 Now let's look at the first question on the back side of your worksheet for today. I've taken the question here and just blown it up a little bit larger tech so it's easier to see on the video. A velocity selector is set up such that the magnetic field is 50.0 Tesla and the desired speed of the particles is 5.5 meters per second. What is the needed electric field? Show your work. So we start off by figuring out what it is we've got as our knowns and our first known is the magnetic field 50.0 Tesla and the second one is our velocity 5.5 zero meters per second. Now the next key here those are our two numbers given in the problem is that this is a problem which is a velocity selector. Now that means you need to go back to either the textbook or the pre lecture videos to understand what's happening here. If you don't know what a velocity selector is none of these equations are going to make sense to you but if you go back and watch those videos we have a very specific equation that relates the velocity and a velocity selector to some other quantities and that shows us that the velocity is the electric field divided by the magnetic field and again you need to go back to the other videos to watch those to see where this equation came from. In this particular case I've got the velocity in the magnetic field so we have to rearrange it doing some algebra to solve for the electric field and if I do that algebra I find that the electric field is going to be the desired velocity times the magnetic field. So at this point we can just sort of plug our values in. I've got 5.5 meters per second and 50.0 Tesla. Now I'm not going to go through here and do the entire math for you. I'm going to let you plug that into your calculator and put that down on your sheet and remember you have to show all your work not just what's on here but include your final solution. But I do want to take a minute to talk to you a little bit about those units. So if we look at those units what we're going to find is that I've got meters per second times a Tesla. Well that doesn't make sense for the electric field until I do some algebra. So what I find is that I can replace Tesla by what I found for the units for Tesla in the very first lecture of this unit. So again you got to go back and watch that to understand where this comes from but we find that a Tesla is a Newton divided by a Coulomb meter per second. Well at this point that meter per second out front and the meter per second on the bottom they're going to cancel each other out leaving me with Newton's per Coulomb which should be our familiar unit for the electric field. So you guys have to finish off this problem multiply it out make sure you're showing all your work and again you really should go back and watch the earlier videos if you haven't done so to understand what the heck of a velocity selector actually is.