Charge
Did you know that the Greeks first noted the transfer of charge? They observed that by rubbing amber with fur, the amber would attract hair. When different substances are rubbed together, electrons can be transferred from one substance to the other.
You may have noticed this phenomenon in the winter when you walk across a carpeted room in your socks. You pick up electrons from the carpet and usually discover it when you touch a metal doorknob and feel the spark. This is similar to a lightning strike, just with a lot less intensity. In the John Travoltage PhET Simulation, did you listen for his reaction when you bring his finger near the doorknob?
The first method, friction, occurs when two different materials are rubbed together. Each materials has a different tendency to either give or receive electrons. When the two are rubbed together, electrons are transferred from one to another. In an example of walking on the carpet with socks on, charge is transferred.
You can try the PhET Balloon and Static Electricity simulation to see how charging by friction works. In the simulation you can click and drag on the balloon. Rub the balloon on the sweater. Notice what happens to the electrons. To get started, click on the image of the simulation.
- You will notice that the electrons move from the sweater to the balloon. The balloon picks up extra electrons. Now it is negatively charged.
- The sweater gives away some electrons. Now it is positively charged.
- The total number of charges is constant before and after this process.
- If you try to move the balloon away from the sweater and let go, it is attracted back to the oppositely charged sweater.
Electric Charge
In this presentation, you will learn about the charge and the process of charging. Click the player to begin.
View a printable version of the interactivity.
You have probably seen a Van de Graaff generator at a science museum or in a science class. A common demonstration is having someone put their hand on the dome and watching their hair raise as they get charged. But how does it work? Watch this video clip from Jefferson Labs, How Does a Van de Graaff Generator Work?, that shows how a Van de Graaff generator works. See how many of the four charging methods you can spot in the video clip.
Which charging methods did you see in use in the Van de Graaff generator?
Friction (when the belt rubs against the wire mesh)
Conduction (when electrons travel from the upper wire mesh onto the dome)
Watch this video clip, Opposites Attract and Likes Repel, from Jefferson Labs. You should have noticed that negatively charged plastic strips are attracted to a nail. This is a type of homemade electroscope. The diagram shows why this occurs. When the metal nail is brought near the strips, the negative charges on the strips repel the electrons in the nail. The tip of the nail becomes positively charged and the other end of the nail becomes negatively charged. This is an example of charging by induction. The total charge of the nail is still zero. The plastic strips are attracted to the positive nail tip and move inward. This demonstration illustrates an important difference between conductors, such as metal, and non-conductors, like plastic or rubber. Electrons are free to move in metals, but not in plastic.
This method of charging where charges move to redistribute charge but are not removed or added is called charging by induction. If the nail is moved away from the negatively charged plastic, its electrons will return to their normal positions.
Watch this video clip, Van de Graaff Confetti Explosion, from Jefferson Labs. You see from the confetti demonstration that like charges repel. What method of charging describes how the charge gets from the dome of the Van de Graaff to the confetti? This method is conduction because the confetti and the dome are in contact.
Charge Practice
In the practice problems below, you will review vocabulary and apply it to examples of charging. Click submit after answering each question. Click the player to get started.