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24. Electric Force & Field; Gauss' Law

Coulomb's Law (Electric Force)

Physics

24. Electric Force & Field; Gauss' Law

Coulomb's Law (Electric Force)

Previous problemNext problem

5:56 minutes

Problem 15b

Textbook Question

Two 10-cm-diameter charged rings face each other, 20 cm apart. The left ring is charged to −20 nC and the right ring is charged to +20 nC. What is the force on a proton at the midpoint?

Verified step by step guidance

Determine the electric field at the midpoint due to each ring. The electric field due to a ring of charge along its axis is given by:

E = \frac{k Q z}{(\sqrt{R^{2} + z^{2}}) 3^{2}}

, where

k

is Coulomb's constant,

Q

is the charge on the ring,

R

is the radius of the ring, and

z

is the distance from the center of the ring to the point of interest.

Substitute the given values for the left ring:

= -20 \, ext{nC}, \, R = 0.05 \, ext{m}, \, z = 0.1 \, ext{m}

. Calculate the electric field magnitude at the midpoint due to the left ring.

Repeat the calculation for the right ring, using

= +20 \, ext{nC}, \, R = 0.05 \, ext{m}, \, z = 0.1 \, ext{m}

. Note that the electric field due to the right ring will point in the opposite direction to the field from the left ring because the charges are opposite.

Add the electric fields from both rings at the midpoint. Since the fields are in opposite directions, their magnitudes will subtract. Determine the net electric field at the midpoint.

Calculate the force on the proton using the formula:

F = q E

, where

q

is the charge of the proton (

= 1.6 \, imes \, 10^{-19} \, ext{C}

) and

E

is the net electric field calculated in the previous step.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Electric Field

The electric field is a vector field surrounding charged particles that exerts a force on other charged objects. It is defined as the force per unit charge experienced by a positive test charge placed in the field. The direction of the electric field is away from positive charges and towards negative charges, which is crucial for understanding how charges interact in this scenario.

Coulomb's Law

Coulomb's Law describes the force between two point charges. It states that the magnitude of the electrostatic force is directly proportional to the product of the magnitudes of the charges and inversely proportional to the square of the distance between them. This law is essential for calculating the forces exerted by the charged rings on the proton at the midpoint.

Superposition Principle

The superposition principle states that the total force acting on a charge due to multiple other charges is the vector sum of the individual forces exerted by each charge. In this problem, the forces from both charged rings on the proton must be calculated separately and then combined to find the net force at the midpoint.

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Multiple Choice

In case 1, a charge q is at the origin, and a charge 5q is 1 m away. In case 2, a charge q is at the origin, and a charge -5q is 1 m away. Is the magnitude of the force exerted on the charge at the origin in case 1 greater than, less than, or equal to the force in case 2?

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Multiple Choice

Two point charges each experience a 1 N electrostatic force when they are 2 cm apart. If they are moved to a new separation of 8 cm, what is the magnitude of the electric force on each of them?

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Multiple Choice

Two 1 kg masses are 1 m apart on a frictionless table. Each has +1 C of charge. What is the magnitude of the electric force on one of the masses according to Coulomb's Law?

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Textbook Question

Two equal point charges 2.5 cm apart, both initially neutral, are being charged at the rate of 5.0 nC/s. At what rate (N/s) is the force between them increasing 1.0 s after charging begins?

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