Point charge Definition and 363 Threads

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. A

    Force exerted by a charged rod on a point charge

    Homework Statement Determine the electric force exerted on a negative point charge –Q located a distance d away from a line segment of charge, extending from –L/2 to +L/2 along the x-axis. The line segment has a non-uniform charge distribution, lambda(x)=lambda_0*x . Homework Equations...
  2. D

    Simple Point Charge Potential Problem

    Homework Statement There are two point charges aligned on the X-axis. Charge A is a distance -d/2 from the origin and Charge B is a distance d/2 from the origin. What is the potential at a distance z above the center of the charge distribution? To further clarify Charge A location at...
  3. J

    How Is the Electric Flux Calculated for a Point Charge Inside a Cylinder?

    Homework Statement A point charge Q is on the axis of a short cylinder at its center. The diameter of the cylinder is equal to its length L (see figure). What is the total flux through the curved sides of the cylinder? [Hint First calculate the flux through the ends. Homework Equations...
  4. S

    Derivation of electric potential due to point charge

    Electric potential is the work done in moving a unit charge from infinity to a point in an electric field. Electric potential due to point charge: V=-\int \vec{E}\cdot d\vec{s} V=-\int E\cdot ds cos \vartheta if the stationary charge is positive and if the test charge is is moved from...
  5. T

    Point Charge and Charged Sphere

    Homework Statement A point charge q1 = -7 μC is located at the center of a thick conducting shell of inner radius a = 2.6 cm and outer radius b = 4.9 cm, The conducting shell has a net charge of q2 = 1.1 μC. What is Ex(P), the value of the x-component of the electric field at point P...
  6. J

    The electric field due to a point charge

    http://www.monmsci.net/~fasano/phys2/Chap22_10.pdf The question is on this PDF File. It's 22.8. I get the logic of it, the electric fields will be 0 only when they both have the same magnitude. The math is shown below the problem, but i don't understand it. Could someone solve this and show me...
  7. T

    Electric potential of a point charge inside a spheric conductor

    I'm trying to find the electric potential inside a thin (not charged) spheric conductor of radius R, containing a point charge of charge +q exactly in the center. To do this, I'm trying to solve the differential equation \Delta V = 0 (harmonic). Together with the boundary condition V(R) = c...
  8. T

    Electrostatics - Conducting torus and a point charge

    Homework Statement We have a metal conducting torus and a point charge that is located on the torus' axis (location on the axis is arbitrary). Calculate the (influenced) charge distribution on the torus and the electrostatic force on the point charge. Homework Equations Equation for...
  9. S

    A point charge on a string in equilibrium with electric field

    Hi everyone, I took a look at previous questions similar to this one and it seems like I am doing everything correctly, but I still get the wrong answer. Any advice to where I am going wrong would be greatly appreciated!Homework Statement A point charge (m=1.0g) at the end of an insulating...
  10. Y

    Help in explaining what the book said in fields of a moving point charge.

    My questions are what the book said after working out the solution that I have no issue of finding. The original question is to find E and B at a field point pointed by r due to a moving point charge pointed by w(t_r) at retard time moving at a constant velocity v. For constant velocity...
  11. Y

    Question in Electric field of a moving point charge

    The electric field at a point P pointed by the position vector \vec r \;\hbox {, from a moving point charge pointed by a position vector }\;\vec w(t_r)\; at the retarded time is given by: \vec E_{(\vec r,t)} = \frac q {4\pi\epsilon_0}\frac{\eta}{(\vec{\eta}\cdot \vec u)^3}[(c^2-v^2)\vec u +...
  12. A

    Deriving an equation for the electric field at a point charge

    Homework Statement Use V=kq/r, E(x) = V/x, E(y) = V/y, E(z) = V/z to derive an expression for the electric field at a point charge q. E(r) = ? Homework Equations E= F/Q
  13. A

    Deriving an equation for the electric field at a point charge

    Homework Statement Use V=kq/r, E(x) = V/x, E(y) = V/y, E(z) = V/z to derive an expression for the electric field at a point charge q. E(r) = ? Homework Equations E = F/Q
  14. W

    Electric field due to point charge

    Homework Statement what is the magnitude or a point charge whose electric field 50cm away has the magnitude 2nc? Homework Equations E=1/4pi.epsilon sub zero q absolute/r sqrt2 1/4pi.epsilon sub zero=8.99x10sqrt9 The Attempt at a Solution that gives me 71.92 and my solution in the...
  15. W

    Electric Field Due to a Point Charge

    Homework Statement Two particles are fixed to an x axis: particle 1 of charge q1= 2.1 * 10^-8 and particle 2 of charge q2= -4.00q1 at x=70 cm. At what coordinate on the axis is the net electric field produced by the particles equal to zero? Homework Equations Enet = E1 + E2 E=F/q F=k...
  16. C

    How Does Capacitance Change Affect Current in a Nearby Charged Object?

    I have a small electrode (5x5mm) connected to 200V via a 1Meg resistor. A charged object is passed 1mm away from the electrode to produce a signal of 100mV. The static capacitance changes from 25fF to 150fF. How much of the 100nA drawn from the electric field is due to the change in...
  17. A

    Electric Field at Point 36.0 cm from Rod Center

    1. A rod 16.0 cm long is uniformly charged and has a total charge of -24.0 µC. Determine the magnitude and direction of the electric field along the axis of the rod at a point 36.0 cm from its center. 2. How is this problem done? 3...
  18. Y

    Help in steps on derivation of potential involve moving point charge.

    I mainly have problem is in the final step of the derivation, but I am going to list the steps leading to that: The question is: Find the potentials of a point charge moving with constant velocity. Relevant equation: \vec w(t) = \;\hbox { is position of q at time t.} | \vec r - \vec...
  19. Y

    Find charge distribution of a point charge at origin.

    This is part of the problem in the exercise to find charge distribution of a point charge at origin. I know \nabla \cdot \vec E = \frac {\rho}{\epsilon_0} \rho = \epislon \nabla \cdot \hat r \frac q {4\pi\epsilon_0 r^2} \;\hbox { where } \vec r = \hat x x + \hat y y + \hat z z \nabla...
  20. R

    Point Charge Electric Field Problem

    I've got a test coming up on this, and got quite a few of the questions on the related homework wrong. Attached is the problem, my solutions, and the book(correct) solution, please try to find what I'm doing wrong, this is driving me nuts. Homework Statement Two charges exist along the...
  21. P

    Electric potential of point charge

    Homework Statement A charged particle is fixed in place at the origin. A second particle of charge +10^{-6} C is released from rest from very far away (\approx (\infty, 0)). The second particle passes the point (9 m, 0) with a kinetic energy of 1.0 J. Find the electric potential due to...
  22. J

    Electrostatics - Point charge question

    1. Three equal positive point charges of magnitude Q = 5.00μ C are located at three corners of a square of edge length d = 7.8 cm. A negative charge -15.00μ C is placed on the fourth corner. At the position of the negative charge, what is the magnitude of the electric field due to the three...
  23. U

    Can a time-varying magnetic field emulate the electric field of a point charge?

    A point charge generates electric field lines that are radially directed from the source in spherical symmetry. Similarly, a line of charge generates electric field lines that are radially directed from the line of charge in cylindrical symmetry. Is there any way that a time-varying magnetic...
  24. A

    Electric potential energy due to a point charge

    Hey friends, I am stuck up in the derivation of the electric potential energy due to a charge 'Q' at the origin on test charge 'q' at a point p (position vector 'rp'). The derivation is shown below, am just struggling with a minus sign... Electric Potential Energy at P = Work done to move...
  25. S

    Point charge and finite dielectric slab

    Hi All, I want help to solve a problem: A point charge is placed at a distance 'L' from the surface of an infinite dielectric slab (permittivity: k1 and thickness 'd'). The entire region of space (outside the slab has a permittivity k2). First, I need to solve for the energy of the system...
  26. S

    How to Solve for Energy of a Point Charge Near a Dielectric Slab?

    Hi All, I want help to solve a problem: A point charge is placed at a distance 'L' from the surface of an infinite dielectric slab (permittivity: k1 and thickness 'd'). The entire region of space (outside the slab has a permittivity k2). First, I need to solve for the energy of the system...
  27. B

    Calculate electromagnetic momentum given a solenoid and point charge

    Homework Statement A point charge q is a distance a > R from the axis of an infinite solenoid (radius R, n turns per unit length, current I). Find the linear momentum and the angular momentum in the fields. (Put q on the x axis, with the solenoid along z; treat the solenoid as a nonconductor...
  28. S

    How Do You Calculate the Charge Between Two Points Given Voltage Difference?

    Homework Statement Location A is 3.04m to the right of a point charge q. Location B lies on the same line and is 3.87m to the right of the charge. The potential difference between the two locations is VB - VA = 45.0V . What is the magnitude and sign of the charge? Homework Equations...
  29. J

    Charge on the Surface of a Spherical Shell with a point charge inside with

    Homework Statement A right solid conducting cylinder has a total charge of +5.10 mC. Inside the cylinder a charge q = -1.55 mC rests at the center of a hollow sphere as shown in the diagram below. What is the charge on the surface of the hollow sphere? Homework Equations The Attempt at a...
  30. A

    Point charge electric field help find direction?

    Homework Statement Ok i got the hard parts, a and b. For some reason I'm stuck on part c, the direction of the electric field. Consider point a which is 70 cm north of a -4.2 µC point charge, and point b which is 76 cm west of the charge...
  31. C

    Point charge and electric flux

    Homework Statement You measure an electric field E at a distance x from a point charge. What is the electric flux through a sphere whose center is at that distance and whose radius is less than x from the charge? Homework Equations E*dA = (q_enc)/(e_0) The Attempt at a Solution I'm...
  32. R

    Gauss's Law - Cube with point charge at centre

    Homework Statement a)A charge Q=10nC is placed at the origin, which is the centre of a cube with side lengths a=1cm whose faces are perpendicular to the x,y and z axis. What is the total flux through the box and what is the average of the perpendicular component of the electric field on the...
  33. T

    How Do You Calculate the Resultant Force on a Charge in an Electric Field?

    Homework Statement Three charges, q1 = +2.10 10-9 C, q2 = -3.09 10-9 C, and q3 = +1.08 10-9 C, are located on the x-axis at x1 = 0, x2 = 10.0 cm and x3 = 20.0 cm. Find the resultant force on q3. I am unsure of how to even attempt this problem...
  34. A

    Electric Fields & Potentials: A Point Charge Surrounded by a Spherical Shell

    Homework Statement A point charge +Q is surrounded by a spherical shell of inner radius a and outer radius b. The spherical shell has charge density \alphar. a) What is the electric field for r < a? b) What is the electric field for a < r < b? c) What is the electric field for r > b? d)...
  35. M

    Infinite energy in a point charge

    What? W = \frac{\epsilon_{0}}{2} \int E^2 d \tau = \frac{\epsilon_{0}}{2 \left( 4 \pi \epsilon_{0} \right)^{2}} \int \frac{q^{2}}{r^{4}} r^{2} sin \theta dr d \theta d \phi = \frac{q^{2}}{8 \pi \epsilon_{0}} \int_{0}^{\infty} \frac{1}{r^{2}} dr = \infty Griffith explains this infinite energy...
  36. M

    What is the force on point charge A due to other charges?

    Homework Statement What is the force on the 1.0nC charge in the middle of the figure due to the four other charges? http://filesmelt.com/dl/question.JPG Homework Equations F = K*q1*q2 / r^2 K = 8.99x10^9 The Attempt at a Solution Initial observation: The symmetry of the...
  37. jegues

    Did I Misunderstand the Direction of Force on the Electron in an Electric Field?

    Homework Statement See figure attached for problem statement. Homework Equations The Attempt at a Solution I think my main source of confusion is this problem is how the force is acting on the electron. The picture shows that the electric field is pointing straight up...
  38. X

    Electric Field & Point Charge Potential Energy

    Homework Statement A uniform electric field of magnitude 8.5×105 N/C points in the positive x direction. Find the change in electric potential energy of a 8.0 \muC charge as it moves from the origin to the point (6.0 m , 0). Homework Equations 1 N/C = 1 V/m \DeltaU = q\DeltaV E =...
  39. E

    Point charge moving in an electric field

    A point charge accelerating through space generates an electric field and radiates energy. But how is this influenced if the point charge is moving along the field lines of an external electric field? Specifically how is calculation of radiated energy effected? Would you add the external field...
  40. W

    Force on a Point Charge in an Electric Field

    Homework Statement A negative point charge of 10 ^(−6 )C is situated in air at the origin of a rectangular coordinate system. A second negative point charge of 10^(-4) C is situated on the positive x-axis at a distance of 50 cm from the origin. What is the force on the second charge...
  41. Q

    Spherical cavity in a dielectric with a point charge inside

    Homework Statement I am trying to find the force acting on a charge placed a distance r0 from the center of a spherical cavity of radius a. The entire cavity is immersed in a dielectric material, epsilon2. Homework Equations The Attempt at a Solution Here's my quandary: I can find the...
  42. M

    Can Gauss' Law be applied to point charges on non-spherical surfaces?

    2 questions: 1. A point charge of 1.84 microC is at the center of a cubical Gaussian surface 55cm on edge. Find \Phi_E through the surface. So here I was thinking, well the shape doesn't matter so the surface can be a sphere, so I calculated it for a sphere and it was correct (taking the...
  43. D

    Conducting spherical shell with point charge

    Hi, I received this question on my exam today and it stumped me... Conducting spherical shell of inner radius 0.045m and outer radius 0.18m, which has a net charge of zero. A charge Q of 5.5x10-5 is placed at the center, find the electric fields at point A (0.03m), point B (0.06m), and...
  44. Q

    Point charge at a boundary between dielectrics

    Homework Statement A point charge Q is at the boundary plane of two infi nite, homogeneous dielectrics with dielectric constants \epsilon_1 and \epsilon_2. Calculate the electric potential, the electric field and the displacement vector at any point in space. Homework Equations...
  45. A

    Electric field of an infinite line charge and a point charge

    Electric field of an infinite line charge and a point charge! Homework Statement An infinite line charge with linear charge density \lambda=2.0\muC/m lies along the y-axis. A point charge of 8.0 \muC is locate at (x,y)=(-1.0m, 2.0m). Find (a) the electric field at (x,y)=(1.0m, 2.0m) (b) Find...
  46. AlexChandler

    Electric Potential near a point charge.

    Hello, I am having trouble in proving the equation of the electric potential near a point charge Q. There are two different ways I can think to go about deriving the equation V=q/(4*Pi*E0*r). The first way is to calculate the change in potential energy of a test charge q0 in coming from...
  47. C

    Point Charge Problem (Not in a straight line)

    Point charge 2.0 uC is located at X= 0, X= 0.30m , point charge -2.0uC is located at X= 0 Y= -0.30m . What are (a)the magnitude and (b)direction of the total electric force that these charges exert on a third point charge Q= 5.0 uC at X= 0.40 , Y= 0? I drew a diagram of what was...
  48. C

    Point Charge Problem: Force Exerted by Q1 & Q2 on Q3

    Two point charges are placed on the x-axis as follows: one positive charge, Q1= 3.96nC , is located to the right of the origin at x = 0.195m , and a second positive charge, Q2= 5.05nC , is located to the left of the origin at x = -0.295m . What is the total force (magnitude and direction)...
  49. B

    Infinite Plane with Point Charge Above - Method of Images - Uniqueness Dogma

    Homework Statement You have point charge a distance "d" above infinite conducting plane held at V = 0. What is the potential when you remove charge to infinity? Homework Equations The Attempt at a Solution I think I incorrectly used Coulomb's law between the charge (+q...
  50. L

    Point charge inside solid sphere

    I'm curious. Say you have a solid, positively charged sphere with a uniform charge distribution. Now suppose you place a single, negatively charged electron inside the solid sphere. How will the electron behave? I assume the electrostatic forces exerted on the electron will ultimately tend it...
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