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AQA A Level Physics > Electric Fields > Flashcards

Electric Fields Flashcards

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1
Q

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

A

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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2
Q

Electrostatic force between 2 point charges, F, =

A

1/4πε0 x Q1Q2 / r^2

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3
Q

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

A

From the centre

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4
Q

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

A

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

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5
Q

Is the electrostatic force between 2 opposite charges positive or negative

A

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

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6
Q

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

A

Air is treated as a vacuum

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7
Q

Uniform spherical conductor

A

One where its charge is distributed evenly

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8
Q

Electric field lines around a point charge

A

Radial
Direction depends on the charge.

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9
Q

Direction of field lines around a positively charged spherical conductor

A

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

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10
Q

Direction of field lines around a negatively charged spherical conductor

A

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

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11
Q

What are field lines used to represent

A

Direction and magnitude of an electric field

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12
Q

Which direction are field lines in an electric field directed in

A

From the positive charge to the negative charge.

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13
Q

Field lines in a uniform electric field

A

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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14
Q

Field lines in a radial electric field

A

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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15
Q

What does field lines being closer together mean

A

Stronger field

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16
Q

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

A

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

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17
Q

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

A

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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18
Q

Electric field strength

A

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

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19
Q

Electric field strength =

A

Force / Charge

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20
Q

Units for electric field strength

A

NC^-1

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21
Q

Is electric field strength a vector or scalar

A

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

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22
Q

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

A

Potential difference between plates / Separation between plates

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23
Q

What does the equation E = V/d show

A

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

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24
Q

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

A

They have a radial field

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25
Q

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

A

0V

26
Q

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

A

Equate the two equations for electric field strength

E = F/Q = V/d

27
Q

Derivation of work done moving charge between plates

A

E = F/Q = V/d

Fd = VQ
W = Fd
W = VQ

28
Q

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

A

Parallel to the electric field lines

29
Q

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

A

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

30
Q

What does the amount of deflection of the particle depend on

A

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

31
Q

What does less deflection look like

A

Path has a smaller curve

32
Q

Electric field strength due to a point charge =

A

F/q = Q / 4πε0r^2

33
Q

Are electric force and field strength vectors or scalars

A

Vectors

34
Q

Direction of electric field strength if the charge is negative

A

Negative.
Points towards the centre of the charge

35
Q

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

A

Each field can be combined by vector addition

36
Q

Similarities between gravitational and electrostatic fields

A

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

37
Q

Differences between gravitational and electrostatic fields

A

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

38
Q

Electric potential of a point charge

A

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

39
Q

Is electric potential vector or scalar

A

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

40
Q

What does electric potential depend on

A

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

41
Q

Electric potential when around an isolated positive charge

A

Has a positive value

42
Q

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

A

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

43
Q

Electric potential when around an isolated negative charge

A

Has a negative value

44
Q

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

A

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

45
Q

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

A

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

46
Q

Electric potential around point charge =

A

Q / 4πε0r

47
Q

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

A

Each potential can be combined by addition

48
Q

Potential gradient of an electric field

A

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

49
Q

Gradient of V-r graph

A

Electric field strength at that point

50
Q

Equation relating V to E

A

E = - change in V / change in r

51
Q

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

A

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

52
Q

What does of V against r

A

An L shape and is reflected in the x axis

53
Q

Key features of the graph of V against r

A

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

54
Q

What does the area under a E-r graph represent

A

The potential difference between those 2 points

55
Q

What does a E-r graph look like

A

A L shaped curve

56
Q

Key features of a E-r graph

A

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

57
Q

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

A

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

58
Q

Electric potential energy

A

Q1Q2 / 4πε0r

59
Q

Work done on a point charge is equal to

A

The change in electric potential energy

60
Q

When is work done in an electric field

A

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

61
Q

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

A

Concentric circles around the charge
Progressively further apart with distance

AQA A Level Physics (15 decks)

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