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Electric Charge and Coulombs Law

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

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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.

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

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

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Coulombs Law – Gives the electric force between two point charges. k = Coulombs Constant = 9.0x10 9 Nm 2 /C 2 q 1 = charge on mass 1 q 2 = charge on mass 2 r = the distance between the two charges The electric force is much stronger than the gravitational force. Inverse Square Law

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If r is doubled then F is : If q 1 is doubled then F is : If q 1 and q 2 are doubled and r is halved then F is : ¼ of F 2F 16F Two charges are separated by a distance r and have a force F on each other. q1q1 q2q2 r F F Example 1

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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 3μC 40g

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The electric force is much greater than the gravitational force

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5μC5μC - 5μC Three charged objects are placed as shown. Find the net force on the object with the charge of -4μC. - 4μC F2F2 F1F1 F 1 and F 2 must be added together as vectors. 20cm 45º Example 3

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F1F1 F2F2 45º 2.3cos sin451.6 F 1 = F 2 = + F net = θ 3.31N at 209º 29º

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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 q 20º L = 30cm FEFE FEFE r =2(30sin10º)=10.4cm 10º 30sin10º r

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Draw a force diagram for one charge and treat as an equilibrium problem. FEFE F g =.08N T q Tsin80º Tcos80º 80º

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