Atreyi SahaPhysics 132Prof. HatchFebruary 24, 2016Unit 2, Lecture 4: Electric Fields- Electric Fieldo How does one charge know that another charge exists and experiences a force? Electric force field - another charge modifies the space which it is occupying A given charge experiences the electric field that is createdo Coulomb's law tells us how to find the force between two point charges. The forceacts over a distance so the charges do not need to be in contact with each other. This leads to the question: does the presence of the charge somehow modify the space surrounding it? Yes, and this leads to the concept of the electric fieldo Challenge with Coulomb's Law is that you can only deal with 2 charges at a time. In the real world, there are a lot more than two charges. Coulomb's law is NOT feasible.o Thinking about electric fields A charge is obviously doing something such that it is letting the other charges know it is there There is a force between two charged particles and the two particles communicate with each other. How do they communicate? One charged particle sends out an electric field/force field by modifying space around it such that when we bring in asecond charge, the second charge sees the electric field created by the first charge- Mapping out an electric fieldo Charge of interest is at the center in redo Then, we use test charges to map out the field. Forconsistency sake, we always use a positive as a testcharge. We see what force the test charge experienceso The closer you are to the charge of interest, the bigger themagnitude of the repulsion force. Farther away you are,the smaller the magnitude (small arrow, small vector) ofthe repulsion force is.o Patterns Force field radiates away in a nice symmetric patterns As you get further away, the repulsive force gets weakero The overall electric field points away (radial outfield) The farther you are away, the field lines get fartherapart. Thus, the field is weaker at the extremities. The field lines are closer together closer to the charge ofinterest and therefore the field is stronger.- Mapping out an electric field for a negatively charged particleof interesto test charge is always positiveo force field appears to be pointing towards the particleof interest (a negative charge)- Summary so far....1. Electric fields point away from positive charges2. Electric fields point toward negative charges- Definition of an electric fieldo Equals to = the force our test charge experiences / the charge This allows you to figure out what force is there ...then you don't have to deal with the other charge anymore You can just deal with the force it experiences and itself and the charge that is experiences the force Force being experienced by particle of interest is equal to E = strength of the electric field multiplied by whatever charge of the test particle- The number of lines is a representation of the amount of chargeo By convention, the more field lines that you have, the stronger the electric field. Outside extremities its a weaker electric field than closer to the charged particle ofinterest.o In the example below, charged particles is twice as strong (on right) and thus has more electric field lines- What do electric field lines look like when we have two charges of interest? Here is a schematic of a dipole-dipole interactiono Electric field lines leave the positive charge and head towards the negative chargeo Free charges follow the field lines of the electric field as shown by the positive charge drawn in red Free charges EXPERIENCE a force in theSAME DIRECTION as the field lines- Positive free charge would follow the field lines andwould experience a force in the direction of the field lineso Positive charges experience a force in the samedirection as the field lineso accelerates if the direction of the particle is thesame as the direction of the fieldo slows down if the direction of the particle isopposite to the direction of the field- Negative charges (as shown in blue) experience a forcewhose direction is against the field- Two positive charges with equal size, the fields will cancel out inthe middle - there will be no electric field at the points ofcancellation- Sample problem: Point charge in the middle, what is t the fieldstrength at point X created by that charge?o E = F/Qoo F = coulomb's law qo is the charge of the test particle - what youcare about, what force it experiences? q is the thing creating the chargeo Substituted formula - the electric field for a pointcharge used to find field strength at the black dot r = distance from q to the black dot- Sample
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