Chapter 11 Lesson 2, "Conductors and Insulators"

Materials:

Objective:

Anticipatory Set:* Show the students a picture of electrical wires and ask what the cords are made out of. Why are they made out of these materials. Show several other items. A gold bar, an egg, a plastic army man, and explain that everything is made of atoms with neutrons, protons, and electrons. However some types of atoms give up elections easily and some hold their electrons very tightly and don't give them up. The gold bar contains atoms of gold. It gives up electrons very easily. The army man is made of atoms of carbon and hydrogen. Neither of these give up electrons very easily.

Teacher Input:* Look at the cord again. You see that the center is made of a type of metal. Most metal will give up electrons easily. This gives electricity (a surge of electrons) the ability to pass along the material. This is a conductor. Common conductors used in wires is silver, copper, aluminum, and gold. Since gold and silver are very expensive they are used in very limited situations. Looking back at the electrical wire you see that metal surrounded by a rubber or plastic. Both rubber and plastic are made of molecules that hold electrons. Because of this electricity (the flow of electrons) will not pass through it from molecule to molecule. This allows you to touch a wire and not have the electrons in the wire flow into your body. These types of materials are called insulators. Good insulators are things like glass, rubber, wood, and plastic. Some of these materials can crack or break. In a spark plug we want the electricity to jump from one part to another. If there is a crack that allows the electricity to flow between the two sides an engine will not run. Sometimes cracks a can occur in the insulation around a wire. If that happens the electrons may be able to jump through the crack and flow to your body. This baby bit through the insulation of a wire and was severely shocked. So make sure all your electrical cords are in good shape and not crimped to tightly.

Remember that every material would fit somewhere on a line from being a great conductor to being a great insulator. A material is not just one or the other. The measure of how easily electric current will flow through a material is known as resistance. Resistance is measured with a unit called an Ohm This is a 4.7 K resistor Resistance is a lot like a water faucet. Lets imagine that you have two different faucets hooked up to the same pump. The larger the faucet the easier the water can flow and the more water that can flow. The smaller faucet does not allow water to flow as easily and the pump can not pump as much water. With this in mind, conductors with the lowest resistance allow the most electrons to move and insulators with more resistance allow the fewest electrons to move. You can measure resistance with a special meter.

The resistance of a wire depends on three things:

  1. The material the wire is made out of. gold has very little resistance, copper has a little more resistance and tungsten has a little more resistance even though they all allow electrons to travel through them.
  2. The length of the wire. The longer the wire the more resistance. Short Wire. So if you try to run something like a welder or refrigerator with a long extension cord you may not get enough electricity.
  3. The thickness of the wire. A thick wire can carry more electricity than a thin wire.

All of these situations have the ability to cause a fire. Resistance causes electrical energy to change into heat and light energy. If you try to surge too many electrons through a conductor with too much resistance you will get heat. Most of our electric appliances work by using resistance to do work with electrical energy. You see this in a light bulb. The tungsten filament is very thin and extra long with the coil action. It actually could create a fire if it were exposed to oxygen. This is one of the reasons for the glass bulb around the filament. If you break the glass and the light is on you can get a shock from the filament and wires and it can start a fire. Nichrome is a metal with a high resistance. A toaster, curling iron, hairdryer, and iron all have Nichrome in them. The Nichrome wires have electricity sent through them and because they resist the large number of electrons they heat up. You have to be careful with these because if you were to use a knife to remove a piece of toast and you touch one of those wires you could get a shock.

Guided Practice:* Students will take turns or work together to answer the questions on Page 319.

Independent Practice:* Students will read lesson 2, Conductors and Insulators. Students will also complete the worksheet. 11-02, "Conductors and Insulators"

Check for Understanding:* Students will take the quiz, 11-02, "Conductors and Insulators".

 

Duration:

30 minutes + 10-15 for Independent Practice (Could be longer if taken for homework).

Alaska Content Standards Addressed in this lesson: