Tribal Challenge Review Question! Two identically charged small spheres, of negligible mass, are separated by a center-to-center distance of 2.0m. The force between them is 36N. Calculate the charge on each sphere. (3 points – first Tribe with a correct answer) (1 point – every Tribe with a correct answer after that)

Tribal Challenge Review Question! Two identically charged small spheres, of negligible mass, are separated by a center-to-center distance of 2.0m. The force between them is 36N. Calculate the charge on each sphere. Solution: 1.3 x 10-4 C.

SPH4U – Grade 12 Physics Unit 1 Electric Fields SPH4U – Grade 12 Physics Unit 1

Electric Fields Electric charges exert forces that can attract and repel each other even when they are not in direct contact. An electric field is defined as the region in which a force is exerted on an electric charge. An electric field has both a magnitude and a direction, and it is therefore a vector. The symbol for electric field is:

Electric Fields Electric charges exert forces that can attract and repel each other even when they are not in direct contact. An electric field is defined as the region in which a force is exerted on an electric charge. An electric field has both a magnitude and a direction, and it is therefore a vector. The symbol for electric field is: Electric Fields are measured in N/C

Electric Fields Every charged object creates an electric field of force in the space around it and any other charged object in that field will feel a force of attraction or repulsion. The electric field is represented by drawing “field” lines. Field lines show the direction of the electric force on a positive test charge placed at every point in the field. Field lines closer together indicate a stronger field.

Properties of Electric Fields We know that a charged particle will feel a force if it is in the presence of an electric field (either repulsion or attraction). The magnitude of this force is given by: The electric force is parallel to , with the direction dependent on whether or not the charge is positive or negative. If the charge is positive the force is in the same direction. If it is negative, the force is in the opposite direction. F – force (in N) q – charge (in Coulombs) ε – electric field (in N/C)

Properties of Electric Fields We can also use the formula, to find the magnitude of the electric field at a distance r from a point charge q2. ε – electric field (in N/C) k = 8.99x109 N-m2/C2 r = distance (in meters) q = charge (in Coulombs) q2 r

Example 1 An electric force with a magnitude of 2.5N, directed to the left, acts on a negative charge of -5.0C. Determine the electric field in which the charge is located.

Electric Dipoles An electric dipole consists of two equal but opposite charges separated by some small distance. An electric dipole makes for a more complex type of electric field, especially in the area very close to the two charges. They will produce field lines that will bend towards or away from the other charge.

Some examples of electric dipole situations. Recall that electric field lines will always point from positive to negative charges. Also note that field lines never cross. Note that for two identical charges there is a region in the middle that will have zero electric field.

Note that the field around two charges where one of them is a stronger charge than the other is even more complex. The symmetry of this field situation is altered.

Example 2 Two charges, one of 3.2x10-9C, the other of -6.4x10-9C, are 42cm apart. Calculate the net electric field at a point P, 15cm from the positive charge on the line connecting the charges.

Uniform Electric Fields Not all electric fields are caused by point charges. The electric field between two parallel plates of charge is uniform and perpendicular to the plates. The electric field in the region outside the plates is zero (except for a slight bulging of the field near the edges of the plates called edge effects).

Uniform Electric Fields At any point between the plates, the electric field has the same magnitude and direction. As long as the separation between the plates is much smaller than their surface area the electric field between the plates remains uniform. The magnitude of the electric field depends only on the amount of charge q, the area of the plates, and the material between the plates.

Example 3 The electric field strength midway between a pair of oppositely charged parallel plates is 3.0x103 N/C. Find the magnitude of the electric field midway between this point and the positively charged plate.

Homework Read Sections 7.3 Make additional notes to supplement the lesson notes. In particular you need to read about the Earth’s electric field and electrostatic precipitators. Complete the following questions: Pg. 345 # 2, 3, 6