1. 2 SENSING Basic electrical relations
Review equations relating charge, voltage, current and power
Understand how ion beams can be used in micromachining

2. Micro-machined structures
A human hair with holes drilled in it by a laser beam
Part of a miniature electric motor, compared for scale with a human hair
Part of a silicon pressure sensor, made by depositing layers and then etching material away chemically.

3. Micro-machined structures
Silicon nitride bridging strips over a channel, forming part of a sensor to measure flow rates. Circuitry is also built into the device as it is made.

4. Machining using Ion Beams
Argon ions are made by firing electrons at argon atoms. The ions are then accelerated towards a negatively charged grid to create a beam of fast moving ions. This beam of ions is directed at the material to be machined and knocks atoms out of the material, the shape of the material produced depends on the angle of the beam.
This technique can be used to produce sharp diamond tips as small as 0.1μm across, mechanical polishing techniques can only produce tips 5μm across.

5. Show demo: 80S difference between ion beam and current in a wire.
Ion Beams
Key facts:
Beam carries energy (its moving).
When moving particles knock into things they deliver some of their energy to the target.
Beam carries an electric current as the particles are charged and moving.
Beams are accelerated by a potential difference acting on the ions....
Show demo: 80S difference between ion beam and current in a wire.

6. Potential difference (voltage)
For charged particles to move they need other charged particles repelling or attracting them.
Potential difference (voltage)
The energy converted per unit charge moving.
V = ΔE/ΔQ
V- Potential difference (volts)
ΔE – energy (joules)
ΔQ – charge (coulombs)
Current, A
Ohm’s law
Provided the temperature is constant, the current through an ohmic conductor is directly proportional to the voltage across it.
P.D. , V

7. The basic equations Symbols and units Q = charge (coulomb C)
I = current (ampere A)
V = potential difference (volts V)
E = energy (joule J)
P = power (watt W)
t = time (second s)

8. More equations

9. Q1. (a) 50 Coulombs of charge flow through an ammeter in 10 seconds
Q1. (a) 50 Coulombs of charge flow through an ammeter in 10 seconds? What is the current as measured by the ammeter?
(b) The charge on a single electron is 1.6 x C. Use this information and your answer to Q 1(a) to work out the number of electrons that pass through the ammeter every second.

10. How many electrons flow out of a battery in one minute, if the current is 0.2 A?
Take the charge on an electron, e = 1.6 x C

11. Sensors
Investigate the properties of a range of sensors

12. A potential divider
Q1. What happens to the resistance of the thermistor when the temperature increases? Q2. What happens to the current that flows as a result of this change? Q3. What happens to the potential difference across the resistor as a result of this change in current? (Hint: V = IR)

13. Voltage in an electrical circuit
Use different models to improve your
understanding and your ability to explain the role of voltage in an electrical circuit
Models to be used: treadmill model

14. What does each element in this circuit model represent?
How would you model the effect of increasing the battery voltage?
What effect would this have on what happens in the circuit?
Electric circuit model.

15. You can think of an electric current as a flow of charges, which are present in all materials (and free to move in conductors), moving round a closed conducting loop, pushed by the battery.

16. Questions to really test your understanding of voltage
Explain what controls how fast the charges move around the circuit.
What happens to energy in the circuit?
Explain how energy is related to charge and voltage.
Why is the first bulb not brighter than the second?
Why does a voltmeter have to be placed in parallel?
Explain in words why the power is equal to the current multiplied by the voltage.
Work in pairs, take it in turns to answer the questions, with the listener forcing the other person to explain properly… “what do you mean by the word…..”, “does that mean that….?”, “so how does the…”, “what would happen if…?”

17. Active and passive sensors
Active sensors are sensors that transmit some kind of
energy ( microwave , sound , light , .... ) into the
environment in order to detect the changes that occur
on the transmitted energy . That means it transmits and
detects at the same time .
Passive sensors don't transmit energy but only detects
the energy transmitted from an energy source.

18. The basic equations Symbols and units Q = charge (coulomb C)
I = current (ampere A)
V = potential difference (volts V)
E = energy (joule J)
P = power (watt W)
t = time (second s)

19. More equations

20. Series and parallel resistance
Determine rules for equivalent resistance and conductance in series and parallel circuits

21. Conductance and resistance
CH 2 - Sensing
Conductance and resistance

22. In series... Potential difference adds up in series
CH 2 - Sensing
In series...
Potential difference adds up in series
Current same for both
Resistances add up

23. In parallel... CH 2 - Sensing Potential difference same for both
Currents add up
Potential difference same for both
Conductance add up
Reciprocal of resistances add up

24. CH 2 - Sensing
Have a go...
For each of the following find the current drawn from the power source:-

25. CH 2 - Sensing
Have a go...
Find I for this one!

26. Electrical power
Derive and use equations for power dissipation in electric circuits
Starter:
If a train delivers 10 wagons of coal every hour to a power station, which burns the coal, and each wagon contains 50 tons of coal, what is the rate at which coal is burned in the power station?
How do the three variables in this example relate to the electrical quantities current, potential difference and power?

27. Power = P.D. x Current P = V x I W = V x A J s-1 = J C-1 x C s-1
Variable
Electrical equivalent
Unit
Rate at which wagons arrive at power station
Current (charge passing per second)
C s-1 = A
Coal per wagon
Potential difference (energy per coulomb of charge)
J C-1 = V
Rate at which coal is dumped at power station
Power (energy per second)
J s-1 = W
Power = P.D. x Current
P = V x I
W = V x A
J s = J C-1 x C s-1

28. By combining the equations
P = V I and R = V / I,
and eliminating the appropriate variables,
can you obtain:
An expression for P in terms of I and R (no V) ?
An expression for P in terms of V and R (no I) ?

29. There are two other equations we can get for power from this.
We know equation: P = IV
There are two other equations we can get for power from this.
(hint use R = V/I)

30. Have a go... CH 2 - Sensing Don’t forget your the resistance rules!
What is the power dissipated in the 12Ω resistor?
What is the power dissipated in the 5Ω resistor?

31. Have a go... CH 2 - Sensing Don’t forget your the resistance rules!
Find the power dissipated in each resistor and the total power of the circuit.

32. Current-voltage relationships
Investigate and explain current-voltage relationships for a range of components

33. R = V/I is a statement of Ohm’s law
CH 2 - Sensing
Recap of - Ohm’s Law
Current, A
Ohm’s law
Provided the temperature is constant, the current through an ohmic conductor is directly proportional to the voltage across it.
P.D. , V
R = V/I is a statement of Ohm’s law

34. Instructions: On computers....
CH 2 - Sensing
On computers....
Instructions:
Working in pairs or on own work through the following software activities on :-
(You will need paper to answer the questions)
Activity 160S ‘Conductance and resistance in a filament lamp’
Activity 170S ‘Conductance and resistance in a neon lamp’
Activity 180S ‘Conductance and resistance in a silicon diode’
Activity 190S ‘Conductance and resistance in an ohmic resistor’

35. CH 2 - Sensing
I-V graphs
What do the I-V graphs look like for a filament lamp, a neon lamp, a silicon diode and an ohmic resistor?
And why do they look like they do?
I (A) ?
V (V)

36. CH 2 - Sensing
I-V graphs
Diodes behave like ohmic resistors when the current is travelling through them in the correct direction. However, if the current is reversed the resistance of the diode is extremely high.
The temperature of a filament lamp increase as current increases. Increasing temperature increases resistance.

37. Neon lamp shows non-ohmic behaviour.
CH 2 - Sensing
I-V graphs
Obeys ohms law completely!
Ohmic resistor
Neon lamp shows non-ohmic behaviour.

38. The potential divider
Investigate and explain the resistance-voltage relationship for potential divider circuits

39. CH 2 - Sensing
A potential divider
Remember potential differences divide in series in same ratio as the resistances R1
Vout
Vin R2

40. Used to control a potential difference.
CH 2 - Sensing
A potentiometer
Used to control a potential difference.

41. CH 2 - Sensing
A chain of resistors

42. CH 2 - Sensing
Rotary types

43. CH 2 - Sensing
Questions 170S...
A series circuit is connected as shown in the diagram.
1.What is the potential difference between A and B?
2.An additional resistor of 100 ohms is connected in series between the 50 ohms resistor and the cells. What is the potential difference between A and B now?
3.The additional 100 ohms resistor is now connected in parallel with the first 100 ohms resistor. What is the potential difference between A and B now?
4.A potential divider is made from a 4 kilohm and a 6 kilohm resistor connected in series with a 20 V supply. Draw a diagram of the arrangement. What three values of potential difference can be tapped off?

44. Questions 170S cont... CH 2 - Sensing
5. A student puts a 12 ohms variable resistor in series with a 6 V battery, expecting to get a variable potential difference.
The voltmeter is a high resistance digital multi meter. Explain why the circuit won't work. Draw a circuit which would work.
6. B is the wiper of a 100 ohms rotary potentiometer.
What is the full range of the potential difference that can be tapped off between A and B?

45. EMF and internal resistance
Explain the meaning of these terms
Measure EMF and internal resistance for a 1.5 V dry cell

46. Batteries have resistance
CH 2 - Sensing
Batteries have resistance
Resistance comes from electrons colliding with atoms and losing energy. This happens inside the battery as well as rest of circuit.
Internal resistance
Load resistance (R)

47. Batteries have resistance
CH 2 - Sensing
Batteries have resistance
Internal resistance (r) - resistance of the power source.
Load resistance (R) - Total resistance of all components in rest of the circuit.
Internal resistance (r)
Load resistance (R)

48. Electromotive force (EMF)
CH 2 - Sensing
Electromotive force (EMF)
Note: It’s not actually a force!
V = ε - Ir
Internal resistance (r) A V
Load resistance (R)
EMF (ε) – Amount of electrical energy the battery produces for each coulomb of charge (measured in volts).

49. V = ε – Ir ε = I(R + r) V = ε - v ε = V + v Key equations
CH 2 - Sensing
Key equations
Only this one on formula sheet!
V = ε – Ir ε = I(R + r)
V = ε - v ε = V + v
Where:
ε – emf
I - current
R – load resistance (external)
r – internal resistance
V – terminal p.d.
v – ‘lost’ p.d.

50. Low and high internal resistances
Equation of straight line: y=mx+c

51. Use the graph to estimate the EMF and internal resistance for the battery. What would the graph look like for a battery of higher internal resistance? Explain why the p.d. across the load resistance falls as the current increases.

52. High resistance source

53. Low resistance source

54. Digital thermometer
Build and test a digital thermometer using a potential divider and thermistor
Determine its sensitivity and resolution
Starter: Draw a potential divider circuit containing a variable resistor, thermistor and voltmeter that will give a voltage output that increases with increasing temperature. Assume that the thermistor’s resistance goes down with increasing temperature
How would you determine the best resistance setting for the variable resistor?

55. A temperature sensing circuit is set up with a 6
A temperature sensing circuit is set up with a 6.0 V battery of negligible internal resistance, a thermistor and a fixed resistor as shown.
The resistance of the thermistor in the range oC is given by :
R (Ω) =
T (oC)
Q1. Calculate the thermistor resistance at 10 oC and 100 oC.
Q2. Which of the following would be the best choice for the fixed resistor, 300 kΩ, 300 Ω or 3 Ω? Explain your answer.
Q3. Calculate the output voltage when the temperature is 55 oC, if a 500 Ω fixed resistor is used.
Q4. It is desired to have an output voltage of 4.0 V when the temperature is 20 oC. Calculate the thermistor resistance at this temperature and hence determine the size of the fixed resistor required in this case.

56. A piezoelectric crystal generates voltages in response to applied stress. The maximum voltage that can be generated is 100 mV, and the crystal has an internal resistance of 1 MΩ.
Which of the following is the most suitable voltmeter for measuring the voltage generated by the crystal? Give reasons for your choice.
(Hint: Draw the circuit, or see p44 of textbook)
Moving coil meter with a full scale deflection of 0.5 V and an internal resistance of 200 Ω.
Digital voltmeter with full scale reading of 10 V and internal resistance of 1000 Ω.
Digital voltmeter with full scale reading of 0.5 V and internal resistance of 10 MΩ.

57. Key features of the digital thermometer
Estimate or measure the following for your digital thermometer: SENSITIVITY (IN V OC-1 OR mV OC-1) RESPONSE TIME RESOLUTION (assuming voltmeter uncertainty is +/-0.01 V) SYSTEMATIC ERROR (if present) PRECISION

58. Strain gauges and response time
Explain the meaning of the term response time and explain its importance in the context of sensing.
Explain how a resistive strain gauge works.

59. Many simple sensing circuits use a sensor in series with a resistor.

60. Response time
Length of time a sensor takes to reach its final reading following a sharp change in input.
Note:
Needs to be short enough to detect any important changes.
Changes more rapid than response time will be averaged out.

61. Sensitivity
Ratio of change of output to change of input.

62. Strain gauges

63. Strain gauges

64. Galvanometer measures when current is zero very accurately.
Wheatstone bridge
Galvanometer measures when current is zero very accurately.
If you know R1, R2, Rv and you know there is zero current, you can work out Ru very accurately.
e.g.: strain gauge