Topic C Flashcards
What are dielectrics?
Dielectrics are insulators, there are no free charges carriers to produce an electrical current.
A dielectric contains bound charges, which cannot move freely but will
displace through small distances
when affected by an electric field.
What are the properties of dielectrics?
If a capacitor has capacitance C_0 when the space between its two
conductors is a vacuum, filling this space with a dielectric increase the capacitance to a new
value Cm.
What is the relative permittivity of the dielectrics
?
The ratio Cm/C0= ϵ_r
Why does a dielectric reduce the size of the E-field?
- Dielectric are insulators.
- Some charges are able to move around freely but not very far
- The positive and negative charges may be able to move slightly (but in opposite
directions) when an E-field is applied - This sets up a series of electric dipoles whose magnitude
is proportional to E - The E-field produced by these dipoles acts in the opposite direction to
the applied field and so the overall effect is to decrease the magnitude of the total E-field within the dielectric
What is a conductor?
A conductor contains free charges that can move through the material.
What happens when an E-field is applied to a conductor?
Initially, the field penetrates inside the conductor.
The field causes free charges to move in such a way that they set up an additional field in the opposition to the applied field
Equilibrium is quickly reached when the new field exactly cancels the applied field so that the total field with the conductor is zero.
What is the capacitance for a parallel plate capacitor?
C = ε_r ε_0 A/ d
What is polarisation and what causes it?
An applied E-field causes the atoms or molecules in a dielectric to become small electric
dipoles. These are referred to as induced dipoles. It is the electric
dipole moment per unit volume.
