More Fields

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

Charge

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:

t

q

q

FE

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.

21

r

kqE

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 109 Nm2/C2

• r is the distance from the point charge (q)• q1 is the charge of the source of field (C)

21

r

kqE

Example 1

• What is the intensity of the electric field 2.60cm from a charge of 1.50 x 10-6 C?

• 1.99 x 107 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).

• 1.22 x 108 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 107 N/C [E45’N]

Practice Problems

• Questions 26 to 30

• Page 655

Gravitational Field Intensity near a Point Mass

21

r

Gmg Formula:

G is universal gravitation constant: 6.67 x 10-11 Nm2/kg2

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 300.0 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