Electric Force and Coulombs Law Basic Introduction

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This is an electrostatics introduction. We'll introduce the property of electric charge and discuss the electric force. This will include covering Coulomb's law and the basics of how to setup problems where you may be asked to find the electric force on a point charge due to one or more other charges. • • Timestamps: • 0:00 - Introduction: Electric Charge • 1:05 - Notation • 2:24 - Electric Force and Coulomb's Law • 3:48 - Direction of Electric Force • 5:45 - Force from Multiple Charges (1D) • 8:15 - Force from Multiple Charges (2D) • In the next video we will cover electric fields and see how they are related to the electric force and caused by electric charges. • Electric charge is a property of matter. There are two types of electric charge, positive charge and negative charge. Charges interact, or exert forces, on other charges. Opposite charges attract and like charges repel. • All matter is made of particles; protons which are positively charge, electrons which are negatively charged, and neutrons which are electrically neutral. When an object, made of many particles, has a net charge, it is because of unequal amounts of protons and electrons. • We denote charge mathematically with a q. A capital Q is often used to represent the overall charge, or net charge, on an object. Lowercase q is often a single charge, like a point charge. The units of electric charge is coulombs named after Charles Coulomb. The electron and proton have the same magnitude of charge, or 1.602 x 10^-19 C. This is the smallest amount of charge possible, since all charge ultimately comes from electrons and protons, and is often called the fundamental charge or electronic charge. • The fundamental charge of an electron is often denoted with a lowercase e, but I will use q along with a subscript e to not get confused with the constant 'e' from mathematics. Also, the reason we don't attribute the fundamental charge to the proton even though it has the same magnitude of charge as the electron is because protons are composite particles, meaning they are made of smaller particles and are not fundamental. • The electric force between electric charges is described using Coulomb's law. The force is proportional to the product of the two charges and inversely proportional to the distance between them squared. The constant out front has a quantity called epsilon-naught, the permittivity of free space or the vacuum. This is a fundamental constant we measure in nature related to electric fields which will discuss later. Sometimes this whole constant out front is condensed to be written as a single constant k, sometimes called Coulomb's constant. Again, this is just a number and makes our units work out nicely, so if you see Coulombs law written different ways, know they are equivalent. • Coulomb's law only gives the magnitude, or strength of the electric force between point charges. The direction of force in Coulomb's law must be put in by hand. This means it is useful to draw a picture. Draw both charges and make a straight line connecting them. If the forces are the same, the force on both charges should point outward away from the other charge along this straight line. Similarly, if the charges are opposite, the force on each charge will point in toward the other charge along this straight line. • Taking the exact same charge and placing them the exact same distance apart will lead to the same magnitude of electric force between them. However, if we charge the orientation of the two charges, say rotate them 30 degrees, now both forces point in a different direction in space. Coulomb's law does not account for this direction. It also gives the magnitude of force on both charges, which is always equal. But there is no way to choose which charge you want the force direction for in Coulomb's law. • We often have cases where there are more than one or two charges. For this we use the principle of superposition of electrostatic force, the same way we would for any other force or vector quantity. This simply means that to get the overall force on a single charge, we can add the individual forces on that charge due to the other charges. So basically we get a copy of Coulomb's law for each individual charge that exerts a force. • In two dimensions, we do the same thing but now we must be careful of vector components. This means, after we use Coulomb's law to find the magnitude of each force, we still need to find the x- and y-components of each force. This may include multiplying by sine or cosine of some angle, which can often be written as a ratio of distances already given. • • • This video is part of a lecture series that will accompany a Physics II course. Though the course will involve some calculus, the conceptual videos and many problems will be relevant for those taking a trigonometry and algebra only physics course.

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