Math Problem Statement

Point charges of 5.00µC5.00µC and –3.00µC–3.00µC are placed 0.250 m apart. (a) Where can a third charge be placed so that the net force on it is zero? (b) What if both charges are positive?

Solution

To solve this problem, we need to find the position where a third charge can be placed so that the net force on it is zero. We will consider both scenarios: when the charges are of opposite signs and when both are positive.

Given:

  • Charge 1 (q1q_1) = +5.00μC+5.00 \, \mu\text{C}
  • Charge 2 (q2q_2) = 3.00μC-3.00 \, \mu\text{C}
  • Distance between q1q_1 and q2q_2 = 0.250m0.250 \, \text{m}

Let's denote the third charge as q3q_3, and assume it's placed at a distance xx from q1q_1. The distance from q3q_3 to q2q_2 would then be 0.250x0.250 - x.

(a) When the charges have opposite signs

Finding the position xx:

The net force on q3q_3 will be zero when the magnitudes of the forces exerted by q1q_1 and q2q_2 on q3q_3 are equal. Using Coulomb's Law:

F1=F2F_1 = F_2

kq1q3x2=kq2q3(0.250x)2k \frac{|q_1 \cdot q_3|}{x^2} = k \frac{|q_2 \cdot q_3|}{(0.250 - x)^2}

Since kk and q3q_3 cancel out, we have:

q1x2=q2(0.250x)2\frac{q_1}{x^2} = \frac{|q_2|}{(0.250 - x)^2}

Substituting the values:

5.00μCx2=3.00μC(0.250x)2\frac{5.00 \, \mu\text{C}}{x^2} = \frac{3.00 \, \mu\text{C}}{(0.250 - x)^2}

Now, solve for xx.

(b) When both charges are positive

If both charges are positive, the third charge can be placed outside the segment joining the two charges because the forces will repel each other.

We'll also need to solve for xx in this case.

Let's compute the results for both scenarios.### (a) When the Charges Have Opposite Signs

The possible solutions for the position xx where the net force on the third charge would be zero are:

  • x=0.141mx = 0.141 \, \text{m} from the +5.00μC+5.00 \, \mu\text{C} charge (between the two charges).
  • x=1.109mx = 1.109 \, \text{m} from the +5.00μC+5.00 \, \mu\text{C} charge (beyond the 3.00μC-3.00 \, \mu\text{C} charge on the right side).

However, only the position between the two charges (x=0.141mx = 0.141 \, \text{m}) is physically meaningful in this context because the other solution would place the third charge beyond the first charge, which is not typical for such problems.

(b) When Both Charges Are Positive

The possible solutions for the position xx are:

  • Left side of the +5.00μC+5.00 \, \mu\text{C} charge: x=1.109mx = -1.109 \, \text{m} (which places the third charge 1.109 meters to the left of the +5.00μC+5.00 \, \mu\text{C} charge).
  • Right side of the +3.00μC+3.00 \, \mu\text{C} charge: x=0.141mx = 0.141 \, \text{m} (which is the same position as in part (a), but now on the right side and outside the segment).

In this case, the third charge must be placed either to the left of the +5.00μC+5.00 \, \mu\text{C} charge (1.109 m away) or to the right of the +3.00μC+3.00 \, \mu\text{C} charge (0.141 m away).

Would you like any further details or have additional questions?

Here are five related questions to explore this topic further:

  1. What would happen if the distance between the two charges was doubled?
  2. How does the magnitude of the third charge affect the location where the net force is zero?
  3. What if one of the charges was much larger than the other, say 10 times larger?
  4. How would the problem change if the charges were placed in a medium with a dielectric constant other than 1?
  5. Can the third charge be at an infinite distance and still experience zero net force?

Tip: When solving problems with forces acting in opposite directions, consider the relative magnitudes of the forces and how distance affects each.

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Math Problem Analysis

Mathematical Concepts

Electrostatics
Coulomb's Law
Force on Point Charges

Formulas

Coulomb's Law: F = k * |q1 * q2| / r^2

Theorems

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Suitable Grade Level

High School