Knowledge-15 electric fields

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Summer Flitcroft

Cards (29)

Electric field exist around all objects with charge. Any object with charge will experience a force if placed in electric field of another charged object.
A charged sphere can be considered to have all its charge at its centre. The size of the force between two point charges in a vacuum is given by Coulombs law:
F = 1 / 4 pie e0 x Q1 Q2 / r^2
Each charge will experience the same size force but in opposite directions. The electric force between any two point charges:
can be attractive or repulsive depending on the charges of the objects.
is proportional to the product of their charges.
obeys inverse square law.
depends on the permittivity of the material around them
The strength of an electric field at any point in a field is the force per unit charge on a positive test charge placed at that point:
E = F / Q
The radial field strength for an electric field at distance r from a point charge Q :
E = 1 / 4 pie e0 x Q /r^2
electric field can be represented by field lines where :
the arrows on field lines show the direction of the force acting on a positive charge placed in the field.
the separation of field lines indicates the strength of the field , the closer together the lines the stronger the field.
A uniform electric field can be produced by applying a potential difference between two parallel metal plates.
In a uniform field , the field strength and direction are the same at every point , indicated by the field lines being parallel to one another and equally spaced.
The magnitude of an electric field strength at any point in a uniform field is:
E = V/d
To derive the magnitude of electric field strength in a uniform field:
F = EQ
W = F x d
W = EQ x d
W = QV
V = W/Q = EQd /Q = Ed
E = V/d
A charged particle entering a uniform field in a direction initially perpendicular to the field will experience a constant force:
if the particle is positively charge, the direction of the force will be in the same direction as the field lines.
the force on a moving particle in a uniform field will produce a parabolic trajectory for the particle inside the field.
The absolute electric potential V at a point in an electric field is defined as the work done per unit charge to bring a small positive test charge from infinity to that point.
the magnitude of absolute electric potential at that point in a radial field caused by charge Q is :
V = 1 / 4 pie e0 x Q/r
The work done in moving charge Q between two points in an electric field is :
work done = charge x electric potential difference
Gravitational vs electric fields (differences)
G fields are caused by and act on an object with charge , electric fields are caused by and act on an object with charge.
G fields are always attractive , E fields can be either attractive or repulsive.
Gravitational vs electric fields (similarities)
both fields potential vary with 1/r
field lines can represent either fields
potential is defined similarly for both
both follow inverse square law
An equipotential surface is one where electric potential is the same at all points.
Graphs of electric field strength look similar but E is a straight line (1/r^2) , whilst V is a 1/r line
The electric field strength is related to potential difference by:
E = change in V / change in r
epsilon 0 represents the permittivity of free space.
When calculating the force between two particles air can be treated as a vacuum.
For a charged sphere charge is assumed to be at the centre of the the sphere.
The electrostatic force on particles is stronger than the gravitational.
Electrical field lines always go from positive to negative charge.
Electric field strength is the force per unit charge acting at a point in an electric field.
The magnitude of E in a uniform electric field is the potential difference between plates.
The trajectory of a particle entering a uniform field at right angles is a parabolic.
Electric potential is the work done per unit charge to move a point charge from infinity to a point.