Sensing Flashcards
What are electronic sensors?
Things designed to sense things we either can’t or don’t want to and so any change in whatever the sensors detecting will change the current in the connected circuit and the current is processed to give a reading
What is current?
The rate of flow of charged particles
(no. coulombs transferred per second)
What is the defining equation for current?
I = Q/t
How do you define one coulomb?
The amount of charge that passes a point in one second when the current is ones amp
How do you measure current and how do you connect it in circuit?
Ammeter attached in series
What direction does conventional current flow?
From positive to negative
(Conventional current is in the opposite direction to electron flow)
How you make electric charge flow through a conductor?
Do work on it
What is potential difference?
Energy transferred per unit charge
How do you define one volt?
When you convert one joule of energy moving one coulomb of charge through the component
How do you measure voltage and how is it connected in a circuit?
Voltmeter connected in parallel
What is power?
The rate of transfer of energy
How do you calculate power in electrical circuits?
P=IV
What is the defining equation of power?
P=W/t
What does the amount of current you get for a particular p.d. depend on?
Resistance
What is a simplified definition of resistance?
How difficult it is for current to flow
What is the defining equation of resistance?
R=V/I
What is power dissipation?
The rate at which a component converts electrical energy into other types of energy
What does a shallow gradient of an I-V graph mean?
Component has a high resistance
What does a curved line on an I-V graph mean?
The resistance changes with the potential difference across it
How do you investigate the I-V characteristic of a graph?
Use a variable resistor to alter the potential difference across a component and therefore its current, and then record V and I.
Repeat measurements and take averages to reduce the effects of random errors
Plot a graph of the results and you can then use the I-V graph to see how resistance changes
What are ohmic conductors?
Conductors that obey ohms law, meaning that resistance is always constant
What is Ohm’s law?
Provided external factors such as temp. are constant, the current through an ohmic conductor is directly proportional to the potential difference across it
Describe the I-V graph of an ohmic conductor
Straight line through the origin
Describe the I-V graph of a filament lamp?
A curve which starts steep and gets shallower as potential difference increases
Explain the shape of the I-V graph of a filament lamp?
Current flowing through the lamp increases its temperature so its resistance increases as current increases.
Filament lamp is just a coiled up length of metal wire so temp. increases resistance as metal ions in the wire vibrate more making it more difficult for electrons to flow
What is a thermistor and how are they used?
A resistor whose resistance depends on its temperature so they can be used as temp. sensors
How does the resistance of a NTC thermistor vary with temperature?
Resistance decreases as temp. increases
What is the shape of the I-V graph of a thermistor?
A curve with increasing gradient (starts shallow and gets steeper)
What is an LDR?
Light dependant resistor so the more light falls on it the lower its resistance
What is a diode?
Only allows current to flow in one direction
What direction do diodes allow current to flow?
In the direction that the circuit symbol points to
What is the shape of the I-V graph of a diode?
Nothing to the left of axis and then at around 0.6V (threshold voltage) a straight line graph
What three things determine the resistance of a wire?
length
cross sectional area
resistivity of the material
What does a longer wire mean for resistance?
Greater resistance as more difficult to get current to flow
What does a greater cross sectional area mean for resistance?
The wider it is the easier it is for electrons to flow so the lower the resistance
What is the resistivity of a material and its units?
The resistance of 1m length with a 1m2 cross sectional area (Ωm)
What is the conductivity of a material and its units?
The conductance of a 1m length with 1m2 cross sectional area (Sm-1, siemens per metre)
What is conductance?
The opposite of resistance so a measure of how easy it is for electrons to flow
How do you find the resistivity of a wire?
Measure the cross sectional area of the wire by measuring the diameter in three different places with a micrometer and taking an average and using A=πr2
Then connect the wire in a circuit with a voltmeter and ammeter and vary the length of wire connected to the circuit by moving one of the crocodile clips
Record the length of the wire and the ammeter and voltmeter readings
Then plot a graph of R against L and draw a line of best fit. The gradient = resistivity/area
What assumptions do you need to make about a wire when finding its resistivity?
The wire is cylindrical with uniform cross sectional area
What must be kept constant when finding the resistivity of wire?
Temperature
What does how conductive a material depend on?
The number density of mobile charge carriers
What makes a good conductor?
High density of mobile charge carries
Why are metals good conductors?
In a metal the charge carriers are delocalised electrons meaning they have a high density of mobile charge carriers
How does temperature affect the conductivity of metal and why?
As temp. increases the lattice vibrates more which increases the electron scattering so they are slightly less free to move meaning conductivity decreases (resistivity increases)
What are conductors?
Materials (such as metals) which have a high proportion of mobile charge carriers and so can conduct electricity well
What are semiconductors?
Materials with a low proportion of mobile charge carriers but often the no. increases with a factor such as light or temp which enables them to conduct electricity. However under normal conditions they do not conduct very well
What are insulators?
Materials with no or very few mobile charge carriers meaning they don’t conduct electricity
What is a perfect insulator?
Has no mobile charge carriers
What are some examples of semiconductors?
Thermistors - heat liberates electrons
LDRs - light liberates electrons
Where does resistance come from?
Electrons colliding with atoms and losing energy to other forms
What does a battery do?
Uses chemical energy to make electrons move
(converts chemical energy to electrical energy)
What is internal resistance?
When electrons collide with electrons inside the battery creating resistance. This causes batteries and cells to warm up when they’re used
What is load resistance?
The total resistance of all the components the external circuit
What is the e.m.f?
Electromotive force is the amount of electrical energy the battery produces per coulomb of charge
What is the terminal p.d?
The potential difference across the load resistance (the energy transferred per coulomb that flows through the load resistance)
When is terminal pd the same as the emf?
When there is no internal resistance
What are lost volts?
The energy wasted per coulomb in overcoming the internal resistance
What is emf equal to?
ε = V + v
V = terminal pd
v = lost volts
How do you work out the total emf of multiple cells in series?
Total emf is found by adding all the individual emus (given they’re all connected in the same direction)
How do you find the total emf of multiple identical cells in parallel?
For identical cells in parallel the total emf is the same size of the emf of each individual cell
What is an ideal ammeter and why?
Resistance is negligible so that the voltage across it is negligible so that the ammeter doenst affect the pd across other components
What is an ideal voltmeter and why?
Very high internal resistance so that the current through them is so low you can assume its negligible, this is so the voltmeter doesn’t affect the current through the component its connected to
Why do we include switches in circuits?
To turn off the current when circuit not in use to reduce the effect of heating in the wires on the resistance of the circuit
How can you investigate internal resistance and emf?
Set up a circuit with a battery, variable resistor, ammeter and voltmeter.
Use the variable resistor to change the value of the load resistance and measure the different pds (this value is the terminal pd) and currents
Plot a graph of V against I and using the equation V = ε -Ir we know the yintercept is ε and the gradiet is r
How do you decide the values for the load resistance when investigating emf and internal resistance?
A low load resistance will give a large current which will reduce percentage uncertainty in the ammeter reading but large currents causes significant heating the wires which will invalidate the results so you need a compromise
What is an easy way to measure the emf of a power source?
Connect a voltmeter across its terminals (voltmeter has negligible current so any difference between actual value and measured value will be so small its insignificant)
What is conserved in an electrical circuit?
Energy and charge
How is charge conserved in a circuit?
Charge doesn’t get used up or lost
What is Kirchhoff’s first law?
The total current entering a junction = total current leaving it
Charge is conserved
What is the difference between emf and potential difference?
emf is the energy transferred to charge and pd is the energy transferred from a charge
What is true about emf and pd in a closed loop?
They are the same (as energy is conserved)
What s Kirchhoff’s second law?
total emf around a series = the sum of the pds across each component
ε = ΣIR
In series circuit what is true of current, p.d and resistance?
current is the same at all points (as there are no junctions)
p.d. is split between components (Kirchhoff’s second law)
total resistance is the sum of all the resistance
Why is current the same at all points in a series circuit?
There are no junctions in a series circuit
What is true of current, pd and resistance in parallel circuits?
Current is split at each junction
Same pd across all components
1/Rtotal = 1/R1 + 1/R2 + …
Why do we use potential dividers?
To get a fraction of an input voltage
What is a potential divider put simply?
A circuit with a voltage source and a couple of resistors in series
What is a use of potential dividers?
Calibrating voltmeters
Why can’t resistors in a potential divider circuit have a low resistance?
Then you effectively have two resistors in parallel which will have a total resistance less than the resistance of the resistor which means Vout will be less than youve calculated
How can potential dividers be made into sensors?
Include components whose resistance changes with external factors eg LDRs, thermistors
What is a potentiometer?
Uses a variable resistor to give a variable voltage
When you make a potential divider that works as a sensor what must you do and why?
Calibrate the circuit so you know how the voltage across the component and Vout varies as external factors change
What goes on the axis of a thermistor calibration curve and how can it be used?
Plot temperature against voltage
You can then find the temperature if you know the voltage across it effectively making it a thermometer
Why is there a potential difference across the plates of a battery?
The two terminals have different charge built up on each plate which is stored as electrical potential energy meaning there is a potential difference
What is the mean drift velocity of electrons in a wire?
The average distance of an electron travelled along the wire per second. The electrons move slowly in one direction as they are constantly colliding
