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Learning Introductory Physics with Activities

Paul J. Emigh, Rebecka Tumblin, Kathryn Hadley, Danielle Skinner

Section 25.8 Conductors

Activity 25.8.1. Warm-Up.

The diagram shows a large positive charge next to an electrical conductor. A conductor is a material that has a lot of free charges that can move around in response to external fields: most metals are good electrical conductors. The conductor in the figure is neutral, so its net charge is zero.
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A long rod is positively charged and is held next to a conductor represented as a square.
Figure 25.8.1. A large positive charge to the left of a conductor.
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(a)

Predict how you expect the charges in the conductor to distribute themselves in the presence of the large positive charge. Make a charge diagram showing your prediction.
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(b)

Together, the large positive charge and the charges in the conductor will create a net electric field everywhere in space. Predict the direction of this net electric field at the center of the conductor.
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(c)

Click on this link to view a video demonstrating what happens in case like the one described above.
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(d)

Has your thinking about the electric field at the center of the conductor changed? If so, explain how and why it has changed.
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Activity 25.8.2. Sensemaking Check.

Consider the charge diagram below of a conductor in an external electric field.
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A circular gray conductor in an upward pointing electric field. Positive charges line the top of the conductor and negative charges line the bottom. Points A and B are separated vertically inside the circle.
Figure 25.8.3. A charged conductor and an external electric field.
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(a)

What direction is the net electric field at each point inside the conductor? Explain your reasoning.
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(b)

Do you think the electric potential difference between points A and B is positive, negative, or zero? Explain your reasoning.
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Subsubsection Key Ideas

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Definition 25.8.5. Electrical Conductor.

An electrical conductor is an object that possesses free electric charges that can move in response to external electric fields. When the conductor is in Electrostatic Equilibrium it has the following properties:
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  • Inside a conductor \(\vec{E} = 0\text{.}\) This is true for every point in the interior of the conductor.
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  • The conductor is an equipotential surface. For any two points inside the conductor, \(\Delta V\) must be equal to zero.
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  • The external electric field is perpendicular to the surface. This is true locally for the entire surface, but once you get some distance from the surface, the electric field can point in other directions.
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  • All surplus charge exists at the surface. All points inside the conductor, not at the surface, will have a net charge equal to zero.
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  • Charge density is largest near corners. As a result of this, the magnitude of the electric field will also be largest near corners. The sharper the corner, the larger the charge density and the field.
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References References

[1]
"MIT Physics Demo -- Faraday’s Cage" YouTube, uploaded by mittechtv, 20 November 2009, https://www.youtube.com/watch?v=WqvImbn9GG4&ab_channel=mittechtv
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