7.5 - Electric Potential

Question 1

What is the definition of electric potential at a point?

Answer 1

The work done per unit positive charge in bringing a point test charge from infinity to a defined point.

Question 2

Is electric potential energy a vector or scalar?

Answer 2

A scalar quantity as it has no direction.

Question 3

If electric potential is a scalar quantity why can it have positive and negative sign?

Answer 3

positive - around an isolated positive charge
negative - around an isolated negative charge
zero - at infinity

Question 4

When a positive test charge moves closer to a negative charge, does its electric potential increase or decrease?

Answer 4

Decrease.

Question 5

When a positive test charge moves closer to a positive charge, does its electric potential increase or decrease?

Answer 5

Increase.

Question 6

What is the equation for electric potential in a radial field due to a point charge?

Answer 6

V = Q/4πε₀r
V = electric potential (V)

Question 7

What does the equation for electric potential show? (+ and - charges)

Answer 7

(for positive charge) - As the distance from the charge r decreases, the potential V increases (more work is done on a positive charge to overcome repulsive force)
(for negative charge) - As the distance from the charge r decreases the potential V decreases (less work has to be done on a positive test charge since the attractive force makes it easier)

Question 8

What is the equation for electric field strength in terms of electric potential?

Answer 8

E = ΔV/Δr
E = Electric field strength (V/m)
ΔV = change in potential (V)
Δr = displacement in the direction of the field (m)

Question 9

What is the written definition of an electric field in terms of the variation of electric potential at different points in the field?

Answer 9

The electric field at a particular point is equal to the gradient of a potential-distance graph at that point.

Question 10

What is the potential gradient in an electric field defined as?

Answer 10

The rate of change of electric potential with respect to displacement in the direction of the field.

Question 11

What are 5 key features of an electric potential / separation graph?

Answer 11

• values for V are all negative for a negative charge
• all + for +charge (reciprocal graph)
• as r increases, V against r follows a 1/r relation for a positive charge and a -1/r relation for a negative charge
• gradient = E (at that point)
• graph has a shallow increase/decrease as r increases

Question 12

What are 4 key features of an electric field strength against r graph?

Answer 12

• All values for E are positive
• As r increases, E against r follows a 1/r² relation
• The area under the graph is ΔV
• The graph has a steep decline as r increases

Question 13

If the electric field lines are pointing outward, what is the charge of the particle and which way is electric potential decreasing?

Answer 13

• charge is positive
• V decreases going outwards

Question 14

What is the work done in moving a charge?

Answer 14

W =qΔV
W = work done (J)
q = charge (C)
ΔV = change in potential difference (J/C)

Question 15

What is work done on a charge equal to?

Answer 15

The change in electric potential energy
(When V=0 EPE=0

Question 16

What is the equation for change in EPE/work done for a point charge q at a certain distance from larger charge Q?

Answer 16

ΔEPE = Qq/4πε₀ (1/r₂ - 1r₁)
Q = charge that is producing the electric field (C)
q = charge moving through field (C)
r₁ = first distance of q from centre of W (m)
r₂ = second distance of q from the centre of Q (m)

Question 17

When is work done when a point charge is moving in the electric field of a larger charge?

Answer 17

• when a positive charge in an electric field moves AGAINST the electric field lines
• when a negative charge moves WITH the electric field lines

Question 18

What are electrostatic equipotential lines?

Answer 18

Lines that are perpendicular to the electric field lines in both radial uniform fields.
- represented by dotted lines (field line are solid lines with arrows)

Question 19

What do equipotential lines look like in a radial field?

Answer 19

Concentric circles around the charge.

Question 20

What do equipotential lines look like in a uniform field?

Answer 20

Horizontal straight lines, parallel and equally spaced.

Question 21

When moving along an equipotential line or surface, is work done?

Answer 21

Nah, work is only done when moving between equipotential lines/surfaces.

Question 22

What is the change in electric potential when moving along equipotential lines?

Answer 22

0