
The magnitude of electric force is a fundamental concept in electromagnetism and physics. It is calculated by multiplying the charge by the electric field strength, or F = qE. The electric field is defined as the ratio of the Coulomb force to the test charge. The electric field depends on the charge and the distance, and the force is calculated using Coulomb's law. The direction of the force depends on the charge signs, with positive charges resulting in a repulsive force and negative charges resulting in an attractive force. The magnitude of the electric force can be determined by using vector analysis and considering the principles of superposition.
| Characteristics | Values |
|---|---|
| Formula to find the magnitude of electric force | F = qE |
| Formula to find the electric field | E = kQ/r^2 |
| Formula to find the electric field | E = F/q |
| Formula to find the magnitude of electric force acting on an electron | F = qE = -1.6 × 10-19 coulombs x 1.4 × 105 newtons/coulomb = -2.24 × 10^-14 newtons |
| Magnitude of electric force at point P | 102N |
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What You'll Learn

Using Coulomb's law
Coulomb's law, or Coulomb's inverse-square law, describes the electrostatic force acting between two charges. It can be used to calculate the magnitude of the electric force between two objects. The law states that the force is proportional to the product of the charges and inversely proportional to the square of the distance between them.
The magnitude of the force can be calculated using the following formula:
> F = ke x qe x qp/r^2
Where:
- F is the force between the charges
- Ke is Coulomb's constant
- Qe and qp are the magnitudes of the charges
- R is the distance between the charges
The direction of the force is along the line joining the centres of the two objects. If the two charges are of the same sign, the force is repulsive, and if they are of opposite signs, the force is attractive. This can also be determined from the result of the calculation: a positive result indicates a repulsive force, while a negative result indicates an attractive force.
It's important to note that Coulomb's law applies only to charged objects that are stationary relative to each other. Additionally, the charges must be distinct and have a minimal distance between them.
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Electric field strength
Any electrically charged object produces an electric field. The field exerts a force on other charged objects in the vicinity, repelling like charges and attracting opposite charges. The electric field strength of a source object is measured at specific vector points within an electric field. Each vector point reflects both the magnitude and direction of the electric force at that point.
To measure the field strength, a test charge is positioned at one of the vector points within the electric field of the source object. This test charge should have a small positive charge that does not interfere with the source charge. The test charge makes it possible to measure the electric field strength of the source charge at the vector point where the charge is positioned. The electric field strength at a specific vector point is directly proportional to the electric charge of the source object in coulombs (C).
The magnitude of the electric force can be calculated using the formula F=qE, where F is the force, q is the charge, and E is the electric field strength.
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Charge distribution
Discrete Charge Distribution:
A discrete charge distribution is composed of individual point particles, each contributing to the overall charge. In this scenario, each particle's charge is distinct and separate from the others. To calculate the electric force, we consider the cumulative effect of these individual charges. This approach is particularly useful when dealing with a large number of discrete charges, allowing us to simplify our calculations.
Continuous Charge Distribution:
On the other hand, a continuous charge distribution differs from a discrete one in that it has at least one nonzero dimension. In this case, we can generalize the definition of the electric field. We do so by dividing the charge into infinitesimally small pieces and treating each piece as a point charge. While charge quantization means there is no "truly" continuous distribution, in practical scenarios, the vast number of charges often justifies treating the distribution as continuous.
Now, let's explore some specific scenarios and strategies for calculating electric fields arising from charge distributions:
Symmetry in Charge Distributions:
Symmetry is a powerful tool for simplifying calculations. When dealing with symmetrical charge distributions, we can take advantage of this property to make our computations more manageable. By identifying the symmetry within the distribution, we can reduce the complexity of the problem and apply mathematical techniques more effectively.
Infinite Plane Charge Distributions:
Consider the scenario of two infinite planes with equal and opposite charges. Outside these planes, the electric fields cancel each other out due to their opposing nature. However, in the region between the planes, the electric fields add up. This principle of superposition allows us to calculate the resulting electric field by considering the contributions from each plane.
Uniform Line Charge Distributions:
When dealing with a uniform line charge distribution, we can calculate the electric field at a specific distance above the midpoint of the line. By breaking the line into differential pieces, we can determine the differential field created by each piece. Then, by applying the principle of superposition and integrating over the length of the line, we can find the complete electric field expression.
In summary, charge distribution plays a fundamental role in determining the magnitude of electric force. By understanding the nature of the distribution, whether discrete or continuous, we can employ various strategies, such as symmetry considerations and the principle of superposition, to calculate the resulting electric fields and, consequently, the electric force.
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Vector analysis
To find the magnitude of electric force, we can use the formula F = qE, where q is the charge and E is the electric field intensity in newtons per coulomb. This formula is derived from the definition of electric field E = F/q.
When dealing with multiple charges, Coulomb's law and the principle of superposition can be used to calculate the magnitude of the electric force. Coulomb's law states that the force exerted by a charge is directly proportional to the product of the charges and inversely proportional to the square of the distance between them. The principle of superposition states that the total force on an object is the vector sum of the individual forces acting on it.
For example, consider three charges located at the vertices of a cube. To find the magnitude of the electric force at a point in the cube, we can use the principle of superposition. Let's denote the charges as A, B, and C, and the electric forces they exert as F_A, F_B, and F_C, respectively. The net electric force F_net at a point in the cube is given by the vector sum of the individual forces: F_net = F_A + F_B + F_C.
If the vectors F_A and F_B are perpendicular, as in the example given, then the magnitude of their sum can be easily found using the Pythagorean theorem. In this case, the magnitude of the net electric force at point P in the cube is equal to the square root of (F_AP^2 + F_BP^2), where F_AP and F_BP are the magnitudes of the electric forces exerted by charges A and B, respectively.
In another example, we are given a charge of -22.0 μC at the origin and asked to find the electric field at point A (8.00 nm, 6.00 nm). The electric field vector E at point A has the same direction as the force vector on a positive charge placed at point A. Since the charge at the origin is negative, the force on a positive charge at point A would be directed towards the origin. Thus, the electric field vector E at point A has the same direction as the vector from point A to the origin (AO). The magnitude of the electric field vector E is given by the formula E = kq/r^2, where k is Coulomb's constant, q is the charge, and r is the distance from point A to the origin.
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Electric field intensity
The electric field intensity is characteristic of the electric field. The force that a unit test positive charge experiences when placed in an electric field is regarded as the electric field intensity. The electric field intensity is a ratio of the force experienced by the test charge upon the charge of the test charge. The formula for electric field intensity is given as:
> Electric field intensity = Force/Charge
> In symbol, its form, this can be represented as: E = F/q
The standard unit of force is Newton, and the charge is generally measured in Coulomb. Hence, the unit of electric field intensity is Newton/Coulomb or N/C. The formula for electric field intensity can also be derived using Coulomb's law.
The electric field is understood to be in the same direction as the force. The magnitude of the electric force can be calculated using the formula F= qE, where q is the charge of the electron. The negative sign in the result indicates the direction opposite to the electric field.
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Frequently asked questions
The magnitude of electric force can be found using the formula F = qE, where q is the charge of the electron and E is the electric field intensity in newtons per coulomb.
If you don't know the charge of the electron, you can use Coulomb's Law, which states that the force it exerts on a test charge q is F = k q Q /r^2, where k is Coulomb's constant, Q is the charge, and r is the distance.
To find the magnitude of the electric force from multiple charges, you need to use Coulomb's Law and the principle of superposition. First, calculate the magnitude of the forces exerted by each charge using Coulomb's Law. Then, use the principle of superposition to sum these forces.











































