MAGNETIC FORCE ON A CURRENT-CARRYING WIRE Pre-Lab Questions Page 1. What is the SI unit for Magnetic Field? Name: Class: Roster Number: Instructor: 2. The magnetic field on a wire is 12.0 x 10 5 Gausses, what is the equivalent value of the field in units of Teslas? 3. Aurora Borealis, the Northern Lights, are beautiful auroral displays are a common sight in far northern or southern latitudes. What causes these shimmering curtains of light, and why are they usually visible only near the Earth s North and South poles? 4. Electric charges can be isolated, but can a single magnetic pole be isolated? 5. Draw the magnetic field lines of a bar magnet, making sure to indicate the North Pole and the south pole of the magnet. 0
THIS PAGE LEFT BLANK... 1
Name: Date of Lab: Lab Partners: Roster# MAGNETIC FORCE ON A CURRENT-CARRYING WIRE OBJECTIVE:To show that the magnetic force on a wire is proportional to the current in the wire and to use that force to calculate the magnetic field strength. APPARATUS: Large electromagnet with DC power supply Two ammeters U-shaped wire suspended from balance DC power supply for wire Gaussmeter with transverse probe INTRODUCTION: The apparatus for this experiment is set up as shown in Figure 1. 2
A U-shaped wire forms part of a balance and is also connected to a DC power supply so that a variable current may be passed through it. The wire is placed in the gap between the pole faces of a large electromagnet which provides a uniform magnetic field perpendicular to the plane of the U. The force on the horizontal wire forming the U is downward, while the force outward on one vertical wire cancels the outward force on the other vertical wire. If the current in the U were reversed, the upward force on the balance would still be measurable, but the inward forces on the vertical parts would put the U in unstable equilibrium, making it impossible to keep squarely between the pole pieces. After the weight of the U-shaped wire is measured with no magnetic field present, the magnet is energized. The force due to the current in the wire interacting with the electromagnet can be found by rebalancing, computing the force, then subtracting the weight of the U-shaped wire. At one electromagnet current, Im, a series of force, F, versus U-shaped wire current, Iu, is found. This is repeated for a second electromagnet current. For each of the two-electromagnet currents a graph is drawn to plot F versus Iu. From these graphs and equation 1, the magnetic field strength B is found and compared with the directly measured value. F B = IBl Equation 1 PROCEDURE: 1. Adjust the apparatus so that the horizontal portion of the U-shaped wire is centered between the pole faces and parallel to them. 2. Adjust the sliding weights on the balance to achieve a balance and record the balance reading as Mo. 3. Turn on the electromagnet's power supply and adjust to 1 ampere of current through the electromagnet. Record this value as Im. 4. Turn on the power supply for the U-shaped wire and adjust for a current of 1 ampere through the wire. Rebalance the apparatus: If the new balance reading is less than Mo found in step 2, the polarity is wrong and the current in the U-shaped wire must be reversed now. If the current must be reversed, first turn the current in the wire to zero, disconnect and reverse the wires at the power supply for the U-shaped wire, then increase the current to 1 ampere and rebalance the apparatus. Record the balance reading. 5. Take balance readings for currents of 1, 2, 3, 4 and 5 amperes of current through the U-shaped wire. Record this data. Return the wire current to zero. 3
6. Adjust the current in the electromagnet to 1.5 amperes and repeat step 5 for this new magnet current. 7. Turn off the power supply to the U-shaped wire after reducing to zero. 8. Use the Gaussmeter supplied to measure the magnetic field between the pole faces of the electromagnet for both a current of 1.5 and 1.0 amperes. 9. Slowly reduce the current in the electromagnet to zero and turn off this power supply. 10. Record the horizontal length of wire. This number is posted near the apparatus. CALCULATIONS: 1. Multiply all balance readings by the acceleration due to gravity to compute the force exerted on the balance for each measurement. Subtract the force due to Mo from each of the other computed forces. This is the magnetic force on the wire for each trial. 2. Using the Graphical Analysis program, make plots of F versus Iul for each electromagnet current on the same graph. To do this, place F values in the first column and Iul values in the second. The first data set should include the values for the U-shaped wire and the current of 1 ampere through the wire. To create a second data set: click on the Data Menu New Data Set. Make sure you put in correct units. 3. Find the experimental values of B for each electromagnet current (1.0 and 1.5 amperes) and compare these values to the measured values using the Gauss meter. (%Error = A E x 100) Note: you A should do the %Error twice, one for each of the values found in procedure 8. Questions: 1. In this experiment, what effect would reversing the direction of the current through the wire have on the magnetic force? 4
2. At any given instant, a proton moves in the positive x-direction in a region where a magnetic field is directed in the negative z-direction. What is the direction of the magnetic force? Does the proton continue to move in the positive x-direction? 3. A current carrying conductor (wire) experiences no magnetic force when placed in a certain manner in a uniform magnetic field. Explain. 4. What is the maximum work that a constant magnetic field B can perform on a charge q moving through the field with velocity v? Explain. 5. A wire carries a steady current of 2.40 Amps. A straight section of the wire is 0.750 m long and lies along the x-axis within a uniform magnetic field of magnitude B = 1.60 T in the positive z direction. If the current is in the +x direction, what is the magnetic force on the section of wire? Don t forget the direction. 6. If the length of a current carrying wire were doubled, what affect would this have on the Magnetic Field? Assume all other variable remain constant. 5