Deflecting Charged Particles: Electron Beam

• An electron gun shoots a beam of electrons onto a screen, similar to a TV picture tube
• The screen is coated with a fluorescent material that emits light upon electron impact
• The screen has lines drawn horizontally and vertically at 1 cm intervals, resembling graph paper
• High voltages applied to parallel plates deflect the electron beam
• The beam's deflected path is a parabola due to a constant force, like a projectile

Deflecting a Beam of Charged Particles

• Other methods exist to deflect a beam of charged particles

Time Base Function

• The electron beam normally hits one spot
• A time base can be activated to make the beam scan the screen from side to side repeatedly
• This creates a steady sweep across the screen
• When the sweep is very rapid, a horizontal line appears instead of a moving spot
• Adjusting the deflection voltage on the e/m tube controls how the beam sweeps up and down
• A slow time base allows the trace sweeping to be easily observed
• With a time-base of 4 ms/div, the beam moves 1 cm in 4 milliseconds
• With a time-base of 2 ms/div, the trace moves twice as fast, taking 2 ms to cover 1 cm

Oscilloscope Connections

• The oscilloscope's Y-input terminals connect to two points on the front panel
• The Y-input is connected to the Y-plates via an amplifier
• The Y-plates control vertical movement of the trace
• Voltage across the device's terminals causes the beam to move vertically
• A dry cell connected to the Y-input with the time base on produces a straight line above the zero axis
• Reversing the cell connections results in a straight line below the zero axis
• The Y-plates are oriented either horizontally or vertically

Oscilloscope Observations with a Cell

• With a cell connected across the Y-input and the time-base switched off, a spot will appear
• Reversing the connections will move the spot to the opposite side

Oscilloscope Activity 3

• An oscilloscope with a time base of 0.5 ms/cm

Activity Questions

• To calculate how long the beam takes to cover an 8 cm wide screen: (8 cm) * (0.5 ms/cm)
• The beam instantaneously restarts at the left side after reaching the end of its horizontal path and how many times per second that the beam scans the screen
• If the beam deflection is proportional to the voltage, a graph of voltage versus time should be sketched
• This graph is a sawtooth or ramp waveform, not sinusoidal

Alternating Sinusoidal Voltage with Y-Input

• With the time base off, a vertical line is present
• With the time base on, a sine wave appears

Calculations

• If a vertical line is 4.2 cm high with a VOLTS/DIV setting of 5
• The 4.2 cm represents twice the peak value
• The peak value represented by 2.1 cm or 2.1 divisions
• Peak voltage (Vmax) is calculated as 2.1 div * 5 V/div

Sinusoidal Voltage Trace Analysis

• A sinusoidal voltage trace crosses the zero-volt line at three consecutive points (A, B, C)
• With an onscreen width AC of 5.1 cm and a timebase of 50 ms/div
• The period (T) is 5.1 div * 50 ms/div
• The frequency (f) is 1/T

An electron gun emits a beam onto a fluorescent screen with a grid. High voltages deflect the beam, creating a parabolic path. A time base function enables the beam to scan the screen horizontally, forming a sweep or line.