AP Physics B : Electricity
Study concepts, example questions & explanations for AP Physics B
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Example Questions
Example Question #1 : Electricity And Magnetism
Charges A and B are placed a distance of 
A student places these two charges in a vacuum medium and measures 
The dielectric constant of glass is 
You can simplify this question tremendously by using the definition of a dielectric constant. Dielectric constant is defined as the ratio of the electrostatic force in vacuum to the electrostatic force in the medium (in this case glass).
The question is asking for the reciprocal of this value: the ratio of the force in glass to the force in the vacuum. Our answer is calculated by taking the reciprocal of the dielectric constant of glass.
Example Question #1 : Electricity And Magnetism
How much work is done by the electric field moving an electron along an equipotential surface with a potential of 
We must know the strength of the electric field to solve
Moving a charge along an equipotential surface will involve no work done by the electric field. The potential is constant throughout an equipotential surface; therefore, the potential difference experienced by the electron will be zero. Remember that energy is dependent on the potential difference.
If the potential difference is zero then the energy (and work) will be zero.
Example Question #2 : Coulomb's Law
Charges A and B are placed a distance of 
What is the ratio of 
This question is very simple if you realize that the force experienced by both charges is equal.
The definition of the two electrostatic forces are given by Coulomb's law:
In this question, we can rewrite this equation in terms of our given system.
It doesn’t matter if the charges of the two particles are different; both particles experience the same force because the charges of both particles are accounted for in the electrostatic force equation (Coulomb's law). This conclusion can also be made by considering Newton's third law: the force of the first particle on the second will be equal and opposite the force of the second particle on the first.
Since the forces are equal, their ratio will be 
Example Question #1 : Coulomb's Law
An excess charge of 
Two principal realizations help with solving this problem, both derived from Gauss’ law for electricity:
1) The excess charge on an ideal conducting sphere is uniformly distributed over its surface
2) A uniform shell of charge acts, in terms of electric force, as if all the charge were contained in a point charge at the sphere’s center
With these realizations, an application of Coulomb’s law answers the question. If 
In this equation, 
Using the given values in this equation, we can calculate the generated force:
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