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