Electric Fields Flashcards
What is charging by friction?
The action of rubbing causes a transfer of electrons from the surface atoms of one of the rubbed materials to the other. The material which gains electrons acquires a negative charge and that which loses electrons becomes positively charged
How does a conductor work?
contains free electrons, therefore electrons gained by a conducting material are able to move throughout the material, and if electrons are lost those which remain redistribute in the material. If the conductor is connected to Earth, the charge acquired can flow to the Earth
How does an insulator work?
Doesn’t contain free electrons, therefore if one region of an insulating object becomes charged it does not cause a movement of electrons within the body. The charge stays in the region in which it has been placed (it is static).
What are electric fields?
- an electric field is the region of space in which a charged body exerts an influence
- the direction of an electric field is the direction of the force experienced by a positively charged particle, a ‘test charge’, when placed at that point in the field
Define electric field strength
The electric field strength, E, at a certain point is the force per unit charge acting on a positive charge placed at that point in the field
E = F/Q
What is the unit for electric field strength?
Newton per coulomb (N/C)
or
Volt per metre (V/m)
Explain electric field lines
In an electric field, the shape of the field can be shown using electric field lines. These are the paths along which a small point positive charge would move it if was to be placed in the field and allowed to move. The closer the spacing of the lines, the greater the strength of the field
State Coulomb’s law
The force between two charged particles is directly proportional to the product of the two charges and inversely proportional to the square of their separation
F ∝ Qq/r²
What is the constant of proportionality added to Coulomb’s law, and what equation does it produce?
Constant of proportionality = 1/4πε
Full equation - F = Qq/4πε₀r²
What is the permittivity of free space (ε₀)?
ε₀ = 8.85 x 10^-12 F/m
How do you calculate the permittivity of a material?
ε = εᵣε₀ , where εᵣ is the relative permittivity (dielectric constant) and ε₀ is the permittivity of free space
What equation is formed by combining E = F/Q and F = Qq/4πε₀r²?
E = Q/4πε₀r²
State the relationship between potential and electric field strength
E = -V/d , where V is the potential difference between 2 plates, and d is the separation of the plates
What happens to a charged particle entering a field perpendicular to the field lines?
The charged particle will experience a force parallel to the field lines
- positively charged particles experience a force in the same direction as the field lines
- negatively charged particles experience a force in the opposite direction to the field lines
What path will a particle moving through a uniform field follow?
As the field lines are uniform, but the direction of the particle changes, the particle will follow a parabolic path through the field.
State the two equations for capacitance
C = Q/V
C = εA/d
Define electric potential
The electric potential at a point is defined as the work done in bringing unit positive charge from infinity to that point; electric potential is zero at infinity
What are the two equations for electric potential, and how can they be combined to form an equation for energy?
V = Q/4πε₀r
V = E/q , where E is energy transferred
Combines to form E/q = Q/4πε₀r
Rearranges to form energy = Qq/4πε₀r
What is an equipotential?
An equipotential surface is one over which the potential in an electric field remains constant
What are properties of equipotentials?
- no work is done in moving a charge along an equipotential
- equipotentials always cut across lines of force at right angles
Close to the charge, equipotentials are close together, showing a large potential gradient
Define electric potential energy
The electrical potential energy, E, is equal to the work done in bringing two positively charged particles from infinity to a separation, r
