What is Point charges: Definition and 359 Discussions

A point particle (ideal particle or point-like particle, often spelled pointlike particle) is an idealization of particles heavily used in physics. Its defining feature is that it lacks spatial extension; being dimensionless, it does not take up space. A point particle is an appropriate representation of any object whenever its size, shape, and structure are irrelevant in a given context. For example, from far enough away, any finite-size object will look and behave as a point-like object. A point particle can also be referred in the case of a moving body in terms of physics.
In the theory of gravity, physicists often discuss a point mass, meaning a point particle with a nonzero mass and no other properties or structure. Likewise, in electromagnetism, physicists discuss a point charge, a point particle with a nonzero charge.Sometimes, due to specific combinations of properties, extended objects behave as point-like even in their immediate vicinity. For example, spherical objects interacting in 3-dimensional space whose interactions are described by the inverse square law behave in such a way as if all their matter were concentrated in their centers of mass. In Newtonian gravitation and classical electromagnetism, for example, the respective fields outside a spherical object are identical to those of a point particle of equal charge/mass located at the center of the sphere.In quantum mechanics, the concept of a point particle is complicated by the Heisenberg uncertainty principle, because even an elementary particle, with no internal structure, occupies a nonzero volume. For example, the atomic orbit of an electron in the hydrogen atom occupies a volume of ~10−30 m3. There is nevertheless a distinction between elementary particles such as electrons or quarks, which have no known internal structure, versus composite particles such as protons, which do have internal structure: A proton is made of three quarks.
Elementary particles are sometimes called "point particles", but this is in a different sense than discussed above.

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  1. K

    What are point charges?Why we use point charges for coulomb's law?

    Homework Statement What is a point charge? and what is the purpose of it when the definition of coulomb's law says,"force b/w TWO POINT CHARGES is directly proportional to their magnitude,and inverse to the square of their distance..."...I mean what is the use of it,why only point charges...
  2. M

    Electric field lines between 20/30 V point charges vs. 10/0 V

    Homework Statement What would the electric field lines / equipotential lines look like between point charges that are set up with potentials of 20 volts and 30 volts versus point charges that are set up with potentials of 0 volts and 10 volts? The Attempt at a Solution At first I thought the...
  3. C

    Calculating Work and Potential Energy in a System of Point Charges

    Homework Statement Two point charges are located on the x-axis, q1 = -e at x = 0 and q2=+e at x=a.(a)Find the work that must be done by an external force to bring a third point charge q3 = +e from infinity to x = 2a. (b) Find the total potential energy of the system of three charges.The Attempt...
  4. K

    Two Point Charges and Magnetic Field

    Homework Statement Find the magnetic forces acting on each electron in the attached image Homework Equations B = \frac{μ0}{4pi}*q*\frac{v x \hat{r}}{r^{2}} F_{m} = qv x B The Attempt at a Solution B_{1} = 10^{-7} * 1.6*10^{-19} *...
  5. F

    Potential Difference between two positive point charges

    Homework Statement Two equal positive charges Q are fixed on the x-axis, one at +a and the other at -a. (a) The electric field E at the Origin O (b) The electric potential V at the origin O Homework Equations E=-dV/dr --> V=kQ/rThe Attempt at a Solution VNet = V0 + V1 I got V0 = -kQ/r...
  6. C

    Help on problem for physics point charges

    Two point charges, one measuring +5μC and the other -6μC are placed .02m apart. What is the force between the two charges? Which point charge exerts the greater force on the other? Is it an attracting or repelling force? 1μC= 1x 10 to the negative sixthC
  7. Roodles01

    Confusion about Coulomb's Law and 3 Point Charges

    Homework Statement 2 questions regarding the answer I have been given for this problem. Attachments are the problem & relevant worked answer I disagree with. Problem Three charges are arranged in the xy-plane as shown in attachment. A charge Q is at the point A with (x, y) coordinates...
  8. Pruddy

    Finding the Net Electric Field of Two Point Charges

    Homework Statement Two point charges Q1 = -58.6 mC and Q2 = +35.4 mC are located so that Q1 is directly above Q2. The coordinates of Q1 is (-12.0 cm, +15.6 cm). The two charges are 42.8 cm apart. Determine the coordinates of the location where the net electric field due to Q1 and Q2 is zero...
  9. B

    0 Electric potential between point charges?

    Can someone explain how you can have a zero electric potential between opposite charges? a charge would move there. I thought electric potential was the measure of energy per charge. wouldn't a 0 electric potential imply that a test charge cannot have any potential energy at this point? how is...
  10. M

    Three Point Charges, find the electric force exerted

    Homework Statement Three charges, each of magnitude 2 nC, are at separate corners of a square of edge length 1 cm. The two charges at opposite corners are positive, and the other charge is negative. Find the force exerted by these charges on a fourth charge q = +3 nC at the remaining (upper...
  11. R

    Finding r using point charges and work done by an electric force

    Homework Statement A positive point charge (q = +8.9 10-8 C) is surrounded by an equipotential surface A, which has a radius of rA = 1.3 m. A positive test charge (q0 = +2.7 10-11 C) moves from surface A to another equipotential surface B, which has a radius rB. The work done by the electric...
  12. N

    Having trouble finding the potential between two positive point charges.

    Homework Statement How to find the potential between two positive point charges? I don't know where I'm going wrong. ==I am adding V_1 and V_2. converted mC to C, cm to m, correct value of k. V1= kq1 / r1 v2= kq2/ r2. ==I am adding both of these, but I'm not getting...
  13. N

    Finding and labeling equilibrium points between multiple point charges.

    Homework Statement How to find equilibrium points between four point charges? How do you determine whether these equilibrium points are stable, unstable, or neutral? I know the sum of the forces should equal zero, but that's it. How do you determine the stability of the equilibrium...
  14. P

    Two Point Charges: Physics Problem Help

    Greetings fellow physics adorers/ mathematicians, I was wondering if someone could help me with my posted problem. All the necessary details from the template are included in the screenshot on the right.
  15. T

    Help with Point Charges and Charge by Contact

    1. Homework Statement Three small identical metal spheres carry electric charges, +Q, +2Q, and +3Q. The centres of the spheres lie in a straight line at fixed positions R, S and T, such that the distance between the spheres at R and S is d1 and that between the spheres at S and T is d2. Assume...
  16. F

    Proton-Proton Interaction at Minimum Separation

    I don't even know why I am having trouble with this problem. Homework Statement Two protons are initially an infinite distance apart. One of the protons is initially at rest, and the other is approaching the other at a certain speed. What kinetic energy must the other proton have so that their...
  17. L

    Point Where Electric Field=0 (3 point charges on x-axis)

    Homework Statement Point charges of -7 µC, 1.0 µC, and +7 µC are located along the x-axis at x = -1.0 cm, x = 0, and x = +1.0 cm, respectively. Locate a point on the positive x-axis where the magnitude of the electric field is zero. Homework Equations E=(kQ)/r^2 E(total)=E(1)+E(2)+E(3)...
  18. M

    Distance between point charges in equilibrium (simple)

    Homework Statement Two fixed charges, +1.07 uC and -3.28 uC, are 61.8 cm apart. Where may a third charge be located so that no net force acts on it? Homework Equations Coulombs law The Attempt at a Solution q1 = 1.07 uC q2 = -3.28 uC q3 = q q1 ----- q2 ----- q q1 and q2...
  19. P

    Finding the Force on One Point Charge Due to Multiple Point Charges

    Homework Statement Two +4 mC charges and two -4mC charges are arranged on a circle of diameter "d" as shown in the figure below. The magnitude and direction of the force on the +4mC charge at the top of the circle is: a) 6.39*10^6 N radially inward b) 1.2 * 10^7 N radially inward c) 2.4...
  20. P

    Finding the Force Between Two Point Charges

    Homework Statement Two point charges with charges of +5μC and -100μC are separated by 15 cm. The magnitude of the force between these charges is: a) 1.99 N b) 4.02 N c) 20 N d) 199 N e) 400 N Homework Equations F = q1q2 * k / r^2 The Attempt at a Solution F =...
  21. A

    Electric potential due to two point charges

    Homework Statement Two point charges (opposite signs, equal in magnitude) are a distance d apart. Point P is a distance z above both charges and horizontally equidistant. Find the electric potential at point P. Homework Equations Kq/r The Attempt at a Solution It's my...
  22. I

    Calculating Electric Field from Two Point Charges

    Homework Statement "A point charge of 3*10^-6 C is at the origin and another point charge of -4*10^06 C is at point 0.1m away along the x axis. What is the magnitude and direction of the electric field at point 0.15m from the origin along the posting y-axis? Homework Equations E=...
  23. N

    Electric Potential Energy, Point Charges and velocity

    Homework Statement A small metal sphere, carrying a net charge of q1 = -2.90 μC , is held in a stationary position by insulating supports. A second small metal sphere, with a net charge of q2 = -7.20 μC and mass 1.70g , is projected toward q1. When the two spheres are 0.800 m apart, q2 is...
  24. N

    Point Charges and Electric Potential Energy with Work Done

    Homework Statement A point charge q1 is held stationary at the origin. A second charge q2 is placed at point a, and the electric potential energy of the pair of charges is + 5.4 x10^-8 J. When the second charge is moved to point b, the electric force on the charge does - 1.9x10^-8 J of work...
  25. N

    Point Charges, Maginitudes and Electric Fields.

    Homework Statement Point charge 2.5 microC is located at x = 0, y = 0.30 m, point charge -2.5 microC is located at x = 0 y = -0.30 m. What are (a)the magnitude and (b)direction of the total electric force that these charges exert on a third point charge Q = 4.5 microC at x = 0.40 m, y = 0...
  26. G

    [Point Charges] Can't figure this out

    Homework Statement In the figure below, six charged particles surround particle 7 at radial distances of either d = 1.0 cm or 2d, as drawn. The charges are q1 = +8e, q2 = +8e, q3 = +e, q4 = +8e, q5 = +8e, q6 = +4e, q7 = +4e, with e = 1.60 10-19 C. What is the magnitude of the net electrostatic...
  27. Pruddy

    Coulombs Law: magnitude of the force between two point charges

    Homework Statement Determine the magnitude of the force between two point charges, Q1 = -2e located at (-3.60 cm, +4.10 cm) and Q2 = -8e located at (-0.60 cm, -8.60 cm). Give your answer in the form "a.bc x 10^(y) N". Homework Equations F = kq1q2/r2 The Attempt at a...
  28. P

    Electric Potential: Two identical + 8.5mu C point charges

    Homework Statement Two identical + 8.5mu C point charges are initially 2.5 cm from each other. If they are released at the same instant from rest, how fast will each be moving when they are very far away from each other? Assume they have identical masses of 2.0mg . Homework Equations...
  29. M

    Three point charges are arranged on a line.

    Homework Statement Three point charges are arranged on a line. Charge q3 = +5.00nC and is at the origin. Charge q2 = -4.00nC and is at x = 4.50cm . Charge q1 is at x = 2.00cm. What is q1 (magnitude and sign) if the net force on q3 is zero? Homework Equations The Attempt at a...
  30. 1

    Moving point charges - help debugging

    Homework Statement See attachment Homework Equations The Attempt at a Solution I was successful with finding a solution. But, only because I tried the practice version of that (where you can see the answers) and noticed the equation I derived gave angles that, when summed with...
  31. B

    Finding Electric Field at r>= R with Four Point Charges

    Homework Statement Four point charges, two of charge +q and two of charge -q, are placed in an arbitrary way. Each charge is located a distance r_i < R from the origin. Find the electric field at r >= R. Homework Equations According to Gauss's law, the field should be zero, right...
  32. U

    Interaction between two point charges

    Let's suppose, we have two ideal point charges q_{1}, m_{1} and q_{2}, m_{2}. One of them comes from almost infinite distance with relative velocity v_{0}, w.r.t another charge. I'm curious how can we analyze this situation mathematically, i.e the equations of the motions of these particles...
  33. 1

    Point charges on equilateral triangle no idea what's wrong.

    Homework Statement These are really frustrating because they seem so simple, I work them all out without a hitch only to find that I'm wrong half the time and left wondering why. In fig 21-26a, particles 1 and 2 have charge 20 microcoulombs each and are held at separation distance d = 1.5...
  34. R

    Point Charges Composing A Sphere

    I have a quick question about understanding the theory behind point charges and electrostatic potentials. I've not had any classes in electrodynamics, so I lack a comfortable foundation to help me think about these problems. I need to determine the electrostatic potential a certain distance...
  35. R

    Point Charges Composing A Sphere

    Homework Statement I have a quick question about understanding the theory behind point charges and electrostatic potentials. I've not had any classes in electrodynamics, so I lack a comfortable foundation to help me think about these problems. I need to determine the electrostatic...
  36. Z

    Change in energy between system of point charges

    Homework Statement The energy stored by any pair of positive charges is inversely proportional to the distance between them, and directly proportional to their charges. Three identical point charges start at the vertices of an equilateral triangle, and this configuration stores 15 Joules of...
  37. V

    Electric Potential of 2 Point Charges

    Homework Statement The 2.1mm diameter spheres in the figure are released from rest. What are their speeds vc and vd at the instant they collide? Spheres: C has 2nC of charge and a mass of 1g. D has -1nC of charge and a mass of 2g. The starting distance between the two spheres is 10mm...
  38. C

    Question about point charges (electric field strengths)

    Homework Statement A +15μC point charge Q1 is at a distance of 20mm from a +10μC charge Q2. Explain why there is a point along the line between the two charges at which the electric field strength is zero. After this, calculate the distance from this point to Q1 and Q2. The Attempt at a...
  39. M

    Point charges of charge Q placed at each corner of a cube of side r

    Imagine a cube with side lengths 'r', (like a dice) and we put point charges of charge Q at each corner of the cube. Issues with 2 subsections of this problem... First: What is the total electrostatic potential energy of the arrangement? Second: Add a charge -CQ to the center of the cube for...
  40. C

    Relating Point Charges for a Net Zero Force

    Point charges q1 and q2 lie on the x-axis at points x = -a and x = +a, respectively. (a) How must q1 and q2 be related for the net electrostatic force on point charge +Q, placed at x = +a=2, to be zero? (b) Repeat (a) but with point charge +Q now placed at x = +3a=2. Can someone help me...
  41. B

    Electrical Potential at point Y between 2 point charges

    Homework Statement Two charges, one of charge +1.5 x 10^–2 C and the other of charge –2.7 x 10^5 C, are 20.0 cm apart. The positive charge is to the left of the negative charge. (a)Draw a diagram showing the point charges and label a point Y that is 5.0 cm away from the positive charge, on...
  42. C

    How do you work out the potential difference between two point charges?

    I'm having trouble trying to understand what a volt is, and I thought this question might be able to help me understand, so.. For example: If there are 10^6 electrons in one place, and 100 electrons in a different place, what will the potential difference between these two places be. Also...
  43. N

    Difference in potential between point charges

    Homework Statement Below is a graphic representation of the problem. With a point charge at the origin, calculate the difference in potential of point B relative to point A using the path integral. Homework Equations As I recalled, to find the electric potential difference, Vab = - ∫ E ° dl...
  44. G

    Question about calculating electric field made by finite point charges

    Homework Statement Hi! I have a question about calculating electric field made by finite point charges q_{1},q_{2},..., q_{n}. From the book "introduction to electrodynamics", you can see that the electric field E at a point P made by the finite point charges can be calculated by the below...
  45. E

    EPE at a point due to two point charges

    The above equation gives the EPE of two point charges separated by a distance r. Firstly, I do not understand how this formula gives the TOTAL EPE of the system. Secondly, let's say I have three deuterium nuclei moving towards one another with initial speed V. They all stop...
  46. A

    Two point charges. Given one find the other?

    Homework Statement Two point charges are situated at A and B a distance of 35cm apart in a vacuum. The charge A is a positive charge of 16 μC and the field strength at X, a distance of 12 cm from A, is zero. Calculate: (i) The field strength at X due to the charge at A. (I found this to be...
  47. N

    Calculating forces using point charges.

    A small charge of 4 x 10^-6 C is located at the point x=2 m, y=3 m in the xy-plane. A second small charge of -3 x 10^-6 C is at the point x=4 m, y=-2 m. A) Calculate the force that the first charge exerts on the second. Express your answer in both terms of magnitude and angle, and also in i ...
  48. H

    Electrostatics - point charges and work

    Electrostatics -- point charges and work Homework Statement A 3.0 μC point charge and a 9.0 μC point charge are initially infinitely far apart. How much work does it take to bring the 3.0 μC point charge to x = 3.0 mm, y = 0.0 mm and the 9.0 μC point charge to x = -3.0 mm, y = 0.0 mm? (The...
  49. D

    Electric potential between two point charges

    Homework Statement Two point charges are separated by a distance r. If the separation is reduced by a factor of 3/2, by what factor does the electric potential between them change? A. 3/2 B. 2/3 C. 9/4 D. 4/9 E. Sqrt(3/2) Homework Equations The Attempt at a Solution Honestly I...
  50. A

    Coulomb's Law with 3 Point Charges

    Homework Statement The questions asks to consider the following comment about a situation where there are three point charges held fixed along a linear line. "There will be zero net electric force on the charge in the middle due to the other charges. Using Coulomb's law, the force due to...
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