 Hello, my name is Brad in Langdell. I'm here to talk to you today about charge particles in uniform electric fields and we've got a cool little problem where we're going to find the potential difference a particle moves through based on its initial and final velocity. We'll also calculate the direction the electric field is in that the particle is moving through to. So let's try this one out. Okay we've got an electron and it's moving initially at 3.5 times 10 to the 4 meters per second to the left. It goes into a uniform electric field. We don't know the magnitude of the direction of the uniform electric field. When it leaves the electric field the electrons got a speed of 1.7 times 10 to the 2 meters per second to the left. So it slows down but still going in the same direction. What's the magnitude of the potential difference? What's the direction of the electric field? So I'm going to answer the second part first. I'm going to figure the direction electric field and to do that I'm going to make myself a nice little diagram here. So in my diagram I've got the electron initial velocity and final velocity and the first thing I recognize is that it's slowing down. Slowing down means there must be some sort of force acting on it and since the particles moving to the left but it's slowing down the force must act to the right. That's the electric force. Now in order to get the direction of the electric field as being to the left I had to remember something. I wrote it down here in this little paragraph. Electric forces and electric fields always go in opposite directions for negative particles. That's a great thing to remember. If you've got that down you've pretty much answered the second part of the question. Now you know which way the electric field is pointing to slow this particle down and although it seems kind of weird the electric field has to point to the left to slow a particle down which is moving to the left. Kind of seems backwards that's the way it works. Now other part of the question what's the potential difference? Well I got to start thinking about some calculations now. First of all what the heck is potential difference is how much the energy changes per unit of charge. So I got to figure out how much the energy of this particle changes by and since I'm given velocities and I know the mass and charge of an electron because I can read that off my data sheet I'm going to go and find kinetic energy. One half mv squared I did it once for the initial velocity and I got a second calculation for the kinetic energy of the final velocity down here subtracted the two of them to find how much the energy changed by and I did this because I wanted to find the delta e the change in energy between the initial and final velocity because if I know how much the energy of a particle changes by I can then go through and find its potential difference by taking that energy change and dividing it by the charge. Keep me in mind potential difference how much the energy changes per unit of charge. So in a problem like this what I'm hoping you're doing is you're thinking about hey we've got velocities or speeds I have mass from that I can get kinetic energy kinetic energy is going to lead me to potential difference so the velocities the little v and the potential difference the big v they're pretty closely related. So try these problems out for more information on these topics for more videos for more problems check out the website LDindustry.C