Electricity is everywhere, powering our daily lives. While difficult to visualize, the basics of electricity rely on the idea of electric charge and the interactions of charged objects.

This unit introduces the concepts of electric charge and electric force. These concepts serve as the basis for developing electric circuits in Unit 9 as well as being foundational to the magnetism topics in AP Physics 2. These topics will account for ~4-5% of the AP exam questions and will take approximately 3-5 45 minute class periods to cover.

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Unit 8 Overview: Electric Charge and Electric Force

Certain quantities are conserved, in the sense that the changes of those quantities in a given system are always equal to the transfer of that quantity to or from the system by all possible interactions with other systems.

Electric charge is a property of an object or a system that affects its interactions with other objects or systems containing charge.

Electric charge is conserved. The net charge of a system is equal to the sum of the charges of all the objects in the system.

There are only two kinds of electric charge. Neutral objects or systems contain equal quantities of positive and negative charge, with the exception of some fundamental particles that have no electric charge.

The smallest observed unit of charge that can be isolated is the electron charge, also known as the elementary charge.

You may remember from a chemistry class discussing the idea that protons have a positive charge, and electrons have a negative charge. But what exactly is charge? Charge is a fundamental property of matter, like mass. The smallest amount of charge measurable is called the 

 (e) and is the charge found on a proton (+1e) or an electron (-1e). 

In addition, we use a larger unit, the Coulomb (C) for describing large amounts of charge (1e = 1.6x10^-19 C). A key idea here is that although Coulombs of charge are very large, they must be equal to an integer value of elementary charges, since we can’t split an electron in half to get ½e.

When charge remains in a single location, it’s referred to as static charge. This is the reason you see or feel shocks and sparks on a dry winter’s day, or have a bad hair day due to static electricity.

Charge that is allowed to flow in a loop is called a circuit. The rate of flow of charge through a loop is referred to as 

is the amount of charge (in coulombs) and 

 is the time. The unit for current is the Ampere (A). For more information on circuits, and their behaviors check out the Unit 9 Study Guide.

The conservation of charge is a fundamental principle of physics. Charge can’t be created or destroyed and in any interaction, the total amount of charge must be the same before and after.

Conservation of Charge

Charged objects will attract or repel other objects. These interactions follow a simple rule: Likes Repel, Opposites Attract. If two objects have similar charges (both negative or both positive) they will repel each other. If the objects have opposite charges (one positive and one negative) they will attract each other. A neutral object will 

 be attracted to a charged object. To explore why this is, check out this 

: AP Physics 1 doesn’t cover the idea of electric fields. This is included as a basis for discussing electric force in the next key idea. AP Physics 2 goes into much greater depth with electric and magnetic fields.

The region around a charged object can be described by an electric field. Electric field lines show the direction a positive charge will travel when placed near the charged object. If placed near a positive object it will be repelled, therefore the electric field lines point away from a positive object. 

Likewise, the test charge will be attracted to a negative object and the field lines will point in. This is useful in visualizing the direction the electrostatic force will be acting in. The images below show some common arrangements, or check out this 

Electric Charge

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