CHAPTER 18 & 19 ELECTRIC FIELD, ELECTRICAL ENERGY and CAPACITANCE. Michael Faraday developed the concept of electric field. A charge creates an electric field about it in all directions. When another charged object enters this electric field, it experiences an electric force.

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CHAPTER 18 & 19 ELECTRIC FIELD, ELECTRICAL ENERGY and CAPACITANCE

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Electric field Lines are imaginary lines drawn in such a manner that their direction at any point is the same as the direction of the electric field at that point. The strength of the electric field is indicated by the spacing between the lines. The closer the electric field lines the stronger is the electric field.

Q Problem #4 Find the electric field intensity at a point P, located 6 mm to the left of a point charge of 8C. What are the magnitude and direction of the force on a –2nC charge placed at a point P. 6mm p 8 x 10-6 C

B Problem #5 Determine the electric field intensity at the midpoint between a –60 C charge and a + 40 C charge. The charges are 70 mm apart in air. EA p A EB -60 x 10-6 C 40 x 10-6 C

B A Problem #6 Find the electric field intensity at a point 30 mm to the right of a 16 x 10-9 C charge and a 40 mm to the left of a 9 x 10-9 C charge. EA p 30mm 40mm EB 16 x 10-9 C 9 x 10-9 C

Problem #7 Two charges of +16 x 10-6 C and +8 x 10-6 C are 200 mm apart in air. At what point between the two charges is the field intensity equal to zero?

A good conductor contains charges that are not bound to any atom and are free to move about within the material.When no net motion of charges occurs within a conductor, the conductor is said to be in electrostatic equillibrium. CONDUCTORS IN ELECTROSTATIC EQUILIBRIUM

3. The electric field just outside a charged conductor is perpendicular to the conductor’s surface. AN ISOLATED CONDUCTOR HAS THE FOLLOWING PROPERTIES 4. On an irregularly shaped conductor, the charge tends to accumulate at sharp points.

ENERGY & ELECTRIC POTENTIAL Point B Point A Consider a fixed positive charge placed at a point A and a fixed negative charge at point B. There is a force of attraction between the two charges.

ENERGY & ELECTRIC POTENTIAL Point B Point A Point B Point C Therefore to bring the charge at B to a point C a distance (r) from B, will require work to be done against the force of attraction. Thus the charge at B will undergo a change in potential energy.

Example The electric field intensity between the two charged parallel plates is 8000 N/C. The plates are 0.05 m apart. What is the potential difference between the two plates?

Problem #1 A voltmeter reads 500 V when placed across two charged, parallel metal plates. The plates are 0.02 m apart. What is the electric field between them?

Problem #2 What work is done on a 5 C charge that is raised by a potential of 1.5 V

Problem #3 If 120 J of work is done to move one Coulomb of charge from a positive plate to a negative plate, what voltage difference exists between the plates?

Problem #4 How much work is done to transfer 0.15 C of charge through a potential difference of 9 V?

Problem #5 An electron is moved through a potential difference of 500 V. How much work is done on the electron?

Problem #6 A force of 0.053 N is required to move a charge of 37 x 10-6 C a distance of 25 cm in an electric field. What is the size of the potential difference between the two points?

Parallel Plates When two parallel plates are connected across a battery, the plates will become charged and an electric field will be established between them.

The direction of an electric field is defined as the direction that a positive test charge would move. So in this case, the electric field would point from the positive plate to the negative plate. The field lines are parallel to each other and hence this type of electric field is uniform and is calculated with the equation E = V / d.