Current Electricity Flashcards
what is current
the rate of flow of charge
what is potential difference/voltage
the energy transferred per unit charge between two points in a circuit
what is resistance
a measure of how difficult it is for charge to pass trough a component
what does ohm’s law state
that for an ohmic conductor current is directly proportional to the potential difference across it given that physical conditions are kept constant
what is an example of an ohmic conductor
a resistor
why does a diodes graph look like that
because it allows current to flow through easily one way but nit the other way so the potential difference and current is very low to begin with as the current cannot flow through easily due to the resistance
why does a filament lamp graph look like that
because it contains a metal wire which heats up so at the beginning as it is not very hit current can easily flow through as there is a low resistance how the hotter it gets the higher the resistance so it is more difficult for the current to flow through causing the gradient to decreased
what can we assume about ammeters and voltmeters resistance
ammeters have zero resistance and voltmeters have have infinite resistance so no current can flow through it so their measurement of potential difference across a component is exact
what is resistivity
is a measure of how easily a material conducts electricity
what does resistivity measure
is a measure of how easily a material conducts electricity
what will resistivity give the value of
the value of resistance through a material of length 1m and cross sectional area 1m^2
why is resistivity useful
because you can compare materials even though they might nit be the same size
what is resistivity also dependent on
environmental factors such as temperature
what will happen the resistance if the temp of a metal conductor increases
resistance also increases
why does the resistance increase if the temp of a metal conductor increases
because the atoms of the metal gain more kinetic energy and move more which causes the electron (charge carriers) to collide with the atoms more frequently causing them to slow down therefore current decreases and resistance increases
in a thermistor what happens to the resistance as the temp increases
the resistance decreases
why in a thermistor does resistance decrease when the temp increases
increasing the temp causes electrons to be emitted from atoms therefore the number of charge carriers increases and so current increases causing resistance to decrease
what is one application of a thermistor in a circuit
is a temperature sensor which can trigger an event to occur once the temp drops or reaches a certain value
what is a superconductor
is a material which below a certain temp known as the critical temp has zero resistivity
what does the critical temp of a superconductor depend on
the material it is made out of
which temp do most well known superconductors have a critical temp at
0K (-273 Degrees Celsius)
what are the applications of super conductors
- power cables which would reduce energy loss through heating to 0 during transmission
- strong magnetic fields which would not require a constant power source (could be used in maglev trains where there would be no friction between the train and the rail and in certain medical applications
how do you add resistances in a series circuit
Rt = R1 +R2 +R3 +….
how do you add resistances in a parallel circuit
1/Rt = 1/R1 + 1/R2 + 1/R3 + ….
what is power
is the energy transferred over time (rate if energy transfer)
what happens to the current and p.d in a series circuit
- the current is the same throughout the circuit
- the battery p.d is shared across all elements in he circuit, therefore the total um of the voltages across all the elements is equal to the supply p.d
what happens to the current and p.d in a parallel circuit
- the sum of the currents in each parallel set of branches is equal to the total current
- the p.d across each branch is the same
in a series circuit what is the total voltage across the cells equal to
the sum of the individual voltages of the cells
in a parallel circuit what is the total voltage equal to if the cells are identical and why
the total voltage is equal to the voltage of one cell this is because the current is split equally between the branches, therefore the overall potential difference is the same as if the total current was flowing through a single cells
in DC circuits what is always conserved and what are the name of the laws which describe how this is achieved
charge and energy are always conserved and the laws are called kirchoffs laws
what is Kirchoff’s 1st law and what does it show
the total current flowing into a junction is equal to the current flowing out of the junction
it shows that no charge is lost at any point in the circuit
what is Kirchoff’s 2nd law and what does it show
the sum of all the voltages in a series circuit is equal to the battery voltage
this shows that no energy is lost at any point in a circuit
what is a potential divider
is a circuit with several resistors in series connected across a voltage source, used to reduce a required fraction of the source potential difference, which remains constant
how can you make a potential divider supply a variable potential divider supply
use a variable resistor as one of the resistors in series, therefore by varying the resistance across it, you can vary the potential difference output
what could you replace a variable resistor with to form a temp or light sensor
a thermistor or LDR
when a LDR resistance decreases what happens to the light intensity
it increases
what has an internal resistance
batteries
what causes internal resistance
caused by electrons colliding with atoms inside the battery, therefore some enrgy is lost before electrons even leave the battery
what is the electromotive force
is the energy transferred by a cell per coulomb of charge that passes through it
what is emf the product of
the total resistance and the current of the circuit
what is the p.d across a resistor called
terminal p.d
what is the p.d across a the resistor for internal resistance called and why
the lost volts because this value is equal to the energy wasted by the cell per coulomb of charge
what is emf the sum of
the terminal p.d and the lost p.d
how can the emf of a battery be measured
by measuring the voltage across a cell using a voltmeter when there is no current running through the cell, which means its an open circuit
