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Physics Lab

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The purpose of this lab is to examine the motion of an electron, when it encounters a constant magnetic and electric field. We will also observe when the electric field and magnetic field will cancel each other out. This will lead to the electron having no net force acting upon it. By adjusting the values for the magnetic and electric fields, we will be able to check the different paths the electron follows. From this data we will be able to calculate the charge-to-mass ratio. Then, using the accepted value, we can calculate the percentage error.

This lab requires the use of two important pieces of equipment: the Helmholtz Coils and the deflection tube. The Helmholtz coils are used to generate the constant magnetic field. The power supply sends a current through the coils. This current generates a constant magnetic field which acts on the electron. The deflection tube is the device that emits the ray of electrons. The electrons get emitted because a filament wire becomes heated and releases the electrons. The electron beam becomes visible on the mica sheet. There are also two deflection plates on the top and bottom which create a potential difference between them. Because of this potential difference there is an electric field that is produced and acts on the electron.

6.3.1 Magnetic Deflection

Objective: To determine the charge-to-mass ratio for an electron by using a magnetic field. We will run a current through the Helmholtz coils to create a magnetic field. We will alter the values for the accelerating voltage (Va) and the current through the Helmholtz coils (Ib) and measure the radius of the trajectory. With our calculated values for e/m we will compare it to the accepted values.

Data and Explanations:

Positive Deflection

Va(V) x (m) y (m) Ib (A) B (T) R (m) e/m (C/kg)

2500 0.10 0.02 0.152 6.43 x 10^-4 0.26 1.79 x 10^11

3000 0.07 0.01 0.167 7.06 x 10^-4 0.25 1.93 x 10^11

3500 0.08 0.0.12 0.136 5.80 x 10^-4 0.325 1.97 x10^11

Negative Deflection

Va (V) x (m) y (m) Ib (A) B (T) R (m) e/m (C/kg)

2500 0.08 -0.01 0.152 6.43 x 10^-4 0.325 1.14x10^11

3000 0.09 -0.01 0.133 5.63 x 10^-4 0.41 1.13x10^11

3500 0.08 -0.01 0.169 7.15 x 10^-4 0.325 1.97x10^11

Mean e/m (C/kg) Standard Deviation Standard Deviation of the Mean

1.54x10^11 +/- 1.618x10^10 3.963x10^10 1.618x10^10


To obtain the e/m ratio we varied the accelerating voltage. When we varied this voltage we changed the velocity at which the electrons traveled. We also adjusted the current

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