Two identical conducting spheres, \(A\) and \(B\) carry charges \(+Q\) and \(-Q\), respectively. A third, identical conducting sphere \(C\) carries charge \(Q=0\). Sphere \(A\) is touched to sphere \(C\) and separated. Next, sphere \(B\) is touched to sphere \(C\) and separated. Finally, \(A\) is touched to \(B\) and separated. What is the final charge on each sphere? (A) \(A=Q ; B=-Q ; C=0\) (B) \(A=Q / 2 ; B=Q / 2 ; C=Q / 4\) (C) \(A=Q / 8 ; B=Q / 8 ; C=-Q / 4\) (D) \(A=Q / 2 ; B=-Q / 4 ; C=-Q / 4\) (E) \(A=Q / 4 ; B=-Q / 8 ; C=-Q / 4\)

Short Answer

Expert verified
Answer: The final charges are A: \(+Q/8\), B: \(+Q/8\), and C: \(-Q/4\).

Step by step solution

01

Sphere A touches Sphere C

When two identical conducting spheres come in contact, they will share their charges uniformly. Sphere A has charge \(+Q\) while sphere C has charge \(Q = 0\). After touching, the charges will be redistributed equally between them. The total charge \((+Q)\) is divided by 2 (\(n=2\)). So, the charge on each sphere will be \(Q/n = Q/2\). After separating, Sphere A has charge \(+Q/2\) and Sphere C has charge \(+Q/2\).
02

Sphere B touches Sphere C

Sphere B has charge \(-Q\) and Sphere C has charge \(+Q/2\). When they touch each other, their charges will also redistribute uniformly. The total charge in this case is \((-Q) + (Q/2) = -Q/2\). Again, since there are two spheres (\(n=2\)), the charge on each sphere after redistribution will be \((-Q/2) / 2 = -Q/4\). After separating, Sphere B has charge \(-Q/4\) and Sphere C has charge \(-Q/4\).
03

Sphere A touches Sphere B

Finally, Sphere A with charge \(+Q/2\) touches Sphere B with charge \(-Q/4\). The total charge in this case is \((Q/2) + (-Q/4) = Q/4\). Since there are two spheres (\(n=2\)), the charge on each sphere after redistribution will be \((Q/4) / 2 = Q/8\). After separating, Sphere A has charge \(+Q/8\) and Sphere B has charge \(+Q/8\).
04

Final charge on each sphere

After performing all these interactions, the final charges on the spheres are: 1. Sphere A: \(+Q/8\) 2. Sphere B: \(+Q/8\) 3. Sphere C: \(-Q/4\) Thus, the correct answer is (C) \(A=Q/8 ; B=Q/8 ; C=-Q/4\).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Electric Charge
Electric charge, often denoted by the symbol 'Q', is a fundamental property of matter that comes in two types known as positive and negative. This property is the basis for the electromagnetic interaction, which is one of the four fundamental forces in physics. Charge is quantized and occurs in discrete amounts; the elementary charge, carried by electrons and protons, is approximately equal to \(1.6 \times 10^{-19}\) coulombs.

In the context of the given problem, we look at conducting spheres, which are capable of transferring charge to one another on contact. The redistribution of charge is a result of the conductive material attempting to reach an equilibrium, where the potential difference between the touching objects is minimized. A key characteristic of conductors is that they allow charge to move freely across their surfaces. This is why when two charged conductive spheres come into contact, the charges redistribute until each sphere reaches an equal amount of charge per unit area, given that they are identical in size.
Conductivity in Physics
Conductivity in physics is defined as the ability of a material to allow the flow of electric charge. This is most commonly seen in metals, whose structure allows for free electrons—also known as charge carriers—to move within the material. Good conductors, like copper or silver, have a high density of these carriers, enabling efficient charge transport.

When considering our problem, the conductivity of the spheres allows for the charge to be transferred and distributed uniformly across the spheres' surface upon contact. The nature of conductive materials is essential in understanding how Sphere A, initially charged with \(+Q\), transfers half of its charge to Sphere C when they touch, due to their conductive properties. Similarly, Sphere B's interaction with Sphere C involves the flow of charge to reach a new equilibrium.
Charge Conservation
Charge conservation is a principle stating that the total electric charge in an isolated system remains constant over time. This fundamental conservation law means that the amount of charge is the same before and after any process, such as the transfer or redistribution of charge between objects. It is important to note that while charges can move and redistribute among conductive objects, the system's total charge will not change unless there is an external influence.

In our problem, this principle is at play when the spheres touch and redistribute their charges. At each step, the total system charge of Sphere A, B, and C remains constant because no external charge is added or removed. Understanding charge conservation is key to correctly determining that, after all interactions, the final charges on spheres A, B, and C must still sum to the original total, which is zero.

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Most popular questions from this chapter

Two negative charges with magnitude \(q\) and \(2 q\) sit at points \((1,0)\) and \((0,1)\) on the \(x\) - and \(y\)-axis, respectively. Which figure best represents the total electric field at the origin? (A) \(\mathrm{A}\) (B) \(\mathrm{B}\) (C) \(\mathrm{C}\) (D) \(\mathrm{D}\) (E) \(\mathrm{E}\)

Metal sphere \(A\) has a radius of \(5 \mathrm{~cm}\) and metal sphere \(B\) has a radius of \(10 \mathrm{~cm}\). Sphere \(A\) carries a charge of \(9 \mathrm{nC}\) and sphere \(B\) carries a charge of \(18 \mathrm{nC}\). If the surfaces of \(A\) and \(B\) are \(185 \mathrm{~cm}\) apart, the potential energy between them is (A) \(7.29 \times 10^{-17} \mathrm{~J}\) (B) \(7.88 \times 10^{-9} \mathrm{~J}\) (C) \(7.88 \times 10^{-7} \mathrm{~J}\) (D) \(7.29 \times 10^{-9} \mathrm{~J}\) (E) None of the above

A test charge \(+q\) and a test charge \(-q\) are released midway between the two plates. Let the voltage of the top plate be \(V\) and the voltage of the bottom plate be 0 . The distance between the two plates is \(d\). Which of the following statements about the test charges are true? i. Both charges have gained a kinetic energy of \(|q| E d / 2\) when they hit the plates. ii. Charge \(+q\) is initially at a positive voltage and charge \(-q\) is initially at a negative voltage. iii. Charge \(+q\) initially has a positive potential energy and charge \(-q\) initially has a negative potential energy. iv. Both charges lose potential energy. (A) i only (B) i and ii (C) i and iii (D) iii and iv (E) i, iii and iv

The Millikan oil-drop experiment was the first experiment that attempted to determine the charge on the electron. Today students perform modern versions of the experiment in various ways, usually involving small latex spheres. The spheres are injected between two conducting plates held at a potential difference \(V\). Assume that the plates themselves are contained in a vacuum chamber. The spheres are of identical mass but may carry different and unknown amounts of charge. The voltage is adjusted until a selected sphere rises a constant velocity and the calculated charge on the sphere is recorded. This procedure is repeated many times for many different spheres. Eventually a graph of the results is produced: a) Write down the condition that the spheres rise with constant velocity as a function of voltage and charge. b) What shape should the theoretical curve of the charge versus the voltage be? c) How might you physically account for the lack of data between \(1000 \mathrm{~V}\) and \(2000 \mathrm{~V}\) ? d) If the distance between the plates is \(5 \mathrm{~cm}\), what is the mass of the latex spheres?

What potential difference between the two plates would be needed to accelerate a hydrogen ion (a proton) from rest to a speed of \(10^6 \mathrm{~m} / \mathrm{s}\) ? (A) \(100 \mathrm{~V}\) (B) \(500 \mathrm{~V}\) (C) \(1000 \mathrm{~V}\) (D) \(5000 \mathrm{~V}\) (E) \(10,000 \mathrm{~V}\)

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