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Using the Two-Charge Model of Electric Charge

Chapter 2 /  Lesson 6
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Instructor: Matthew Bergstresser
Electric charge has two varieties, positive and negative. In this lesson we will explore how the two-charge model of electric charge was created and see how it is applied.

Electric Charge

Have you ever taken clothes out of the dryer and found them stuck together? Maybe you noticed that when you peeled them apart you heard a soft ripping sound, and you felt that the clothes wanted to stay together. The ancient Greeks noticed something similar after they rubbed amber (solidified tree resin) with a piece of cloth -- the amber had the ability to attract small pieces of dried leaves or particles of dust.


Amber
amber

At the end of the 1800s, it was thought that electricity was a type of fluid that was transferred between objects and caused attraction and repulsion between the objects. We now know that the transfer of electrons to and from objects creates the two types of electric charge, negative and positive. Electrons (whose name comes from elektron, the Greek for amber) are negatively charged. They exist in the outer part of atoms, and are electrically balanced by the positively charged nucleus, making the atom electrically neutral. Negatively charged particles (anions) have an excess of electrons, and positively charged particles (cations) have a deficit of electrons. When an object has more anions, it is negatively charged. You can move electrons around by performing some simple experiments.

If you take some wool and rub it onto a plastic rod, the plastic rod will strip electrons away from the wool, leaving it positively charged. The amount of negative charged gained by the plastic rod is the exact amount of positive charge the wool now possesses.

Rubbing silk on a glass rod has the opposite effect. The silk strips off electrons from the glass, and becomes negatively charged, while the glass becomes positively charged. In both cases, the total electric charge is conserved. No new charge is ever created, it is only transferred from one object to another. Benjamin Franklin came to this realization, and it is called the conservation of electrical charge.

In terms of electric charge, opposites attract and likes repel. If two positively charged particles are brought near each other, the electric force will cause these two particles to repel each other. If a positively charged particle and a negatively charged particle are brought close together, they will attract each other. In Diagram 1, the proximity of the positively charged rod to the metal plate on top of the jar causes the electrons in the metal strips inside the jar to move to the metal disk. This leaves the metal strips positively charged, and they move away from each other.


Diagram 1. Negative charges move towards the positive charge.
electrostatic

Lightning

A cloud-to-ground lightning bolt is a transfer of charge. Billowing clouds have updrafts and downdrafts of air. The colliding particles of water in these air streams cause electrons to be stripped off the ascending particles and accumulate on the descending particles. Thus, the bottom of the cloud starts to build up an enormous amount of negative charge. Air allows this initial separation of charge because it acts as an insulator, which prevents electrons from moving through it. On the ground underneath the cloud, electrons are repelled by the negatively charged air column, causing an area of positive charge. Eventually, the insulating properties of the air between the cloud and the ground break down, and the electrons from the cloud reach the ground in the form of a lightning bolt.


Cloud-to-ground lightning
lightning

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You are viewing lesson Lesson 6 in chapter 2 of the course:

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AP Physics 2: Exam Prep

27 chapters | 158 lessons | 13 flashcard sets

Ch 2. AP Physics 2: Electric Charge
Ch 3. AP Physics 2: Mass &...

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