Electric Force

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This page contains information on the electric force on a point charge. Electric force is created by an external Electric Field.

The Coulomb Force Law

The formula for the magnitude of the electric force between two point charges is:

[math]\displaystyle{ |\vec F|=\frac{1}{4 \pi \epsilon_0 } \frac{|{q}_{1}{q}_{2}|}{r^2} }[/math]

where [math]\displaystyle{ {q}_{1} }[/math] and [math]\displaystyle{ {q}_{2} }[/math] are the magnitudes of charge of point 1 and point 2 and [math]\displaystyle{ r }[/math] is the distance between the two point charges. The units for electric force are in Newtons.

Direction of Electric Force

The electric force is along a straight line between the two point charges in the observed system. If the point charges have the same sign (i.e. both are either positively or negatively charged), then the charges repel each other. If the signs of the point charges are different (i.e. one is positively charged and one is negatively charged), then the point charges are attracted to each other.

Derivations of Electric Force

The electric force on a particle can also be written as:

[math]\displaystyle{ \vec F=q\vec E }[/math]

where [math]\displaystyle{ q }[/math] is the charge of the particle and [math]\displaystyle{ \vec E }[/math] is the external electric field.

This formula can be derived from [math]\displaystyle{ |\vec F|=\frac{1}{4 \pi \epsilon_0 } \frac{|{q}_{1}{q}_{2}|}{r^2} }[/math], the electric force between two point charges. The magnitude of the electric field created by a point charge is [math]\displaystyle{ |\vec E|=\frac{1}{4 \pi \epsilon_0 } \frac{|q|}{r^2} }[/math], where [math]\displaystyle{ q }[/math] is the magnitude of the charge of the particle and [math]\displaystyle{ r }[/math] is the distance between the observation location and the point charge. Therefore, the magnitude of electric force between point charge 1 and point charge 2 can be written as:

[math]\displaystyle{ |\vec F|=\frac{1}{4 \pi \epsilon_0 } \frac{|{q}_{1}{q}_{2}|}{r^2}=|{q}_{2}|\frac{1}{4 \pi \epsilon_0 } \frac{|{q}_{1}|}{r^2}=|{q}_{2}||\vec{E}_{1}| }[/math]

The units of charge are in Coulombs and the units for electric field are in Newton/Coulombs, so this derivation is correct in its dimensions since multiplying the two units gives just Newtons. The Newton is the unit for electric force.

Examples

Example 1

Problem: Find the magnitude of electric force on two charged particles located at [math]\displaystyle{ \lt 0, 0, 0\gt }[/math]m and [math]\displaystyle{ \lt 0, 10, 0\gt }[/math]m. The first particle has a charge of +5 nC and the second particle has a charge of -10 nC. Is the force attractive or repulsive?

Step 1: Find the distance between the two point charges.

[math]\displaystyle{ d=\sqrt{(0 m-0 m)^2+(0 m-10 m)^2+(0 m-0 m)^2}=\sqrt{100 m}=10 }[/math]m.

The distance between the two points is 10 m.

Step 2: Substitute values into the correct formula.

[math]\displaystyle{ |\vec F|=\frac{1}{4 \pi \epsilon_0 } \frac{|{q}_{1}{q}_{2}|}{r^2}=\frac{1}{4 \pi \epsilon_0 } \frac{|(5 nC)(-10 nC)|}{(10m)^2} }[/math]

[math]\displaystyle{ |\vec F|=4.5e-9 }[/math] N

The magnitude of electric force is [math]\displaystyle{ |\vec F|=4.5e-9 }[/math] N.

Step 3: Determine if force is attractive or repulsive.

Since the first particle is positively charged and the second is negatively charged, the force is attractive. The particles are attracted to each other.

Example 2

Problem: Find the electric force of a -3 C particle in a region with an electric field of [math]\displaystyle{ \lt 7, 5, 0\gt }[/math]N/C.

Step 1: Substitute values into the correct formula.

[math]\displaystyle{ \vec F=q\vec E }[/math]

[math]\displaystyle{ \vec F=(-3 C)\lt 7, 5, 0\gt }[/math]N/C

[math]\displaystyle{ \vec F=\lt -21, -15, 0\gt }[/math]N

The electric force vector for this particle is [math]\displaystyle{ \lt -21, -15, 0\gt }[/math]N.

History

French physicist Charles-Augustin de Coulomb discovered in 1785 that the magnitude of electric force between two charged particles is directly proportional to the product of the absolute value of the two charges and inversely proportional to the distance squared between the two particles. He experimented with a torsion balance which consisted of an insulated bar suspended in the air by a silk thread. Coulomb attached a metal ball with a known charge to one end of the insulated bar. He then brought another ball with the same charge near the first ball. This distance between the two balls was recorded. The balls repelled each other, causing the silk thread to twist. The angle of the twist was measured and by knowing how much force was required for the thread to twist through the recorded angle, Coulomb was able to calculate the force between the two balls and derive the formula for electric force.

See also

External links

http://www.physicsclassroom.com/class/estatics/Lesson-3/Coulomb-s-Law

References

Matter & Interactions, Vol. II: Electric and Magnetic Interactions, 4th Edition

https://en.wikipedia.org/wiki/Coulomb's_law