Electron Charge-to-Mass Pre-lab Assignment

Your name: _____________________

Print this page, record your answers on it, and show it to your lab TF at the start of your lab session. In the experiment you will make a very fundamental measurement - the ratio of the charge of the electron to its mass. To do that you will use a magnetic field to cause electrons to follow a circular path. If you know v, the speed of the electrons, B, the magnitude of the field, and r, the radius of the circular path, you can find the charge-to-mass ratio.

What is the equation that gives the charge-to-mass ratio in terms of v, B, and r?

q/m =

The electrons in the lab are accelerated through a potential difference that you control. What is the speed of an electron that is released from rest and accelerated through a potential difference of 100 volts? Hint: consider conservation of energy.

A larger potential difference corresponds to a higher speed. By what factor should you multiply the potential difference by to double the speed of the electrons?

Click the button above to run the simulation. Negative charges are given an initial horizontal velocity to the right. When you adjust the magnetic field with the slider you will affect the path followed by the charges. The grid is marked in integer units. Set the magnetic field so the charges follow a path that passes through one of the intersections on the grid.

A positive setting for the magnetic field generally corresponds to a magnetic field in one direction, while a negative setting reverses the direction of the field. What direction corresponds to positive settings for the field in the simulation? Circle the correct direction.
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In the table below, record the magnetic field and the X,Y coordinates of the grid point the charge passes through. Then record the field required to have the field pass through the point X,-Y. Repeat for two additional X,Y and X,-Y sets.

Point # X-coordinate Y-coordinate Positive Magnetic Field Negative Magnetic Field
1 ___________ ______________________ ___________
2 ___________ ______________________ ___________
3 ___________ ______________________ ___________

One thing you should find is that the magnitudes of the two fields necessary to have the charge pass through X,Y and X,-Y are different. This is because there is an additional field, not under your control, much like the Earth's magnetic field that affects your data in the real experiment.

In what direction is this additional field? Circle the correct direction.
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What is the magnitude of this additional field in the simulation? The units are arbitrary.