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Ch. 23 - Electric Potential
Giancoli Douglas - Physics for Scientists and Engineers 5th edition
Giancoli Douglas5th editionPhysics for Scientists and EngineersISBN: 9780137488179Not the one you use?Change textbook
Chapter 22, Problem 83a

Two point charges are fixed 4.0 cm apart from each other. Their charges are Q₁ = Q₂ = 6.5 μC and their masses are m₁ = 2.5 mg and m₂ = 3.5 mg. If Q₁ is released from rest, what will be its speed after a very long time?

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Understand the problem: Two point charges are fixed 4.0 cm apart, and we are tasked with finding the speed of Q₁ after it is released and allowed to move due to the electrostatic force. The charges are Q₁ = Q₂ = 6.5 μC, and their masses are m₁ = 2.5 mg and m₂ = 3.5 mg. Assume the system is isolated and energy is conserved.
Step 1: Use the principle of conservation of energy. Initially, the system has only electrostatic potential energy, and as the charges move apart, this potential energy is converted into the kinetic energy of the charges. The total energy of the system remains constant.
Step 2: Write the expression for the initial electrostatic potential energy between the two charges: U = (1 / (4πϵ₀)) * (Q₁ * Q₂ / r), where ϵ₀ is the permittivity of free space, Q₁ and Q₂ are the charges, and r is the initial separation (4.0 cm = 0.04 m).
Step 3: At a very large distance (effectively infinity), the potential energy becomes zero, and all the initial potential energy is converted into the kinetic energy of the two charges. The total kinetic energy is given by K_total = (1/2) * m₁ * v₁² + (1/2) * m₂ * v₂², where v₁ and v₂ are the final speeds of Q₁ and Q₂, respectively.
Step 4: Use the principle of conservation of momentum to relate the velocities of the two charges. Since the system is isolated, the total momentum is conserved: m₁ * v₁ = m₂ * v₂. From this, express v₂ in terms of v₁: v₂ = (m₁ / m₂) * v₁.
Step 5: Substitute the expression for v₂ into the kinetic energy equation and solve for v₁. Combine this with the initial potential energy equation to find v₁. This will give the speed of Q₁ after a very long time.

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Key Concepts

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

Coulomb's Law

Coulomb's Law describes the electrostatic force between two point charges. It states that the 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 fundamental principle is essential for calculating the force acting on the charges and understanding their interactions.
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Coulomb's Law

Conservation of Energy

The principle of conservation of energy states that the total energy in a closed system remains constant. In this scenario, the potential energy due to the electrostatic force between the charges will convert into kinetic energy as Q₁ moves. This relationship allows us to determine the final speed of Q₁ after being released from rest.
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Kinetic Energy

Kinetic energy is the energy possessed by an object due to its motion, calculated using the formula KE = 1/2 mv², where m is the mass and v is the velocity. In this problem, once the potential energy is converted into kinetic energy, we can use this formula to find the speed of Q₁ after it has been released and has moved away from Q₂.
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