AP Physics B : Understanding Electrostatics

Study concepts, example questions & explanations for AP Physics B

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Example Questions

Example Question #1 : Ap Physics 2

Charges A and B are placed a distance of  from one another. The charge of particle A is  whereas the charge of particle B is . Charge B experiences an electrostatic force of  from charge A. Similarly, charge A experiences an electrostatic force of  from charge B.

A student places these two charges in a vacuum medium and measures . A second student places these two charges in a glass medium and measures . What is ratio of the force measured in the glass medium to the force measured in the vacuum?

The dielectric constant of glass is .

Possible Answers:

Correct answer:


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 ?

Possible Answers:

We must know the strength of the electric field to solve

Correct answer:


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.

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