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In a vacuum diode, electrons are “boiled” off a hot

Introduction to Electrodynamics | 4th Edition | ISBN: 9780321856562 | Authors: David J. Griffiths ISBN: 9780321856562 45

Solution for problem 53P Chapter 2

Introduction to Electrodynamics | 4th Edition

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Introduction to Electrodynamics | 4th Edition | ISBN: 9780321856562 | Authors: David J. Griffiths

Introduction to Electrodynamics | 4th Edition

4 5 1 346 Reviews
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2
Problem 53P

Problem 53P

In a vacuum diode, electrons are “boiled” off a hot cathode, at potential zero, and accelerated across a gap to the anode, which is held at positive potential V0. The cloud of moving electrons within the gap (called space charge) quickly builds up to the point where it reduces the field at the surface of the cathode to zero. From then on, a steady current I flows between the plates.

Suppose the plates are large relative to the separation (A ≫ d2 in Fig. 2.55), so that edge effects can be neglected. Then V, ρ, and v (the speed of the electrons) are all functions of x alone.

(a) Write Poisson’s equation for the region between the plates.

(b) Assuming the electrons start from rest at the cathode, what is their speed at point x, where the potential is V(x)?

(c) In the steady state, I is independent of x. What, then, is the relation between ρ and v?

(d) Use these three results to obtain a differential equation for V, by eliminating ρ and v.

(e) Solve this equation for V as a function of x, V0, and d. Plot V(x), and compare it to the potential without space-charge. Also, find ρ and v as functions of x.

(f) Show that

and find the constant K. (Equation 2.56 is called the Child-Langmuir law. It holds for other geometries as well, whenever space-charge limits the current. Notice that the space-charge limited diode is nonlinear—it does not obey Ohm’s law.)

Step-by-Step Solution:

Solution 53P 

Step 1 of 9:

In this question, we need to write Poisson’s equation for region between the plates

In part b, we need to find the speed of electrons at point , where the potential is  assuming the electrons start from rest at the cathode

In part c, we need to find the relation between  and  in the steady state, if current  is independent of

In part d, we need to obtain differential equations for  by eliminating  and

In part e, we need to solve the equation for V as a function of  and plot a graph  and compare this potential to potential without space charge. Also, find  and  as function of

In part f, we need to show

Step 2 of 7

Chapter 2, Problem 53P is Solved
Step 3 of 7

Textbook: Introduction to Electrodynamics
Edition: 4
Author: David J. Griffiths
ISBN: 9780321856562

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In a vacuum diode, electrons are “boiled” off a hot