# Electric Charge and Coulomb’s Law

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Electric Forces, Fields, Potential and Energy. Fundamental Charge: The charge on one electron. e = 1.6 x C Unit of charge is a Coulomb (C)

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Electric Charge and Coulomb’s Law

Fundamental Charge: The charge on one electron.
e = 1.6 x C Unit of charge is a Coulomb (C)

Two types of charge: Positive Charge: A shortage of electrons.
Negative Charge: An excess of electrons. Conservation of charge – The net charge of a closed system remains constant.

Number of electrons > Number of protons -2e = -3.2 x 10-19C
Nucleus - - + n - n + + n n + n - + + n - - - Negative Atom Number of electrons > Number of protons -2e = -3.2 x 10-19C Neutral Atom Number of electrons = Number of protons Positive Atom Number of electrons < Number of protons +2e = +3.2 x 10-19C

Electric Forces + + - Like Charges - Repel F Unlike Charges - Attract

Coulomb’s Law – Gives the electric force between two point charges.
Inverse Square Law k = Coulomb’s Constant = 9.0x109 Nm2/C2 q1 = charge on mass 1 q2 = charge on mass 2 r = the distance between the two charges The electric force is much stronger than the gravitational force.

Example 1 Two charges are separated by a distance r and have a force F on each other. q1 q2 r F If r is doubled then F is : ¼ of F If q1 is doubled then F is : 2F If q1 and q2 are doubled and r is halved then F is : 16F

Example 2 Two 40 gram masses each with a charge of 3μC are placed 50cm apart. Compare the gravitational force between the two masses to the electric force between the two masses. (Ignore the force of the earth on the two masses) 3μC 40g 50cm

The electric force is much greater than the gravitational force

F1 and F2 must be added together as vectors.
Example 3 Three charged objects are placed as shown. Find the net force on the object with the charge of -4μC. - 5μC 20cm 45º F1 5μC - 4μC F2 F1 and F2 must be added together as vectors.

+ F1 = < - 4.5 , 0.0 > F2 = < 1.6 , - 1.6 >
2.3cos45≈1.6 - 2.9 F2 45º 29º θ - 1.6 3.31 2.3sin45≈1.6 F1 = < , 0.0 > + F2 = < 1.6 , > Fnet = < , > 3.31N at 209º

Example 4 Two 8 gram, equally charged balls are suspended on earth as shown in the diagram below. Find the charge on each ball. q 20º L = 30cm FE 10º 30sin10º r =2(30sin10º)=10.4cm r

Draw a force diagram for one charge and treat as an equilibrium problem.
Tsin80º FE 80º Fg = .08N Tcos80º

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