Superposition of Forces

The superposition rule (superposition property) claims that for every linear forces the full force is a vector amount of individual forces.

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Learning Objectives

Apply the superposition rule to identify the net an answer caused by 2 or more stimuli


Key Takeaways

Key PointsThe superposition principle suggests that the net response at a provided place and also time resulted in by 2 or an ext stimuli is the amount of the responses i m sorry would have been resulted in by every stimulus individually.Total Coulomb pressure on a check charge as result of a team of dues is same to the vector sum of all the Coulomb forces in between the test charge and also other separation, personal, instance charges.The superposition of forces is not limited to Coulomb forces. It applies to any types (or combinations) the forces.Key TermsLorentz force: The force exerted top top a charged bit in an electromagnetic field.unit vector: A vector with size 1.electrostatic force: The electrostatic interaction between electrically charged particles; the amount and also direction that attraction or repulsion between two charged bodies.

The superposition principle (also known as superposition property) claims that: for all straight systems, the net an answer at a given place and also time resulted in by two or much more stimuli is the amount of the responses i beg your pardon would have been caused by every stimulus individually. For Coulomb’s law, the stimuli space forces. Therefore, the principle suggests that total force is a vector sum of separation, personal, instance forces.

Coulomb Force

The scalar form of Coulomb’s regulation relates the magnitude and sign of the electrostatic pressure F, acting at the same time on two suggest charges q1 and q2:

|\\boldsymbol\\textF|= \\frac14\\pi ar\\epsilon_0\\frac\\textr^2,


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Lorentz pressure on a relocating Particle: Lorentz force f ~ above a charged particle (of fee q) in motion (instantaneous velocity v). The E field and also B field vary in space and time.


where r is the separation distance and also ε0 is electrical permittivity. If the product q1q2 is positive, the force between them is repulsive; if q1q2 is negative, the force between them is attractive. The rule of linear superposition allows the expansion of Coulomb’s legislation to encompass any number of point charges—in stimulate to derive the pressure on any type of one point charge by a vector enhancement of these individual forces acting alone top top that allude charge. The resulting force vector wake up to be parallel to the electric field vector at that point, through that suggest charge removed.

To calculate the pressure on a tiny test fee q at position \\textr, as result of a mechanism of N discrete charges:

\\beginalign \\boldsymbol\\textF(\\textr) &= \\frac\\textq4 \\pi ar \\epsilon_0 \\sum_\\texti=1^\\textN \\textq_\\texti \\frac\\boldsymbol\\textr-\\textr_\\texti \\ &= \\frac\\textq\\textr 4 \\pi ar \\epsilon_0 \\sum_\\texti=1^\\textN\\textq_\\texti\\frac\\boldsymbol\\widehat\\textR_\\texti \\endalign,

where qi and ri room the magnitude and position vector the the i-th charge, respectively, and also \\boldsymbol\\widehat\\textR_\\texti is a unit vector in the direction that \\boldsymbol\\textR_\\texti = \\boldsymbol\\textr - \\boldsymbol\\textr_\\texti (a vector pointing indigenous charges qi come q. )

Of course, our discussion of superposition of forces uses to any types (or combinations) of forces. Because that example, once a charge is moving in the existence of a magnetic field and also an electric field, the charge will certainly feel both electrostatic and also magnetic forces. Full force, affecting the motion of the charge, will certainly be the vector amount of the two forces. (In this certain example the the relocating charge, the force as result of the existence of electromagnetic field is jointly called Lorentz force (see ).

Spherical distribution of Charge

The charge distribution approximately a molecule is spherical in nature, and creates a kind of electrostatic “cloud” about the molecule.


Key Takeaways

Key PointsThe force in between two objects is inversely proportional to the square that the distance between two objects.The attraction or repulsion pressures within the spherical circulation of fee is stronger closer to the molecule and becomes weaker together the distance from the molecule increases.This law additionally accounts because that the forces that tie atoms together to type molecules and also for the pressures that bind atoms and molecules with each other to type solids and also liquids.Key Termselectrostatic force: The electrostatic interaction between electrically charged particles; the amount and direction the attraction or repulsion in between two charged bodies.

Through the work-related of researchers in the late 18th century, the main attributes of the electrostatic force —the existence of two varieties of charge, the monitoring that prefer charges repel, unequal charges attract, and the diminish of pressure with distance—were at some point refined, and expressed as a mathematical formula. The mathematics formula for the electrostatic force is referred to as Coulomb ‘s law after the French physicist Charles Coulomb (1736–1806), that performed experiments and very first proposed a formula to calculate it.



Modern experiments have actually verified Coulomb’s regulation to great precision. Because that example, it has been presented that the pressure is inversely proportional come distance between two objects squared (F∝1/r2) come an accuracy that 1 component in 1016. No exception have ever been found, also at the little distances in ~ the atom.

Coulomb’s law holds even within the atoms, appropriately describing the force between the positively charged nucleus and each of the negatively charged electrons. This an easy law also correctly accounts for the forces that tie atoms together to form molecules and also for the pressures that tie atoms and also molecules with each other to form solids and also liquids.

Generally, as the distance in between ions increases, the energy of attraction viewpoints zero and also ionic bonding is much less favorable. Together the size of opposing dues increases, power increases and also ionic bonding is an ext favorable.

An electric field is a vector ar which associates to each allude of the space the Coulomb force that will experience a test unity charge. Provided the electric field, the strength and direction that a pressure F on a quantity charge q in an electric field E is established by the electrical field. Because that a positive charge, the direction the the electric field points follow me lines directed radially away from the place of the point charge, if the direction is in the direction of for a an unfavorable charge.

This distribution about a charged molecule is spherical in nature, and creates a sort of electrostatic “cloud” roughly the molecule. The attraction or repulsion forces within the spherical circulation of fee is stronger closer come the molecule, and becomes weaker as the street from the molecule increases.

This image shows the outer electron cloud of a neutral water molecule. The charge circulation of the oxygen molecule is negative, and also attracts the two confident hydrogen molecules. The attraction between the 2 opposing charges develops a neutral water molecule. It is a polar molecule since there is quiet a permanent charge separation due to the fact that the electron spend much more time near the oxygen 보다 the hydrogens.

Solving problems with Vectors and Coulomb’s Law

Coulomb’s Law, which calculates the electric force between charged particles, have the right to be written in vector notation together \\textF(\\textE) = \\frac\\textkq_1\\textq_2\\textr^2 r+.


Key Takeaways

Key PointsThe vector notation that Coulomb ‘s Law can be supplied in the basic example of two suggest charges where only one of which is a source of charge.The total force top top the field charge because that multiple point source charges is the sum of these individual forces.Coulomb’s Law can be additional simplified and also applied come a fixed number of charge points.Key Termscoulomb’s law: the math equation calculating the electrostatic pressure vector in between two charged particleselectrostatic force: The electrostatic interaction between electrically charged particles; the amount and also direction of attraction or repulsion in between two charged bodies.

Solving difficulties with Vectors and also Coulomb’s Law

Electric Force between Two suggest Charges

To resolve the electrostatic forces among electrically fee particles, very first consider two particles with electric charges q and also Q, be separated in empty an are by a distance r. Suppose that we desire to find the electric force vector on charge q. (The electrical force vector has both a magnitude and a direction. ) We can express the location of charge q together rq, and also the ar of charge Q together rQ. In this method we deserve to know both how strong the electric force is on a charge, but likewise what direction that force is directed in. Coulomb’s law using vectors deserve to be created as:

\\mathbf\\textF_\\textE = \\frac\\textkqQ(\\mathbf\\textr_\\textq - \\mathbf\\textr_\\textQ)^3

In this equation, k is same to \\frac14\\pi\\varepsilon_0\\varepsilon ,where \\varepsilon_0 is the permittivity of cost-free space and \\varepsilon is the family member permittivity the the product in which the charges space immersed. The variables \\mathbf\\textF_\\textE, \\mathbf\\textr_\\textq, and \\mathbf\\textr_\\textQare in bold since they room vectors. Thus, we need to uncover \\mathbf\\textr_\\textq - \\mathbf\\textr_\\textQ through performing typical vector subtraction. This method that we must subtract the corresponding materials of vector \\mathbf\\textr_\\textQ indigenous vector \\mathbf\\textr_\\textq.

This vector notation deserve to be provided in the an easy example that two suggest charges wherein only one of which is a resource of charge.




Note the coordinate system that has actually been chosen. Every one of the fees lie top top the corners that a square, and the beginning is liked to collocate through the reduced right source charge, and also aligned with the square. Since we can have just one origin of coordinates, no much more than among the resource points have the right to lie in ~ the origin, and also the displacements native different source points come the field suggest differ. The total force ~ above the ar charge q is due to applications the the force explained in the vector notation that Coulomb’s legislation from every of the source charges. The complete force is because of this the sum of this individual forces.



Applying Coulomb’s legislation three times and also summing the results offers us:

\\mathbf\\textF_\\textE_\\textq = \\frac\\textkq\\cdot \\textq_1(\\mathbf\\textr_\\textq - \\mathbf\\textr_\\textq1)\\mathbf\\textr_\\textq - \\mathbf\\textr_\\textq1 + \\frac\\textkq\\cdot \\textq_2(\\mathbf\\textr_\\textq - \\mathbf\\textr_\\textq2)\\mathbf\\textr_\\textq - \\mathbf\\textr_\\textq2 +\\frac\\textkq\\cdot \\textq_3(\\mathbf\\textr_\\textq - \\mathbf\\textr_\\textq3).

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This equation can further be simplified and also applied to a fixed number of charge points.

\\mathbf\\textF_\\textn = \\sum_\\texti \\ne \\textn \\frac\\textq_\\textn \\textq_\\texti(\\mathbf\\textr_\\textn - \\mathbf\\textr_\\texti)4 \\pi \\epsilon_0