Electricity Flashcards
Electric current
The amount of charge passing a point
per second
Electric current is measured in…
Amps
Charge (current) =
current x time
Charge
is a property of some particles, e.g. protons and electrons
Unit for charge
Coulumbs (C)
Electrons have a charge of
1.6 x10^-19 C
Charge (electrons) =
number of electrons x electron charge
1C of charge =
6.25 x10^18 electrons
Conventional current
defined as moving from positive to negative terminal. This is what is marked on all diagrams and used in calculations.
Electric field lines show:
- Which way a positive charge will move
- The direction of the electric field
Objects have gravitational potential energy due…
- to the gravitational force acting on mass
Objects have electric potential energy due…
to their position in an electric field, and the electric force acting
Potential difference (or voltage)
is the difference in electric potential energy between two points for 1C of charge
When current passes through components:
energy is transferred from the charges:
- The charge lose electric potential energy
- The potential difference drops
When current passes through batteries/cells:
energy is transferred to the charge:
- The charges gain electric potential energy
- The potential difference increases
- We say the batteries/cells provide an electromotive force e.m.f (it just means a positive change in p.d.).
P.D.
is the energy transferred per coulomb from
charge carriers to components.
E.M.F.
is the energy transferred per coulomb to
charge carriers from sources (batteries/cells)
Resistance:
- Charge carriers collide with the materials
they flow through, whether it’s a lattice or
a liquid. - Each collision transfers KE from the
carriers to thermal energy in the material
Ohm’s law:
- The current through a conductor at
constant temperature is directly
proportional to the voltage supplied - For each conductor this constant is its
resistance.
Resistance and temperature
- Resistance increases with temperature
- As temperature of a material increases, its
atoms/molecules vibrate more. - This causes more frequent and disruptive
collisions with charge carriers, causing
greater transfer of energy.
Ohmic conductors:
Anything with a fixed resistance - which therefore follows Ohm’s law:
- Fixed resistors
- Wires
- Most metal components
Non-ohmic conductors
Anything with a variable resistance - which therefore shows non-linear relationships on I-V graphs:
- Filament lamps
- Diodes
Resistance of a wire depends on:
- Length (more wire to travel through, more resistance)
- Area (more charge carriers (or more pathways), less resistance)
- Resistivity
R =
ρL/A
Resistivity
Measure of the intrinsic resistance of amaterial, due to electron density, electron structure and lattice structure
Temperature dependance
- Regular conductors (as resisitivity increases temp increases), increase in energy goes to vibrations in the lattice
- Semiconductors (as resistivity increases temp decreases), increase in energy frees more electrons for current
free electrons:
- Free electrons behave like a gas inside
conductors - The more free electrons, the more
conductive a material
electrons drift
- Electrons drift through circuits at
about 0.1 mm s-1. - Electrons bump into the lattice and
each other – they don’t travel in
straight lines, so their drift velocity
varies. - Electric signals travel faster than this
because once a potential difference is
applied to a circuit, all electrons
move at once
current (drift velocity) =
I = nqvA
Semiconductors
- semiconductors are materials (including the elements on the right) that have a conductivity less than conductors but greater than insulators
- as temperature rises lattice ions release electrons, increasing number density (n) and so decreasing resistivity
Holes
the positive ‘holes’ left behind by free electrons themselves move (the opposite way to electrons), also increasing the current
In semiconductors electrons absorb
energy, moving from the…
valence band to the conduction band of an atom, leaving behind a hole.
valence band
is the last electron shell of atom
conduction band
contributes to the sea of delocalised electrons that cause current
Insulators (band theory)
the conduction band is quite empty and there is a large energy gap
Semiconductors (band theory)
the conduction band is quite empty but the energy gap is small.
Conductors (band theory)
the bands overlap, so the conduction band has lots of electrons
Thermistor
Resistance decreases with temperature:
- Heat energy transferred to free electrons, increasing charge carrier density
- (Regular conductors have positive temperature coefficient: R ∝ T )
Used to monitor temperature, e.g. thermostats, air-conditioning, incubators, electrical devices, preventing overheating
Light-dependent resistor (LDR)
- Made from a semiconductor material in which incident light frees more charge carriers, increasing charge carrier density
- Used for automatic lighting
Diodes
Non-ohmic component formed of two types of semiconductor that allows current in one direction only
Superconductors
- For these materials at very low temperatures (<30K) resistance drops to zero.
- They undergo a phase transition in which
electrons form pairs (Cooper pairs) due to a quantum-mechanical attraction. - The electron pairs behave like a superfluid in the lattice, flowing without viscosity - not losing any KE to collisions.
Why do electrical signs travel faster?
- All e- move at once
- e- pass charge through collisions to move charge quickly
Potential dividers
A component (resistor) that divides the potential energy
Potential dividers equation:
v1 / v2 = r1/ r2
3 electrical power equations
- P = V x I
- P = I^2 x R
- P = v^2 / R
More brightness means
more power
