb uses the most power.

Calculating Voltage and Current and Resistance


There is a very easy way to determine how much current will flow through a circuit when the voltage and resistance is known. This relationship is expressed in a simple equation (don't let the word scare you... this is going to be easy as "pie"...

Lets say you have 200Volts hooked up to a circuit with 100 Ohms of resistance. How much current would flow?

Let's start with the "pie"...

This circle will help you to know how to figure out the answer to these electrical problems. The three letters stand for...

Since our "unknown" value in this problem is the current, then we put our finger over the "I". What you see is "E over R". This means you take the Voltage and divide it by the Resistance. This is 200 Volts divided by 100 Ohms. The result is 2 Amps.


What if we wanted to find out the voltage in a circuit when we know the current and resistance? Go back to the "pie" and cover up the E. You're now left with I times R.

How much voltage would you need in a circuit with 50 ohms and 2 amps? E=IxR... E=2x50... E=100 Volts.


Finally, if you had a circuit with 90 Volts and 3 amps, and you needed to find the resistance, you could cover up the R... the result is E over I (Volts divided by Current). R=E/I... R=90/3... R=30 Ohms. This circuit would have 30 Ohms of resistance if it was hooked up to 90 Volts and 3 amps flowed through the circuit.

Ohm's Law

This relationship between voltage, current, and resistance is known as Ohm's Law. This is in honor of the man who discovered this direct relationship (his last name was Ohm). The relationship described in Ohm's Law is used when working with almost any electronic circuit.

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