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:
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)
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)
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 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
what are thermistors made up of?
SENSING
thermistors are made up of semiconductors whose conductivity change with temperature, as shown:
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)
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 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 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
what is the equation that allows you to choose resistances correctly to get the voltage output you want across a component?
SENSING
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 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
