1. Lab #3: An Electron Beam in E and B fields verify the lorentz force law. learn how to work safely with high voltages practice error propogation gymnastics

2. SAFETY One hand in pocket! Any students with pacemakers or heart arythma? Read the precaution in the lab manual carefully! You are dealing with large voltages that can deliver Dangerous currents.

3. Lorentz Force

4. E field Hold a constant potential difference across the plates. What will the direction of the E field be? Where will the E field be biggest? Smallest? Electron gun: accelerate electrons through a potential V A 0V VDVD

5. E

6. Electron Gun cathode Hot filament 0V -VC-4.5V Do electrons feel a force towards more positive or more negative potentials? More positive -VB Secondaries at the grids marked with dashes Net change in V -> VB+VC -VC-VB

7. Stray Fields If we want to reduce their effects, how should we align our apparatus? Apparatus will be basically upright. Move side to side and up/down to get beam centered. If beam goes straight, must be parallel to earth’s field.

8. Deflection See equation III-4

9. 174 refresher If you have made two measurements of the same thing, how do you check to see if they agree within errors -> Is their difference zero within errors? Calculate chi2 and prob of having a difference that big or bigger…

10. Hints BEWARE of the battery! It is at 400 V!!!! If the epoxy is coming off, do not proceed!!! battery: one lead bigger than other (keyed) 2 mm per division! X terminals in figure III-3 need to be shorted to the ground on the power supply for the first part. VB tends to accelerate VC tends to focus follow instructions about B field. The setup should be basically vertical. Rotate a little left and right, up and down until beam is centered. VD moves the beam VA is VB + VC

11. Hints part B, setting up VD put both powersupplies at their max. pat c “maximum deflection” means up, not down ammeter: use 300 mA setting (mA) ignore systematic errors don’t do any of the sections with magnetic field (VIII and on)

12. Magnetic Fields with E Fields What will the direction of the field be in the CRT? What will be the direction of the magnetic force? (the electron beam is straight out of the page)

13. Deflection