1. The problem statement, all variables and given/known data The news just announced that aliens armed with either electron- or proton-guns have invaded the city. A quick glance reveals that there is indeed an alien on the street just below my top-floor, north-facing window. When the alien shoots his rifle vertically at the sky, the needle of the compass I keep on the window ledge points to the left. Which of the two kinds of gun is the alien using? (Explain your answer, if you like with the help of a sketch). 2. Relevant equations 3. The attempt at a solution I have this sketched on paper, but I'll just describe it because I can't attach. I have a square notated N E S W. In the middle, I have a circle with a dot to signify that the gun is pointing up. According to the right hand rule, my magnetic field is rotating counterclockwise. A compass always points in the direction of a magnetic field. And if I'm correct, doesn't positive electric charge produce a magnetic field which is orientated at a right angle to charges direction of motion. So it should be a protongun?