PHYS272_Lab1

=Physics for Scientists and Engineers - Lab 1 =

Objectives:

 * To determine the lines of constant potential and map the electric fields produced by various arrangements of charged conductors.
 * To determine magnetic field lines produced by a bar magnet.

Equipment:

 * 10-V Power Supply
 * Conducting Paper
 * Small Permanent Magnets
 * Non-conductive Paper
 * Voltmeter
 * Small Compasses
 * Linear and Point conductors
 * Masonite Board
 * Connecting wires, banana clips

Physical Principles:
The electric field vector at some point in space, E, is a vector defined as the force per unit charge at that point due to some distribution of charges at other locations. That is, math \displaystyle \vec{E}=\frac{\Delta V}{\vec{\Delta d}} math where F is the force that a positive test charge, q 0, would experience at that point in space. The direction of the electric field is in the same direction as the force would be on a positive test charge. Electric field lines are a convenient visual aid for representing the electric field. The electric field vector, **E**, is tangent to the electric field lines at each point and the density of lines is proportional to the strength of the electric field in the region.  Electrical potential (or voltage), V, at some point in space is a scalar quantity defined as the potential energy per unit charge, i.e., math \displaystyle V=\frac{PE}{q_0} math  In a region of space where the electric field is unchanging over some small distance Δd, the potential difference is related to the constant electric field by math \displaystyle \vec{E}=\frac{\Delta V}{\vec{\Delta d}} math  Equipotential lines are lines drawn on a two-dimensional surface where the potential difference between any two points on the line is zero. They are lines of constant potential. No work is required to move a charge along an equipotential line. These lines are analogous to the lines of constant altitude on a topographical map where travel along the line does not involve movement either uphill or downhill. The electric field lines and the equipotential lines are related to one another. Electric field lines are always perpendicular to the equipotential lines. Furthermore, the electric field lines must be everywhere perpendicular to the surface of a conductor (otherwise currents would flow along the surface of the conductor) and the surface of a conductor must be an equipotential surface. Magnetic fields are produced by moving charges. In a permanent magnet, aligned spins of electrons in the atoms produce the strong field. The direction of the magnetic field, B, at any point in space is taken to be the direction that the north pole of a compass needle would point at that location. 

Procedure:
 Attach the conductive paper to the masonite board. Using the screws, make holes in the paper, approximately 15 cm apart. Then tighten the screws and paper to the board using wingnuts. Connect one wingnut to the positive lead and the other to the negative lead, and turn the voltage up to 10 Volts.  Ground the common lead of the voltmeter to the negative side of the power supply. The potential at any location on the paper may now be measured by pressing the probe (from the positive input to the voltmeter) down on that point. Using 1 volt increments, locate points that have the same potential. Connect these points for a given voltage level to create equipotential lines.  //Hint: You will get more consistent readings if you hold the probe exactly vertical and press down firmly!//  The electric field lines may now be drawn by following the rule that electric field lines are perpendicular to each conductor line and perpendicular to each equipotential line they cross. Put arrows on the electric field lines to indicate their direction. They begin on the conductor with high potential and end on the conductor with low potential. The pattern of electric field lines and equipotential lines should resemble what you would expect from an electric dipole.  <span style="color: #333333; display: block; font-family: Arial,sans-serif; font-size: 14px;"> Using a new conducting sheet, attach the aluminum bar conductors to the board using the wingnuts. Attach the electric leads to the bars to simulate the electric field between two plates with a potential difference between them. Note both the uniformity of the electric field between the two plates and the edge effects where the electric field “bulges” out at the ends of the lines. <span style="color: #333333; display: block; font-family: Arial,sans-serif; font-size: 14px;"> <span style="color: #333333; display: block; font-family: Arial,sans-serif; font-size: 14px;"> Tape an ordinary piece of paper to your drawing board. Place a small magnet in the center of the page, oriented such that the north pole of the magnet is facing either East or West. Trace the outline of the magnet. Place a small compass near the north pole end of the magnet. The south pole of the compass points toward the north pole of the magnet. Put a pencil mark both where the south pole tip of the compass needle would intersect the edge of the magnet and at the point on the compass edge where the north pole tip of the needle is directed. Now shift your compass so that the center of the compass is centered over your previous mark and mark the position of the north pole tip again. <span style="color: #333333; display: block; font-family: Arial,sans-serif; font-size: 14px;"> <span style="color: #333333; display: block; font-family: Arial,sans-serif; font-size: 14px;">//**Be careful not to rotate your board during this process.**// <span style="color: #333333; display: block; font-family: Arial,sans-serif; font-size: 14px;"> Continue this procedure until you have either reached the south pole of the magnet or have gone off the page. Connect the dots to form a magnetic field line and put an arrow on it to indicate the direction the north pole of the compass needle was pointing. Do this for several lines and you should obtain a pattern of lines somewhat similar to the electric field lines obtained for the electric dipole of step I. As you get further from the magnet, the magnitude of the Earth’s magnetic field will become comparable in magnitude to the magnetic field of the magnet. In fact, there will be two locations where the two magnetic fields will be exactly equal and opposite on your paper. Locate these two points of zero magnetic field. <span style="color: #333333; display: block; font-family: Arial,sans-serif; font-size: 14px;"> <span style="color: #333333; display: block; font-family: Arial,sans-serif; font-size: 14px;"> Repeat the magnetic field line tracing for the magnetic north pole facing either North or South. Again, determine the points where there is zero magnetic field. Explain the differences that you see in the magnetic field lines from the previous experiment.
 * Electric dipole** <span style="color: #333333; display: block; font-family: Arial,sans-serif; font-size: 14px;">
 * Parallel plates:** <span style="color: #333333; display: block; font-family: Arial,sans-serif; font-size: 14px;">
 * Magnetic field lines** <span style="color: #333333; display: block; font-family: Arial,sans-serif; font-size: 14px;">
 * Further Investigation** <span style="color: #333333; display: block; font-family: Arial,sans-serif; font-size: 14px;">