Electric Fields Flashcards
Define Electric Field
A region of space in which charged particles are subject to an electrostatic force.
What shape of field do point charges have?
Radial fields
How can you model uniformly charged spheres?
As a point charge at the centre of the sphere.
What do field lines show?
The path a positive test charge would take when placed in an Electric Field.
Which direction do the field lines point?
Positive to negative - the lines always point away from a positive charge and towards a negative charge
What effect does distance have on the
strength of the electrostatic force?
The greater the distance, the weaker the force.
How is the strength of an Electric Field
represented in a diagram?
By how close together the field lines are - the closer the lines, the stronger the field.
Define Electric Field Strength
Force per unit charge on a positive test charge placed in the field
What is the formula for Electric Field Strength?
E = F ÷ Q E = Electric Field strength (NC-1) F = Force (N) Q = Charge (C)
What is Coulomb’s Law?
The force between any two point charges is proportional to the product of their charges and inversely proportional to the square of the distance between them.
What is the formula for the force
between two point charges?
(Coulomb’s Law)
F = Q1Q2/4π ε0 r^2
Q = chargers of the particles ε0 = permittivity of free space, constant r^2 = distance of between the charges
Define permittivity
The ability of a medium to store energy in an Electric Field (how easily the atoms become polarised).
What is the formula for the Electric Field Strength of a point charge?
E = Q/4π ε0 r^2
Q = chargers of the particle ε0 = permittivity of free space, constant r^2 = distance of between the charges
Can be derived from E=F/Q
Name some similarities between
Gravitational and Electric Fields
Name some similarities between Gravitational and Electric Fields
- Both follow the inverse square law for the force.
- Point masses and point charges both produce a radial field.
- Newton’s law and Coulomb’s law formulae for force are very
similar
- Field strength is defined by force per unit charge/mass.
Name some differences between
Gravitational and Electric Fields
- Gravitational fields are always attractive, Electric Fields can be attractive or repulsive (depending on the charge)
- The constants of proportionality in Newton’s Law and Coulomb’s Law are different:
What is the formula for the work done when moving a charge in an Electric
Field?
Work done = Force x Distance moved
Define the potential at a point in an Electric Field
The work done per unit charge in moving a positive test charge from infinity to that point in the Electric Field. V = W/Q
What is the formula for the potential at a
point in an Electric Field?
V= W ÷ Q
V = Potential (V) W= Work done in moving the particle(J) Q= Charge of the particle (C)
What is the formula for the potential
between two parallel plates?
V = E x d
V= Potential (V) E= Electric Field strength (NC-1) d = distance between the plates (m)
What is the formula for the capacitance of a parallel plate capacitor?
C = A ε0 εr /d
A = area of the plates (m^2) ε0 = permittivity of free space εr = relative permittivity of dielectric d = distance between the plates (m)
What can the motion of charged particles in an Electric Field be modelled as?
Projectile motion: the two components of velocity are independent of each other.
Velocity perpendicular to the field is not affected, velocity parallel to the field is
How do you calculate the parallel component of velocity for a charged particle in a uniform Electric Field?
- First, calculate the time the particle is in the field (using time = distance/speed, where distance = length of charged plates and speed = velocity perpendicular to the field)
- Use a = F/m and F=Eq to calculate the acceleration of the particle while it is in the field (a = Eg/m)
- Substitute these values into V = u + at where u is the initial parallel velocity and V is the final parallel velocity.
What is the formula for the potential near a point charge?
Coulomb’s Law
V = Q/4π ε0 r
Q = charge of point charge ε0 = permittivity of free space r = distance from point charge
What does the force-distance graph for a point/spherical charge look like?
Force I` I.` I...` I......` I.............` I--------------------` radius
Force is inversely proportional to the square of the distance.
What does the area under a force-distance graph for a point/spherical charge represent?
Force I` I.` I...` I......` I.............` I--------------------` radius
Area under graph = work done
The work done in moving the moving
What is the formula for electric potential energy near a point charge?
E= Vxq E = electric potential energy (J) V = potential (V) q = charge of the point charge (C)
What is the formula for the capacitance of an isolated sphere?
Isolated spheres can store charge, so technically they can
be classed as capacitors
Using C= Q/V and the formula for V (in terms of Coulomb’s law), you can derive the formula for capacitance
C = 4π ε0 r
What is the formula for electric potential energy near a point charge?
(Coulomb’s Law)
E = Q1Q2/4π ε0 r
Q = chargers of the particles ε0 = permittivity of free space, constant r = distance of between the charges
