2 SENSING Basic electrical relations

1. 2 SENSINGBasic 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. 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 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? What is the current as measured by the ammeter? (b) The charge on a single electron is 1.6 x 10-19 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 10-19 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?

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

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

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

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

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

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

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

31. Have a go... Don’t forget your the resistance rules! CH 2 - Sensing 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. 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. CH 2 - Sensing P.D. , V R = V/I is a statement of Ohm’s law

34. 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’ CH 2 - Sensing

35. 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? CH 2 - Sensing I (A) ? V (V)

36. 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. CH 2 - Sensing The temperature of a filament lamp increase as current increases. Increasing temperature increases resistance.

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

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

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

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

41. A chain of resistors CH 2 - Sensing

42. Rotary types CH 2 - Sensing

43. Questions 170S... A series circuit is connected as shown in the diagram. 1.What is the potential difference between A and B? CH 2 - Sensing 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... 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? CH 2 - Sensing

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

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

48. Electromotive force (EMF) Note: It’s not actually a force! V =ε - Ir CH 2 - Sensing 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. Key equations Only this one on formula sheet! • V = ε – Irε = I(R + r) • V = ε - v ε = V + v CH 2 - Sensing • 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