E fields and B fields
1) Draw the electric field lines surrounding a stationary, negative, point charge.
2) Draw the electric field lines surrounding two stationary point charges next to each other, one positive and the other negative.
3) Draw the electric field lines surrounding two stationary point charges, still next to each other, but this time both charges are positive.
4) The picture below shows a positive charge moving past a second, stationary charge. What is the direction of the magnetic force on the moving charge? What is the direction of the electric force?
5) The picture below shows an electron moving between two parallel metal plates. The bottom plate is positive, and the top plate is negative. First, draw the electric field lines between the plates. Second, what is the direction of the electric force on the moving electron? Third, is there a magnetic force?
6) A + 25 mC charge is 0.75 m to the right of a + 37 mC charge. What is the field (magnitude and direction) experienced by the + 25 mC charge?
7) A negative test charge is 0.45 m to the right of a + 9.5 mC charge. What is the field (magnitude and direction) experienced by the test charge?
8) Fire an electron through a magnetic field. The velocity makes an angle with the field lines. Which angles (note the plural) will give the maximum magnetic force? Which angles give the minimum?
9) A charged particle moves at 7.5 ´ 104 m/s through a magnetic field of 5.9 ´ 10-5 T, and experiences a 1.5 ´ 10-4 N force. The angle between the particle’s velocity and the field lines is 35°. What is the magnitude of the charge?
10) A proton moves into a 0.25 T magnetic field. The proton moves perpendicular to the field which causes it to move in a circle of radius 0.25 m. First, find the speed of the proton. Second, find the centripetal force exerted on the proton. The charge on a proton is 1.60 ´ 10-19 C, and the mass of a proton is 1.67 ´ 10-27 kg.
11) It is impossible for a constant magnetic field to do work on a moving charge, even though the field exerts a force on the charge. Please explain. (Use the right hand rule!)
12) You’ve been given the equation for the magnetic force on a moving charge (i.e. F = qvBsinq). However, sometimes you have to deal not with individual point charges, but with large numbers of point charges. In these cases, it’s much easier to think in terms of currents rather than point charges. Current is defined as I = q/t where I is the current. Using these equations, and your old friend v = d/t, derive the eqution for the force on a length of wire, starting from the equation for the force on a point charge.