1. Lesson 1: 8 step challenge building circuits
Equipment: Rheostats, ammeters, voltmeters, 6v cells (3v max), 10ohm resistors, clip component holders, leads.

2. A human hair with holes drilled in it by a laser beam

3. Part of a miniature electric motor, compared for scale with a human hair

4. Part of a silicon pressure sensor, made by depositing layers and then etching material away chemically.

5. 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. The bridging strips are made to vibrate; the frequency of vibration depends on the rate of flow of gas past the strip.

6. Charge, current and potential difference…and anything else.
Chapter 3: Sensing
Charge, current and potential difference…and anything else.

7. Learning Objectives
Review how current, potential difference, energy and charge are inter-related and link to movement of charges in beams.
Friday, 05 April 2019 7

8. Learning Outcomes
Recall what is meant by current, charge and potential difference.
Give definitions for the above in terms of charge and energy transfer.
Make links between these quantities and use homogeneity to verify these links.
Grade E
Grade C
Grade A
Friday, 05 April 2019 8

9. Electrical Circuits: Key Definitions
Chapter 3: Sensing
Electrical Circuits: Key Definitions
Quantity
Symbol
Unit
Definition
Key formula
Other additional info…
To begin: start a new page in your notes and draw out this table (don’t put a “bottom” on it as we will keep adding to it)
Complete for current, charge, potential difference, power and resistance.

10. What can you tell me about this circuit?
BY THE END OF THE LESSON: Recall what is meant by current, charge and potential difference. Give definitions for the above in terms of charge and energy transfer. Make links between these quantities and use homogeneity to verify these links.

11. You can’t have these on their own – they are all carried by other things:
Charge
Beauty
Confidence
Momentum
possessed by electrons and protons
possessed by people, paintings, ornaments, flowers, etc.
possessed by people
possessed by objects
‘Charge’ is a rather abstract idea. An electron is not charge; it carries charge. You cannot have pure charge on its own any more than you can have pure mass or pure beauty – all are characteristics of objects.

12. If you are into a bit of cross-curricular experience (I am), you might like to tell the story of Helen of Troy. The following four slides are all depictions of her.
In ancient Greek myth, Helen of Troy, daughter of the god Zeus, was said to be the most beautiful woman in the world. She had many suitors, and married King Menelaus of Sparta. She was later abducted by (or some versions of the story say she ran away with) Paris, prince of Troy. A fleet of a thousand ships was launched to attack Troy and get Helen back – and so the Trojan War began.
Scientist and writer Isaac Asimov came up with the tongue-in-cheek unit of beauty, the Helen: the beauty required to launch a thousand ships. Mere mortal women only have a fraction of a Helen’s-worth of beauty each.

13. It takes many women to make a Helen…
x10
makes up 1 Helen
(1 H)
1 thousandth of a Helen (1 mH)

14. It takes many electrons to make a coulomb…
of a Coulomb
x a gazillion (or so)
makes up one Coulomb

15. Charge & Current Charge is measured in coulombs.
The charge on one electron is 1.6 x 10-19C
While charge moves you get a current.
Current is the rate of flow of charge or the quantity of charge that flows per second.
I = ΔQ Δt
Current is measured in amperes.
1 Ampere = one coulomb per second (1Cs-1)
BY THE END OF THE LESSON: Recall what is meant by current, charge and potential difference. Give definitions for the above in terms of charge and energy transfer. Make links between these quantities and use homogeneity to verify these links.

16. Question If the charge on one electron is e=1.6 × 10-19 C,
how many electrons are needed to make up 1 C
of charge?
BY THE END OF THE LESSON: Recall what is meant by current, charge and potential difference. Give definitions for the above in terms of charge and energy transfer. Make links between these quantities and use homogeneity to verify these links.

17. Spooning charge demo
Calculate the number of electrons on A) the teaspoon B) the dessert spoon
link to earth socket
5 kV supply
internal 50MW resistor
bare 4mm plug
044
coulomb meter
insulating handle
metal disk on 4mm plug
BY THE END OF THE LESSON: Recall what is meant by current, charge and potential difference. Give definitions for the above in terms of charge and energy transfer. Make links between these quantities and use homogeneity to verify these links.

18. Calculating the number of electrons
Knowing that the charge on an electron is –1.6 x
10–19 C, you can calculate the number of
electrons in a 'spoonful' of charge.
A typical spoonful of negative charge is –2 nC. So
the number of electrons is:
BY THE END OF THE LESSON: Recall what is meant by current, charge and potential difference. Give definitions for the above in terms of charge and energy transfer. Make links between these quantities and use homogeneity to verify these links.

19. Potential Difference and electromotive force.
ANALOGY
Charge carriers are pushed around a circuit by the emf of the cell.
The cell supplies each electron with electrical potential energy.
They do work in the lamp as they pass through it.
Charge is not ‘used up’ but it does lose potential energy.
A cell provides a potential difference and charges move around the circuit from higher to lower potential
The higher the emf across a source of electrical energy the greater the change in potential energy per coulomb of charge moving between its terminals.

20. Both p.d and emf are measured in volts.
The volt is defined as the energy transfer per coulomb of charge as charges move between two points in a circuit.
When 1 Coulomb of charge (6.2x1018 electrons) transfer 1 Joule of energy, it is said that the potential difference is 1 Volt.
So 1 Volt is 1 Joule per Coulomb
Energy transferred
Pd or voltage or emf =
charge

21. Electrical Power Power is the rate at which work is done.
In our circuits, the work done or energy transferred is given by…
W = V Q
and Q = I t
so W = I Vt
The rate of doing work is the power:
P = W / t = I V
BY THE END OF THE LESSON: Recall what is meant by current, charge and potential difference. Give definitions for the above in terms of charge and energy transfer. Make links between these quantities and use homogeneity to verify these links.

22. Drift velocity of electrons.
Drift velocity just means “how fast are they moving?”
BY THE END OF THE LESSON: Recall what is meant by current, charge and potential difference. Give definitions for the above in terms of charge and energy transfer. Make links between these quantities and use homogeneity to verify these links.

23. Worked example: Drift velocity
How would you approach this question?
BY THE END OF THE LESSON: Recall what is meant by current, charge and potential difference. Give definitions for the above in terms of charge and energy transfer. Make links between these quantities and use homogeneity to verify these links.

24. Drift Velocity of Charge Carriers
d = vt
A = cross sectional area of wire.
v = drift velocity of carriers.
n = charge carrier density (m-3)
q = charge of each carrier
In a time, t, these carriers move a distance, d = vt.
Volume moved through by these carriers, V
V = Ad = Avt
How many carriers have we got in V?
Total number of carriers = nV = nAvt
If each of these have a charge, q
Total charge passing through = nAvtq
But we’re interested in current, I (=Q/t)
I = Avnq

25. A current of 2A flows through a wire of cross sectional area 1mm2 containing 1.0 x 1029 electrons per m3. Calculate the drift velocity of the electrons.

26. USE WHEN MEASURING RESISTANCE
USE WHEN MEASURING DIRECT P.D
USE WHEN MEASURING DIRECT CURRENT
USE WHEN MEASURING ALTERNATING P.D
USE WHEN MEASURING ALTERNATING CURRENT
USE WHEN MEASURING RESISTANCE OR P.D
ALWAYS USE!
USE WHEN MEASURING 0.1 – 10A CURRENTS
USE WHEN MEASURING SMALL CURRENTS

27. Homework Tasks 2 & 3 from hwk booklet
NB: This includes work on paper and on Isaac.
BY THE END OF THE LESSON: Recall what is meant by current, charge and potential difference. Give definitions for the above in terms of charge and energy transfer. Make links between these quantities and use homogeneity to verify these links.

28. Copper conducting
As a rough guide, copper wires can conduct about 10 A mm-2 before overheating and there are approximately 1020 free electrons per mm3 in copper.
Questions
1 Find:
a The number of electrons per second required to carry a current of 10 A. b The length of wire with cross section 1 mm2 containing this number of electrons. c The average drift speed of electrons in the wire.
2If the same wire carried a current of only 10 mA what would the drift speed be, and how long would it take a typical electron to drift through 1 mm?

29. Equations so far Charge (C) Current (A) = time (s) Charge (C) =
no. of electrons x charge of one electron (C)
Charge (C)
no. of electrons =
Charge of one electron (C)
Pd (V) =
Energy transferred (J)
Charge (C)
Power (W) =
Energy transferred (J)
Time taken (s)
Power (W) =
Current (A) x P.d (V)
BY THE END OF THE LESSON: Recall what is meant by current, charge and potential difference. Give definitions for the above in terms of charge and energy transfer. Make links between these quantities and use homogeneity to verify these links.

30. BINGO!
BY THE END OF THE LESSON: Recall what is meant by current, charge and potential difference. Give definitions for the above in terms of charge and energy transfer. Make links between these quantities and use homogeneity to verify these links.