3: Sensing Flashcards
Describe an electronic sensor
Any change in the sensor’s detecting will change the current in the connected circuit. The current is processed to give you a reading
What is current?
The rate of flow of charge
Like the rate of flow of water in a pipe
Describe conventional current and electron flow
Conventional: Flows from positive to negative terminal of a power supply.
Electrons are negatively charged and flow from negative to positive terminals - so conventional current is in the opposite direction to electron flow
What is potential difference?
The energy converted per unit charge moved - to make electric charge flow through a conductor you need to do work on it
(Like the pressure forcing water through the pipe)
Do you connect a voltmeter in parallel or series. Why?
Parallel because the p.d across components in parallel is the same
What is power?
The rate of transfer of energy (the rate of work done)
If you put a p.d across an electrical component….
A current will flow
What is resistance?
A measure of how difficult it is to get a current to flow through a component
How can you reduce the power dissipated during transmission of mains electricity?
P=IV
Mains electricity is transmitted at a high voltage and low current to minimise the power dissipated
What does the term ‘I-V characteristic’ mean?
Refers to a graph which shows how the current flowing though a component changes as the p.d across it is increased
The shallower the gradient of a characteristic I-V graph, the [ ] the resistance of the compoenent
Greater
What does a curved line on an I-V graph mean?
Resistance of the component changes with the p.d across it
How can you investigate the I-V characteristic of a component using a test circuit?
1) Use a variable resistor to alter the p.d across the component and the current flowing through it, record V and I
2) Plot a graph of current against p.d difference from your results. This graph is the I-V characteristic of the component
If a conductor obeys Ohm’s Law what it it called?
An Ohmic conductor
What is Ohm’s Law?
Provided the external factor such as temperature are constant, the current through an ohmic conductor is directly proportional to the p.d across it (V=IR)
The gradient of the IV graph is constant (so resistance is constant) and the graph goes through the origin
Describe the I-V characteristic of a filament lamp
Why is it this shape?
A curve, which starts steep but then gets shallower as the p.d rises
Current flowing though the lamp increases its temperature, so its resistance increases
Draw the I-V characteristic and circuit symbol for a filament lamp
Symbol: Cross in a circle
Graph: In 1st quadrant - steep then flat (like y = √x) and opposite of that in 3rd quadrant
Describe the relationship between a thermistor’s resistance and temperature
As the temperature increases the resistance decreases
What is the circuit symbol for a thermistor?
And what is the I-V characteristic?
Resistor symbol with a line going diagonally across it
1st quadrant: curve where the gradient increases. The 3rd quadrant is the opposite
Why can thermistors be used as temperature sensors?
A thermistor is a resistor with a resistance that depends on its temperature
What type of thermistor do we look at?
NTC negative temperature coefficient, as the temperature increases the resistance decreases
Why does the resistance of a thermistor decrease as the temperature increases?
Increasing the current through the thermistor increases the temperature. As the thermistor heats up, the resistance decreases
Describe an LDR and its circuit symbol
Light dependent resistor, sensitive to light
The more light falls on it, the lower the resistance
Symbol: resistor in a circle with 2 arrows outside the circle, pointing towards centre of circle
Describe diodes
They are designed to let current flow in one direction only
What does forward bias mean when talking about diodes?
The direction in which the current is allowed to flow
Most diodes require a [ ] voltage of about 0.6 V in the [ ] direction before they will conduct
Threshold
Forward
What happens in reverse bias with diodes?
The resistance of the diode is very high and the current that flows is very tiny
Describe the circuit symbol of a diode and a LED
A diode is a triangle, side on with a vertical line touching the point on the right, in a circle
A LED is a diode with 2 arrows pointing away from the circle, outside the circle
Describe the IV characteristic of a diode
Slightly negative current before threshold voltage where the current increases in a rough straight line
What does the resistance of a length of wire depend on? Explain each one
1) Length. The longer the wire, the more difficult it is to make a current flow
2) Area. The wider the wire, the easier it will be for the electrons to pass along it
3) Resistivity. This depends on the material the wire’s made from, as the structure of the material may make it easy or difficult for charge to flow. Resistivity also depends on external factors like temperature
This experiment finds the ‘X’ of a wire:
Measure the cross sectional area of a wire
Clamp the wire to a ruler with the circuit attached to the wire and attach flying leads to wire
Record length of the wire, the voltmeter reading, and the ammeter reading
Calculate the resistance
What is X?? How do you calculate X, given this data? How do you make the experiment more accurate?
X = resistivity
Repeat this measurement and calculate an average resistance for the length
Repeat with several different lengths
Plot your results on a graph of R against L, and draw a line of best fit
Multiply the gradient by the area to get Resistivity
Explain how to find the cross sectional area of a wire
Assume the wire is cylindrical, and so it’s cross section is circular
Use a micrometer to measure the diameter of the wire in at least 3 different points along its length. Take an average value of the diameter and divide by 2 to get the radius
What it a flying lead? (Used when investigating Resistivity)
The lead is a wire with a crocodile clip at the end to allow connection to any point along the test wire
When calculating the Resistivity of a wire, does it matter that other components in the circuit have resistance?
No, the gradient of the line of best fit (R against L) isn’t affected by the resistance within the rest of the circuit
Describe the affect of temperature on the experiment to find Resistivity. What do you do in the experiment to mitigate this?
The Resistivity of a material depends on its temperature, so you can only find the Resistivity of a material at a certain temperature
Current flowing in the test wire can cause its temperature to increase, so you need to try and keep the temperature of the wire constant, eg. By only having small currents flow through the wire
How can you find the conductivity of a wire using the experiment to find Resistivity?
σ = 1/ρ so just use the same data
What affects how conductive a material is?
It’s number density of mobile charge carriers - the number of free electrons (or ions that are free to move) there are per cubic metre of the material. The more mobile charge carriers a material has per unit volume, the better a conductor it will be
What are the charge carriers in metals?
Free electrons
Why are metals good conductors?
They have loads of free electrons (charge carriers). The number density of mobile charge carriers is high
Explain how a metal’s conductivity is affected by temperature
If you increase the temperature of a metal, the number of mobile charge carriers stays about the same.
As the electrons move, they scatter from the metallic lattice
As the temperature increases the lattice vibrates more, increasing the electron scattering, so the electrons are slightly less free to move
This means that as the temperature increases, the conductivity of a metal will slightly decrease
What are the charge carriers in semiconductors?
Free electrons
Compare the density of charge carriers of metals and semiconductors
Semiconductors have a much lower charge carrier number density (fewer free electrons) than metals, so they have a lower conductivity
Explain how temperature effects the conductivity of a semiconductor
As you increase the temperature, more electrons are freed to conduct. This means that as the temperature increases, the conductivity of a semiconductor rapidly increases
Just as in metals, the semiconductor atom lattice will also vibrate more, scattering the free electrons as they move, but its effect is much smaller than the effect of the huge in increase in charge carriers
What are thermistors and LDRs made out of? What does this mean about their conductivity?
Semiconductors
In thermistors, as temperature increases the conductivity rapidly increases, just like a semiconductor
In LDRs there are semiconductors whose conductivity is mostly controlled by light rather than heart, their conductivity increases with increasing light levels.
Describe the mobile charge carriers in an insulator
A Perfect insulator wouldn’t have any mobile charge carriers, so it wouldn’t be able to conduct at all
Resistance comes from [ ] colliding with [ ] and losing their [ ] to other forms
Electrons
Atoms
Energy
Why do batteries have resistance? What is it called?
In a battery, chemical energy is used to make electrons move. As they move, they collide with atoms inside the battery.
Internal resistance
[ ] is what causes batteries and cells to warm up when they’re used
Internal resistance
What is load resistance?
The total resistance of all the components in the external circuit. Also called external Resistance
What produces electrical energy in the battery?
Chemical reactions
What is e.m.f?
The amount of electrical energy the battery produces for each coulomb of charge
What is the terminal p.d?
The potential difference across the load resistance. It is the energy transferred when one coulomb of charge flows through the load resistance
What would happen if there was no internal resistance in the battery?
The terminal p.d would be the same as the e.m.f. However in real power supplies, there’s always some energy lost, as heat energy, overcoming the internal resistance
What are lost volts?
The energy wasted per coulomb overcoming the internal resistance
Energy per coulomb supplied by the source = energy per coulomb transferred in load resistance + [ ]
Energy per coulomb wasted in internal resistance
How do you find the total emf of two cells in series pointing in different directions?
Subtract emf of the one in the opposite directions
Why are all emfs in parallel equal?
The current will split equally between identical cells. The charge only gains emf from the cells it travel through - so the overall emf in the circuit doesn’t increase
How can you investigate internal resistance and emf?
1) Vary the current in the circuit by changing the value of the load resistance using the variable resistor
2) Measure the p.d for several different values of current
3) Record the data for V and I, and plot the results in a graph of V against I, then calculate emf and internal resistance
Describe the circuit needed to investigate emf and internal resistance
Cell (made up of a cell and resistor) in series with a ammeter, voltmeter and switch (each on one side of a square with the cell at the top, and the others in order going clockwise)
Then in parallel with the voltmeter, a variable resistor
Pg 35
How can you calculate emf and internal resistance from a V against I graph?
Start with V = ε - Ir
Since ε and r are constants, this is an equation of a straight line.
So the intercept with the vertical axis is ε
And the gradient is -r
Why is it hard to choose the value for the load resistance, when investigating emf and internal resistance?
A low load resistance will give a large current, which will reduce the percentage uncertainty in the ammeter reading of the current.
But large currents will cause significant heating in the wires, which will invalidate your results
What assumption fo you make in any experiment using voltmeters and ammeters?
You can assume that the voltmeter has a very high resistance, and the ammeter has a very low resistance
Why does including a voltmeter in the circuit not affect the current through the variable resistor?
(Investigation of emf and internal resistance)
Voltmeters have a very high internal resistance, so the current through them is so low you can usually assume it is negligible
Why does including an ammeter in the circuit not affect the current trough the variable resistor?
(Investigating emf and internal resistance)
The ammeter has a resistance that’s so low it’s negligible, and so the voltage across it is also negligible
What is an easier way of measuring the emf of a power source?
Connect a voltmeter across its terminals.
The current through the voltmeters 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
When charge flows through a circuit, it is always [ ]
Conserved
Whatever charge flows into a junction….
Will flow out again
As current is rate of flow of [ ], it follows that whatever current flows into a junction is the same as the ….
Charge
Current flowing out of it
What is Kirchhoff’s first law?
The total current entering a junction = the total current leaving it
Describe the conservation of energy in a circuit
In electrical circuits, energy is transferred round the circuit. Energy transferred to a charge is emf, and energy transferred from a charge is potential difference
What is Kirchhoff’s second law?
The total emf around a series circuit = the sum of the p.ds across each component
Describe current, emf, and resistance in series circuits
Same current at all points of the circuit
Emf split between components. E = V₁ + V₂ etc.
R = R₁+ R₂ + R₃….
1/G = 1/G₁ + 1/G₂ + 1/G₃
Describe current, voltage and resistance in a parallel circuit
Current is slept at each junction. I = I₁ + I₂ + …
Same p.d across all components
1/R = 1/R₁ = 1/R₂ = 1/R₃
G=G₁+G₂+G₃…
What is the simplest potential divider?
A circuit with a voltage source and a couple of resistors in series
How is the p.d split in a potential divider circuit?
The p.d of the voltage source is divided in the ratio of the resistances
What can you use a potential divider circuit for?
Calibrating voltmeters, which have a very high resistance, because you can choose the resistances to get the voltage you want across one of them
How can potential discovers be made into sensors?
By including components whose resistance changes with external factors, eg. LDRs and thermistors. This means Vₒᵤₜ varies with light or heat, so you can make a potential divider that works as a light or heat sensor
The circuit needs to be calibrated so you know how the voltage across the component and Vₒᵤₜ varies as external conditions change
Describe the process of calibrating an electronic thermometer
1) measure the temperature of the water using the thermometer, and record the voltage across the resistor
2) Heat the beaker gently using the Bunsen burner, and record the temperature and the voltage at regular intervals
3) Plot a graph of temp. against voltage
Describe the circuit needed to calibrate an electronic thermometers
A battery in series with a resistor and a NTC thermistor. With a voltmeter in parallel with the thermistor. The thermistor should be in a beaker of water, above a Bunsen burner
What does a potentiometer do?
It uses a variable resistor to give variable voltage
It has a variable resistor replacing R1 and R2 of the potential divider, but it uses the same idea as a potential divider circuit
How does a potentiometer work?
You move a slider or turn a knob to adjust the relative sizes of R1 and R2. That way you can vary V out (which is across R2) from 0V up to the input voltage, Vin
Why are potentiometers very useful?
The voltage can be varied from 0 V to the input voltage
You can change a voltage continuously
How do you reduce the effect of random errors when investigating the I-V characteristic of a component?
Repeat your measurements and take averages
What can the I-V characteristic of a component tell you about the component?
Use it to see how the resistance changes (as the voltage/current increases/decreases)
When investigating internal resistance and emf, how can you reduce the effect of heating the wires?
Include a switch in your circuit to turn off the current whenever possible to reduce the effect of heating in the wires on the resistance of the circuit
Calibrating an electronic thermometer: How can you use the graph of temperature against voltage to calibrate the thermometer?
This graph is the thermistor’s calibration curve. You can use it to find the temperature of the thermistor from the voltage across it, without needing a thermometer - the thermistor and the calibration curve together are effectively another thermometer
What is the equation linking current, the number of electrons, and the velocity of the electrons?
I = nAve I = current (A) n = n. density of electrons (m⁻¹) A = cross sectional area (m²) v = drift velocity (ms⁻¹) e = charge of an electron (C)
