Electric fields Flashcards
Coulomb’s La…w
F = kQq/r^2
An attractive force is
negative (opposite signs)
A repulsive force is
positive (same signs)
The force acts upon the line
joining the centres of the two particles
An electric field is generated by…
any charged object
Electric fields lines only act on
charged particles
The electric force on a charge in an electric field is
F = qE
Is electric field a scalar of vector
Vector
E=F/Q
force is vector, charge is scalar so E is vector
What is electric field strength, with two equations and units
Electric field strength (N/C) or (V/m) is defined as the force (N) per unit charge (C) acting on a positive charge, it is a vector. E = kQ/r^2 = F/Q
The direction of the electric field at any point is
the direction of the force experienced by a positive charge at that point
Field lines are represented by
drawing lines with arrows
The closer together the field lines the…
stronger the field
Field lines point in the direction
a positive test charge would move e.g. away from another positive charge or towards a negative charge
What is a point charge
a body with charge but no mass
Does not really exist but is a useful approximation for small charged particles
The net field strength inside a hollow charged sphere is…
Outside, the field behaves…
zero
as if all the charge creating it is at a point in the centre of the sphere
The potential at a point in an electric field is defined as
the work done in bringing a test unit positive charge from infinity to that point
The potential of a charge at infinity is always defined to be zero because
If the charge creating an electric field is positive, then since ‘like charges repel, work must be done to bring a test positive charge in from infinity.
However, if the charge creating the field is negative, then as ‘unlike charges attract’, the work done to bring in a test positive charge from infinity is negative – the charge would tend to be attracted in, so work would actually be required to stop it.
The potential due to a positive charge is
positive
The potential due to a negative charge is
negative
Electric potential is
a measure of the potential energy per unit charge: Potential energy (J) = charge(q – C) * electric potential (V)
Is electric potential vector or scalar
Scalar as energy and charge are scalar
The work done when a charge moves through a potential is given by
Work done (J) = Charge (Q-C) * change in potential difference (delta V)
Electrical potential energy increases if:
a positive charge moves to a point of higher potential,
a negative charge moves to a point of lower potential.
Why is no work done when moving in a loop
work done depends only on the potential of the starting and finishing point – the route that the charge follows does not matter
This tells us that if a charged particle is taken around a closed loop in an electric field, no work is done – since the potential of the starting point is the same as the potential of the finishing point
Electric field =
= -gradient of electric potential
Equipotentials are
lines connecting points of equal potential – can be drawn on field line diagrams, and are at right angles to field lines
How are field lines and equipotentials drawn and explain
Field lines are straight arrows drawn from the centre, equipotentials are drawn circularly . In fact, the equipotentials around a point charge will be spherical, centred on the point charge because all points on the sphere are the same distance from the point charge – so the work required to bring a charge to any point on the sphere will be the same
What do equipotentials and field lines tell us
Equipotentials tell us about energy changes – the energy required to move from one equipotential to another. Field lines tell us about forces on an electric field at a particular point
For a point charge Q, V =
kQ/r
How do conductors and insulators work
free electrons
Coulomb’s law states that
the force between two point charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them.
The force between the two charged particles depends on
what is between them.
If anything other than a vacuum comes between them, the force between the charges is reduced and the permittivity is increased.
Two positive charges have electric field strengths of ____ halfway between them because
0
they repel
Field strengths strongest at
centre so equipotentials get further apart away from the centre
How does the gold leaf electroscope work
detects charge which is transferred to electroscope transferring the charge to the stem and the leaf causing them to repel
Any two charged objects exert…
equal and opposite forces without being in contact
Small charges follow…
Small charges follow field lines towards a larger body
The field lines of an electric field are the
lines which positive test charges follow - a direction is that which a positive charge would move along
Oppositely charged objects create
a field
A positive test charge follows
curved paths to a negative point charge
E =
F/Q = V/d
If charge is positive the force acts in the _____ direction as the electric field
same
If charge is negative the force acts in the _____ direction as the electric field
negative
The charge of the test charge must be very much less than one coulomb because
otherwise it would affect the charges that cause the field so altering the electric field strength
Field lines be drawn via
arrows from +ve to -ve and, when not straight, be smooth curves
nC =
10^-9 C
pC =
10^-12 C
The electric potential at a certain position =
work done per unit positive charge on a positive test charge with unit V
A test charge moving along an equipotential has
constant potential energy. No work is done by an electric field on a test charge because the force due to the field is at right angles to the equipotential
The potential gradient at any position in an electric field is the
charge of potential per unit charge of distance in a given direction.
If the field is non-uniform, the potential gradient
varies according to position and direction
The closer the equipotentials are,
the greater the potential gradient is at right angles to the equipotentials
If the field is uniform, the equipotentials between the plates are
equally spaced lines parallel to the plates
Potential gradient is constant and =
V/d
A graph of potential against distance is
y=x
The electric field strength is equal to
the negative of the potential gradient
How does a Van de Graaf generator work
A Van de Graaff generator can easily produce sparks in air – charge created when the rubber belts rubs against a pad is carried by the belt up to the metal dome of the generator. As charge gathers on the dome, the potential difference between the dome and Earth increases until sparking occurs.
A spark suddenly transfers energy from the dome. Work must be done to charge the dome because a force is needed to move the charge on the belt up to the dome. So the electric potential energy of the dome increases as it charges up. Some or all of this energy is transferred from the dome when a spark is created.
When a spherical metal conductor insulated from the ground is charged, the charge spreads out across the surface with the greatest concentration where the surface is most curved.
A parallel plate capacitor has
a uniform field between the plates except for close to the edges where it is curved. For the uniform part, E = V/d
Rectangle with curved sides and arrows pointing vertically from + to -
Oppositely charged plates - parallel plates = uniform field (constant magnitude and direction) from +ve to -ve plate, at right angles to plate, parallel to each other
Graph of electric field strength against distance for a point charge
1/r^2
-ve charges would given negative version of graph
Graph of electric field strength against distance for a hollow charged sphere
1/r^2 + c
where c is the radius of the sphere
Starts at +c at radius of sphere and where E = kQ/r^2
Graph of electric field strength against distance for a parallel plate capacitor
horizontal line from V/D
For a parallel plate capacitor with spacing d and potential V between the plates, the potential decreases linearly between the positive and negative plated while the electric field strength is horizontal
If two points have different electrical potentials then the potential energy of a charge must
change if it moves from one point to another and so work must be done on or by the charge
Graph of potential against distance for a point charge
1/r (starts behind 1/r^2 and finishes above)
Graph of potential against distance for a hollow charged sphere
horizontal line till radius of sphere then 1/r
The surface of a conductior is an ______ so The electric field is _____ to the surface of the conductor
The surface of a conductior is an equipotnetial
so the electric field is perpendicular to the surface of the conductor
Oppositely charged points
lines going from +ve to -ve
straight in middle then above and below getting more curved
lines going out of positive charge
lines going into negative charge
Point near plate
straight lines leaving positive charge
curved lines (straight in centre) going from charge to plate (of opposite charge) and leaving plate
Repulsive field lines
space in centre of diamondy shape
lines bending away from each other
two from inside of each charge curving up and down
loads curved away from outside
straight line leaving outside
Why doesn’t a Van de Graaf generator work well in humid conditions
water molecules in the air will ionise
