1. Electricity
Chapter 34

2. Electric Current Section 1 Lab: What do you need to have a circuit?
What flows in a circuit? .

3. 34.1 Flow of Charge
Charge flows when there is a potential difference, such as from the positive terminal of a batter to the negative terminal of the battery
Remember, the positive and negative sides of the battery have to do with the excess and shortage of electrons.

4. Charge flows until potential is the same
If there was not difference in height, then the water would not flow out of the reservoir in the picture
Similar to heat flow- if there is no difference in temperature than there is not flow of heat

5. 34.2 Electric Current
Electric current is the flow of charge or the amount of coulombs (charge) that flows past a point in a given amount of time
Ampere (A)is the SI unit of electric current
1A= 1C/s
Named after who? What did he do?

6. I = Q/t
I = symbol for current (SI unit of current = Ampere)
Q = symbol for charge (SI unit for charge = Coulomb)
t = symbol for time (SI unit for time = second)

7. Practice Problem:
20 Coulombs of charge passes a point in 10 seconds. What is the current?
2 A
A current of 100A exists. How many coulombs of charge move past a given point in 5 seconds?
500C

8. Net charge in a conducting wire is zero What moves?
Electrons do not enter/leave a conductor, such as a wire.
Electrons transfer energy from one conductor to another.

9. 33.5 Electric Potential
Electric potential is electrical potential energy per charge.
Voltage is potential energy per coulomb available to electrons moving between terminals

10. Voltage, or electrical potential difference, causes current
Volt (V) is the SI unit for potential difference
1 Volt = 1 Joule (or energy) per Coulomb (or charge)
1V = J/C

11. Electric outlets have a potential of 120V
Electric outlets have a potential of 120V. This means there is 120 J of energy per coulomb of charge available
The voltage resulting from rubbing a balloon on hair could be several thousand volts.
Then why does putting your finger in an outlet be potentially deadly but static from rubbing a balloon is not?....Answers to come…

12. 34.3 Voltage Sources
A voltage source is something that provides a potential difference
Must be a continuous potential difference for a current to flow

13. Charged metal spheres are not good at maintaining a potential difference, therefore there is not a continuous current.
Dry cells, wet cells, and generators are good at maintaining a potential difference, therefore will produce a continuous flow

14. Section 2 Lab What is electrical resistance?
How does resistance affect current?

15. 34.4 Electric Resistance Electric resistance is the resistance to flow
Current does not stop, only ‘slowed’

16. Resistance depends on:
Conductivity of the metal
Length: longer the wire the more the resistance
Diameter: thicker wires have less resistance than thin wires
Temperature: higher the temperatures, the more resistance
Superconductors- zero resistance at low temperatures

17. R = ρ L/A R = resistance ρ = resistivity L = length A = area

18. 34.5 Ohm’s Law
“for a given resistance, voltage and current are proportional”
V= IR
V= potential difference
I= current
R= resistance

19. Short Circuiting
Something is ‘short-circuited’ when current by passes an area by following the path of least resistance

20. If the red wire were not present the bulb would light
When the red wire is present, the bulb does not light because the current follows the path of least resistance. It is easier for the current to flow through the wire than the filament because the filament has greater resistance.

21. END OF NOTES. Remember: your Homework: -p. 529 #11-13 -p
END OF NOTES!! Remember: your Homework: -p. 529 # p. 545 #1-9, read p chpt 35 vocabulary (quiz Monday/Tuesday)

22. 34.6 Ohm’s Law
Ohm discovered that current in a circuit is directly proportional to the voltage and inversely proportional to the resistance in a circuit.
V = IR
V= voltage (Volts- V)
I = current (Ampere- A)
R = resistance (Ohm- Ω)

23. Ohms Law Practice Problem
A CD player with a resistance of 40 ohms has a current of 0.1 amps flowing through it.
Sketch the circuit diagram
Calculate how many volts supply the CD player.

25. 34.7 Direct and Alternating Current
Direct Current (DC)
Electrons always flow in one direction
Ex: battery
Alternating Current (AC)
Electrons flow in one direction and then another
Able to transmit electrical energy large distances with easy voltage setups and little heat loss
Ex: power outlets

26. 34.8 Converting AC to DC
Diodes are devices that only allow electrons to flow in one direction
Half of each cycle will pass through
Rough DC produced, because it is off half the time
Capacitors used to smooth the effect on changes in electric currents
See p. 540 Figure 34.12

27. 34.9 The speed of Electronics in a Circuit
Signal of on/off moves at nearly the speed of light, NOT electrons
Wire acts as a guide or pipe for the electron’s energy (DC)
Electrons do not make any notable progress in one direction (AC)

28. 34.10 The Source of Electrons in a Circuit
Source of electrons is the material in the conducting circuit
Power outlets are AC, so electrons move back and forth
Energy from outlet is what is transmitted, not electrons

29. 34.11 Electric Power P= IV P= power I= current V= potential difference
Watt is the SI unit of power
1W= 1J/s
Named after who? What did he do?

30. Sometime use:
kWh for kilowatt hour
(1000W)(3600s)= 3.1 x 106 J

31. Power Practice Problem
A microwave is connected to a 120V outlet. It uses 10A of current to heat food.
How much power does this machine use?
Write this number in terms of kW.

32. Series vs Parallel Circuits
A series circuit is a circuit where current travels through every wire and device in one path.
A parallel circuit is a circuit where current branches off at different points.

33. Voltage for a series circuit and a parallel circuit does not change.
Current for a series circuit and a parallel circuit changes.

34. Kirchoff’s Rules Used for parallel circuits
Junction Rule: the amount of charge entering a junction equals the amount of charge leaving a junction
Loop Rule: the amount of voltage drops across a resistors equals the total amount of voltage from the circuit

35. Resistance and Circuits
Series: as resistors are added to a circuit, total resistance increases
R= R1 + R2 + R3…..
Parallel: as resistors are added to a circuit, total resistance decreases
R= 1/R1 + 1/R2 + 1/R3…..

36. Series Circuit and Resistance
Look at a round light bulb and calculate the resistance one round bulb provides.
Create a series circuit with the two light bulbs.
Calculate the total resistance for the series circuit.
What would happen if to the bulb brightness if another light bulb were added?
What would the total resistance be?

37. Parallel Circuit and Resistance
Look at a round light bulb and calculate the resistance one round bulb provides.
Create a parallel circuit with the two light bulbs.
Calculate the total resistance for the parallel circuit.
What would happen if to the bulb brightness if another light bulb were added?
What would the total resistance be?