1. Electricity and Circuits
Unit 11
Electricity and Circuits

2. Starter If you are given a light bulb,
What other equipment would you need to light it up?
How would you set up the equipment? 2

3. I. Current
Battery acts as the pump to push the charges (electrons) around the circuit 3

4. I. Current B. Current (definition): HOW QUICKLY CHARGE MOVES
Equation (see ref. tabs.)
I = Current
Δq = change in charge
t = time
I = Δq t
Units:
C/s = Ampere (A) 4

5. “It’s the volts what jolts, but it’s the mills that kills.”
I. Current
Note: High currents through your body can cause serious injury or death. Here are a few of the typical consequences of different currents
“It’s the volts what jolts, but it’s the mills that kills.” 5

6. II. Conditions Necessary for Electric Current
CHANGE IN
POTENTIAL
What is needed? A ___________ between ______ points in the circuit 2
Provided by:
Battery or chemical cell
Most types of 9 V batteries consists of V cells added in series 6

7. III. Ohm's Law
A. Potential Difference (V): Change in electric potential energy between two positions
Units:
PROVIDES THE ELECTRICAL “PUSH”
Volts (V)
B. Resistance:
OPPOSES THE FLOW OF CHARGES
Units:
Ohm (Ω) 7

8. C. Ohm’s Law (see ref. tabs.)
R = Resistance (Ω)
V = Potential Difference (V)
I = Current (A)
R = V I 8

9. IV. How to Measure Current:
Device used:
AMMETER
How to set it up:
Place in series (line) with circuit 9

10. IV. How to Measure Potential Difference (voltage):
Device used:
Voltmeter
How to set it up:
Place it across the device you want to measure 10

11. VI. Resistivity
Resistivity: How well a specific conductor allows a current to move through it
Factors that affect the resistance:
1. Area (↑ A, ↓ R)
2. Length (↑ L, ↑ R)
3. Material (resistivity – ↑ ρ, ↑ R) (see ref. tabs)
4. Temperature (↑ T, ↑ R) 11

12. VI. Resistivity
Equation (see ref. tabs) ↑ 12

13. VI. Resistivity
Example: Determine the resistance of a copper wire that has a cross-sectional area of 2 x m 2 and 40 m long. 13

14. VII. Electric Power Mechanics’ Equation for Power: P = W t Units:
J/s = Watt (W)
Electric Power:
P = W = IV t
Alternate Equations (see ref. tabs.):
P = IV = I2R = V2 R 14

15. VII. Electric Power
Example: A microwave draws 12.5 amperes of current and resistance of 9.6 ohms. What is the power dissipated in the resistor? 15

16. VII. Electric Power
Example: A 60 W and 100 W light bulb are plugged into the wall that provides a potential difference of 120 V. Which bulb has the greater resistance? What is the resistance of both? 16

17. VIII. Electrical Energy
Equations:
W = Pt = IVt = I2Rt = V2t R
Units:
Joule (J) 17

18. VIII. Electrical Energy
Example: A television set draws 2 A when operated on 120V.
How much power does the set use?
Calculate how much energy is used if the television is on for 2 hours a day. 18

19. VIII. Electrical Energy
Example: An electric dryer consumes 1.0 x 106 J of energy when operating at 220 volts for 2 minutes. During operation, how much current does the hair dryer draw? 19

20. All parts are connected to provide a ______________ for the current
IX. Series Circuit
All parts are connected to provide a ______________ for the current
SINGLE PATH
Schematic Diagram: Draw a circuit diagram with three resistors set up in series hooked up to a 12 V battery. 20

21. IX. Series Circuit Things to Remember (See Ref. Tabs) 1. Current
Remains constant through each resistor
Equation:
I = I1 = I2 = I3 = … 21

22. IX. Series Circuit Things to Remember (See Ref. Tabs)
2. Equivalent Resistance
Total resistance is equal to the SUM of all resistors
Equation:
Req = R1 + R2 + R3 + … 22

23. IX. Series Circuit Things to Remember (See Ref. Tabs) 3. Voltage Drops
Applied (total) voltage from the power source equals the sum of the voltages across each device (resistor)
Equation:
V = V1 + V2 + V3 + … 23

24. IX. Series Circuit
Example: Draw a circuit diagram for series circuit that contains a 5 ohm, 10 ohm, and 15 ohm resistor that are connected to a 20 V battery. Include an ammeter to read the total current and three voltmeters to measure the potential difference across each resistor.
Resistor V I R P 1 2 3
Total
Calculate the total resistance of the circuit.
Calculate the total current in the circuit.
Calculate the potential difference across each resistor. 24

25. X. Parallel Circuit TWO OR MORE PATHS
Allows ____________________ for the current to flow
TWO OR MORE PATHS
Schematic Diagram: Draw a circuit diagram with three resistors set up in parallel connected to a 12 V battery. 25

26. X. Parallel Circuit Things to Remember (See Ref. Tabs) 1. Current
Sum of the currents in each of the branches is equal to total current
Equation:
I = I1 + I2 + I3 + … 26

27. X. Parallel Circuit Things to Remember (See Ref. Tabs) 2. Voltage
Voltage remains constant across each device
Equation:
V = V1 = V2 = V3 = … 27

28. X. Parallel Circuit Things to Remember (See Ref. Tabs)
3. Equivalent Resistance
As more resistors are added in parallel, total resistance DECREASES
Equation:
= …
Req R1 R2 R3 28

29. X. Parallel Circuit
Example: Draw a circuit diagram for parallel circuit that contains a 10 ohm, 15 ohm, and 20 ohm resistor that are connected to a 20 V battery. Include ammeters to measure the current through each and voltmeters to measure the potential difference across each resistor. 29

30. X. Parallel Circuit
Calculate the equivalent resistance of the circuit.
ii. What is the potential difference across each resistor?
iii. Calculate the reading on each of the ammeters.
iv. Calculate the total current (the current leaving the source).
v. What would happen to the total current if more resistors are added in parallel?
Res. V I R P 1 2 3
Total 30

31. XI. Conservation of Charge
Like energy, there is a conservation of __________ in circuits
CHARGE
Junction Rule:
_______ of the currents entering a junction must ________ the _______ of the currents leaving
SUM
EQUAL
SUM 31

32. XI. Conservation of Charge32

33. DC (Direct Current) (video clip/animation)
XII. AC vs. DC
DC (Direct Current) (video clip/animation)
AC (Alternating Current)
current (charges) flows in the same direction between the + and - terminals
used in many electronic devices (ie – computers)
can be produced by batteries, solar cells, fuel cells
the direction of the current (charges) reverses (alternates)
in the US it does this at a rate of 60/sec or 60 Hz
advantage is that power companies save a lot of money transmitting power at very high voltages over long distances
they convert AC to high voltages for transmission (above 100,000 V) then use transformers to step down to lower volts 33

34. XII. AC vs. DC Great Minds – Nikola Tesla (10 min)
Thomas Edison (DC – Low Voltage) vs.
Nikola Tesla (AC – High Voltage)
Video (6 min)
The high current problem
P = IR2 34

35. XII. AC vs. DC
Sending AC Electricity (How does electricity get to you?)
Probably the biggest advantage of AC is that you can use high voltages with small currents to reduce losses when you transmit power. Remember that lost energy increases the more collisions you have, and reducing current decreases the amount of collisions (and reduces heating in the wires). You can send power with DC, but the DC power transmission loses a lot of energy. You would have to put much more effort into sending DC power over the same distance 35