Phy1203 – Assignment 2 - Electric Field Intensity & Potential
The Electric Field Intensity
Q1. A charge of +2 C placed at a point P in an electric field experiences a downward force of 8
x 10-4 N. What is the electric field intensity at that point?
𝑓 8 × 10−4
𝑁
𝐸= =
=
400
𝑑𝑜𝑤𝑛𝑤𝑎𝑟𝑑
𝑞 2 × 10−6
𝐶
Q2. A charge of –3 C placed at point A experiences a downward force of 6 x 10-5 N. What is
the electric field intensity at point A?
𝐸=
𝑓 −6 × 10−5
=
= 20 𝑁/𝐶
𝑞 −3 × 10−6
Q3. What must be the magnitude and direction of the electric field intensity between two
horizontal plates if one wants to produce an upward force of 6 x 10-4 N on a +60-C
charge? (The upward force on +q means E is also upward.)
𝑓
6 × 10−4
𝐸= =
= 10 𝑁/𝐶
𝑞 60 × 10−6
Q4. Find the electric field intensity at a point P, located 6 mm to the left of an 8-C charge.
What are the magnitude and direction of the force on a –2-nC charge placed at point P?
𝐸=
𝑘𝑄 (9 × 109 )(8 × 10−6 )
=
= 2 × 109 𝑁/𝐶 𝑡𝑜𝑤𝑎𝑟𝑑 𝑄
𝑟2
(6 × 10−3 )2
𝑓 = 𝑞𝐸 = (−2 × 10−9 )(2 × 109 ) = −400 𝑁 𝑎𝑤𝑎𝑦 𝑓𝑟𝑜𝑚 𝑄
Phy1203 – Assignment 2 - Electric Field Intensity & Potential
Q5. Determine the electric field intensity at a point P, located 4 cm above a –12-C charge.
What are the magnitude and direction of the force on a +3-nC charge placed at point P?
Electric field will be downward, since that is the direction a positive charge would move.
Calculating the Resultant Electric Field Intensity
Q6. Determine the electric filed intensity at the midpoint of a 70 mm line joining a –60-C
charge with a +40-C charge.
Q7. An 8-nC charge is located 80 mm to the right of a +4-nC charge. Determine the field
intensity at the midpoint of a line joining the two charges.
Q8. Find the electric field intensity at a point 30 mm to the right of a 16-nC charge and 40 mm to
the left of a 9-nC charge.
Phy1203 – Assignment 2 - Electric Field Intensity & Potential
Q9. What is the electric potential at a point that is 6 cm from a 8.40 C charge? What is the
potential energy of a 2 nC charge placed at that point?
Q10. Calculate the potential at point A that is 50 mm from a –40 C charge. What is the
potential energy if a +3 C charge is placed at point A?
Q11. What is the potential at the midpoint of a line joining a –12 C charge with a +3 C charge
located 80 mm away from the first charge?
Q12. When a positive charge is moved from a point of low potential to a point of high potential,
its potential energy:
a) increases
b) decreases
c) stays the same
d) increases and then decreases
Q13. Point P is 0.42 m from a 0.02 C charge.
i. What is the electric potential at point P?
Phy1203 – Assignment 2 - Electric Field Intensity & Potential
ii. What is the electric potential energy of the system of charges if a 0.003 C charge placed at point
P?
Q14. A 5μC charge is moved from a point where the electric potential is 3 V to a point where the
electric potential is 8 V. How much work is done by the electric field in moving the charge?
Q15. Two +2 nC charges and a -2 nC charge are placed at the corners of a square of side 10 cm, as
shown. What is the electric potential at the corner of the square without a charge?
Phy1203 – Assignment 2 - Electric Field Intensity & Potential
Q16. An electric field intensity of 23 kV/m exists between two parallel plates 3.1 cm apart, in
vacuum. An electron is placed on the negative plate and released. The electron then moves towards
the positive plate. What will the speed of the electron when it hits the positive plate? (mass of an
electron = 9.11x10-31 kg)