Chapter 19 QUIZ

VACUUM TUBES

1. In thermionic emission, why do electrons shoot off the metal surface?

2. Why is the air removed from a vacuum tube?
3. Why is the cathode negative regardless of battery connection?
4. Why does current never flow from plate to cathode?
5. How does a diode act as a rectifier?
6. How does the grid control current in a triode?
7. Why must the grid be biased negatively?
8. How does a triode amplify signals?

Chapter 19 ANSWERS

VACUUM TUBES

1. The heat increases the electrons speed so that proton attraction can no longer hold them.
2. The air molecules would clog up the space around the cathode. They would interfere with
    the emission of electrons.

3. The space charge of electrons determine the cathode polarity. Electrons are negative so
    the cathode is negative.

4. Because the plate has no electrons emitted to make up a current from plate to cathode.
5. The diode only passes current from cathode to plate. When the plate is negative no
    current can flow. Therefore, only the current in the cathode to plate direction is passed.
6. The grid acts as a valve between cathode and plate. The negativeness of the grid
    controls the amount of current that can pass to the plate.
7. The grid would lose control if it became positive. Therefore, a negative bias prevents the
    grid ever becoming positive and losing control.
8. The cathode to plate current is strong. But this current is controlled by very small charges
    in grid potential.

Return to Chapter 19