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EXPERIMENT 1: NAME ELECTRIC POTENTIAL & FIELD MAPS STUDENT ID Scores ROOM:Fundamental teaching Building, 1103 DATE: TABLE NUMBER: PURPOSE: To map and study the equipotential lines and electric field lines around two electrically charged conductors. APPARATUS: DC power supply, voltmeter, field map board, coordinate paper INTRODUCTION: The electric field E at a point is the force per unit charge on a test charge located at that point. It is numerically equal to the force on a unit (one Coulomb) positive test charge. Because similar charge s repel each other, a positive test charge placed near another positive charge experiences a repulsive force, and the field due to a positive charge points away from the charge. The field direction near a negative charged object is towards the object. Electric-field lines: Electric field lines are a way of visualizing the electric field near charged objects. Field lines define the direction of the force that a positive "test" charge experiences near other charges. The field lines are also used to define the magnitude of the electric field. The lines are drawn such that their density (the number of lines that cross a unit area) is proportional to the magnitude of E. An electric field, E, exists wherever an electric force, F, acts on a charge, q . E and F are vector quantities; they have both magnitude and direction. These fields can be represented by lines of force which show the magnitude and direction of the force that would act on a small positive charge placed in the field. Electric field lines follow two rules: 1. Lines of force originate from positive charges or infinity and terminate at negative charges or infinity. 2. Lines of force are close together where the field is strong and far apart where the field is weak. 1/3 ELECTRIC POTENTIAL & FIELD MAPS The electric lines of force between point charges are shown in Figure 1. Force lines for a pair of oppositely charged plates are shown in Figure 2. If you place a positive charge in the field it will move along a field line towards the negative electrode. The field will do work on the charge. If you move a charge from the negative to the positive electrode you will have to do work against the field. Rather than trying to measure electric fields directly, it is easier to measure the electrical potential near the charges. This is an easier measurement because electric potential does not have a direction (it’s a scalar) and can be measured with a voltmeter. The electrical potential is the potential energy of the test charge divided by the test charge. Equipotential lines: Consider a test charge that is made to move in the presence of one or more other charges in such a direction that it does not experience any change in electrical potential energy. The path of this test charge is perpendicular to the electric field at every point of its motion, and is called an equipotential line. No work is done to move the test charge; its potential energy does not change. We define electrical potential as the electrical potential energy divided by the test charge. The unit of electrical potential is the Volt. Work against the electric field happens only when the charge is moved with a component parallel to the E field; thus, equipotential lines are perpendicular to the E field. Devices that measure electric potential are voltmeters which are inexpensive and readily available. The equipotential lines, V, measured with the voltmeter can then be used to m a p the electric field lines, E. From the definitions of E and V, it follows that: • Electric field lines at any location are perpendicular to the equipotential lines. • Electric field lines go from higher to lower potential. • Electric fields are vectors: they define force directions. Some properties of electrically charged conductors in electrostatic equilibrium: 2/3 ELECTRIC POTENTIAL & FIELD MAPS • All points on the conductor's surface are at the same potential. • The electric field at a conductor's surface is perpendicular at each point to the surface (since the entire conductor surface is at the same potential). • The electric field at the surface of a charged conductor is large at a sharp point (a convex region with small radius of curvature). MAPPING PROCEDURE: You will measure electric potential and draw equipotential lines for three different configurations of oppositely charged conductors: 1. two parallel plates (a capacitor); 2. two points; 3. a hollow circular conductor between oppositely charged parallel plates. 5.00V 10 Figure 3 shows the experimental arrangement for simulating the electric field between two point charges. A dc power supply set just under 10 volts is connected across the two so. For the parallel strip electrodes measure potential beyond and to the sides of the electrode region. For the circular electrode between parallel strips make measurements inside the ring to test shielding. For the two points electrode configuration measure behind the points as well as between, to map the dipole (two-pole) shape. In all cases, be careful to take sufficient additional data to define the shape of the equipotential lines and corresponding E field lines close to the conducting electrodes. Connect points at the same potential with smooth lines to produce equipotential lines. Label the potentials. Draw the associated electric-field in a different color. The E field lines should be everywhere perpendicular to the equipotential lines, from the defining relation between E and V. The direction of E is toward decreasing potential V. Indicate the direction of the E field lines on your plot. 3/3