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Date Course Name Instructor Name Student(s) Name Force Fields When in a deli shop, one is always interested in the price per pound of the items so that one can determine the price for a given amount of an item. In the same manner, if you knew the electric force per unit charge at points in space due to a charge configuration or the magnetic force per unit pole or moving charge, then one can easily calculate the electric force or magnetic force an object would experience at different locations. The electric force per unit charge is a vector quantity known as the electric field (E). By determining the electric force on a test charge at different points around a charge configuration, the electric field can be “mapped,” or graphically represented by lines of force. The lines of force are used to visualize the magnitude and direction of an electric field. If a positive test charge is released in the vicinity of a stationary positive charge, it will move along the line of force in a direction away from the source charge. Since a free charge moves in an electric field by the action of the electric force, then work (Work = F d) is done by the field in moving the charge from one point to another. The work per unit charge (W/q o) in moving the charge between two points in an electric field is called the potential difference V between the two points. If a charge is moved from A to B along a path at right angles of perpendicular to the line of force, no work is done since there is no force component along the path. Thus the potential difference is zero, and the potential at A = potential at B. The potential is then constant along paths perpendicular to the field lines. These paths are equipotentials (in three dimensions, it is an equipotential surface). The electric field will be mapped experimentally by determining the equipotential lines using the galvanometer as the detector. When no current flows between two probe points (zero deflection on the galvanometer) there is no potential difference between the points (V = 0), and the points are on an equipotential. Similarly, the magnetic force per unit pole is a vector quantity known as the magnetic field (B). The direction of the force at a particular location is that of the force experienced by a north magnetic pole. The magnetic field is mapped using the north pole of the magnetic needle of a compass. The torque on the compass needle causes the needle to line up with the field and the north pole of the compass points in the direction of the field. If the compass is moves in the direction indicated by the north pole, the path of the compass traces out the field line. In this experiment, you will be investigating the concept of the fields. Electric and magnetic fields will be mapped around different configurations. STUDENT OUTCOMES Through this experiment, students will be able to: - Describe the concept of a force field - Explain lines of force and its physical interpretations - Distinguish between lines of force and equipotentials and their relationships with work MATERIALS Magnetic Field: Two bar magnets One horse shoe compass paper Electric Field Mapping Board and probe Connecting wires Power supply Conducting plates galvanometer paper PRELIMINARY QUESTIONS 1. What is an electric field and what does it tell you?. 2. What are “ lines of forces”? 3. What are equipotentials and how are they experimentally determined? What is their relationship to the electric field lines? 4. What is a magnetic field, how is it defined, and what does it tell you? PROCEDURE Part I: Magnetic Field Place the magnets for each arrangement (below) on a piece of paper. Draw an outline of each arrangement on the paper and label the poles. Using the compass, trace out on the paper the magnetic field lines as smooth curves. The compass needle will align with the magnetic field. So to trace out the field lines, first place the compass near the N pole of the magnet, then put a dot on the paper on both poles of the compass needle. (The front end will be the N pole of the compass needle and the back end will be the S end.) Then move the compass, so that the S pole of the compass needle will coincide with N dot drawn earlier. Place another dot on the new N end. Continue moving the compass until the line ends on the S pole of the magnets on the paper. (See diagram below.) Draw enough field lines so that the pattern can be clearly seen. Draw the directions of the magnetic field lines (N to S).