Electric Fields Flashcards by Khush Damani

Coulomb’s Law for the electrostatic force between 2 point charges

The electrostatic force between 2 point charges is directly proportional to the product of the charges and inversely proportional to the square of their separation

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Electrostatic force between 2 point charges, F, =

1/4πε0 x Q1Q2 / r^2

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Where is charge considered to be acting from in a charged sphere

From the centre

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Is the electrostatic force between 2 charges of the same type positive or negative

Product Q1Q2 is positive so force is positive.
A positive force means the charges experience repulsion

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Is the electrostatic force between 2 opposite charges positive or negative

Product Q1Q2 is negative so force is negative.
A negative force means the charges experience attraction

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Why is the permittivity of free space used when calculating the force between 2 charges

Air is treated as a vacuum

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Uniform spherical conductor

One where its charge is distributed evenly

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Electric field lines around a point charge

Radial
Direction depends on the charge.

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Direction of field lines around a positively charged spherical conductor

If positively charged, the field lines are directed AWAY from the centre of the sphere

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Direction of field lines around a negatively charged spherical conductor

If negatively charged the field lines are directed towards the centre of the sphere

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What are field lines used to represent

Direction and magnitude of an electric field

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Which direction are field lines in an electric field directed in

From the positive charge to the negative charge.

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Field lines in a uniform electric field

Field lines are equally spaced at all points as electric field strength is constant at all points and the force on a test charge has the same magnitude and direction at all points in the field.

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Field lines in a radial electric field

Field lines spread out with distance.
Field lines equally spaced as they exit the surface of the charge
Separation between field lines increases with distance
Magnitude of electric field strength and force on test charge decreases with distance

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What does field lines being closer together mean

Stronger field

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What do electric field lines between two charges of the same type look like

Field lines are directed away from 2 + charges and towards 2 - charges
They are not connected to show repulsion

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What do electric field lines between two charged parallel plates look like

Uniform/ evenly spaced between the plates, moving from positive to negative plate.
Beyond the edges, it is non-uniform and weaker

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Electric field strength

The force per unit charge experienced by a small positive test charge placed at that point.

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Electric field strength =

Force / Charge

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Units for electric field strength

NC^-1

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Is electric field strength a vector or scalar

Vector and is always directed AWAY from a positive charge and TOWARDS a negative charge

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Magnitude of electric field strength in a uniform field between 2 charged parallel plates (Vm^-1) =

Potential difference between plates / Separation between plates

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What does the equation E = V/d show

The greater the voltage between plates, the stronger the field
The greater the separation between plates, the weaker the field

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Why can you not use the equation E = V/d for point charges

They have a radial field

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Voltage of a plate if one of the parallel plates is earthed

How to find E/F when a point charge moves between 2 parallel plates

Equate the two equations for electric field strength E = F/Q = V/d

Derivation of work done moving charge between plates

E = F/Q = V/d Fd = VQ W = Fd W = VQ

Which direction will a charged particle move if it remains stationary in a uniform electric field

Parallel to the electric field lines

Which direction will a charged particle move if a charged particle is in motion through a uniform electric field

It will experience a constant electric force and travel in a parabolic trajectory

What does the amount of deflection of the particle depend on

Mass - greater mass = smaller deflection Charge - greater charge = greater deflection Speed - greater speed = smaller deflection

What does less deflection look like

Path has a smaller curve

Electric field strength due to a point charge =

F/q = Q / 4πε0r^2

Are electric force and field strength vectors or scalars

Vectors

Direction of electric field strength if the charge is negative

Negative. Points towards the centre of the charge

How can you find electric force/field strength at a point due to multiple charges

Each field can be combined by vector addition

Similarities between gravitational and electrostatic fields

The field lines around a point mass and negative point charge are identical The field lines in uniform gravitational and electric fields are identical Both forces follow inverse square law relationships with distance Both gravitational field strength and electric field strength follow an inverse square law (1/r2) relationship with distance in a radial field Both gravitational potential and electric potential both follow an inverse (1/r) relationship with distance The equipotential surfaces are spherical around a point mass or charge and equally spaced parallel lines in uniform fields The work done by either field is equal to the product of the mass or charge and change in potential

Differences between gravitational and electrostatic fields

The gravitational force acts on particles with mass whilst the electrostatic force acts on particles with charge The gravitational force is always attractive whilst the electrostatic force can be attractive or repulsive The gravitational potential is always negative whilst the electric potential can be either negative or positive Gravitational fields are relatively weak compared to electric fields as the gravitational constant G is much smaller than the Coulomb constant k

Electric potential of a point charge

Work done per unit charge in taking a small positive test charge from infinity to a defined point

Is electric potential vector or scalar

Scalar but has a +/- sign to indicate the sign of the charge

What does electric potential depend on

Magnitude of the point charge Distance between the charge and the point

Electric potential when around an isolated positive charge

Has a positive value

What happens to electric potential around an isolated positive charge when a test charge moves closer/further

Increases when test charge moves closer Decreases when it moves further away

Electric potential when around an isolated negative charge

Has a negative value

What happens to electric potential around an isolated negative charge when a test charge moves closer/further

Decreases when test charge moves closer Increases when it moves further away

How can you tell if potential decreases or increases with distance from charge

Using the direction of the electric field lines. Potential always decreases in the same direction as the field lines

Electric potential around point charge =

Q / 4πε0r

How to find the potential at a point caused by multiple charges

Each potential can be combined by addition

Potential gradient of an electric field

The rate of chsnge of electric potential with respect to displacement in the direction of the field

Gradient of V-r graph

Electric field strength at that point

Equation relating V to E

E = - change in V / change in r

Why is there a negative sign in the equation E = -V/r

To indicate the direction of the field strength opposes the direction of increasing potential

What does of V against r

An L shape and is reflected in the x axis

Key features of the graph of V against r

All values of potential are negative for a negative charge All values of potential are positive for a positive charge As r increases, V against r follows a 1/r relation for a positive charge and -1/r for negative The gradient is the field strength

What does the area under a E-r graph represent

The potential difference between those 2 points

What does a E-r graph look like

A L shaped curve

Key features of a E-r graph

All values of field strength are negative for negative charge All values of field strength are positive for a positive charge As r increases, E against r follows a 1/r^2 relation Curve is steeper than V-r graph

Why do 2 points at different distances from a charge have different electric potentials

Electric potential increases with distance from a negative charge and decreases with distance from a positive charge.

Electric potential energy

Q1Q2 / 4πε0r

Work done on a point charge is equal to

The change in electric potential energy

When is work done in an electric field

When a + charge moves against the electric field lines When - charge moves with the electric field lines

Equipotential lines in a radial field such as around a point charge

Concentric circles around the charge Progressively further apart with distance