Learning Resources
Lesson
Before looking at how electrical force is measured, it is helpful to look at one more electrical concept—electrical resistance. When electrons move from one atom to another, it requires energy to get them to make the move. Resistance is the property of materials that resists the flow of electrons from one atom to the next. It is related to the amount of energy required to make electrons move.
Insulators have very high resistance, enough to prevent electrons from flowing. Conductors have less resistance, but they do have some. Gold, for example, has very little resistance. Aluminium or copper, which are commonly used as conductors, have a bit more resistance than gold. Resistance is also affected by the size of the conductor—larger means less resistance.
Resistance is sometimes an important design element in a circuit. In the electric kettle, for example, the heating element is designed with a specific amount of resistance. As electrons flow through the element, the resistance causes electrical energy to be converted to heat energy, which is transferred by conduction to the water, eventually boiling it.
The specific component of a circuit designed to have a pre-determined amount of resistance is called a resistor. Electronic circuits frequently have small resistors in the circuit to control the flow of electricity through different parts of the circuit.
Resistance, then, is an important element of circuits and is used to control the amount of electric current that flows in a circuit. Electrical resistance is measured in Ohms.
Fixed and Variable Resistors
While there are many different types of resistors, there are 2 generalized types used in electronic circuits—a fixed resistor and a variable resistor. A fixed resistor has a resistance that does not change. A variable resistor has a resistance that you can change. The most common type of variable resistors are called potentiometers. A rotary light dimmer and a volume control on a radio are both examples of a variable resistor. This type of variable resistor is adjusted by turning a post. Examples of each are shown below
Figure Fixed Resistor and Variable Resistor
The schematic symbols for both fixed and variable resistors are shown below.
Figure Symbol for Fixed Resistor and Variable Resistor
Photoresistors and Thermistors
Variable resistors that are sensitive to light are called photoresistors. As the light increases or decreases the value of the resistance changes. Brighter light lowers the resistance and dimmer light increases the resistance. This property can be used to make the photresistor act like a switch. When the light reaches a pre-determined level the circuit is 'turned on'.
Variable resistors that are sensitive to temperature changes are called thermistors. Thermistors are used in ovens, toasters, fridges, water heaters, hair dryers, and so on as sensors to help control the temperature of the device.
Resistor Colour Code
Resistors can be made from a variety of materials, with carbon being the most common. They are made of a solid carbon cylinder with a wire attached to each end, and the whole thing encased in epoxy (a type of plastic). Usually they are too small to write the resistance value on them, so a colour code is used. In most cases 4 colour bands are used, as shown below.
Figure Resistor Colour code
The first 3 bands give the resistance value. The fourth band gives the tolerance (accuracy of the value). The code is
1st band is the first number in the value (ie 1, 2, 3, ...)
2nd band is the second number in the value (ie 1, 2, 3, ...)
3rd band is the multiplier (ie. 1, 10, 100, 1000, ...)
4th band is the tolerance (ie, + - 5%, + - 10%)
The figure below shows the resistor colour code
Figure Resistor Colour Code
Tolerance uses a different colour code. Gold designates 5% tolerance and silver designates 10% tolerance.
Based on the code, we can now read the value of the resistor shown in the figure above
Red Green Orange Gold
2 4 X 1000 = 25000 ohms, with a tolerance of 5%.
For More Information
For more information, check these sites
- Interactive Java Tutorial on Molecular Resistance
- Interactive Java Tutorial on Resistance Colour Codes
- Detailed Resistance Tutorial
Activity
Assigned activities
Determine the resistance of the following resistors
- red blue red gold
- red red red gold
- grey black black silver
- blue purple yellow gold
- white yellow white gold
Investigate applications of variable resistors at home, school, in vehicles, and entertainment devices.
Test Yourself
There is no self test for this lesson.