Electric field and electric current
B1 Consider a point 2.0 m away from a −3.0 µC point charge. Calculate
(a) the electric field E and
(b) the electric potential V at this point.
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B2 An electron in a uniform electric field experiences a force of 8.0 × 10−16 N. What is the
magnitude and direction of E at this point?
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B3 A point charge of −36 µC is located at the origin. Find the electric field (a) on the x
axis at x = 2 m, (b) on the y axis at y = −3 m and (c) at the point with coordinates
x = 1 m, y = 1 m.
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B4 A charge of 16 × 10−9 C is fixed at the origin of coordinates, a second charge of unknown
magnitude is at x = 3 m, y = 0, and a third charge of 12 × 10−9 C is at x = 6 m, y = 0.
What is the magnitude of the unknown charge if the resultant field at x = 8 m, y = 0 is
20.25 x̂ N C−1 ?
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B5 A small object carrying a charge −5×10−9 C experiences a downward force of 20×10−9 N
when placed at a certain point in an electric field.
(a) What is the electric field at the point?
(b) What would be the magnitude and direction of the force acting on an electron
placed at the point?
(c) What is the acceleration of the electron?
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B6 A uniform electric field exists in the region between two oppositely charged, plane,
parallel plates. An electron is released from rest at the surface of the negatively charged
plate and strikes the surface of the opposite plate, 2 cm distant from the first, in a time
interval of 1.5 × 10−8 s.
(a) Find the magnitude of the electric field.
(b) Find the velocity of the electron when it strikes the second plate.
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B7 What is the direction of the electric field at the centre C of each of the charge distributions
shown below? The charges are located at the corners of a square.
q
q
q
C
q
C
b
q
−q
q
q
C
b
q
−q
C
b
−q
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−q
2q
b
−q
−q
2q
B8 Three charges are located at the corners of an equilateral
triangle of side a as show. (a) Determine the magnitude and
direction of E at the centre of the triangle. (b) Find the
magnitude and direction of the resultant force on charge −q.
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−q
b
a
q
b
b
q
B9 Two charges q1 and q2 are placed 10 cm apart in vacuum. At what point on the line
joining the two charges is the electric field zero if
(a) q1 = +1 µC and q2 = +2 µC?
(b) q1 = +1 µC and q2 = −2 µC?
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B10 A negative charge −q is fixed to one corner of a rectangle
as shown. What positive charge must be fixed to corner
A and what positive charge must be fixed to corner B
so that the total electric field at the remaining corner is
zero? Express your answers in terms of q.
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2d
A
b
d
−q
b
b
B