Physics Pre lab 212P 8 Template Magnetic Fields and Electric Current Name Section Date Read this answer the questions before coming to lab Pre lab Questions A Force exerted by a magnetic field on a moving charge Suppose an electron charge e mass m is accelerated from rest through a potential difference V It then enters a region of uniform magnetic field B as shown in the figure below The electron is moving in the plane of this page and the magnetic field is perpendicular out of the page Answer the following questions based on this information You will need these answers in the lab activity Potential difference V B Q1 Determine an expression for the speed v of the electron just before it enters the magnetic field Hint use energy conservation Penn State University Physics 212P 8 Written by Redwing and Samarth 1 Penn State University Physics 212P 8 Written by Redwing and Samarth 2 Q2 Describe qualitatively why the electron moves in a circle once it enters the region of uniform magnetic field Does the speed remain constant Q3 Derive an equation for the charge to mass ratio e m The equation should contain the accelerating voltage V the radius r of the electron s trajectory and the magnetic field B Penn State University Physics 212P 8 Written by Redwing and Samarth 3 Q4 Using your result for Q3 what is the slope of the plot 1 r versus B equal to Q5 What approximately is the magnitude of the earth s magnetic field B Magnetic Force on a Current Q6 Use your compass to determine which end of your magnet is North Position your magnet with the N pole immediately below a roughly horizontal straight portion of your wire Note down the direction in which current will flow when the circuit is completed Watch the reaction of the reaction of the wire when you connect the battery for a few seconds Repeat the experiment with a the current direction reversed and b with the S pole of the magnet pointing upwards Describe and explain the reaction of the wire using F iL x B Penn State University Physics 212P 8 Written by Redwing and Samarth 4 Physics Lab 212P 8 Template Moving Charges Magnetic Fields NAME LAB PARTNERS LAB SECTION LAB INSTRUCTOR DATE EMAIL ADDRESS Penn State University Physics 212P 8 Written by Redwing and Samarth 5 Lab Activity 1 A Magnetic Field Exerts A Force On Moving Charges Measuring the Charge to Mass Ratio for Electrons Q1 Record in the table below the current I through the Helmholtz coils and the diameter d of the circular electron path Make sure that you convert the diameter into meters Repeat this procedure for 10 different values of the current I I A Diameter of Circle d m Q2 Plot 1 r vs B Deduce an average value for the ratio e m including standard error by fitting the data to a straight line Use your result from Q4 of the pre lab The standard error is given by SE e m 4Va 1 SE a1 where a1 is the slope of 1 r versus B V is the power supply voltage and SE a1 is the standard error of the slope obtained from your Excel graph Compare your measured value with the established one 1 756 x 10 11 C kg Include your Excel spreadsheet graph and any relevant analysis with your report Penn State University Physics 212P 8 Written by Redwing and Samarth 6 Lab Activity 2 Moving Charges Create A Magnetic Field Magnetic Field Of A Current carrying Wire Q3 For the polarity of the batteries shown above the conventional current in the wire travels vertically upwards Use the Biot Savart Law and or a right hand rule to qualitatively sketch the magnetic field lines produced by this current Q4 While viewing your experimental set up from above complete the following diagram to show the compass needle direction without and with current in the wire One of the compass locations has been filled in for you N Without Current Penn State University With Current Physics 212P 8 Written by Redwing and Samarth 7 Penn State University Physics 212P 8 Written by Redwing and Samarth 8 Q5 By your observations in Q4 did the field produced by the current in the wire have a larger magnitude than the Earth s magnetic field Do your results of Q4 confirm your expectations for the magnetic field produced by the current in the wire Explain clearly why or why not Q6 What do you expect will happen to the magnetic field lines if the current direction is reversed Justify this using a the right hand rule b the Biot Savart Law Describe the results of an experiment that tests your prediction Penn State University Physics 212P 8 Written by Redwing and Samarth 9 Penn State University Physics 212P 8 Written by Redwing and Samarth 10 Q7 Order of magnitude estimate what is the approximate magnetic field in the experiment you just carried out at a distance of 1 cm from the center of the wire Specify your approximations and assumptions Compare this with the magnetic field of the Earth Do your results agree with what you observed in Q4 Explain Penn State University Physics 212P 8 Written by Redwing and Samarth 11 Physics Post lab 212P 8 TEMPLATE Magnetic Fields and Electric Current Name Section Date The following graph is a plot of data from Physics 212P 8 Activity 1 1 r versus B has been plotted Series 1 Fit is a linear fit to the data and is of the form 1 r a0 a1 B with SE a0 and SE a1 being the standard error for the intercept and slope respectively 1 e Q1 Given that r m 1 2V B and e m SE a SE e 1 m da1 d where a1 is the slope of 1 r versus B What is the value of e m including standard error obtained from the above graph Penn State University Physics 212P 8 Written by Redwing and Samarth 12 Penn State University Physics 212P 8 Written by Redwing and Samarth 13 Q2 Does the value you obtained agree with the generally accepted value of 1 756 x 1011 C kg Yes No Explain Penn State University Physics 212P 8 Written by Redwing and Samarth 14