Magnet Force on Current-1 Magnetic Force on a Current-Carrying Wire Warm Up 1. Forces on magnets Assume that we have a magnet of mass m 1 sitting on a scale (force meter 1), situation A. For this configuration the force meter will read the normal force exerted on the magnet by the meter. Situation B, shows second magnet hanging by a string below a scale (force meter 2). In this configuration the force meter will read the tension in the string. For each of the situations draw and label a free-body diagram and solve for the reading on the scale. Situation A Situation B FBD for magnet 1 FBD for magnet 2 Solve for the reading on force meter 1 (normal) Solve for the reading on force meter 2 (tension)
Magnetic Force on Current - 2 Physics Experiments 133 2. Forces on magnets and the force between magnets We put the two previous configurations together to obtain situation C. Now you must include the magnetic force between the two magnets, F m. For the situation below draw and label a free-body diagram and solve for the reading on each scale. Situation C FBD for magnet 1 FBD for magnet 2 Solve for the reading on force meter 1 (normal) Solve for the reading on force meter 2 (tension) 1. How do the readings on the two scales differ? 2. What is the relationship between the increase in one scale and the decrease in the other? Justify your answers using Newton s laws. 3. What happens if the magnets are in the repulsive configuration instead of an attractive configuration? Justify your answers using Newton s laws. OBSERVE THE DEMONSTRATION
Magnetic Force on Current-3 G: Magnetic Force on a Current-Carrying Wire GOAL To measure the magnetic force on a current-carrying wire in a magnetic field. To determine the magnetic force dependence on the current. To determine the magnetic force dependence on the length of the wire. EQUIPMENT Horseshoe magnet Current balance apparatus Different lengths of wire segments DC Power supply (5 A) DC Ammeter THEORY The force on a wire of length L carrying current I in a magnetic field! is given by! =!(!!) Eqn. 1 where! is in the direction of I. In terms of the magnitude of the force we have! =!"# sin! Eqn. 2 with! the angle between the direction of the current and the direction of the field. EXPERIMENT Consider the setup shown below
Magnetic Force on Current 4 Physics Experiments 133 Force vs. Current Keeping the conductor unchanged, vary the current (do not exceed 4.5 A!). Record your data and answer the questions that follow. 1. Before you turn on the power supply, trace the direction of current in the conductor by observing that current should flow from the positive terminal to the negative terminal of the power supply. Based on the direction you traced, obtain the direction of the magnetic force that will be generated on the current-carrying wire. 2. Now turn on the power supply and verify your observations based on the change in reading of the force meter. Provide an explanation that either confirms or disproves your prediction. 3. Plot a graph with current on the horizontal-axis and Force on the vertical-axis. By fitting it to a straight line, record the slope and y-intercept of the graph. 4. What does the slope represent? (Hint: See Eqn. 2.) 5. What does the y-intercept represent? What is its expected value? 6. Repeat steps 1-3 with the direction of current reversed. 7. How should the slopes obtained (based on directions of current) compare? 8. From your calculations, estimate the field strength of the horseshoe magnet. Current (A) Current (reversed) Slope = Y-intercept = Reversed current Slope = Y-intercept =
Magnetic Force on Current 5 Force vs. Length Set the current to 1 Amp and vary the length of the conductor by replacing it with a different conductor after each observation. Record your data and answer the questions that follow. Length (m) 1. Plot a graph with length on the horizontal-axis and Force on the vertical-axis. Record the slope of the graph. What does the slope represent? (Hint: See Eqn. 2.) 2. Based on your data, calculate the field strength of the horseshoe magnet. Is your result consistent with what you obtained previously?