Retrieval Practice — Electric Fields & Potential — Electric Fields & Potential

Q1. State Coulomb's law.

The electrostatic force between two point charges is directly proportional to the product of the charges and inversely proportional to the square of their separation: F = Q1Q2 / (4piepsilon0 \(r^{2}\)).

Q2. What is the difference between attractive and repulsive electric forces in terms of charge signs?

Like charges (both positive or both negative) repel: Q1Q2 > 0, force is positive.
Opposite charges attract: Q1Q2 < 0, force is negative.

Q3. Define electric field strength and state its two equivalent units.

The force per unit charge experienced by a small positive test charge placed at that point.
Units: N C^-1 or V m^-1.

Q4. Write the expression for electric field strength in a uniform field between parallel plates.

E = V/d, where V is the potential difference and d is the plate separation.
Only valid for parallel plates.

Q5. Write the expression for electric field strength due to a point charge.

E = Q / (4piepsilon0 \(r^{2}\)).
This follows an inverse square law.

Q6. Describe the trajectory of a charged particle entering a uniform electric field perpendicular to the field lines.

The particle follows a parabolic path.
It is deflected towards the plate of opposite sign.
Greater mass or speed reduces deflection; greater charge increases it.

Q7. Define electric potential at a point.

The work done per unit charge in bringing a small positive test charge from infinity to that point.
Measured in V (or J C^-1).
Zero at infinity.

Q8. State the equation for electric potential due to a point charge.

V = Q / (4piepsilon0 r).
Positive for a positive charge, negative for a negative charge.

Q9. How is electric field strength related to potential gradient?

E = -DeltaV/Deltar.
The negative sign means the field points in the direction of decreasing potential.
The gradient of a V-r graph gives E.

Q10. State the equation for work done when a charge moves through a potential difference.

W = q DeltaV, where q is the charge being moved and DeltaV is the potential difference between the two points.

Q11. What is the area under an E-r graph equal to?

The potential difference DeltaV between the two distance values.

Q12. State three similarities between gravitational and electric fields.

Both follow inverse square force laws, both have 1/r potentials, and both have spherical equipotential surfaces around point sources.

Q13. State three differences between gravitational and electric fields.

Gravity acts on mass (electric on charge), gravity is always attractive (electric can be repulsive), gravitational potential is always negative (electric can be positive or negative).

Q14. Why can a charged conducting sphere be treated as a point charge?

The charge distributes evenly on the surface.
For any external point, the field and potential are identical to those of a point charge at the centre.