electric Flashcards
current
rate of flow of charge
e
elementary charge
examples of charge carriers
electrons - in metals
ions in electrolytes
conventional current
flow of positive charge, flows from positive to negative
electron flow
current flows from negative to positive
kirchoffs fist law
sum of current entering a junction is the same as the sum of the current leaving a junction
charge is conserved
mean drift velocity
average velocity of the charge carriers due to the applied electric field, it has to be an average because they are often moving randomly in all directions
put conductors, semi conductors and insulators in order from highest n to lowest
highest = conductors
semiconductors
lowest = insulators
potential difference
work done per unit charge, the difference in potential between to points in a circuit
what is the difference between voltage and emf
voltage is the energy transferred per coulomb across a load resistance
emf is the total amount of work done by the battery per coulomb
true or false: the kinetic energy of an electron is multiplied by its charge multiplied by the voltage
true, eV = 1/2 mv^2
resistance
the potential difference required for each unit of current that flows through a resistor / how difficult it is for current to flow through an appliance
R = V/I
1 ohm
when a resistor is subject to a voltage of 1V and allows a current of 1A through, its resistance is 1 ohm
ohmic conductor
conductor that obeys ohms law
ohms law
the current through an ohmic conductor is directly proportional to the potential difference across it, this is only true if the temperature is constant
gradient of potential diff graph
1/R as gradient = rise in current/charge in potential difference
what does an s shaped graph represent
a filament lamp
as current increases resistance increases
why does the current increasing on a filament lamp cause an increase in resistance
- flow of current causes collisions between electrons and the metal lattice
- these collisions increase the temp of lattice
- as temp increases more collisions occur causing the resistance to increase
diode
appliance that only allows current to flow in one direction
graph is flat then curves upwards
LDR
a semiconductor that is sensitive to light: as the light intensity increases the resistance decreases
how does a thermistor work
similar to LDR but as the temp increases the resistance decreases, negative temp coefficient
resistivity
property of a metal which shows how easy or difficult it is is for current to flow in the metal
P= RA/L
experiment to determine resistivity
- measure diameter of wire
- use diameter to calc CSA
- set up circuit
- vary length and record voltage and current
- use R = V/I to work out resistance
- plot graph of resistance against length
- grad = resistivity / area
true/false: the resistivity of both metals and semiconductors increases with temp
false:
- the resistivity of a metal increases with temp because as the metal ions heat up they vibrate more and the electrons bang into them and slow down
- some semi conductors get less resistive as temp increases, because supplying energy actually causes more charge carriers to be released so current can flow more easily
power
rate of energy transfer
why do electricity companies use KWH rather than joules or watts
impractical
kirchoffs second law
for any path of a circuit the sum of all the pd must equal the total emf of the circuit
true/false: adding a resistor to a circuit in parallel will always reduce the total resistance
true
tot resistance drops the more paths there are for current to take
true/false: in parallel circuits the total voltage in each loop is the same
true
Vtot = V1=V2…
how does the current vary between each component of a series circuit
the current through all of the components is the same so the current does not vary
if you connect two cells in series with positive to positive what will the total emf be
e1+e2
true/false: the emf of a source and its terminal voltage will always be the same
false
not all the energy given to the charges in the source makes it out of the cell
internal resistance
resistance with in a power source due to the materials and components which make it up
lost volts
difference in voltage between that supplied by the source and the amount available to the circuit
experiment to determine the internal resistance of a source
set up circuit with a source, ammeter and variable resistor in series and a voltmeter in parallel
vary the resistance and measure the voltage and current
plot graph off V/I
V = -rL + e
internal resiatbce = -m
purpose of potential divider
provide variable potential difference or to provide a constant specific potential difference
how does voltage across component in a potential divider correspond to its resistance
proportion of the total voltage which is dropped across the component is equal to the proportion of the circuit resistance which its contributes
V/Vt = R/Rt
how can a potential divider circuit be used as a sensing circuit
- put resistor that varies with the desired condition in the potential divider
- insert the responding load over either the variable or fixed resistor depending on how the circuit should work
