Physics A: Problem Set 5: Electric potential

classwork

1. An old fashioned television picture tube (also known as a cathode ray tube or CRT) is about a half meter long and has a potential difference of about 10,000 V between the front and the back. Determine the following quantities for an electron in a CRT when it strikes the phosphor coating on the front of the tube.
   1. its kinetic energy
   2. its velocity
   Determine the following quantities inside a typical CRT TV.
   3. the acceleration of an electron
   4. the force on an electron
   5. the electric field strength
   Electrons with this much energy will produce x-rays when stopped abruptly.
   6. Are the x-rays produced by a CRT television a matter for concern? If yes, why? If not, why not?

   Magnify
2. The inside of a human nerve cell is more negative than the outside by about −70 mV. When a nerve impulse propagates down an axon, the polarity reverses and the inside is more positive than the outside by +40 mV. This action potential lasts 2 ms and then the original resting potential is restored. All of this takes place in the space of about 6 nm, the thickness of the cell membrane.
   1. What are the magnitude and direction of the electric field (in V/m) across the membrane of a neuron during…
      1. the resting phase
      2. the action phase
   2. How much work is done moving a single sodium ion (Na⁺) across the cell membrane of a neuron? State your answer in…
      1. joules
      2. electronvolts
   3. What is the power of this microscopic event?

   

homework

3. In a region where the electric field is constant, as it is between two oppositely charged parallel plates, is the voltage also constant? Explain your answer.
4. Two related questions.
   1. What do the electric fields lines look like in a region where the magnitude of the electric field is uniform?
   2. What do they look like in a region where the electric potential is uniform?