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Grade 11 Physics – More Electric Fields
Direction of a Curved Electric Field
Electric Field lines give the direction a positive charge would feel a force at that
location. If the electric field line is curved, the direction is constantly changing. The
__________________________ to a curved electric field line gives the direction of the
field at exactly that location.
Note: the force on a positive charge is in the ________________ direction as the electric
field and the force on a negative charge is in the __________________ direction as the
electric field.
Example: Indicate the direction the following positive and negative charges would feel a
force in the following electric field.
Calculating the Strength of an Electric Field
The strength of an electric field can be found by placing a small, charged object at some
location in the electric field, this is called the test charge. A test charge must be small
enough that it does not affect the other charges.
To calculate the strength of an electric field, the following formula is used:

 Fe
E
q
Remember: the direction of the electric field is the direction of the force on a positive
test charge.
K. Byrnes
PHY30S
Calculating the Charge
We know that one electron or one proton has a charge of ±1.6 x 10-19C or ±1ec. This
means that any object that has lost or gained multiple electrons or protons will have a
charge that is a multiple of 1.6 x 10-19C or ±1ec.
To calculate the charge of an object, the following formula is used:
q  Ne
Example: Calculate the force on a +0.040C test charge in an electric field with a strength
of 800.0N/C [West].
Example: In an electric field with a strength of 4.8 x 1018 N/C [South], a charged object
feels a force of 12N [South].
(a) What is the magnitude of the charge on the object?
(b) How many electrons has the object lost or gained?
K. Byrnes
PHY30S