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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:
F
E
q
qt
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.
 kq1
• Formula: E 
r
2
• 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:

kq1
E  2
r
• 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)
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
Formula:
 Gm1
g  2
r
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