Problem Find the electric potential and the electric field at a point P located on the axis of a uniformly charged ring of radius a and total charge Q. The plane of the ring is chosen perpendicular to the x axis (Fig. | 25.16).

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Problem Find the electric potential and the electric
field at a point P located on the axis of a uniformly
charged ring of radius a and total charge Q. The plane
of the ring is chosen perpendicular to the x axis (Fig.
25.16).
da
Va? + a²
Strategy Figure 25.16 helps us to visualize the source
of the potential and conceptualize what the potential
might look like. We expect the potential to be
symmetric around the x axis and to decrease for
increasing values of x. We categorize this problem as
one involving a continuous distribution of charge on
the ring rather than a collection of individual charges.
Figure 25.16 A uniformly charged ring of radius a,
whose plane is perpendicular to the x axis. All
elements dq of the ring are at the same distance
away from any point P on the x axis.
Solution
To analyze the problem, let us take P to be at a distance x from the center of the ring as in Figure 25.16. The
charge element dq is at a distance equal to r = vx2 + a? from point P. Hence, using Equation 25.20, we can
express V as follows.
dq
V = ke
= ke
x2+a?
In this case, each element dq is at the same distance from P. The term Vr2 + a? can therefore be removed
from the integral, and V reduces to what is given below. (Use the following as necessary: ke, Q, a, and x.)
ke
V =
dq :
=
+a²
Transcribed Image Text:Problem Find the electric potential and the electric field at a point P located on the axis of a uniformly charged ring of radius a and total charge Q. The plane of the ring is chosen perpendicular to the x axis (Fig. 25.16). da Va? + a² Strategy Figure 25.16 helps us to visualize the source of the potential and conceptualize what the potential might look like. We expect the potential to be symmetric around the x axis and to decrease for increasing values of x. We categorize this problem as one involving a continuous distribution of charge on the ring rather than a collection of individual charges. Figure 25.16 A uniformly charged ring of radius a, whose plane is perpendicular to the x axis. All elements dq of the ring are at the same distance away from any point P on the x axis. Solution To analyze the problem, let us take P to be at a distance x from the center of the ring as in Figure 25.16. The charge element dq is at a distance equal to r = vx2 + a? from point P. Hence, using Equation 25.20, we can express V as follows. dq V = ke = ke x2+a? In this case, each element dq is at the same distance from P. The term Vr2 + a? can therefore be removed from the integral, and V reduces to what is given below. (Use the following as necessary: ke, Q, a, and x.) ke V = dq : = +a²
Let us now address the electric field. The only variable in the expression for V is x. From the symmetry, we
see that along the x axis E can have only an x component. We can therefore use Equation 25.16 to find the
magnitude of the electric field at P. (Use the following as necessary: ke, Q, a, and x.)
d
–k„Q– (x² + a²)-1/2 = -k,Q
(2² + a²) -3/² (2.x)
dr
E,
To finalize, note that V decreases as x increases, as we expected from our mental representation. If the point
Pis very far from the ring (x >> a), then a in the denominator of the expression for V can be ignored and V ×
kQ/x. This expression is just the one you would expect for a point charge. At large values of x, therefore, the
charge distribution appears to be a point charge of magnitude Q as you should expect. Also notice that this
result for the electric field agrees with that obtained by direct integration.
Exercise 25.5
Hints: Getting Started | I'm Stuck
What is the electric potential at the center of the uniformly charged ring? (Use the
following as necessary: ke, Q, and a.)
V =
Transcribed Image Text:Let us now address the electric field. The only variable in the expression for V is x. From the symmetry, we see that along the x axis E can have only an x component. We can therefore use Equation 25.16 to find the magnitude of the electric field at P. (Use the following as necessary: ke, Q, a, and x.) d –k„Q– (x² + a²)-1/2 = -k,Q (2² + a²) -3/² (2.x) dr E, To finalize, note that V decreases as x increases, as we expected from our mental representation. If the point Pis very far from the ring (x >> a), then a in the denominator of the expression for V can be ignored and V × kQ/x. This expression is just the one you would expect for a point charge. At large values of x, therefore, the charge distribution appears to be a point charge of magnitude Q as you should expect. Also notice that this result for the electric field agrees with that obtained by direct integration. Exercise 25.5 Hints: Getting Started | I'm Stuck What is the electric potential at the center of the uniformly charged ring? (Use the following as necessary: ke, Q, and a.) V =
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