1. Introduction to Electrostatics The study of electrostatics is essentially the study of electric charges at rest. As you recall from Chemistry class, charges may be positive or negative. The basic building blocks of matter, atoms, are composed of electrons, protons, and neutrons. Electrons have what we define as a a negative charge, protons have a positive charge, and neutrons have zero net charge. From Chemistry you may also recall that:  Charges cannot be created nor destroyed. Charges may move from one location to another, but within the overall system, charges are conserved.  Like charges repel. Meaning positively charged objects repel other positively charged objects, and negatively charged objects repel other negatively charged objects.  Unlike charges attract. Positively charged objects attract both negatively charged objects AND neutrally charged objects, and negatively charged objects attract both positively charged objects AND neutrally charged objects.  A positively charged object has more protons than electrons, and a negatively charged object has more electrons than protons. Charge is a measure of the excess of protons or electrons. Quantifying charges In 1896 J.J. Thomson discovered the electron. One of his students, J.S. Townsend, determined the value of the elementary charge to be 1 x 10 -19 Coulombs. This was a rough estimate, however. In 1911, Robert Millikan conducted his famous oil drop experiments in which he determined the quantity of the elementary charge as 1.60 x 10 -19 C. Basically, Millikan used an atomizer to spray oil droplets into an electric field. The atomizer not only created a fine mist of oil droplets, it also provided a charge (due to friction) to those oil droplets. Due to the force of gravity, all of the oil droplets were drawn downwards. Some of those oil droplets fell into a hole in a conducting plate. Those droplets that fell into the hole then fell down into a space between two charged conducting plates. Millikan had a microscope placed so that he could observe the falling droplets. Based on the terminal velocities of the falling droplets, Millikan determined the mass of the droplets. Millikan also controlled the potential difference between the two conducting plates. As oil drops fell between the plates, Millikan adjusted the strength of the electric field until an oil drop was suspended. WOW! Now the net force on the drop was zero, and guess what?? That meant the electrical force acting on the droplet was equal to the gravitational force acting on the droplet. As he calculated the charge on each oil droplet from that relationship, he determined that every charge was a multiple of 1.60 x 10 -19 C. CHARGE WAS QUANTIZED. THE ELEMENTARY CHARGE WAS DETERMINED TO BE 1.60 x 10 -19 C!!!!!! Resources Cutnell, John and Kenneth Johnson. Physics Sixth Edition http://www.swarthmore.edu/NatSci/cpurrin1/posteradvice.htm http://www.osti.gov/em52/workshop/em52-gl.html Zitzewitz, Paul, Mark Davids, Robert Neff and Kelly Wedding. Merrill Physics: Principles and Problems. Electric Force The electric force of attraction or repulsion between two charges is directly proportional to the quantity of each of the charges and inversely proportional to the square of the distance between those two charges. This is known as Coulomb's Law, named after French mathematician and military man Charles Coulomb (1736-1806). F = k q 1 q 2 / d 2 Force, as always, is measured in Newtons and is a vector quantity, meaning it has magnitude and direction. So, if there are many charges interacting, the net electric force acting on ONE of those charges will be the vector sum of all of the Coulomb forces acting on that charge. Remember, like charges repel (push away from each other), and unlike charges attract (pull toward each other). Coulomb is a new unit that is not derived from any of our previous units. One Coulomb (C) of charge is very large. The charge on one electron is -1.60 x 10 -19 C. The charge on one proton is +1.60 x 10 -19 C. Calculations and Applications F = k q 1 q 2 / d 2 Coulomb's Law allows us to calculate the force between two charges. q 1 and q 2 represent the magnitude of each charge in Coulombs, d is the distance between the two charges, and k is the experimentally derived constant that relates the ratio of charges over distance squared to force. k = 8.99 x 10 9 Nm 2 /C 2 Notice the similarity between Coulomb's Law and Newton's Law of Universal Gravitation. Notice also the difference in scale between k and G. This tells you something about the relative strength of the two forces and why gravitational force is considered the weakest of the fundamental forces. E = F/q where E is electric field strength at a particular location, F is the force on a charge placed at that location, and q is the value of the test charge – NOT THE VALUE OF THE SOURCE CHARGE. Electrostatics and Electric Fields N Najmi Department of Physics, RMHS, 01867 Figure 2. On the left, charged balloons are attracted to the neutral wall and therefore stick to it. On the right, a diagram indicates how electrical force and distance have an inverse square relationship, while charge magnitude and force have a direct relationship. The reason that the force increases fourfold when the charge doubles, is because EACH of the charges will double in magnitude. NEWTON'S THIRD LAW OF MOTION: for every applied force (by object A onto object B), there is a force equal in magnitude and opposite in direction that is applied (onto object A by object B). IF A ATTRACTS B, THEN B ATTRACTS A. Wouldn't it be nice if life imitated science? List of Picture Sources: http://en.wikibooks.org/wiki/Electrodynamics/Electric_Field http://www.allaboutcircuits.com/vol_1/chpt_1/1.html http://www.britannica.com/eb/art/print?id=53247&articleTypeId=0 http://www.gpc.edu/~pgore/PhysicalScience/electric-charge.html http://www.physicsforums.com/showthread.php?t=123612 http://www.phys.unt.edu/~klittler/demo_room/e&m_demos/5a20_20.html http://www.sciencemadesimple.com/static.html http://www.techlib.com/science/electrostatics.html Electric Fields Electric field lines provide a way to visualize electric fields. While electric fields are “real”, electric field lines are a model for the electric field. Imagine a small positive test charge placed in space. By measuring the electrical force acting on that small positive test charge, we can determine the strength of the electric field at that point. The direction of the electric field lines indicate the direction of the electrical force on that small positive test charge at that location. The strength of the electrical field is indicated by the density of the field lines. The closer the field lines, the stronger the electric field strength. In the figure below, a lone positive charge shows electric field lines directed away from that positive source charge. A lone negative charge will generate an electric field in which the electric field lines are directed toward the negative source charge. Without the concept of electric fields, physicists flounder at the concept of forces acting at a distance – without physical contact or interaction. The concept of electric fields allows physicists to consider the force per unit charge value at a particular location. While the true electrical force will be equally dependent on the magnitude of the test charge, the magnitude of the electric field will depend only on the source of the electric field, and the location within that electric field. The figure below shows the electric field lines for two identical positive charges.