Electricity Flashcards
Basic info about the electron
Charge: -1.602x10-19. Smallest possible chunk of charge. Cannot be split.
Basic info about charge
Net charge is conserved. Units: coulombs (C)
Coulomb’s law
Understand how polarisation can be induced
Electron clouds are shifted slightly by an external charge. Or free electrons are shifted (in a conductor)
An understanding of electron orbitals including a)size of Hydrogen, b)relative size of shell, c)orbital shape d)why electrons are in set orbitals
a) Hydrogen is approx 100 pm diameter, b)if the nucleus is enlarged to a golf ball the first shell is approx 1km away, c) shape depends on orbital and orbitals don’t have edges d) electrons form standing waves around the nucleus
An understanding of comparative strengths of electrostatic force and gravity
It takes a balloon (with a few billion electrons) to induce a charge on a small piece of paper and counteract the pull of the whole earth
Electric field equation
Electric field units
NC-1
How to convert between F and E
F12=q1E
Electric field in words
Electric force per positive unit charge at a point (or the force a charge WOULD feel if it were there)
Three rules for electric field lines
1) They point in the direction a positive charge would go, 2) in systems with a net charge of 0 all field lines begin on a positive charge and end on a negative charge, 3) the number of field lines per unit area through a surface perpendicular to the field lines is proportional to the magnitude of the field in that region
What is permittivity in words?
How easy it is for the electron clouds in a material to absorb energy (or ‘resistance to forming an electric field’)
How do you get the total permittivity?
What is the difference bewteen relative susceptibility and permittivity?
They are almost the same: relative susceptibility + 1 = permittivity
What effect does high permittivity have on the force or electric field?
High permittivity (means a lot of energy will be used shifting electrons) results in a smaller force or electric field
To understand how to find electric field at a point from multiple charges
Simply add the electric field from each charge together
To understand how to find the electric field from a continuous charge
Split volume into tiny sections dV. Add up the total charge from all of these in an integration
Charge from a volume dV
dq=ρ dV (where ρ is the charge density in the volume)
To understand the difference between summation and integration
When we split a volume, area or line into smaller sections then integration must be used to make sure we add up all the dVs. When finding the total charge it is not necessary to split into dVs when the charge density is constant throughout - then we can just multiply ρ and V. But if finding the electric field y(with constant density) you would still have to integrate as each section dq is a different distance from the point.
Give equation for dot product and explain where this comes from
|a||b|cosθ Put the two vectors base to base so one is along the x axis and find the lengths of the x and y components of each. Then multiply the two x components and add that to the multiple of the two y components.
What does ‘flux’ mean in general?
The total amount of SOMETHING passing through an area
What is electric flux in words?
It corresponds to the total number of field lines penetrating a surface
In terms of field lines, what is the difference btween electric field and flux?
Electric field is proportional to the number of field lines per unit area, whereas flux is proportional to the total number of field lines
Give an equation relating flux and electric field (for a surface perpendicular to E)
Understand why the flux/electric field equation needs to be modified when the area is not perpendicular
If the area is not perpendicular to E then it will give a smaller reading for Φ than it should
Give an equation relating flux and electric field (for a surface NOT perpendicular to E)
Be able to derive Gauss’s law from Cloumb’s law for a point charge
Be able to derive Gauss’s law from Coulomb’s law for a point charge contained within a non-symmetrical surface
The same as for a point charge, since the non-symmetrical surface can be replaced with a sphere for the purposes of calculating the net flux - as the net flux through both is the same (the area from the ‘E’ and from the ‘da’ cancel)
To understand why Gauss’s law has ‘Q_internal’ (sometimes written ‘Q_total’) rather than just Q
When multiple charges are contained within the surface the principle of superposition can be used to show that ‘Q’ becomes ‘Q_internal’. This is then used as the more generalised formula.
To remember Gauss’s law
To explain why Gauss’s law is valid for all surfaces and charge distributions
Because the NET flux is not affected by where the charges are or what shape surface you use.
To explain why Gauss’s law is independent of the distance from the charge we draw the Gaussian surface
The NET flux through the surface is not affected by the distance from the surface.
To understand why there is no electric field in a hollow space inside a conductor
There is no charge in a hollow space - hence (due to Gauss’s law) no net field inside
To be able to use Gauss’s law to find the electric field inside and outside a charged solid sphere
For outside the sphere, same as a point charge. Same derivation as Gauss’ law from coulomb but dont sub in E.
For inside the sphere, set RHS of Gauss’ eq. to charge density x volume. LHS is E x surface area. Equate and solve for E.
To be able to use Gauss’s law to find the electric field inside and outside a charged hollow sphere
Inside is just zero as RHS of Gauss’ is zero.
Outside is just a point charge
To be able to use Gauss’s law to find the electric field a distance from a charged line
Using cylinder as charge radiating radially.
Top and bottom are zero as parallel to E.
LHS of Gauss’ becomes Eda=E 2Pi r l.
RHS=Q/epsilon=lambda x length/…
To be able to use Gauss’s law to find the electric field a distance from a charged plane
Cylinder byt opposite of line charge
To understand when we need the differential form of Gauss’s law
When the electric field is not constant at all points on the Gaussian surface and it is not possible to take the ‘E’ to the front of the integral in Gauss’s law
To be able to follow the derivation of the differential form of Gauss’s law
See lecture notes
To remember the differential form of Gauss’s law
To be able to describe divergence in words
If there is more stuff (could be particles, electric field lines etc.) leaving an area than is going in, the that area is acting as a source and the divergence is positive. If there is more stuff going in than leaving then the area is acting as a sink and there is convergence (or negative divergence).
To be able to take the divergence of a function eg.
To know the equation for electric potential energy
To know the equation for electric potential
To know how to go about finding the equation for potential energy in words
To find the work done (energy) in bringing a charge close to another charge
To be able to show how potential relates to electric field ie.
Start by finding the work done to bring two charges together - see lecture notes.
To know an equation for electric force using Grad
To know an equation for electric field using Grad
To know the Grad equation for E in cartesian co-ordinates
To know the Grad equation for E in cylindrical co-ordinates