sensing Flashcards
how are most sensors powered?
SENSING
many sensors are powered using electricity
how do electronic sensors work? (simple)
SENSING
1) when using electronic sensors, any change in whatever the sensor is detecting will change the current in the connected circuit
2) the current is then processed to give a reading
what is current and its units?
SENSING
current is the rate of flow of charge[-d particles]
units: Amperes or Amps, A
you can also think of current as the measure of the number of charged particles that move past a point in a wire in a given time
what is the equation for current?
SENSING
I = ΔQ/Δt
what apparatus do you use to measure the current flowing through a part of a circuit, and what is its circuit symbol?
SENSING
you use an ammeter to measure the current flowing through a part of the circuit
the circuit symbol for an ammeter is:
what is the unit for charge and its symbol?
SENSING
the unit for charge is coulomb, C
1 coulomb is the amount of charge that passes a point in the circuit in 1 second when the current is 1 ampere
the symbol for charge is: Q
what is the conventional current and in what direction does it flow?
SENSING
conventional current is the direction scientists thought current flowed before discovering current is usually caused by the flow of electrons
conventional current flows from the positive terminal to the negative terminal of a power supply
what direction does unconventional current flow?
SENSING
unconventional current flows from the negative terminal to the positive terminal of a power supply
(unconventional current is how current actually flows in reality)
why does current actually flow via unconventional current
SENSING
current flows from the negative terminal to the positive terminal bc:
⋅ electrons are negatively charged
⋅ as opposites attract, this means electron will flow to positive terminal
what is the relationship between conventional current and electron flow?
SENSING
conventional current flows in the opposite direction to electron flow
what is potential difference, its symbol and its units?
SENSING
potential difference is the energy converted[/energy transferred/work done] per unit charge moved from one point in a circuit to another
units: Volts, V
what do you need to make electric charge flow through a conductor?
SENSING
to make electric charge flow through a conductor, you need to do work on the charge
what is the equation for potential difference?
V = ΔE/Q
or
V = W/Q
where
V = voltage or potential difference (they’re interchangeable)
E = energy transferred in moving charge
W = work done in moving charge
Q = charge
what is 1 volt?
SENSING
1 volt = when you convert 1 joule of energy in moving 1 coulomb of charge through a component
what apparatus is used to measure voltage[/pd] and what is its circuit symbol?
SENSING
a voltmeter is used to measure voltage[/pd]
the circuit symbol for a voltmeter is:
where should a voltmeter be connected in a circuit and why?
SENSING
a voltmeter should be connected in parallel with the component you’re investigating the pd across
• this is bc the pd across components in parallel is the same
• so when the voltmeter is connected in parallel to the component, both the component and the voltmeter experience the same pd across them
are potential difference and voltage usually interchangeable?
SENSING
yes
how do you describe the pd and the current for a certain component?
SENSING
⋅ there is a potential difference ACROSS the component
⋅ there is a current flowing THROUGH the component
what is power, its symbol and its units?
SENSING
power is the rate of the transfer of energy (or rate of work done bc work done == energy transfer)
the symbol for power is P
the units for power is: Watts, W or Joules per second, J s^-1
what is 1 watt equal to?
SENSING
1 watt = 1 joule per second
what is the equation for power?
SENSING
P = W/t
or
P = E/t
where:
P = power
W = work done
E = energy transferred
t = time
what is the equation for power in ELECTRICAL CIRCUITS?
SENSING
P = IV
where:
P = power
I = current
V = voltage
how can you explain the equation P = IV
SENSING
P = IV is explained by the fact that:
⋅ pd is the energy transferred per coulomb [moved…]
⋅ current is the number of coulombs transferred per second
⋅ so therefore pd x current is the energy transferred per second, i.e. power
I x V = E/Q X Q/t = E/t (bc Qs cancel out)
what makes it easier when calculating energy?
SENSING
• energy is easy to calculate if you know the power
• bc P = E/t
what is the equation for total energy transferred (aka total work done)?
SENSING
total energy transferred (aka total work done) = power x time
W = Pt
so
W = VIt
does everything have resistance?
SENSING
yes
what happens if you put pd across an electrical component?
SENSING
if you put pd across an electrical component, current will flow
what determines how much current flows through a component for a particular pd?
SENSING
how much current flows through a component for a particular pd depends on the resistance of the component
what is resistance, its symbol and its units?
SENSING
resistance is the measure of how difficult is it is for a current to flow through a component
the symbol for resistance: R
the unit for resistance is: Ohms, Ω
when does a component have a resistance of 1 Ω?
SENSING
• a component has a resistance of 1 Ω if a pd of 1 V across the component makes a current of 1 A flow through the component
• think of R = V/I
what is the equation for resistance?
SENSING
R = V/I
(the equation DEFINES resistance also)
or
V = IR
where:
R = resistance
V = voltage
I = current
what is the inverse of resistance?
SENSING
the inverse of resistance is conductance
what is conductance, its symbol and its units?
SENSING
conductance is the measure of how easy it is for a current to flow through a component
the symbol for conductance is: G
the unit for conductance is: Siemens, S or Ω^-1
what is the equation for conductance?
SENSING
G = 1/R
so
G = I/V
what other equations of power can you obtain by subbing V = IR into P = IV?
SENSING
P = (I^2)R
(this equation works out power dissipation)
P = (V^2)/R
what is power dissipation and what is the equation used to work it out?
SENSING
power dissipation is the rate at which a component converts electrical energy into other types of [unwanted] energy (eg, heat)
the equation used to work out power dissipation is:
P = (I^2)R
how can you minimise power dissipation when transmitting mains electricity?
SENSING
to minimise power dissipation when transmitting mains electricity, mains electricity is transmitted at a high voltage (so current would lower, provided resistance stays constant) (+ low I) to minimise power dissipated during transmission (bc I has lowered due to high voltage, P lost as power dissipation would decrease also)
what do I-V characteristic graphs show? (simple)
SENSING
I-V characteristic graphs show how the resistance of a component varies
what do I-V characteristics show? (longer)
SENSING
⋅ ‘I-V characteristic’ refers to a graph which shows how the current (I) flowing through a component changes as the pd (V) across the component is increased
how can you investigate the I-V characteristic of a component? (simple)
you can investigate the I-V characteristic of a component using a test circuit like this one:
INPUT DIAGRAM
how can you investigate the I-V characteristic of a component? (long/method)
SENSING
to investigate the I-V characteristic of a component using a test circuit:
1) set up equipment as shown below:
2) use a variable resistor to alter the pd across the component and thus the current flowing through the component
3) record varying pds and their corresponding currents in a table
4) take multiple measurements for a particular pd and take an average to reduce the effect of random errors on your results
5) plot a graph of current against pd from your table
6) this graph is the I-V characteristic of the component and you can use it to see how the resistance of a component changes
INSERT DIAGRAM
for an ohmic conductor, does resistance change?
SENSING
for an ohmic conductor, R is constant
what are ohmic conductors and can you include examples?
SENSING
ohmic conductors are conductors that obey ohm’s law
ohmic conductors are mostly metals and examples include: metal wires and resistors
what is Ohm’s law?
SENSING
Ohm’s law states that:
“Provided external factors such as temperature are constant, the current through an ohmic conductor is directly proportional to the potential difference across it.”
what is Ohm’s law in formula form?
SENSING
V ∝ I
so
V = kI
which is just
V = IR
can you show and describe the I-V characteristic of an ohmic conductor?
SENSING
description of I-V characteristic of an ohmic conductor:
⋅ as you can see from graph, doubling the pd doubles the current (bc pd ∝ I according to ohm’s law)
⋅ the gradient is constant, which means that resistance is constant
notes:
⋅ remember, Ohm’s law is only true for ohmic conductors where external factors like temperature are constant
⋅ non-ohmic conductors don’t have this correlation (ohm’s law) between current and pd
can you show and describe the I-V characteristic graph of a filament lamp?
SENSING
description of the I-V characteristic graph of a filament lamp:
⋅ the I-V characteristic graph for a filament lamp is a curve, which starts steep but gets shallower as pd rises
⋅ as the current flowing through the lamp increases, the temperature of the lamp will increase, so the lamps resistance will increase (which is shown by curve becoming more shallow as current increases)
note:
⋅ a filament lamp does NOT have the same I-V characteristic graph as a metal conductor even though the filament in a filament lamp is just coiled up metal wire
what is a thermistor and what is its circuit symbol?
SENSING
a thermistor is a semiconductor with a resistance that depends on the temperature
a thermistor’s circuit symbol is:
[you only need to know about NTC (Negative Temperature Coefficient) thermistors]
what does a thermistor’s resistance depending on the temperature mean they can be used as?
SENSING
thermistors can be used as temperature sensors bc their resistance depends on the temperature
how does the resistance of an NTC thermistor change with temperature?
SENSING
the resistance of an NTC thermistor decreases as temperature increase (hence the NEGATIVE temperature COEFFICIENT, (-)coefficient acts as gradient)
how can you tell from an I-V characteristic that the resistance of an NTC thermistor decreases as temperature increases?
SENSING
⋅ if you increase the current through a thermistor (or any component), it increases the temperature of the component
⋅ if you look at the graph, the increasing gradient of the I-V characteristic tells you that resistance is decreasing as current increases
linking stuff together to help understanding:
[⋅ so if temp increases as current increases and resistance decreases (bc R is high when grad is low) as current increases, this means the resistance of the thermistor decrease as the thermistor heats up]
what is a light dependant resistor (LDR) and what is its circuit symbol?
SENSING
a LDR is a semiconductor with a resistance that depends on the amount of light falling on it
⋅ the more light that falls on it, the lower the LDR’s resistance
an LDR’s circuit symbol is:
how do thermistors + LDRs work? (electrons being liberated)
what are diodes and can you show their circuit symbol?
SENSING
diodes are components that let current flow in only one direction (their forward bias)
an example of a diode is a light emitting diode (LED)
what is the forward bias of a diode?
SENSING
⋅ the forward bias of a diode is the direction that the diode allows current to flow
⋅ a diode’s forward bias is the direction that the triangle of its circuit symbol points in
what do most diodes need before they will conduct?
SENSING
most diodes require a threshold voltage of about 0.6 V in the forward direction (direction of the forward bias) before they will conduct
can you show and describe the I-V characteristic of a diode?
SENSING
description of the I-V characteristic of a diode:
⋅ in reverse bias (the opposite direction to which a diode lets current flow) the resistance of the diode is very high and any current that flows in reverse bias is very tiny (it’s negligible)
⋅ the graph starts increasing once the threshold voltage is reached (once x-axis passes certain value)
what three things does the resistance of a simple electrical component (eg, length of wire) depend on and why?
SENSING
the resistance of a material depends on:
⋅ length, L: the longer the component, the more difficult it is to make a current flow through the component
⋅ cross-sectional area, A: the wider the component, the easier it will be for electrons to pass along it (bc less of component for electrons to collide with)
⋅ resistivity, ρ: the resistivity of the component depends on the material the component is made from, as the structure of the material may make it easier or more difficult for a charge to flow
what is resistivity, what is its symbol and what is its unit?
SENSING
the resistivity of a material is the resistance of a 1 m length of the material with a 1 m^2 cross-sectional area
the symbol for resistivity is: ρ
the unit for resistivity is: Ohm metre, Ω⋅m
(NOT OHMIC METRE)
what is the equation for resistivity?
SENSING
resistivity = (resistance x cross-sectional area)/length
ρ = RA/L
or
RA = Lρ (remember “ralp”)
what size are typical values of resistivity for conductors?
SENSING
typical values of the resistivity for conductors are really small
eg) copper (at 25 °C) resistivity = 1.72 x10^-8 Ohm metres
does the resistivity of a material depend on the temperature the material is currently at?
SENSING
• yes
• the resistivity of the material depends on its temperature, so you can only find the resistivity of the material for/at that certain temperature
what is the inverse of resistivity?
SENSING
the inverse of resistivity is conductivity
σ = 1/ρ
what is conductivity, its symbol and its units?
SENSING
the conductivity of a material is the conductance of a 1 m length of the material with a 1 m^2 cross-sectional area
the symbol for conductivity is: σ
the unit for conductivity is: Siemens per metre, S m^-1
what is the equation for conductivity?
SENSING
conductivity = (conductance x length)/cross-sectional area
σ = GL/A
or
GL = Aσ (remember “glass”)
(bc σ = sigma, makes “ss” noise)
what do you need to find to find the resistivity of a wire?
SENSING
to find the resistivity of a wire you need to find its resistance first
how to find the resistivity of a wire? (method)
SENSING
1) set out circuit as shown below
2) using MICROMETER, measure DIAMETER of wire in at least 3 different points along its length + take an AVERAGE. using ave diameter, find cross-sectional area of wire using A = πr^2
3) CLAMP TEST WIRE to ruler, with circuit attached to wire where ruler reads zero
4) attach FLYING LEAD to test wire (flying lead is just wire with crocodile clip on end to allow connection to any point along test wire)
5) record LENGTH of test wire CONNECTED in circuit, VOLTMETER READING + AMMETER READING
6) use your readings to calculate RESISTANCE of length of wire, using R = V/I
7) repeat this measurement at least 3 times + calculate average resistance for length
8) repeat for several DIFFERENT lengths, eg) between 0.10 and 1.00 m
9) plot results on graph of R against L, + draw LINE OF BEST FIT
10) GRADIENT of line of best fit R/L is equal to ρ/A, so MULTIPLY GRADIENT of line of best fit by CROSS-SECTIONAL AREA of wire to find resistivity of wire material
note:
⋅ other components of circuit also have resistance, but gradient of line of best fit isn’t affected by resistance within rest of circuit (bc it would be zero error?, + zero errors don’t affect line of best fit gradients bc effects all data points by same amount)
⋅ current flowing in test wire can cause temperature to increase, so you need to try to keep temperature of test wire CONSTANT to calculate correct resistivity (eg. by only having small currents flow through wire) bc resistivity is affected by temperature
what experiment do you conduct to find the conductivity of a wire?
SENSING
do the same experiment as when you are finding the resistivity of a wire, but then do 1/(final calculated resistivity) bc σ = 1/ρ
why is finding the ρ for a single length of wire using the resistivity formula a bad idea?
SENSING
you could find the ρ for a single length of wire, using the resistivity formula, but this will give you an overestimate as it’ll include the resistance of the ammeter, all the wires, and their connections
how does the number of charge carriers in different materials change?
SENSING
different materials have different numbers of charge carriers
what determines how conductive a material is?
SENSING
⋅ how conductive a material is depends on its number density of mobile charge carriers (aka charge carrier density)
⋅ the more mobile charge carriers a material has per unit volume, the better a conductor it will be
what is charge carrier density (aka number density of mobile charge carriers)
SENSING
charge carrier density is the number of mobile charge carriers (usually free electrons or ions that are free to move) there are per cubic metre of material
describe the quality of metals as conductors
SENSING
⋅ in metals, the charge carriers are free (aka delocalised) electrons
⋅ metals are good conductors bc they have a high number density of mobile charge carriers
how does increasing the temperature of a metal affect the number of its mobile charge carriers
SENSING
if you increase the temperature of a metal, the number of mobile charge carriers stays mostly constant
what happens to the resistivity of a metal as temperature increases?
SENSING
as the temperature increases, the resistivity (thus resistance bc resistance depends on resistivity) of a metal slightly increases and the conductivity (thus conductance - same logic as before) slightly decreases
describe the quality of semiconductors as conductors
SENSING
⋅ in semiconductors, the mobile charge carriers are free electrons
⋅ semiconductors have a much lower charge carrier number density (so as they have fewer free electrons than metals, they have a lower conductivity)
how does increasing the temperature of a semiconductor affect the number of its mobile charge carriers
SENSING
⋅ as you increase the temperature of a semiconductor, more electrons are freed to conduct
what happens to the resistivity and conductivity of a semiconductor as temperature increases?
SENSING
⋅ bc as you increase the temperature of a semiconductor, more electrons are freed to conduct
⋅ this means that as temperature increases, the conductivity of the semiconductor rapidly increase
describe the quality of insulators as conductors
SENSING
perfect insulators wouldn’t have any mobile charge carriers, so the insulator would not be able to conduct at all
do batteries have an internal resistance?
SENSING
yes
what are thermistors made up of?
SENSING
thermistors are made up of semiconductors whose conductivity change with temperature, as shown:
what are LDRs made up of?
⋅ LDRs are made up of semiconductors whose conductivity is mostly controlled by light (rather than heat like thermistors)
⋅ the semiconductors’ conductivity increases with increasing light levels
where does most electrical resistance come from?
SENSING
resistance in general comes from electrons colliding with other atoms and losing energy to other forms
where does internal resistance in batteries / power supplies come from?
SENSING
⋅ in a battery, chemical energy is used to make electrons move
⋅ as the electrons move, they collide with atoms inside the battery
⋅ so batteries must have resistance
why do batteries and cells warm up when they’re used?
SENSING
internal resistance - when electrons collide with other atoms in a battery, they lose energy as heat
what is the circuit symbol for a resistor?
SENSING
what is the circuit symbol for batteries that include internal resistance?
SENSING
remember to draw the box around the battery and the resistor next to it, to signify the resistor is the internal resistance (box can also be drawn as a box with a border of short intermittent lines that act as dots)
what is the EMF, its symbol and its unit?
SENSING
EMF (electromotive force) is:
⋅ the amount of electrical energy the battery produces for each coulomb of charge
⋅ the total power/energy input by the power supply
⋅ the total energy gained by the electrons [when passing through the power supply]
⋅ the electromotive force is the terminal potential difference when the cell is in an open circuit (no current flows)
the symbol for the emf is: ε
the unit for the emf is: Volts, V
is the emf (electromotive force) an actual force?
SENSING
NO
what is the potential difference across the load resistance?
SENSING
⋅ the pd across the load resistance is called the terminal pd
⋅ the potential difference across the load resistance is the energy transferred when 1 coulomb of charge flows through the load resistance
what is load resistance, R?
SENSING
⋅ load resistance (R) is the total resistance of all the components in the external circuit
⋅ (so basically R of circuit sans internal resistance of power supply)
what would happen if there was no internal resistance in a circuit? and why can this not actually happen?
SENSING
if there was no internal resistance (so an ideal power supply), terminal pd would be the same as emf
⋅ however, in real power supplies, there’s always some energy lost (as heat energy) in overcoming the internal resistance
what are lost volts, v?
SENSING
lost volts (v) is the energy wasted per coulomb in overcoming the internal resistance [of power supply]
state the law of the conservation of energy, and knowing the law, what does it tell us about the voltage in a circuit?
SENSING
the conservation of energy tells us that:
the energy per coulomb [voltage] supplied by the source = the energy per coulomb [voltage] transferred in the load resistance + energy per coulomb [voltage] wasted in the internal resistance
emf = terminal pd + lost volts
ε = V + v
state all the equations for emf
SENSING
ε = V + v or V = ε - v
ε = I(R + r)
V = ε - Ir
⋅ all of these equations can be used to work out each other, knowing that V = IR and v = Ir
how do you work out the total emf of multiple cells in series [circuit]?
SENSING
⋅ to work out the total ε in a series circuit of emfs:
total ε = ε1 + ε2 + ε3 + …
(bc each charge goes through each of the cells [power supplies] + so gains ε [electrical energy] from each one
⋅ this requires all the cells to be connected in the same direction
⋅ if one is connected in the opposite direction, you subtract its emf from the total ε rather than adding it
how do you work out the total emf of multiple cells in parallel [circuit]?
SENSING
⋅to work out the total ε for identical cell in parallel:
total ε = ε1 = ε2 = ε3 = …
(bc current will split equally between identical cells [remember rules of how I and V slow in parallel circuits], a charge only gains ε from cells it travels through - so the overall ε in the circuit doesn’t increase)
⋅ the total ε of the combination of the parallel cells is the same size as the ε of each of the individual cells
how to investigate the internal resistance of a battery? (method)
SENSING
1) set up circuit as shown
2) VARY CURRENT in circuit by changing value of LOAD RESISTANCE (R) using variable resistor + MEASURE PD (V) for several different values of CURRENT (I)
⋅ include SWITCH in your circuit to TURN OFF current whenever possible to REDUCE effect of HEATING in wires on RESISTANCE of circuit
3) record your data for V and I in table + PLOT RESULTS in graph of V against I
4) to find EMF and INTERNAL RESISTANCE: y-intercept of graph = ε and m = -r (so to work out actual internal resistance, multiply gradient by -1)
⋅ analysing the graph:
1) to find EMF and internal resistance of cell, start with equation: V = ε - Ir
2) rearrange to give V = -rI + ε (this is in form y = mx + c, where y = V, m = -r, x = I and c + ε)
(we can draw this equation as equation of straight line bc ε and r are constants)
3) y-intercept of graph = ε and m = -r (so to work out actual internal resistance, multiply gradient by -1)
what are some difficulties you may encounter when investigating the emf and the internal resistance of a battery?
SENSING
an example of a difficulty you may encounter when investigating the internal resistance of a battery is that: ⋅ choosing values for the load resistance is difficult
1) a low load resistance will give you a large current, which will then reduce the percentage uncertainty in an ammeter reading of the current
2) but large currents will also cause significant heating in wires, which will invalidate your results (bc heating will change R of wires, thus changing R of load resistance which will invalidate your results)
3) so a middle ground must be reached
what are some assumptions that you make when investigating the emf and the internal resistance of a battery?
SENSING
some of the assumptions that you make when investigating the internal resistance of a battery are:
1) the voltmeter has a very high internal resistance, so the current through the voltmeter is so low it’s negligible (assume it’s zero)
⋅ this makes sure that the voltmeter in the circuit does not affect the current through the variable resistor
⋅(bc current won’t want to flow through smth with really high R (V = IR -> I = V/R so high R will result in small current), so almost all current flows through variable resistor + so voltmeter doesn’t affect current)
2) the resistance of the ammeter is very low (negligible), so the pd across it is negligible
⋅ this makes sure that including the ammeter in the circuit doesn’t affect the pf across the variable resistor)
what is an easier way to investigate the emf of a power source?
SENSING
⋅ an easier way to measure the emf of a power source is by connecting a voltmeter across the power source’s terminals
⋅ as in the other experiment, the current through the voltmeter is assumed to be negligible and so any difference between your measurements and the emf will be so small that the difference isn’t usually significant
is charge in a closed circuit conserved?
SENSING
⋅ yes, charge is conserved - it does not ‘leak away’
what happens to charge at a junction in a closed circuit and what does this logic make up the basis of?
SENSING
⋅ as charge flows through a closed circuit, it cannot be used up or lost
⋅ this means that whatever charge flows into a junction will flow out
⋅ since the current is the rate of flow of charge, it makes sense that whatever current flows into a junction is the same as the current flowing out of it
⋅ this logic makes up the basis for Kirchhoff’s first law
what is kirchhoff’s first law?
SENSING
kirchhoff’s first law states:
“the sum of the current entering a junction is equal to the sum of the current leaving the junction”
what is kirchhoff’s first law about?
SENSING
kirchhoff’s first law is about the law of the conservation of charge
what happens to energy in a closed circuit?
SENSING
⋅ in closed electrical circuits, energy is transferred around the circuit
⋅ the energy transferred to the charge is the emf…
⋅ …and the energy transferred from the charge is the pd
⋅ so in a closed circuit energy is conserved
⋅ so in a closed loop:
energy transferred to a charge and the energy transferred from the charge must both be equal to each other if energy is to be conserved
what is kirchhoff’s second law?
kirchhoff’s second law states that:
“the total emf around a series circuit is equal to the sum of the potential differences across each component”
or the law can be written as ε = ΣIR
what is kirchhoff’s second law about?
SENSING
kirchhoff’s second law is about the law of the conservation of energy
how do you combine currents, voltages, resistances or conductances in a series circuit?
in a series circuit:
1) there is the same current at all points of the series circuit (since there are no junctions), so :
total I = I1 = I2 = I3 = …
2) the emf is split between components in series circuit (by kirchhoff’s second law), so:
ε = V1 + V2 + V3 + …
or
total V = V1 + V2 +V3 + …
3) bc V = IR if I is constant, results in IR = IR1 + IR2 + IR3 + … (basically subbed V = IR into total V equation), you can then cancel out the Is bc I is constant in a series circuit, so:
total R = R1 + R2 + R3 + …
4) 1/total G = 1/G1 + 1/G2 + 1/G3 + …
(remember to do reciprocal of 1/total G to get total G)
what can you use a potential divider for?
SENSING
you use a potential divider to get a fraction of the input voltage in some areas of the circuit
what is a potential divider at its simplest?
SENSING
at its simplest, a potential divider is a circuit with a voltage source and a couple of resistors in series
how is the pd of the voltage source (eg, power supply) divided in a potential divider circuit, how can this be written in equation form and what does this mean?
EMG
⋅ the pd of the voltage source is divided into the ratio of resistances in a circuit
⋅ this can be written as V1/R1 = V2/R2
⋅ or as V1:R1 = V2:R2
⋅ this means that you can choose resistances to get the voltage output you want across one of the components
what is the equation that allows you to choose resistances correctly to get the voltage output you want across a component?
SENSING
what can a potential divider be used for in a more practical sense?
SENSING
a potential divider circuit can be used for calibrating voltmeters, which have a very high resistance
what should you NOT connect across R2 in a potential divider circuit?
SENSING
⋅ DO NOT connect something with a RELATIVELY LOW RESISTANCE across R2
⋅ if you do, the component and R2 will effectively act as 2 resistors in parallel
⋅ two resistors in parallel will always have a total resistance that is LESS than R2
⋅ so the actual V out will be less than what you’ve calculated and will depend on what is connected across R2
what can potential dividers be made into and give examples?
SENSING
⋅ potential dividers can be made into sensors by including components whose resistances change with external factors (like LDRs and thermistors)
⋅ this would mean that V out varies with light or heat, so you can make a potential divider that works as a light or heat sensor
⋅ if you add an LDR to the potential divider, you can make a light sensor
⋅ if you add a thermistor to the potential divider, you can make a heat sensor
what must you do first before conducting an experiment that uses a potential divider?
SENSING
⋅ before you conduct an experiment that uses a potential divider, you must first calibrate the circuit so you know how the voltage across the component and the V out varies as external factors change
⋅ eg) knowing the voltage across the thermistor at a given temperature
how do you conduct an experiment to calibrate an electronic thermometer (using a potential divider circuit that includes an NTC thermistor)? (method)
1) set up equipment as shown, then measure TEMPERATURE of water using THERMOMETER, + record VOLTAGE across resistors
⋅ pick fixed resistor carefully - if its resistance is too high, V out won’t vary enough with temperature, + if it’s too low, V out might vary over bigger range than your voltmeter can handle
2) HEAT beaker GEANTLY using bunsen burner (make sure water is well-stirred), + record temperature + voltage at REGULAR INTERVALS over SUITABLE RANGE (eg. at 5 °C intervals over range of 0-100 °C)
3) plot GRAPH of temperature against voltage from your results - this graph is thermistor’s CALIBRATION CURVE + you can use it to find temperature of thermistor from voltage across it, without needing thermometer
how do potentiometers work?
SENSING
⋅ a potentiometer uses a variable resistor to give a variable voltage
⋅ a potentiometer has a variable resistor instead of the R1 and R2 of a potential divider, but it uses the same idea
⋅ sometimes people call a potentiometer a potential divider too (even though it isn’t archetype one) just to confuse things, so be wary
what is an example of a potentiometer circuit in a practical setting?
what do filament lamps, thermistors, LDRs and diodes all have in common and how can you tell this?
⋅ they are all non-ohmic conductors
⋅ you can tell by looking at their I-V characteristic graphs: their graphs are not linear and/or they do not go through the origin so the I and V are not directly proportional
⋅ for things to be directly proportional, the graph’s line must be linear, go through the origin, and have the variables change in the same way (eg. increase in x, increase in y OR decrease in x, decrease in y)
what is a water analogy that is helpful for understanding current, voltage and resistance and the relationships between them?
concepts of them:
⋅ think of current in wire like flow rate of water in pipe
⋅ just as flow rate is measure of how much water goes through pipe in given time interval, current is measure of number of charged particles that move past point in wire in given time
⋅ for pd, pd is like pressure that’s forcing water along pipe
relationships:
⋅ so knowing V = IR and after looking at picture, you can understand that:
⋅ I ∝ 1/R
⋅ and V ∝ I
⋅ but it does NOT mean V ∝ R
what does the the gradient of an I-V characteristic show?
SENSING
⋅ the shallower the gradient of the I-V characteristic, the greater the resistance of the component (bc less current is flowing through the current for a particular voltage, so current increases only a small amount as voltage increases, meaning it is MORE DIFFICULT for current to flow through component = high resistance)
⋅ a curved I-V characteristic line shows that the resistance of a component changes as the pd across it changes
how do you calculate resistance from an I-V characteristic?
SENSING
• pick a point on the line of the graph
• read the current and the voltage at this point from the graph
• divide the voltage by the current
• this gives you the resistance of the component AT THIS POINT
• the resistance of a component is not the gradient of the whole graph (bc R doesn’t stay constant sometimes, which is reflected in I-V characteristic graphs curving)
why does the resistivity of a metal increase as temperature increases?
SENSING
1) as electrons move, they scatter from the metallic lattice
2) as the temperature increases, the lattice vibrates more, increasing electron scattering, so electrons are slightly less free to move (bc now they keep bumping into each other)
3) this means that as the temperature increases, the conductivity of the metal will slightly decrease (thus resistivity will slightly increase)
why does the conductivity of a semiconductor increase as the temperature increases?
SENSING
1) just as in metals, the semiconductor’s atomic lattice will vibrate more, scattering free electrons as it moves
2) but its ‘increase in resistivity’ effect is much smaller than the huge increase in mobile charge carriers allowing more current to flow (also due to the increase in temperature)
work out v i r g in parallel circuit
