Electric fields

Question 1

Define an electric field

Answer 1

Force field region produced by charged particles where a charged object placed in the field experiences an attractive or repulsive force

Question 2

What is Coulomb’s law

Answer 2

• Relates force of attraction or repulsion between the charges with magnitude of charges and distance between them

F = kQ1Q2 / r^2

Question 3

How does Coulomb’s law show an inverse square law

Answer 3

F ∝ 1/r^2

so e.g. if r doubles F decreases by 4x

Question 4

Define electric field strength

Answer 4

The force experienced by a +1 C charge if it was placed in the field

so E = F/Q

Question 5

Which direction does the vector E point

Answer 5

The direction which a positive charge would move in the field i.e. same as field lines

Question 6

What is the formula for E in a radial field

Answer 6

E = k Q / r^2

(another inverse square law)

Question 7

How can a uniform electric field be produced

Answer 7

By placing parallel plates at the positive terminal and negative terminal and applying a pd across them. This causes parallel equally spaced field lines from + to -

Question 8

How does electric field strength vary in a uniform electric field

Answer 8

Constant at all points

Question 9

How can you calculate a value for the E in a uniform field

Answer 9

E = V / d

where V is voltage applied across plates and d is distance between plates

Question 10

Explain the motion of a charged particle placed in a uniform field

Answer 10

Positive charge would be attracted towards negative plate
Negative charge would be attracted towards positive plate
Would bend towards the plate with projectile motion

Question 11

Define electrical potential at a point in the electric field

Answer 11

Work done per unit charge in moving a positive test charge from infinity to that point

Question 12

What is the formula for V

Answer 12

V = k Q / r where Q is the charge of the point charge causing the field

Question 13

Explain why V is negative for a negative charge and V is positive for a positive charge using the formula

Answer 13

V = k Q/r
so if Q is -ve V is -ve and if Q is +ve V is +ve

Question 14

Explain why V is negative for a negative charge and V is positive for a positive charge with the theory

Answer 14

• For a negative charge, a +ve test mass would be attracted so need to put negative work in to move charge from infinity to that point so V is negative
• For a positive charge, a +ve test mass would be repelled so need to put work in to move from infinity to that point so V is positive

Question 15

Describe the graphs of V against r for a repulsive force

Answer 15

• Repulsive means Q is positive
• V is a high positive value close to surface and approaches 0 as r tends to infinity
• Graph in the form y = k/x

Question 16

Describe the graphs of V against r for an attractive force

Answer 16

• Attractive force means Q is negative
• V is a large negative value close to surface and approaches 0 as r tends to infinity
  Graph in form y = - k/x

Question 17

Describe the graph of E against r

Answer 17

E = kQ / r^2
so graph in the form y=k/x^2 where x>0

Question 18

How can you calculate E from a graph of V against r and V from a graph of E against r

Answer 18

E is the gradient of the graph of V against r

V is the area under the graph of E against r

Question 19

What is the formula for work done when moving across an electrical potential difference

Answer 19

W = QΔV

Question 20

Describe the equipotential for a radial electric field

Answer 20

• Circles around the centre as same distance from centre at every point
• Passes through field lines at right angles

Question 21

Describe the equipotential for a uniform field

Answer 21

Straight line parallel to plates so distance from plates is constant

Perpendicular to field lines so parallel and equally spaced

Question 22

What are the similarities between electric and gravitational fields

Answer 22

• Field strength is force per unit (property) for both
• G and E potential are both potential energy per unit (property) and 0 at infinity
• Both have equipotential which are perpendicular to field lines
• Work done to move across potential difference = property x Δ V

Question 23

What are the differences between electric and gravitational fields

Answer 23

• G is always attractive but E can be attractive or repulsive
• F due to gravity is far weaker in the nucleus which is why we need SNF