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Electric potential questions
Electric potential carries over from gravitational potential almost word for word: zero at infinity, work as charge times potential difference, and equipotentials along which no work is done. Two things differ. The sign depends on the sign of the charge, and the data booklet's notation needs handling carefully.
19 original questions · 51 marks · the electric potential notes · Electric fields
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Define the absolute electric potential at a point in an electric field.
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Work done per unit positive charge (1); in bringing a small test charge from infinity to the point (where the potential at infinity is zero) (1).Calculate the electric potential at a distance of 0.10 m from a point charge of 5.0 × 10−9 C.
k = 8.99 × 109 N m2 C−2Mark scheme
V = kQ/r = (8.99 × 109 × 5.0 × 10−9)/0.10 (1)
V = 449.5 V (1)Describe the equipotential surfaces around an isolated point charge and state how they are related to the electric field lines.
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Concentric spheres centred on the charge (1); everywhere perpendicular to the radial field lines (and no work is done moving a charge along an equipotential) (1).A charge of 5.0 μC is moved 0.40 m along an equipotential surface. State the work done on the charge.
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Zero: the potential difference along an equipotential is zero (1).The electric potential at a distance r from a point charge is 800 V. State the potential at a distance 2r, and at a distance 4r, from the charge.
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V ∝ 1/r, so at 2r the potential is 400 V (1) and at 4r it is 200 V (1).State what a potential difference of one volt means, in terms of energy and charge.
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One joule of energy is transferred per coulomb of charge moved through it (1).Calculate the electric potential at a distance of 0.20 m from a point charge of −3.0 × 10−9 C.
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V = kQ/r = (8.99 × 109 × (−3.0 × 10−9))/0.20 (1)
V = −135 V (negative because the charge is negative) (1)A charge of +2.0 × 10−9 C is moved from a point 0.30 m from a fixed point charge of 5.0 × 10−9 C to a point 0.10 m from it. Calculate the potential difference between the two points and the work done on the charge.
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V at 0.10 m = 449.5 V (1)
V at 0.30 m = 149.8 V (1)
ΔV = 299.7 V (1)
W = QΔV = 2.0 × 10−9 × 299.7 = 5.99 × 10−7 J (1)In a uniform electric field the potential changes by 200 V over a distance of 0.050 m along a field line. Calculate the electric field strength.
Two point charges, each of +4.0 μC, are placed so that each is 0.20 m from a point P. Determine the total electric potential at P.
Determine the distance from a point charge of +12 nC at which the electric potential is 450 V.
k = 8.99 × 109 N m2 C−2In a vacuum tube, an electron moves from a point at a potential of +150 V to a point at a potential of +650 V. Calculate the kinetic energy the electron gains, in J and in eV.
e = 1.60 × 10−19 CA small charge is moved from point X to point Y in the field of a fixed point charge, once along a straight line and once along a much longer curved path. State and explain how the work done in the two cases compares.
A charge of +5.0 μC is placed at a point where the electric potential is 2000 V. Calculate the electric potential energy of the charge and state the work done in bringing it to this point from infinity.
A +6.0 μC charge and a −6.0 μC charge are placed so that each is 0.30 m from a point P. Calculate the total electric potential at P and explain your result.
Explain how the electric field strength at a point can be found from a graph of electric potential against distance.
A charge of +8.0 nC is fixed at point A and a charge of −2.0 nC is fixed at point B, 0.50 m from A. Determine the distance from A of the point on the line between the charges at which the total electric potential is zero.
Two protons are held 1.0 × 10−12 m apart in a vacuum. One is kept fixed and the other is released from rest. Calculate the speed of the released proton when it is very far from the fixed one. Assume that only the electrostatic force acts.
k = 8.99 × 109 N m2 C−2, e = 1.60 × 10−19 C, mp = 1.67 × 10−27 kgA bird perches with both feet on a single bare overhead cable at a potential of 25 kV. Suggest why the bird is unharmed.
The same practice on paper: the printable workbook for this topic, questions and a worked answer book.
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