Transcript Faraday*
Faraday… • What was his insight about the reason the current-carrying wire could make the compass needle move? What did he “invent” in his explanation? And what’s the connection between this and the knitting needle we just observed? Force at a distance • Can we see an electric field? Only its effects Word description Picture description; draw the gravitational force or the electric force at the points. Draw the arrows with the correct relative lengths. Draw the following… Represent with arrows the gravitational force that the Earth (the source mass) exerts on a small object (test mass) at the points shown. Represent with arrows the electric force that the object with a large negative charge (the source charge) exerts on a small object that has a positive charge (called test charge) at the points shown. Represent with arrows the electric force that the object with a large positive charge (the source charge) exerts on a small positively charged object (test charge) at the points shown. + A field can be represented with field lines. They have the following properties: • Field lines start on positively charged objects and end on negatively charged objects. They follow the direction a positive charge would move. • Lines closer together mean a stronger field. Lines further apart mean a weaker field. • Number of lines indicates magnitude of charge. • Lines are perpendicular to surface. • Lines never cross. • E = F/q (force per unit of test charge) • What if I tell you that the electric field strength is not dependent upon the quantity of charge on the test charge? How is this possible? After all, the quantity of charge on the test charge (q) is in the equation for electric field. So how could electric field strength not be dependent upon q if q is in the equation? • Think about Coulomb’s Law: Felec = kq1q2/r2 • Regardless of what test charge is used, the electric field strength at any given location around the source charge will be measured to be the same. • Electric Field Strength: E = F on q’/q’ Several electric field line patterns are shown in the diagrams below. Which of these patterns are incorrect? Explain what is wrong with all incorrect diagrams. Erin Agin drew the following electric field lines for a configuration of two charges. What did Erin do wrong? Explain. Use your understanding of electric field lines to identify the charges on the objects in the following configurations: Back to the Van de Graaff: What do you think the environment is like INSIDE the Van de Graaff’s sphere? • http://en.wikipedia.org/wiki/File:Faraday_cag e.gif http://www.youtube.com/watch?v=qHrBhgwq __Q Can the forces exerted by an electric field do work? • What evidence do we have? + PUSH! So we exert a force over a distance. What does that mean? We did WORK. Where does that energy go that we input into our system? Let’s look at gravitational force. + X The same is true for electrical potential. If we put the charge at a higher electric potential, we do work. This energy is stored as an increased electrical potential energy. When it “falls” down to a lower electrical potential, it can gain kinetic energy or do useful work. • Gravitational Potential energy of a mass depends upon…. • Electrical potential energy of a charge depends upon… Magnitude of charge, strength of electric field, and distance V = energy / charge = Work/q = F∙ d/q q = F/E so V = F∙ d ÷ F/E = E∙ d Fgravity is to gravitational potential energy as Felectrical is to voltage (voltage is same as potential difference).