Electric fields Flashcards
State Couloumbs law
The electrostatic force between two point charges is proportional to the product of the charges and inversely proportional to the square of their separation
Draw a diagram for, and state what the components stand for, for the coulombs law equation
What does the 1/r2 part of the coulombs’s equation mean for charge separation?
This means that when the separation of two charges doubles, the electrostatic force between them reduces by (½)2 = ¼
What does ε0 show
the capability of a vacuum to permit electric fields
What does it mean for the electrostatic force when charges are opposite or the same?
- 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
What is a uniform spherical conductor and give one example
a point charge, where its charge is distributed evenly
e.g a charged sphere
What is a uniform electric field and give one example
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
What is a non-uniform electric field
a field that has varying electric field strength throughout
How do field lines represent a stronger/weaker electric field
A stronger field is represented by the field lines closer together
A weaker field is represented by the field lines further apart
is an electric radial field uniform or non-uniform and why
a non-uniform field
the electric field strength E is different depending on how far you are from a charged particle
Draw the electric field lines between a positive and negative point charge
What is an electric field
a region of space in which an electric charge “feels” a force
Electric field strength def
The electrostatic force per unit positive charge acting on the charge at that point
What do the components of the electric field strength equation mean
is electric field strength a vector or scalar quantity and why
A vector quantity
because it is always directed:
- Away from a positive charge
- Towards a negative charge
What do the components of the electric field strength (of a uniform field) mean
What does the electric field strength (of a uniform field) say about field strength, in regards to voltage and separation
- The greater the voltage between the plates, the stronger the field
- The greater the separation between the plates, the weaker the field
Describe how the derivation of the work done/electric field strength of a uniform field is found
Draw a diagram of the parabolic trajectory of a charged particle which initially moved perpendicular to a uniform electric field.
How will a charged particle move if it remains still in a uniform electric field
it will move parallel to the electric field lines (along or against the field lines depending on its charge)
Describe the movement of a positive/negative charge if initially moving perpendicular to a uniform electric field AND what influences the amount of deflection
- 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
What do the components of the equation describing the field strength for a radial field mean?
What does the equation describing field strength of a radial field describe
- 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
What are the similarities between electric fields and gravitational fields
- 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
What are the differences between electric fields and gravitational 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
electric potential def
The work done per unit positive charge in bringing a point test charge from infinity to a defined point
is electric potential a scalar or vector quantity
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
What is a test charge
A charge with a magnitude so small that placing it at a point has a negligible effect on the field about that point
How does electric potential change when a positive/negative charge moves closer to another positive/negative charge
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
What do the components of the electric potential equation mean
What does the electric potential equation show
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
What do the components of the E, V, R equation signify
How can an electric field be defined in terms of the variation of electric potential
The electric field at a particular point is equal to the gradient of a potential-distance graph at that point
Potential gradient in an electric field def
The rate of change of electric potential with respect to displacement in the direction of the field
How does electric potential change with distance (from a positive and negative charge)
The electric potential around a positive charge decreases with distance and increases with distance around a negative charge
How can change in potential energy be calculated using a E-r graph
the area under the graph can be calculated
what happens to electric field strength when distance r doubles
E decreases by a factor of 4
What happens when a mass with charge moves through an electric field
work is done - this is equal to the change in electric potential energy
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
When is work done in an electric field
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
What are the two features of equipotential lines?
- 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)
What is the purpose of equipotential lines
to join together points that have the same electric potential
The potential gradient is defined by the equipotential lines
Draw the equipotential lines for a radial and a uniform electric field
When is work done in terms of equipotential lines
- 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
