electricity Flashcards
what is current
the rate of flow of charge in a circuit
charge {}
what are the units for charge, give two answers
Q = It
coulombs (C)
or
As
define 1 coulomb
amount of charge passing in 1 second when the current is 1 ampere
what do we use to measure current
why must it always be connected in series
ammeter
so that the current through the ammeter is the same through the component
what is potential difference
work done in moving a unit charge between two points
what is the role of a power source in a circuit
does work to move charge around the circuit
charge flows through the power source
where it is “raised” through a potential and energy is transferred to its electrical potential energy
what is 1 volt
when you convert 1J of energy to move 1 coulomb of charge through a component
what can you use to measure potential difference
how does it work
a voltmeter
A voltmeter measures how much voltage is passing between two points.
It does this by measuring the difference between the positive input of voltage and the negative input of voltage.
what is resistance
measure of how difficult it is for current to flow through a component
resistance {}
what are the units for resistance
give two answers
resistance = potential difference / current
ohms
VA^-1
potential difference {} not V=IR
what is the unit for potential difference
give two answers
potential difference = work done / charge
volt
JC^-1
Ohm’s law
provided physical conditions (temp ect.) remain constant the current passing through an ohmic conductor is directly proportional to the potential difference across it
what are ohmic conductors
conductors that obey ohm’s law
what does the IV graph look like for a ohmic conductor
current against potential difference
straight line through origin
resistance is gradient, therefore is constant
what are some factors which can affect resistance
light level
temperature
shallower gradient on a IV graph means…
greater resistance
how can you investigate the IV characteristic of a component
using a variable resistor to vary resistance
which in turn changes the voltage and current across a component
take a range of current and voltage values
plot an IV graph
what is an ideal voltmeter
assumed to have infinite resistance
( which means no current flows through them )
current takes the path of least resistance
if there is infinite resistance in the voltmeter
all current will flow through the component
what is an ideal ammeter
assumed to have no resistance
(so will have no potential difference across them , so no energy is lost across it)
what does the IV graph for a filament look like
like a weird cubic
an actual cubic would be a VI graph
what is a filament and how does this explain its IV characteristic
a filament is a thin coil of metal wire
as current flows through temperature increases which increases the resistance of the lamp
this is why the curve levels off at high currents
why does resistance increase with temperature
when current flows through a metal conductor some of the electrical energy is transferred to thermal energy and causes the metal to heat up
causes particles in the metal to vibrate more
this makes it more difficult for the charge carrying electrons to get through
more difficult for current to flow so resistance increases
what are diodes
designed to allow current to flow in one direction only
made from semi-conductors
what is forward bias
the direction in which current is allowed to flow in a diode
what is threshold voltage
most diodes require around 0.6v in the forward direction before they will conduct
what is the reverse bias of a diode
where the resistance is very high and current flow is tiny
what is resistivity
what can it be affected by
measure of how much a particular material resists flow of current
it is a property of a material
structure of the material
light intensity
temperature
resistivity {}
what are the units
ρ = AR/L
Ωm = (m^2 * Ω) / m
how is resistance of a wire affected by the length
the longer the wire the more difficult it is to make current pass through it (greater resistance)
resistance is proportional to the length of the wire
how is resistance of a wire affected by the area
the greater the cross-sectional area of the wire the easier it is for electrons to pass through it
less resistance
inversely proportional
what does lower resistivity mean and why would this be beneficial
better at conducing electricity
useful in electrical engineering
what are semiconductors
group of materials that aren’t as good conductors as metals
since they have fewer charge carriers
what makes semiconductors useful
if energy is supplied to a semiconductor more charge carriers can be released
this causes the resistivity of the material to decrease
making them good sensors for detecting change in an environment
what are three common semiconductors
LDRs
(light dependent resistors)
diodes
thermistors
what is a thermistor
a component that has resistance depending on its temperature
what is an NTC thermistor
how do they work
why are they useful
negative temperature coefficient
resistance decreases as temperature goes up
warming the thermistor gives electrons energy to escape their atoms
more charge carriers available
lower resistance
makes them good temperature sensors
high accuracy
investigating the resistance of a thermistor
water bath
keep pd constant
record current for changes in temperature
calculate resistance and plot against temperature
should give a decreasing curve
what is superconductivity
the ability of certain materials to conduct a direct electric current (DC) with practically zero resistance
how can you lower the resistivity of a material
cooling them down
what is the critical temperature of a material
if you cool the material down to below its critical temperature
resistivity disappears
becomes a superconductor
no resistance so electrical energy not turned to heat, “not wasted”
what are 3 uses of superconductors
power cables that transmit energy without loss of power
strong electromagnets - MRI scanners
electrical circuits working with minimal energy loss
what is the catch to superconductivity
critical temperatures tend to be very low
lowering temperature that much is expensive and difficult
REQUIRED PRACTICAL
finding the resistivity of a material
find cross sectional area of wire
measure diameter with micrometer
set up wire in circuit
voltmeter across
measure current and pd through wire for different lengths
plot average resistance against length
gradient of the graph is R/L
ρ/A = R/L
so gradient * A is the resistivity
what is current due to
the flow of charged particle
charge carriers
metals - electrons
salt solution - ions
how does current flow through an insulator
electrons in an insulator are attached to their atoms and cannot move away
no electrons are free to move
so no current flows
how does current flow in a metallic conductor
delocalised electrons act as charge carriers
when a potential difference is applied the electrons are attracted to the positive terminal of the metal
what is the emf of a source of electricity
the electrical energy produced per unit charge passing through the source
how does the resistance of an LDR vary
resistance decreases with increasing light intensity
what is a battery
combination of cells
what is a resistor
a component designed to have a certain resistance
what is an intrinsic semiconductor
An intrinsic semiconductor is an undoped semiconductor, or semiconductor in the raw form without any impurities to add current carriers to the basic semiconductor material.
how does resistance vary for a thermistor
resistance decreases with increasing temperature
why is it important to keep the temperature constant when doing an experiment to find the resistivity of a material
suggest how this can be achieved
resistivity is dependent on temperature
as current flows through a wire, temperature increases
the temperature can be kept constant by using a small current
a switch can be used
define power
rate of transfer of energy
Power {}
give four formulae
what are its units
P = E/t
Watts = Joules/seconds
P = IV
P = V^2/R
P = I^2 R
how do you calculate the total energy transferred
use P=E/t
E = Pt
P = IV
ect.
what is internal resistance
In a battery chemical energy is used to make electrons move
when they move they collide with atoms in the battery
causes batteries and cells warm up when used
what is e.m.f
electromotive force
amount of electrical energy the battery produces and transfers to each coulomb of charge
the electrical energy transferred by a power supply per unit charge
e.m.f {}
give its units
ε = E/Q
volts
terminal pd
the potential difference across the terminals of the cells
( when one coulomb of charge flows through the load resistance )
if there was no internal resistance would equal emf
why does terminal pd differ from the emf
energy is always lost in overcoming the internal resistance of power supplies
lost volts
energy wasted per coulomb overcoming the internal resistance
what are the equations linking emf and internal resistance
ε = I(R + r)
ε = V + v
V - terminal pd - IR
v - lost volts - Ir
describe a practical to investigate internal resistance and emf
set up circuit with variable resistor
set variable resistor to max resistance and record current and potential difference
repeat for different load resistances
plot a VI graph
gradient = -r
y intercept = emf
what is the load resistance in a circuit
the total resistance of all the components in the external part of the circuit
not including the internal resistance
what is Kirchoff’s first law
the total current entering a junction = the total current leaving it
accounts for conservation of charge
since current is rate of flow of charge
what is Kirchoff’s second law
the total emf around a series circuit = the sum of pds in each loop of a circuit
ε = ΣIR
accounts for conservation of energy
in a series circuit how do properties add up
current is the same everywhere
emf is split up proportionally based on the resistance of the components as V = IR
always adds up to emf
total resistance = ΣR
how do properties add up in a parallel circuit
current split up at each junction
same pd across components in parallel
1/Rt = 1/R1 + 1/R2 + 1/R3 +…
how can you work out the emf of a battery
add up the emf of each cell making up the battery
how do two cells in parallel behave in comparison to 1 in series
two cells in parallel behave as a single cell (with the same emf) but with half the
internal resistance / reduced internal resistance
what is a potential divider and what is its purpose
a circuit with a voltage source and a few
resistors in series
to provide a varying potential difference
or a constant specific potential difference
why are potential dividers useful
they can be used to supply a specific potential difference between zero and the the pd of the supply
potential divider {}
V(out) = (R2/R1+R2) * V(supply)
in the potential divider equation which resistance is assigned to R1
R1 - whichever resistor the current travels through first
R2 - the resistor that you’re measuring the voltage across
using a potential divider, how could you vary the output pd
replacing R1 with a variable resistor
how are light and temperature sensors made
using a potential divider
use LDR or a thermistor instead of R1
V(out) varies with light levels or temperature
what is a potentiometer
has a variable resistor replacing both R1 and R2 (two resistors in series)
you move a slider to adjust the relative sizes of R1 and R2
volume control on a stereo
what is the resistance on a reverse-biased diode
infinite resistance
State what is meant by an emf of 1.5 V [2]
work done to move a unit charge through the cell
1.5J of work is done moving 1C through the cell
what effect does decreasing current have on the terminal pd of a cell
Decreased current through cell decreases lost volts
this increases the reading of terminal pd
what is a superconductor
a material that has zero resistivity below a critical temperature
