Sensing

What is current?
Presence of protons and movement of electrons gives metals electrical properties. Current is flow of charged particles where I=dQ/dt and Q=Nq
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What's Kirchoff's 1st law?
In series charges with sane speed along same way so current is the same along all points. Kirchoff's first law - It = I1 + I2 - charge is conserved
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How is the cell charged?
In a cell chemical reactions produce an electrical potential energy difference. Repulsion at negative terminal results in electrons moving to +ve terminal.
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What's potential difference and it's formula?
Potential difference is potential energy difference per unit charge. W=VQ where work is dEnergy.
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What's joule heating?
Potential energy can be dissipated as heat in the wires of circuits, Joule Heating, where P=IV
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What are intensive properties?
Intensive properties resistivity and conductivity rely only on the material being used.
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What is formula for resistance linking resistivity?
Resistance in series adds so if length is doubled, resistance doubles. R=pL/A where p is resistivity.
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What is formula for conductance linking conductivity?
Conductance doubles in parallel so if A doubles, conductance G does. G=oA/L where o is conductivity.
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What scale is used to show conductivity and resistivity?
A logarithmic scale compares conductivity and resistivity and is very vast due to the range of values
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What should be considered when measuring conductivity or resistivity?
For an insulator to find conductivity, use small length L, large A, large PD, and sensitive ammeter. To find resistivity of metals use large A, small A, and beware of pd drops and Joule heating.
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What's Ohm's law?
Electrical resistance stops charges accelerating. R=V/I. I is proportional to V for a constant resistance in metals - Ohms law.
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What's conductance?
Metals are better conductors, conductance G=I/V.
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Why is PD constant in parallel?
PD is constant in parallel as charges travel down same potential hill.
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How are conductance and resistance added in parallel?
In parallel conductance adds and 1/RT = 1/R1 + 1/R2 ...
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What makes conductors non-Ohmic?
Wires dissipate energy through Joule heating which raises temperatures and resistance. Filament lamps are an example of a non-ohmic conductor. Known variation of R with T can make a sensor
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Define PD
Potential difference V in circuits is the potential energy difference per coloumb as charges move downhill
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Where are electrons released and absorbed?
In battery cells chemical processes liberate electrons at the cathode and consume them at the anode. Energy lifts electrons up potential hill.
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What's Kirchoff's 2nd law?
Kirchoffs 2nd law = emf E is the sum of all PDs across the resistance of a circuit
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What is EMF, r, and how does terminal PD fall?
Emf is the energy a source provides per coulomb of charge. Chemicals resist current in cell resulting in a PD drop (lost volts). As load R falls I increases and lost volts increases reducing terminal PD
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What is formula for emf?
E = V+Ir = I(R+r)
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What reduces reaction time?
Reaction time less when sensor is larger, plastic coated, has greater heat capacity and smaller A:V ratio
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What is potential divider formula ? What would make Vout increase?
Vout = Vin x R1/RT - for sensor circuit to have Vout increase with temperature use a fixed resistor for the potential divider, Choose the value for fixed resistor with greatest range - sensitivity
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How do you calibrate?
A sensor involves a variable and fixed resistor. Measure environmental values and compare to Vout to make calibrating graph. Modify scale of ammeter voltmeter and use marker for corresponding temperature
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What is sensitivity, resolution, and response time?
Sensitivity is change in Vout for an input. Resolution is the smallest change detectable. Use smallest change in V/sensitivity. Response time is time for a change in property to be obtained
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What are LDRs and thermistors?
Thermistors are semiconductors where with T rising electrons are liberated and resistance R falls rapidly. LDRs behave similarly as photons liberate electrons
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How are potential dividers used as sensors?
Variable resistors used to switch an electronic switch at a fixed voltage. First calibrate and then when V drops or rises switching point will be reached resulting in a switch being turned.
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What formula can be resolved to find drift velocity?
I = vAnq
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What are conductors and semi-conductors?
Electrical conductors don't have free charges. Glass has immobile ions so when melted conducts better. Semi-conductors have few free charges but as temperature rises more atoms are ionised and conductivity rises.
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Explain why resistance rises for conductors with temperature?
Metals consist of positive ions with delocalised electrons creating a metalic bond. Electrons have random velocities and drift when the PD is applied. As T rises KE of ion cores rises resulting in greater obstruction of electron paths.
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Other cards in this set

Card 2

Front

What's Kirchoff's 1st law?

Back

In series charges with sane speed along same way so current is the same along all points. Kirchoff's first law - It = I1 + I2 - charge is conserved

Card 3

Front

How is the cell charged?

Card 4

Front

What's potential difference and it's formula?

Card 5

Front

What's joule heating?