5 Electricity Flashcards
what is current
rate of flow of charge
which way do electrons flow
negative to positive terminal
which way does current flow
positive to negative
(opposite to flow of electrons)
what is p.d
potential difference of a circuit is the amount of work done to move the charge around the circuit, per unit charge
the ‘push’ that allows the current to flow around the circuit (supplies the power)
what are the properties of an ohmic conductor
resistance is constant
current is directly proportional to voltage
current-voltage graph is a straight line
what does a diode do
a diode only allows current to flow in one direction unless there are extreme conditions
(unless voltage is other direction is very large)
what is reverse bias
the direction in which current is not allowed to flow
what is required for current to flow in reverse bias
a very large voltage
what is breakdown voltage
the voltage at which current will flow in reverse bias
what is forward bias
the direction that current is allowed to flow
what is threshold voltage
the point at which the diode will conduct in the forward direction
(0.6V)
what is resistivity
the resistivity of a material is a measure of how much resistance a material provides
it is independent of a materials shape or size
what are thermistors
electrical components which change their resistance based on their temperature
what is the relationship between temperature and resistance of a negative temperature coefficient thermistor
the higher the temperature, the lower the resistance
what are superconductors
material with a resistivity of zero below a critical temperature
how do superconductors work
because the resistance of a piece of a superconductor is zero, there are no heat losses for currents through them
most superconductors must be cooled to zero, only a few kelvin above absolute zero - this is very expensive and limits the use of superconductors
what are superconductors used in
magnets needing high currents (MRI machines)
when do superconductors occur
when free electrons combine to form cooper pairs at low temperatures
these cooper pairs collide with the other atoms in the material and, if the energy of the collision is low, then the cooper pairs do not split and lose energy
so resistivity is zero
what material is used to cool MRI magnets
liquid helium
what is the resistance in a parallel circuit
the total resistance is less than the smallest resistor in the parallel circuit
1/Rtotal = 1/R1 +1/R2 +…
what is the resistance of a series circuit
the total resistance is the sum of each individual resistor
what is kirchhoff’s first law
the total current arriving at a junction is equal to the total current leaving a junction
‘current splits up’
what is kirchhoff’s second law
the sum of voltage drops in a closed loop must be equal to zero
energy is conserved
what is current convention
the current in a circuit flows from positive to negative
what is power
the rate of transfer of energy
when is work done in a circuit
when current flows through a component with resistance
what do potential dividers do
supply constant or variable potential differences from a power supply
how do potential dividers work
a potential divider uses the resistance of a resistor to change the potential difference across a component
what is emf
electromotive force is the total voltage produced by the cell
how does internal resistance effect a circuit
internal resistance reduces the voltage output of the cell before it reaches the circuit
