1. Electric Circuits and Electric Energy

2. https://phet.colorado.edu/en/simulation/balloons
Charge
Electricity has something to do with electric charge. What is electric charge? Positive charges attract negative charges and repell positive charges.
Exercises: Do exercise 1,2,4 on page 98 in Netzwerk Physik 8.

3. Circuits
Let us start with introducing circuits. What is a circuit? What do you remember from year 7?

4. Current is the flow of electric charge.
When a circuit is complete an electric current flows. What actually travels around the circuit is electric charge.
Current is the flow of electric charge.

5. Current, Definition
Electron flow: Direction that the actual charges flow.
Conventional current: The direction that the current would flow, if the charges were positive.
Quantity
Symbol
Unit
Current I
A (Ampère)
Charge Q
C (Coulomb)
Time t
s (seconds)

6. Current Questions
A current of 10 A passing a point means that 10 C of charge flows past every second.
1. If a current of 50 A flows around a circuit for 5 seconds how much charge flows?
2. A current of 150A flows around a circuit for 1 minute. How much electric charge flows around the circuit in this time?
3. If 20 C of charge pass a point in a circuit in 1 second, what current is flowing?
4. The charges that are flowing around the circuit are electrons. The charge of one electron is 1.602x10-19 C (elementary charge). How many electrons flow thorugh a wire per second, if the current is 1 A?

7. Voltage
So, electric charges flow through the wire. On their way through the wire the electric charges deliver energy to the components, say, a bulb. The energy delivered to the components are called voltage.

8. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Voltage
Voltage is a measure of the potential difference accross the circuit. +
High potential energy +
Low potential energy

9. Voltage
Voltage is a measure of the potential difference accross the circuit.
Quantity
Symbol
Unit
Voltage U
V (Volt)
Charge Q
C (Coulomb)
Energy E
J (Joule)

10. Voltage, Questions
1. Discuss the picture in the lower left corner of page 102 in Netzwerk Physik 8.
2. Why is the sentence: „Die Birne verbraucht Strom“ nonsence? What does a battery run out of, when it eventually goes flat?
3. What energy form is converted to what other energy form, when a battery is placed in a circuit making a bulb shine?

11. Simple Circuits

12. C O M P O N E N T S

13. Series and Parallel circuit
Series circuit: the current goes through all components.
Parallel circuit: the current is split between the different paths.
This is Kirchhoff‘s first rule

14. Circuits, Exercises
Does it matter, where the ampère meter is placed in a series circuit? Why?
Evaluate Ix in the situations below:
Consider two light bulbs placed in a series circuit. The potential difference over one of these is U1, while the potential difference over the other is U2. Argue that the total potential difference over the circuit is U1 + U2.
If the two light bulbs in question 3 are placed in parallel, argue that U1=U2.

15. Revision Lets pause and review the topic:
What is charge? What is the unit of charge?
What is current, and what is the difference between the conventional current and the electron flow?
If a current of 50 A flows around a circuit for 10 seconds how much charge flows?
Why do batteries run flat?
What is voltage?
Explain Kirchhoff‘s first law?
What is the difference between series and parallel circuits?

16. Resistance, Ohm‘s law
When the electrons travel through a component, they deliver energy. The ratio between U and I is called resistance, R.
The unit of resistance is Ohm denoted by the letter Ω.

17. Ohm‘s Law, Questions
Calculate the resistance, if the potential difference over a component is 5V and the current is 0.1 A.
Calculate the current, if the potential difference over a component with a resistance of 100 Ω is 5V.
Calculate the potential difference over a component, if the current in the circuit is 0.1 A and the resistance 500 Ω.

18. Colour Coding

19. U-I-Diagrams
U (V)
Every component has a characteristic U-I graph (Kennlinie). Take a look at the graph above. Describe the behaviour of the restistance of the component. What is the resistance of the component in Ω?

20. Ohm‘s Law
If the current and the voltage are proportional, as is the case on the previous slide, the resistance is constant. This proportionality is called Ohm‘s law.
Do exercise 1 and 2 on page 111 and A3 on page 143 in Netzwerke Physik 8.

21. Series Circuits
Consider a two components that are placed in a series circuit. The resistance of the two components are R1 and R2, respectively. For series circuits
So, since
And since the current is the same everywhere in series circuits,

22. Series Circuits
1. Calculate the total resistance in the circuits below
2. Calculate the total resistance in the circuits below

23. Parallel Circuits
Consider a two components that are placed in a parallel circuit. The resistance of the two components are R1 and R2, respectively. For parallel circuits
So, since
And since the potential difference is the same everywhere in parallel circuits,

24. Parallel Circuits
1. Calculate the total resistance in the circuits below
2. Calculate the total resistance in the circuits below

25. Resistivity Any material has a specific so-called resistivity, ρ:
Here l is the length of the cable and A is the cross sectional area.
Exercise: What is the
unit of resistivity?

26. Temperature Dependence of the Resistance
On their way through the material, the electrons collide with the ions in the grid of the solid. By this process, they loose energy. This leads to resistance.
When the temperature rises, the ions move faster leading to more collisions. So the resistance rises, when the temperature rises.

27. Temperature Dependence of the Resistance
Kaltleiter U
Ohmsches Verhalten
Heißleiter I
Kaltleiter: Positive Temperature Coefficient
Heißleiter: Negative Temperature Coefficient
What would you make a light bulb of?

28. Temperature Dependence of the Resistance
Do exercise 1 on page 115 as well as A2 on p. 141.
Why do light bulbs tend to blow up, when you turn them on,
and not while they are shining.
3 m of constantan with a cross section of 0.1 mm2 has a
resistance of 15 Ω. Calculate the resistivity of constantan.

29. Super Conductors, R=0
Some materials have no resistance below a certain temperature. Not all though. Normally there is resistance even at 0 K due to irregularities in the grid. Below I have included a graph, showing the resistivity as a function of temperature.

30. Super Conductors, R=0

31. Super Conductors, R=0 Discuss superconductors with ypur neighbour:
What defines a superconductor, and how does a superconductor differ from a normal metal?
Why was the train cooled with liquid Nitrogen?
What is the Meissner effect?
How does the resistivity of a normal metal change due to changes in temperature and why?

32. Super Conductors, R=0 Discuss superconductors with ypur neighbour:
Below the critical temperature R = 0 for superconducting materials. In this phase the Meissner effect occurs. In the case of normal metals, the resistance is even at 0 K not 0. The Meissner effect does not occur.
Because the boiling point of nitrogen lies below the critical temperature of the cheramic superconductor at hand.
There are no magnetic fields inside a superconductor; as a consequence you can make magnets hover above superconductors.
Any asymmetry (such as thermal motion) in the lattice results in resistance. R thus increases with increasing temperature.

33. Power
Remember power from the first term? It is measured in Watt and tells you the amount of energy used/produced per unit time.
Exercise: The potential accross a 60 W light bulb is 250 V. What is the current through the light bulb?
Do exercise 1 and 2 on page 130.

34. The National Grid

35. Engines
In cars, the motor sparks ignite the petrol gas. The explosion of the gas is an expansion of the gas, which moves the cylinder.

36. Power Plants
In power plants, water is boiled by some means and the steam drives a turbine. The mechanical energy is turned into electrical energy by means of induction, which is a topic of year 9.