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Conceptual questions

Explain how the displacement current maintains the continuity of current in a circuit containing a capacitor.

The current into the capacitor to change the electric field between the plates is equal to the displacement current between the plates.

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Describe the field lines of the induced magnetic field along the edge of the imaginary horizontal cylinder shown below if the cylinder is in a spatially uniform electric field that is horizontal, pointing to the right, and increasing in magnitude.

Figure shows a cylinder placed horizontally. There are three columns of arrows labeled vector E across the cylinder. The arrows point right. The column to the left has the shortest arrows and that to the right has the longest.
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Why is it much easier to demonstrate in a student lab that a changing magnetic field induces an electric field than it is to demonstrate that a changing electric field produces a magnetic field?

The first demonstration requires simply observing the current produced in a wire that experiences a changing magnetic field. The second demonstration requires moving electric charge from one location to another, and therefore involves electric currents that generate a changing electric field. The magnetic fields from these currents are not easily separated from the magnetic field that the displacement current produces.

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Problems

Show that the magnetic field at a distance r from the axis of two circular parallel plates, produced by placing charge Q ( t ) on the plates is
B ind = μ 0 2 π r d Q ( t ) d t .

B ind = μ 0 2 π r I ind = μ 0 2 π r ε 0 Φ E t = μ 0 2 π r ε 0 ( A E t ) = μ 0 2 π r ε 0 A ( 1 d d V ( t ) d t ) = μ 0 2 π r [ ε 0 A d ] [ 1 C d Q ( t ) d t ] = μ 0 2 π r d Q ( t ) d t because C = ε 0 A d

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Express the displacement current in a capacitor in terms of the capacitance and the rate of change of the voltage across the capacitor.

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A potential difference V ( t ) = V 0 sin ω t is maintained across a parallel-plate capacitor with capacitance C consisting of two circular parallel plates. A thin wire with resistance R connects the centers of the two plates, allowing charge to leak between plates while they are charging.
(a) Obtain expressions for the leakage current I res ( t ) in the thin wire. Use these results to obtain an expression for the current I real ( t ) in the wires connected to the capacitor.
(b) Find the displacement current in the space between the plates from the changing electric field between the plates.
(c) Compare I real ( t ) with the sum of the displacement current I d ( t ) and resistor current I res ( t ) between the plates, and explain why the relationship you observe would be expected.

a. I res = V 0 sin ω t R ; b. I d = C V 0 ω cos ω t ;
c. I real = I res + d Q d t = V 0 sin ω t R + C V 0 d d t sin ω t = V 0 sin ω t R + C V 0 ω cos ω t ; which is the sum of I res and I real , consistent with how the displacement current maintaining the continuity of current.

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Suppose the parallel-plate capacitor shown below is accumulating charge at a rate of 0.010 C/s. What is the induced magnetic field at a distance of 10 cm from the capacitator?

Figure shows a capacitor with two circular parallel plates. A wire is connected to each plate. A current I flows through the wire. A point below the capacitor is highlighted. This is 10 cm from the centre of the plates.
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The potential difference V ( t ) between parallel plates shown above is instantaneously increasing at a rate of 10 7 V/s . What is the displacement current between the plates if the separation of the plates is 1.00 cm and they have an area of 0.200 m 2 ?

1.77 × 10 −3 A

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A parallel-plate capacitor has a plate area of A = 0.250 m 2 and a separation of 0.0100 m. What must be must be the angular frequency ω for a voltage V ( t ) = V 0 sin ω t with V 0 = 100 V to produce a maximum displacement induced current of 1.00 A between the plates?

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The voltage across a parallel-plate capacitor with area A = 800 cm 2 and separation d = 2 mm varies sinusoidally as V = ( 15 mV ) cos ( 150 t ) , where t is in seconds. Find the displacement current between the plates.

I d = ( 7.97 × 10 −10 A ) sin ( 150 t )

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The voltage across a parallel-plate capacitor with area A and separation d varies with time t as V = a t 2 , where a is a constant. Find the displacement current between the plates.

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Practice Key Terms 2

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Source:  OpenStax, University physics volume 2. OpenStax CNX. Oct 06, 2016 Download for free at http://cnx.org/content/col12074/1.3
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