Answer and Explanation:
The correct pair of scalar quantities among the options provided is:
C. Electric potential and electric potential energy
Electric potential and electric potential energy are scalar quantities. Scalar quantities are those that have only magnitude and no direction. Electric potential represents the amount of electric potential energy per unit charge at a given point in an electric field. Electric potential energy, on the other hand, is the energy associated with the position of a charge in an electric field.
Electric force and electric field (Option A) are not scalar quantities as they have both magnitude and direction. Electric force is a vector quantity that represents the strength and direction of the force experienced by a charge in an electric field. Electric field is also a vector quantity that represents the force experienced by a positive test charge per unit charge.
Potential difference and electric field (Option D) are not a pair of scalar quantities either. Potential difference, or voltage, is a scalar quantity that represents the difference in electric potential between two points in an electric field. Electric field, as mentioned earlier, is a vector quantity representing the force per unit charge experienced by a positive test charge.
Regarding the second part of the question, when the only force acting on an electron is due to a uniform electric field, the electron moves with:
B. acceleration in the direction of the field.
Since the electron has a negative charge, it experiences a force in the opposite direction of the electric field. According to Newton's second law of motion (F = ma), the force on the electron causes it to accelerate in the direction of the field, opposite to the force acting on it.
In summary, the correct pair of scalar quantities is electric potential and electric potential energy (Option C). Additionally, when the only force acting on an electron is due to a uniform electric field, the electron moves with acceleration in the direction of the field (Option B).