6.2 Electric Fields Flashcards
Electric Field
A region of space in which charged particles are subject to an electrostatic force.
How can we prove that static electricity and induced charges exist?
By generating a static charge on an object. For example, rubbing a glass rod with a silk cloth.
A glass rod is rubbed with a silk cloth. What occurs?
The glass rod loses electrons to the silk cloth, becoming slightly positive.
Protons and electrons are charged particles that can be modelled as …?
Point charges.
What type of field do point charges have?
Radial fields (which look like the spokes of a wheel).
In which direction do electric field lines point? What does it represent?
Outwards from a positive charge or inwards towards a negative charge.
This is because the direction of the field represents the direction of the electrostatic force on a positive charge
How does the strength of an electric field change when moving away from the charge?
It decreases (field line density decreases).
Why can the metal sphere of a Van De Graaff generator be modelled as a point charge?
Outside the sphere, the electric field is radial and decreases in strength with distance away from the surface of the sphere.
Electric field strength, E
The force applied per unit charge on an object.
E = F/Q
What are the units for electric field strength?
NC-1 or Vm-1 (these are interchangeable)
Is electric field strength scalar or vector?
Vector
State coulomb’s law
Any two point charges exert an electrostatic force between them that is proportional to the product of their charges, and inversely proportional to the square of the distance between them.
Coulomb’s law is similar to Newton’s law of gravitation. What is different between them?
The charges are replaced with masses.
Gravity is a solely attractive force hence the negative sign used in the gravitation law.
What is permittivity?
A constant that defines the ability of a material to become polarized and store charge.
What is the permittivity of free space?
The permittivity of a vacuum.
Why is the permittivity of free space the lowest value possible?
There is no matter in a vacuum to become polarized.
How can the permittivity be found?
ε = εr X ε0
ε: permittivity
εr: relative permittivity (or dielectric constant)
ε0: permittivity of free space
Use coulomb’s law and the formula for electric field strength to form an expression for the electric field of a point charge.
Imagine bringing a small test charge of charge q towards a large point charge of charge Q, such that the effect of the test charge on the point charge is negligible (ie the test charge doesnt cause the point charge to move).
E = F/q
Using coulomb’s law
E = Q / 4πr²ε₀
What is a uniform field?
A field which has no dependence upon position (ie the field is constant in space)
What type of field is produced between two parallel oppositely charged plates?
A uniform field.
Does the force on a particle between two parallel oppositely charged plates change depending on where it resides between the plates?
No. The force is constant.
Potential difference
The work done per unit charge in moving a charge from one point in an electic field to another.
V = W/Q
Define electric potential at a single point
The work done per unit charge to bring a positive charge from infinity to that point.
What is the electric potential at infinity?
0
What is the potential difference between two points, A and B?
The difference between the electric potential at A and the electric potential at B.
V = Vb - Va
What type of field can E = V / d be used for?
Uniform electric fields.
How do we get the formula for electric energy, using the formula for work done and coulomb’s law?
W = Fd
F = Qq / 4πr²ε₀
Ee = Qq / 4πrε₀
Electric potential is defined as electrical potential energy per unit charge. How do we get an equation for V using this?
V = Ee / q
Ee = Qq / 4πrε₀
V = Q / 4πrε₀
