The University of Sydney School of Physics Ideas to Implementation

Cathode Ray Tubes

Changing the direction of Cathode Rays There are two ways you can change the direction of a cathode ray

1. F = qE An electric field can apply a force to a charged particle: where q is the charge of the particle and E is the strength of the electric field. The force is parallel to the electric field: for a positive charge the force is in the direction of the field, for a negative charge the force is in the opposite direction to the field.

Label the diagram with positive and negative charges.

E field

2. F = qvB sin The force on a particle moving in a magnetic field is in a direction perpendicular to both the field and the direction of motion of the particle. where q is the charge of the particle, v is its velocity, B is the magnetic field strength and is the angle between the direction that the charge is moving and the direction of the magnetic field. The direction of the force is given by the right hand rule.

Label the diagram with Force, Velocity, Path of charged particle and Magnetic field.

The University of Sydney School of Physics Ideas to Implementation Measurement of e/m You can use the apparatus here to measure the ratio e/m. The radius of the circular path the electrons take in the tube depends on how fast they are going and how strong the magnetic field is.

Measure the current and voltage from the front of the power supplies. You measure the electron path radius by seeing where the beam crosses the scale that sits behind the bulb.

Diameter

The current in the coils is: I = ______(A) (This produces the magnetic field) The accelerating voltage is: V = ______(V) (This accelerates the electrons out into the bulb) The radius of the circular path is: r = ______(m)

Use these measurements to calculate e/m in units of Coulombs/kg

−4 2 B = 7.80 ×10 I (Wb/m )

Which measurement contributes the most uncertainty to this calculation? How have we € adjusted for parallax error in this experiment?