More Fields Electric Fields at a Location vs. Electric Fields around a Point Charge

Electric Fields at a Location: Measures force on a test charge at a fixed location. It is a vector quantity. Formula:

Electric Fields at a Location vs. Electric Fields around a Point Charge Electric Fields around a Point Charge: Measures field intensity in all directions at a given distance from the centre of a point charge. Formula: It is a scalar quantity.

NOTE You can use either formula to determine field intensity. If it is at a point/location, it is a vector. If it is around a point, it is a scalar (as it radiates outward).

Electric Fields around a Point Charge This equation allows us to calculate the intensity of an electric field (in N/C) at a known distance “r” from a point charge. Formula: k is still 8.99 x 10 9 Nm 2 /C 2 r is the distance from the point charge (q) q 1 is the charge of the source of field (C)

Example 1 What is the intensity of the electric field 2.60cm from a charge of 1.50 x C? 1.99 x 10 7 N/C

Example 2 Find the electric field intensity midway between a +5.0 μC charge and a -3.5 μC charge. Assume they are 5.0cm apart. ***NOTE: Since these charges are attractive we call this COMPLIMENTARY FIELDS. To find the NET electric field, we ADD the field intensities together (use magnitudes- ignore the “sign” of the field) x 10 8 N/C

Practice Problems Questions 20-25, page 655

Example 3 Three charges, A (+6.0 μC), B (-5.0 μC), and C (+6.0μC) are located at three corners of a square as shown below. What is the net field intensity at the 4th corner? NOTE: The 4th corner is NOT a charge, just a point in space.

Step 1: Create an FBD to represent individual fields as vectors at the 4th corner. (Assume positive test charge) Step 2: Find the field intensities at each corner. Step 3: Use vector addition to find the net field intensity at D. (Pythagorean theorem and inverse tan).

Enet = 2.15 x 10 7 N/C [E45’N]

Practice Problems Questions 26 to 30 Page 655

Gravitational Field Intensity near a Point Mass Formula: G is universal gravitation constant: 6.67 x Nm 2 /kg 2 r is the distance from centre (m) m1 is the mass of the source of field (kg)

Example 1 Calculate the gravitational field intensity at a height of km from Earth’s surface. ** Convert km into m ** remember to find height from CENTRE of Earth g = 8.94 N/kg

Practice Problems Page 657, questions 31, 32, 33, 37 Can also do page 649, q18 easily now