Coulomb's Law

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Torsion Balance

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An instrument used to measure minute forces such as electrostatic or magnetic attraction and repulsion.

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Charles-Augustin De Coulomb

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A well-renowned 18th-century French physicist.

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Coulomb's Law

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A fundamental law of electromagnetism and publish it in 1785.

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Coulomb’s Law quantifies the force of electric attraction or repulsion between two electrically charged objects.

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It states:

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The electrostatic force between two charged objects is directly proportional to the product of the quantity of charge on the objects and inversely proportional to the square of the separation distance between the two objects.

The magnitude of the electrostatic force is equal to Coulomb's constant multiplied by the absolute value of the charge of each object divided by the distance between the centre of both the objects squared.

THE FORMULA FOR COULOMB'S LAW:

THE FORMULA FOR COULOMB'S LAW:

The electrostatic force between two charged objects is directly proportional to the product of the quantity of charge on the objects (q1 x q2).

THE FORMULA FOR COULOMB'S LAW:

The electrostatic force is inversely proportional to the distance between the charged objects squared.

• Fe represents the magnitude of the electrostatic force measured in Newtons (N).
• To determine the direction of the force you use the law of electrical charges. The value of Fe provides only the magnitude of the force.
• Law Of Electrical Charges:
• Like charges repel. (+ + or - -)
• Unlike charges attract. (+- or -+)
• Keeping the Law Of Electrical Charges in mind, to determine the direction you will look at the signs of q1 and q2.
• Further examples will be shown later.

BREAKING DOWN THE FORMULA FOR COULOMB'S LAW:

• Q1 and Q2 represent the charges of the object you are given.
• Q1 and Q2 is measured Coulomb's (C). When using the formula you want to deal with Coulombs.
• Conversions to Coulombs:
• 1 Coulomb (C) = 10^6 microCoulombs (uC) → TO CONVERT TO COULOMBS MULTIPLY THE microCoulombs you are given by 10^-6
• 1 Coulomb (C) = 10^9 nanoCoulombs (nC) → TO CONVERT TO COULOMBS MULTIPLY THE nanoCoulombs you are given by 10^-9
• When substituting the value of q1 and q2 in the formula, you want to take the absolute value. DO NOT INCLUDE THE SIGNS.
• You ONLY use the signs to determine the direction of Fe, as we talked about earlier using the Law of Electric Charges.

BREAKING DOWN THE FORMULA FOR COULOMB'S LAW:

• When you determine the direction of Fe you MUST use Law of Electric Charges. The formula only gives you the magnitude of the force.
• Law Of Electric Charges:
• We know that like charges attract and unlike charges repel.
• The charges of the objects as we know are determined by looking at q1 and q2.
• Examples:
• Q1 = +3C and Q2 = -5C. The force will be attractive as they are unlike charges.
• Q1 = +7C and Q2 = +9C. The force will be repulsive as they are like charges.

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DETERMINING THE DIRECTION FOR THE ELECTROSTATIC FORCE (Fe)

• R represents the distance of separation between the centres of the two charges objects.�
• R is measured in meters (m). Be mindful when reading questions, you may have to convert the units.

BREAKING DOWN THE FORMULA FOR COULOMB'S LAW:

• K represents Coulomb's Constant.�
• Coulomb’s Constant is dependent on the medium that the charged object is immersed in. �
• Unless YOU ARE EXPLICITLY TOLD THE CONSTANT IS ANOTHER VALUE, the value for Coulomb’s Constant (K) is:
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BREAKING DOWN THE FORMULA FOR COULOMB'S LAW:

DERIVING THE UNITS OF COULOMBS CONSTANT (K):

FORMULA COMPARISON: Coulomb’s Law vs. Newton's Equation For Universal Gravitation

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Coulomb’s Law

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Newton's Equation For Universal Gravitation

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• Fe is attractive or repulsive.

• Fe is proportional to q1 x q2.

• Fg is ONLY attractive.

• Fg is proportional to m1 x m2.

APPLICATIONS OF COULOMB'S LAW:

• Inkjet Printer & Laser Printers
• Uses electrostatic force to print an image.�
• Chemistry - Understanding Atoms
• Uses electrostatic force to explain protons, electrons, and atoms in general.�
• Power Coating
• A process which applies electrostatic to the object being coated and the spray mechanism.�
• Xerox Machine (Photocopier)
• Uses electrostatic force to make copies of paper.