M4 Electricity Flashcards
what’s an electric current?
rate of flow of charge
negative to positive
measured in amps
define coulomb
one coulomb is the charge which passes a point when a current of 1A flows for 1s
what does the equation Q = It mean
give units
charge = current X time
charge - coulombs
current - amps
time - seconds
what’s an electrolyte
a solution which conducts electricity. it contains positive and negative ions
Ions move in opposite directions when connected to a cell
what are charge carriers
any particle with electric charge. particles which contribute to an electric current
what is the elementary charge
1.6 X 10^-19 C
the charge of an electron
what is the difference between electric current in a metal and an electrolyte
Electric current is the net flow of charged particles. In a metal, the charge carriers are free/delocalised electrons. In an electrolyte the charge carriers are ions. Positive ions move to the cathode, negative ions move to the anode, both contributing to electric current.
define potential difference
Energy transferred across a component per unit charge (energy lost per unit charge by charges passing through a component).
define volt
Two points in a circuit differ in potential by 1 volt if 1 joule of energy is needed to move 1 coulomb of charge between them.
what is electromotive force
The EMF of a battery or cell is the energy transferred per unit charge (energy gained per unit charge passing through a supply)
what is the difference between the EMF and PD in terms of energy transfer
PD is transferring electrical energy into other forms (heat, light, sound).
EMF is transferring other forms of energy (chemical in a battery) into electrical energy.
Energy is lost in potential difference but gained in EMF.
define an ammeter
a meter that measures the flow of electric current in amperes. must be connected in series
define voltmeter
a meter that measures the electrical potential difference between two points in an electric current. must be connected in parallel. if places across the power supply it measures emf of the supply. if placed across a resistor it will measure the pd
series circuit facts
current flowing is the same at all points pd shared between components of a circuit
parallel circuit facts
current shared between branches in circuit pd across each branch is same as the supply voltage or emf
define conventional current
flow of electrical charge from positive to negative
name some components in a circuit and draw their symbol
open switch closed switch bulb cell battery resistor ammeter voltmeter variable resistor LED diode thermistor LDR capacitor fuse
define resistance
opposition to the passage of current within a component
define ohm
the resistance of a component when a potential difference of 1 volt is produce per amp
whats the equation for resistance
voltage / current
R = V / I
a conductor that obeys ohm’s law is called a…
ohmic conductor
give and example of an ohmic component and then an example of a non-ohmic component
ohmic - fixed resistor & wire at constant temp
non-ohmic - bulb, diode & LED
if an I-V graph is straight what does it mean
component obeys ohm’s law - ohmic conductor
if an I-V graph is not a straight line what does it mean
non-ohmic conductor
describe a graph for an I-V graph with a diode
on the negative side of the graph it is a straight line as resistance is very high
Then PD increases the resistance gradually starts to drop - called threshold pd
Then the resistance drops sharply for every small increase pd
what are the benefits and uses of using a LED
lights up when current flows - used as indicators on appliances
efficient source of light - long-lasting
how do find out the total resistance in a series circuit
each resistance added together
how do you find out the total resistance in a parallel circuit
1 over each resistance added together
then 1 over that to get the total resistance
what is number density
number of free electrons per cubic metre of material.
units = m^-3
conductors have a larger number density
what does number of charge carriers equal
number of charge carriers per unit volume X volume of the section
= ALn
what does total charge carried by charge carriers equal
number of charge carriers X charge on each carrier
Q = ALne
what is mean drift velocity and what does the equation I = Anev mean?
the average velocity of electrons as they move through a wire
current = cross-sectional area X number density of electrons X elementary charge X mean drift velocity
why is mean drift velocity slow
there is a large number of random collisions with the fixed positive ions
what is the difference between conductors, semiconductors and insulators in terms of number density
conductors - greater number of free electrons per m cubed = 10 to the 28 m-3
semi-conductors - lower number of free electrons compared to conductors = 10 to 17 m-3
insulators - none or very few free electrons per m3
why do semi-conductors increase in temperature?
they have a much lower number density than metals and so to carry the same current the electrons in a semiconductor need to move much faster. this increases the temp.
what is resistivity
it is a measure of how much a particular material resists current flow.
Measured in ohm metres
what does resistivity depend on?
structure of the material
temperature
light intensity
why does resistivity increase as temperature increases in metals
charge is carried through metals by free electrons in a lattice of positive ions. Heating the metal causes the ions to vibrate more which makes electrons collide more. This causes energy to be lost, usually as heat, and increases the overall resistance of the component.
what is internal resistance?
the resistant created in a power source when electrons collide with atoms inside the power source and lose energy
this is what makes batteries warm up
what are lost volts
energy wasted per coulomb overcoming the internal resistance
how do you calculate the total emf in a series circuit
in a parallel circuit?
series: adding all the individual emf’s
parallel: total emf = emf on first branch = emf on second branch ……
what is Kirchoff’s first law
what is Kirchoff’s second law
1st) total current entering a junction = total current leaving it
2nd) total emf around a series circuit = the sum of the pd’s across each component
when semi-conductors rise in temperature, what does resistivity do and why
resistivity decreases
As temperature increases more electrons can break free of their atoms to become free electrons. The number density (n) increases and so the material becomes a better conductor.
At the same time there are more electron-ion collisions bu this effect is small compared to the increase in number density.
define terminal pd
the amount of electrical energy delivered to the circuit
A cell of emf 1.5V has a closed circuit terminal pd of 1.25V, when a load resistor of 25 ohms is connected to calculate it.
1) what is the current
2) internal resistance
3) closed circuit terminal pd is load resistor changes to 10 ohms
1) I = V/R
= 1.25/25 = 0.05A
2) r = (E-v)/I
= (1.5 - 1.25) / 0.05 = 5 ohms
3) E = I(R + r)
1.5 / (10+5) = 0.1A
V = IR
= 0.1 X 10 = 1V
define power
what are the units
the rate at which energy is transferred
the amount of electrical energy it converts into other forms each second
watts = 1Js^-1
what does power depend on
The current in the component.
The potential difference across the component.
if a kettle has a resistance of 20 ohms and a current of 12 A
what is the rate of energy transfer?
P = I^2 X R
12^2 = 144
144 X 20
= 2880 W
define fuse
the electrical device that can interrupt the flow of electric current when it is overloaded.
it will protect the wiring from excessive current.
it helps protect the device. it breks the circuit so prevents it overheating - safety
what are the fuse ratings
why are they there
1A, 5A, 13A, 30A
they are the maximum current the fuse will let through. this stops the appliance overloading
define kilowatt-hour
the energy transferred by a 1kW operating for 1 hour
how much energy is there in 1 kWh
1kW = 1000 Js^-1
1hr = 3600 s
1kWh = 3.6 X 10^6 J
= 3.6 MJ
when is maximum power delivered to a load resistor
when the resistance of the load is equal to the internal resistance of the power supply
what is a potential divider
A component in a circuit that produces an output voltage that is a fraction of its input voltage. It can reduce voltage. Varying the ratio of a pair of resistors change the Vout in a circuit and allows you to control the Vout.
What is V in
What is V out
Vin - supply pd
Vout - selected pd and is dependent on the ratio of the resistors
How do you calculate V out
1) Use the potential divider equation
2) Using ohm’s law:
Work out I from V/R in main circuit
Work out V from IR in V out circuit
what are working potential dividers
there is a slider so you can change the V out
the lower section of the resistor acts as R2
if the sliding contact is at the extreme negative side of the resistor then V out = 0 and at the extreme positive side Vout = Vin
At the midpoint Vout = Vin/2
what is the graph for a thermistor when the y axis is resistance and the x axis is temperature
curve slope downwards
what is the graph for a LDR when the y axis is resistance and the x axis is light intensity
curve slope downwards
why would you put a thermistor in a potential divider circuit
an NTC thermistor can be used in a potential divider circuit to provide an output voltage, Vout, which depends on temperature
why would you put a LDR in a potential divider circuit
An LDR can be used in a potential divider circuit to provide an output voltage, Vout, which depends on the intensity of light.
why would you include a variable resistor with an LDR or thermistor
allows for sensitivity
allows different levels of light or temperature
why do we need a resistor in series with an LED
The resistor limits current in the circuit otherwise it would overheat and burnout.
whats the advantage of using data loggers to see physical changes
quicker
more accurate
reduces human error
how can you increase the current through a wire
increase the number of electrons (by increasing number density or cross sectional area)
increase the mean drift velocity of the electrons
derive the equation I = Anev
number of charge carriers = charge carriers per unit volume X volume of section = ALn
total charge carried = charge carriers X cjarge on each charge carrier so Q = ALne
v = d / t so time = length of section / mean drift velocity t = L / v
sub in I = Q / t
I = ALne / (L/v) = Anev
what does the work done on the particle equal when it accelerates by a pd
the kinetic energy it has gained
so W = VQ
VQ = 0.5mv^2
what does a graph look like with resistance on the y axis and temperature on the x axis for a NTC thermistor
NTC thermistors are negative temperature coefficient thermistor
slope downwards
so as temp increases, resistance decreases
whats the graph for an LDR with resistance on y axis and light intensity on x axis
slope downwards
so as light intensity increases resistance decreases
why does resistance decrease for thermistors and LDR’s as temp/light intensity increases
electrons (photons for LDR) gain more energy as temp or light intensity increases
so they escape from the atoms
this means there are more charge carriers available
so resistance is lower
draw an IV graph for a filament lamp
S shape through he origin
draw an IV graph for a thermistor
flipped S shape
through the origin
if you have a graph of voltage on the y axis and current on the x axis what is the emf and the internal resistance?
emf is the y intercept
internal resistance is the negative gradient
How much does it cost to use an 800W microwave oven for 6 minutes? Electricity costs 16.1p per unit
6minutes = 0.1 hours 800W = 0.8kW
W = Pt = 0.8 X 0.1 = 0.08kWh
Cost = 0.08 X 16.1 = 1.288 = 1p (1sf)
Why should we try and reduce electricity use?
environmental impacts
Burning fossil fuels releases carbon dioxide into the atmosphere which adds to the greenhouse effect and global warming.
Sulphur dioxide is also released causing acid rain which damages plants and buildings.
Nuclear power stations produce radioactive waste which is dangerous to the environment and human health if not disposed safely.
Renewable energy sources like biofuels requires lots of land to produce them.
