3.2 Electric Charge

5 min read•june 18, 2024

Krish Gupta

Daniella Garcia-Loos

AP Physics 2 🧲

61 resources

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Electric Charge

Charge is a fundamental property of subatomic particles. There are 2 types of charges: positive and negative.

A proton has a positive charge, while an electron is negatively charged. The amount of charge on an object can be measured in either Coulombs (C for large amounts of charge or elementary charges (e) for small amounts of charge. If you've taken chemistry already, you've most likely talked about the charge of atoms or ions as 1, 2, etc. ✨ These charges are measured in elementary charges. For this course, most of the time, we'll be dealing with "larger" amounts of charge and will use Coulombs as our primary unit of charge.

Common Charges

Particle/Object	Charge (Coulombs)	Charge (Elementary Charges)
Proton	+1.6x10^-19 C	+1 e
Electron	-1.6x10^-19 C	-1 e
Neutron	0	0

Law of Electrostatics

Simply put, "likes repel, opposites attract". 2 positive charges will repel each other, and likewise, 2 negative charges repel. Different charges (+ and -) will attract.

Try using the PhET simulation to see how a charged balloon can stick to the sweater as well as to the wall.

The law of electrostatics is a set of principles that describe the behavior of electric charges at rest. Here are some key points about the law of electrostatics:

Electric charges: Electric charges are properties of certain particles that are associated with the presence of an electric charge. Examples of particles that can be charged electrically include electrons, protons, and ions.
Coulomb's law: Coulomb's law is a fundamental principle that describes the force between two electric charges. According to Coulomb's law, the force between two electric charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between the charges.
Electric force: Electric force is the force that is exerted between two electric charges due to the presence of an electric charge. Electric force can be attractive or repulsive, depending on the nature of the charges.
Electric field: An electric field is a region of space in which an electric charge experiences a force. The strength of an electric field is determined by the magnitude of the electric force experienced by a test charge within the field.
Electric potential: Electric potential is a measure of the potential energy of an electric charge within an electric field. Electric potential is often measured in volts and is a measure of the energy per unit charge.
Electric potential energy: Electric potential energy is the energy that is stored in an electric field due to the presence of electric charges. Electric potential energy is a measure of the work that can be done by an electric charge as it moves within an electric field.
Electric flux: Electric flux is a measure of the flow of electric charge through a surface. Electric flux is often used to describe the strength of an electric field at a particular location.

Attract or Repel?

Type of Charge	Attracted to	Repelled by
Positive Charge (+)	Negative & Neutral	Positive
Negative Charge (-)	Positive & Neutral	Negative
Neutral Object	Positive & Negative	Neither

Practice Question:

A metal paper clip is attracted to a positively charged metal ball ⚽️ . What could be the charge on the paper clip?

Answer:

Negative or neutral. As long as the ball is positive it will attract either a negatively charged or a neutral object!

Practice Question 2:

Two charged particles are separated by a distance of 1 meter. The first particle has a charge of +3 Coulombs and the second particle has a charge of -4 Coulombs. What is the electric force between the two particles?

Solution:

According to Coulomb's law, the electric force between two charged particles is given by the equation: F = k*(q1*q2)/r^2, where F is the electric force, k is the Coulomb constant, q1 and q2 are the charges of the two particles, and r is the distance between the particles.

In this problem, the Coulomb constant is 8.988 x 10^9 N*m^2/C^2, the charges of the two particles are +3 Coulombs and -4 Coulombs, and the distance between the particles is 1 meter.

Therefore, the electric force between the two particles is: F = 8.988 x 10^9 Nm^2/C^2(3 Coulombs*(-4 Coulombs))/1 m^2 = -1.199 x 10^9 N

This means that the electric force between the two particles is -1.199 x 10^9 N.

Practice Question 2:

A charged particle is placed in an electric field with a strength of 100 N/C. What is the electric force experienced by the particle if it has a charge of +2 Coulombs?

Solution:

The electric force experienced by a charged particle in an electric field is given by the equation: F = q*E, where F is the electric force, q is the charge of the particle, and E is the strength of the electric field.

In this problem, the charge of the particle is +2 Coulombs and the strength of the electric field is 100 N/C.

Therefore, the electric force experienced by the particle is: F = 2 Coulombs*100 N/C = 200 N

This means that the electric force experienced by the particle is 200 N.

Practice Question 3:

A charged particle is placed in an electric field with a strength of 200 N/C. What is the electric potential at the location of the particle if it has a charge of +3 Coulombs?

Solution:

The electric potential at a location in an electric field is given by the equation: V = q*E, where V is the electric potential, q is the charge of the particle, and E is the strength of the electric field.

In this problem, the charge of the particle is +3 Coulombs and the strength of the electric field is 200 N/C.

Therefore, the electric potential at the location of the particle is: V = 3 Coulombs*200 N/C = 600 N/C

This means that the electric potential at the location of the particle is 600 N/C.

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