Electric fields

Question 1

State Couloumbs law

Answer 1

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

Question 2

Draw a diagram for, and state what the components stand for, for the coulombs law equation

Answer 2

Question 3

What does the 1/r2 part of the coulombs’s equation mean for charge separation?

Answer 3

This means that when the separation of two charges doubles, the electrostatic force between them reduces by (½)2 = ¼

Question 4

What does ε0 show

Answer 4

the capability of a vacuum to permit electric fields

Question 5

What does it mean for the electrostatic force when charges are opposite or the same?

Answer 5

• If there is a positive and negative charge, then the electrostatic force is negative
  This can be interpreted as an attractive force
• If the charges are the same, the electrostatic force is positive
  This can be interpreted as a repulsive force

Question 6

What is a uniform spherical conductor and give one example

Answer 6

a point charge, where its charge is distributed evenly

e.g a charged sphere

Question 7

What is a uniform electric field and give one example

Answer 7

a field that has the same electric field strength throughout the field and this is represented by equally spaced field lines

For example, the field between oppositely charged parallel plates

Question 8

What is a non-uniform electric field

Answer 8

a field that has varying electric field strength throughout

Question 9

How do field lines represent a stronger/weaker electric field

Answer 9

A stronger field is represented by the field lines closer together

A weaker field is represented by the field lines further apart

Question 10

is an electric radial field uniform or non-uniform and why

Answer 10

a non-uniform field

the electric field strength E is different depending on how far you are from a charged particle

Question 11

Draw the electric field lines between a positive and negative point charge

Answer 11

Question 12

What is an electric field

Answer 12

a region of space in which an electric charge “feels” a force

Question 13

Electric field strength def

Answer 13

The electrostatic force per unit positive charge acting on the charge at that point

Question 14

What do the components of the electric field strength equation mean

Answer 14

Question 15

is electric field strength a vector or scalar quantity and why

Answer 15

A vector quantity

because it is always directed:
- Away from a positive charge
- Towards a negative charge

Question 16

What do the components of the electric field strength (of a uniform field) mean

Answer 16

Question 17

What does the electric field strength (of a uniform field) say about field strength, in regards to voltage and separation

Answer 17

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

Question 18

Describe how the derivation of the work done/electric field strength of a uniform field is found

Answer 18

Question 19

Draw a diagram of the parabolic trajectory of a charged particle which initially moved perpendicular to a uniform electric field.

Answer 19

Question 20

How will a charged particle move if it remains still in a uniform electric field

Answer 20

it will move parallel to the electric field lines (along or against the field lines depending on its charge)

Question 21

Describe the movement of a positive/negative charge if initially moving perpendicular to a uniform electric field AND what influences the amount of deflection

Answer 21

• A positive charge will be deflected towards the negative plate
• A negative charge will be deflected towards the positive plate

The force on the particle is the same at all points and is always in the same direction

WHAT INFLUENCES THE AMOUNT OF DEFLECTION:

• Mass – the greater the mass, the smaller the deflection and vice versa
• Charge – the greater the magnitude of the charge of the particle, the greater the deflection and vice versa
• Speed – the greater the speed of the particle, the smaller the deflection and vice versa

Question 22

What do the components of the equation describing the field strength for a radial field mean?

Answer 22

Question 23

What does the equation describing field strength of a radial field describe

Answer 23

• Electric field strength in a radial field is not constant
• As the distance from the charge r increases, E decreases by a factor of 1/r2
• This is an inverse square law relationship with distance. This means the field strength E decreases by a factor of four when the distance r is doubled

Question 24

What are the similarities between electric fields and gravitational fields

Answer 24

• The magnitude of the gravitational and electrostatic force between two point masses or charges are inverse square law relationships
• The field lines around a point mass and negative point charge are identical
• The field lines in a uniform gravitational and electric field are identical
• The gravitational field strength and electric field strength both have a 1 / r2 relationship in a radial field
• The work done in each field is either the product of the mass and change in potential or charge and change in potential

Question 25

What are the differences between electric fields and gravitational fields

Answer 25

- 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

Question 26

electric potential def

Answer 26

The work done per unit positive charge in bringing a point test charge from infinity to a defined point

Question 27

is electric potential a scalar or vector quantity

Answer 27

scalar However, you will still see the electric potential with a positive or negative sign. This is because the electric potential is: Positive around an isolated positive charge Negative around an isolated negative charge Zero at infinity

Question 28

What is a test charge

Answer 28

A charge with a magnitude so small that placing it at a point has a negligible effect on the field about that point

Question 29

How does electric potential change when a positive/negative charge moves closer to another positive/negative charge

Answer 29

Positive work is done by the mass from infinity to a point around a positive charge and negative work is done around a negative charge. This means: - When a positive test charge moves closer to a negative charge, its electric potential decreases - When a positive test charge moves closer to a positive charge, its electric potential increases

Question 30

What do the components of the electric potential equation mean

Answer 30

Question 31

What does the electric potential equation show

Answer 31

for a positive (+) charge: - As the distance from the charge r decreases, the potential V increases - This is because more work has to be done on a positive test charge to overcome the repulsive force For a negative (−) charge: - As the distance from the charge r decreases, the potential V decreases - This is because less work has to be done on a positive test charge since the attractive force will make it easier

Question 32

What do the components of the E, V, R equation signify

Answer 32

Question 33

How can an electric field be defined in terms of the variation of electric potential

Answer 33

The electric field at a particular point is equal to the gradient of a potential-distance graph at that point

Question 34

Potential gradient in an electric field def

Answer 34

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

Question 35

How does electric potential change with distance (from a positive and negative charge)

Answer 35

The electric potential around a positive charge decreases with distance and increases with distance around a negative charge

Question 36

How can change in potential energy be calculated using a E-r graph

Answer 36

the area under the graph can be calculated

Question 37

what happens to electric field strength when distance r doubles

Answer 37

E decreases by a factor of 4

Question 38

What happens when a mass with charge moves through an electric field

Answer 38

work is done - this is equal to the change in electric potential energy

Question 39

What is the equation that describes: work done for a point charge q at a distance r1 from the centre of a larger change Q, to a distance of r2 away

Answer 39

Question 40

When is work done in an electric field

Answer 40

when a positive charge in an electric field moves against the electric field lines or when a negative charge moves with the electric field lines

Question 41

What are the two features of equipotential lines?

Answer 41

- Perpendicular to the electric field lines in both radial and uniform fields - Represented by dotted lines (unlike field lines, which are solid lines with arrows)

Question 42

What is the purpose of equipotential lines

Answer 42

to join together points that have the same electric potential The potential gradient is defined by the equipotential lines

Question 43

Draw the equipotential lines for a radial and a uniform electric field

Answer 43

Question 44

When is work done in terms of equipotential lines

Answer 44

- No work is done when moving along an equipotential line or surface - Work is only done when moving between equipotential lines or surfaces - This means that an object travelling along an equipotential doesn't lose or gain energy and ΔV = 0