21.1

Motion of a Charge in Magnetic and Elecytric Fields

Student Notes

The electric field is directed upward if positive, and downward if it is negative. The magnetic field is either zero or directed out of the screen. Vectors on the screen show you the direction of the two fields. Leave the charge at -1 x 10-5 C for now.
The initial values have an electric field directed upward, no magnetic field, and a negatively charged object. The initial velocity is upward. What is the direction of the force on the object? Predict how the charge will move, and then check your prediction using the simulation.
How is this motion like that of a ball tossed straight upward? How is it different?
Set the x component of the initial velocity to 5 m/s. Predict the path of the charge, and then check your prediction.
The magnetic field is directed out of the screen, perpendicular to the surface. Set the magnetic field magnitude to 0.8 T, the electric field to zero, and the velocity to 5 m/s directed in the positive x direction. What will be the direction of the magnetic force on the charge as it just starts to move? Will the direction of the force change in time? Predict the path of the charge, and then check your prediction with the simulation.
Set the electric field to +4 V/m. This situation is known as crossed electric and magnetic fields, since they are at right angles to one another. Run the simulation, and note the motion of the charge. Explain what has happened.
Leaving the fields crossed with these special values, increase the magnitude of the charge. How will that alter the motion? Make a prediction and use the simulation to check it.
Finally, what will happen if you increase or decrease the velocity of the charge? Make a prediction and try it.

Self-test

Set the initial x velocity to zero, the initial y velocity to 5 m/s, and the magnetic field to zero. Set the electric field to -2 V/m, and the charge to 10 mC. Predict the motion of the charge. Check your prediction using the simulation.
Now set the initial x velocity to 5 m/s and run the simulation. How would the motion be different if the mass of the charged object were to double? Make your prediction and check it with the simulation.
Set the initial x velocity to zero and the initial y velocity to 5 m/s. Set the electric field to -2 V/m, and the charge to +1 x 10-5 C. Predict the motion of the charge. Check your prediction.
Now set the initial x velocity to 5 m/s and run the simulation. How would the motion be different if the mass of the charged object were to double? Make your prediction and check it with the simulation.

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