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Electric Charge and Coulomb’s Law

Electric Charge and Coulomb’s Law. Electric Charge There are two types of electric charge: positive charge (+) and negative charge (-).

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Electric Charge and Coulomb’s Law

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  1. Electric Charge and Coulomb’s Law • Electric Charge • There are two types of electric charge: positive charge (+) and negative charge (-). Electric charge is a property of matter that can cause attraction or repulsion with other charges.The charge carried by electrons are negative (-e) and the charge carried by protons are positive (+e).

  2. Electric Charge and Coulomb’s Law • The unit of electric charge: e = 1.60 x 10-19 C (coulomb) To provide you with an idea of the magnitude of a coulomb, approximately 0.8 C of charge flows through a 100 watt light bulb every second. Or about 5 million trillion electrons every second. The rate of charges flowing through a conductor is called a current.

  3. Electric Charge and Coulomb’s Law • Charge and Charge Separation • When the numbers of electrons and protons in an object are the same, we say that this object is (electrically) neutral. When they are not, we call it charged. • There are many ways to charge up an object. We can rub away or rub in electrons to make an object positively or negatively charged.

  4. Electric Charge and Coulomb’s Law • Conservation of electric charge • Electric charge is conserved. Charge can move between objects in the system, but the net charge of the system remains unchanged. Charges cannot be created or destroyed in the system. 8 = 10 + (-2), so O-21 decays to Ne-21 plus two electrons.

  5. Electric Charge and Coulomb’s Law • Movement of electric charges in matter • A conductor: An object or material in which charge can flow relatively freely. Example: metal, electrolytes, … • An insulator: An object or material in which charge does not flow freely. Example: plastic, glass, … • The ground: A neutral object that can accept or supply an essentially unlimited number of charges. The Earth functions as an electric ground. Application based on these physics concepts: the lightning rod on tall buildings.

  6. Electric Charge and Coulomb’s Law • Electrostatics: forces between charges. • Unlike charges attract; like charges repel.

  7. Electric Charge and Coulomb’s Law • Forces between charges: charge induction. • The force between charges provides a second way to charge up an object: This process is call induction. • Charging an a metal sphere by induction: • Charge rearrangement in insulators:

  8. Electroscope

  9. Electric Charge and Coulomb’s Law • Coulomb’s Law of forces between two point charges:

  10. Electric Charge and Coulomb’s Law • Discussion of Coulomb’s Law. • The force is inversely proportional to the square of the separation r between the charges and directed along the line joining them • The force is proportional to the product of the charges, q1 and q2, on the two particles • The force is attractive if the charges are of opposite sign. • The force is repulsive if the charges are of like sign.

  11. Electric Charge and Coulomb’s Law • The Superposition Principle • The resultant force on any one charge equals the vector sum of the forces exerted by the other individual charges that are present. • Example: If there are four charges from q1 to q4, the resultant force on q1 is the vector sum of all the forces exerted on it by other charges: Remember to add forces as vectors.

  12. Example 1 Two spheres have charges of q1 = 3.5 x 10-5 C and q2 = 7.0 x 10-5 C and are 60 m apart. Find the magnitude of the electric force. Which sphere will experience the greater force?

  13. Example 2 Two spheres have equal charge have an electric force of 7x10-9 N exerted on each. If the distance between them is 7 m, find the charge of each sphere.

  14. Example 3 A proton (+e = +1.60 x 10-19 C, mp = 1.67 x 10-27 kg) and an electron (-e = -1.60 x 10-19 C, me = 9.11 x 10-31 kg) are 1 m apart. Compare the electric force of attraction to the gravitational force of attraction.

  15. Example 4 Two charges interact via the electric force. If the charge of one is tripled, the other doubled, and the separation distance is quadrupled how does the force change?

  16. Example 5 Three charges are arranged below. What is the force on q2? q3= -5.0 x 10-6 C q1= 3.0 x 10-6 C q2= 4.0 x 10-6 C 3.0 m 1.5 m

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