Electric fields Flashcards
Define an electric field
Force field region produced by charged particles where a charged object placed in the field experiences an attractive or repulsive force
What is Coulomb’s law
- Relates force of attraction or repulsion between the charges with magnitude of charges and distance between them
F = kQ1Q2 / r^2
How does Coulomb’s law show an inverse square law
F ∝ 1/r^2
so e.g. if r doubles F decreases by 4x
Define electric field strength
The force experienced by a +1 C charge if it was placed in the field
so E = F/Q
Which direction does the vector E point
The direction which a positive charge would move in the field i.e. same as field lines
What is the formula for E in a radial field
E = k Q / r^2
(another inverse square law)
How can a uniform electric field be produced
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 -
How does electric field strength vary in a uniform electric field
Constant at all points
How can you calculate a value for the E in a uniform field
E = V / d
where V is voltage applied across plates and d is distance between plates
Explain the motion of a charged particle placed in a uniform field
Positive charge would be attracted towards negative plate
Negative charge would be attracted towards positive plate
Would bend towards the plate with projectile motion
Define electrical potential at a point in the electric field
The electrical potential energy that a +1 C charge would have at that point
What is the formula for V
V = k Q / r where Q is the charge of the point charge causing the field
Explain why V is negative for a negative charge and V is positive for a positive charge using the formula
V = k Q/r
so if Q is -ve V is -ve and if Q is +ve V is +ve
Explain why V is negative for a negative charge and V is positive for a positive charge with the theory
- For a negative charge, a +ve test mass would be attracted so need to put work in to leave field so V is negative
- For a positive charge, a +ve test mass would be repelled so don’t need to put any work to leave field so V is positive
Describe the graphs of V against r for a repulsive force
- 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
Describe the graphs of V against r for an attractive force
- 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
Describe the graph of E against r
E = kQ / r^2
so graph in the form y=k/x^2 where x>0
How can you calculate E from a graph of V against r and V from a graph of E against r
E is the gradient of the graph of V against r
V is the area under the graph of E against r
What is the formula for work done when moving across an electrical potential difference
W = QΔV
Describe the equipotential for a radial electric field
- Circles around the centre as same distance from centre at every point
- Passes through field lines at right angles
Describe the equipotential for a uniform field
Straight line parallel to plates so distance from plates is constant
Perpendicular to field lines so parallel and equally spaced
What are the similarities between electric and gravitational fields
- 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
What are the differences between electric and gravitational fields
- 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
