Electric Fields: Start Here
Electric Fields: Start Here
- Two charged objects can push or pull without touching. We describe this action through the space around a charge.
- The charge that changes the surrounding space is the source charge. A small charge used to examine that space is a test charge.
A force can act through a field
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Definition
1 markWhat is meant by an electric field?
Model answer: A region of space in which an electric charge experiences a force.
- The source charge creates the field even when no test charge is present.
- The test charge does not create the field being measured. It only shows the direction and strength of the existing field.
Electric field strength and force
Definition
2 marksWhat is meant by the electric field strength at a point?
Model answer: The force per unit positive charge on a stationary charge at that point.
“Force per unit charge” and “on a positive charge” receive separate marks in recent papers (9702/41/M/J/24 Q5(a)).
Symbols
- = electric field strength (N C⁻¹)
- = electric force (N)
- = small positive test charge (C)
Symbols
- = force on the charge (N)
- = charge placed in the field (C)
- = electric field strength (N C⁻¹)
Positive charge
Force is in the same direction as the electric field.
Negative charge
Force is opposite to the electric field.
Common mistake
A positive charge of 3.0 nC is placed in a field of strength 2.0 × 10⁴ N C⁻¹. Find the force and state its direction.
Show worked answer
The charge is positive, so the force is in the field direction.
The source charge creates the field. The charge placed in that field experiences the force.