1. Unit 13 Electric Circuits

2. Electrostatics refresher:
Voltage
1. ________ is the force created by a buildup of charge.
And is also called a potential difference.
2. Voltage can make electrons __________.
3. Electron movement allows ____________ to flow.
move
current

3. Common Electrical Units
1. The unit of charge is the COULOMB
Charge has the variable Q, the unit C
2. The unit of voltage is the VOLT
Voltage has the variable is V, the unit is V
3. The unit of current is the AMP
Current has the variable I, the unit is A
4. The unit of resistance is the OHM
Resistance has the variable R, the unit W

4. Think in Pairs Electricity is two things: Voltage and Current
Voltage has two polarities: + and –
Two types of electrical power: AC and DC
Electricity has two purposes: as an energy source (power) and as a signal
Signals can be either analog or digital
Alternating current and direct current
Signals represent information

5. Electricity is…. A good power source Easy to create Easy to control
Efficient to transmit
Easy to measure
Makes everything work – our way of life depends on it

6. What is a schematic?
Diagram or drawing using symbols to build a circuit.
There are two types of circuits.
Series- one path for electrons, if one bulb goes out they all do
Parallel – more than one path for electrons, if one bulb goes out the rest can still shine 6

7. Battery Symbol DC ( Direct Current -1 direction ) Voltage source
The long side is positive
The short side is negative
Consists of one or more cells ( batteries )
Cells ( Batteries ) are in series
Cell voltage adds to give total source voltage
Dry Cell ( Battery ) – chemical paste ( Ex: A, AA, D,
cell phone Nicad, Lantern 6-Volt, 9-Volt )
Wet Cell (Car Battery ) – Sulfuric acid & Distilled H2O

8. DC vs. AC DC ( Direct Current ) electrons travel in 1 direction
EXAMPLES: Wet Cell and Dry Cell Batteries
AC ( Alternating Current ) electrons travel in 2 directions ( back and forth )
EXAMPLE: AC is the type of electricity that you have in your home

9. AC Step-Up Transformer
STEP-UP - raises the voltage of the output of the main generator to the level of the grid transmission lines.
(so e-’s can travel long dist.’s to your house)

10. AC Step-Down Transformer
STEP-DOWN – reduces the voltage so electricity can enter your home for 120V & 220V outlets
---

11. Fuse symbol A Fuse is 1 of the major protection elements
Another protective device is called a circuit breaker
Fuses & Circuit Breakers - Protects the circuit, not the person!
Ground Fault Circuit Interrupter (GFCI) is a type of circuit breaker
GFCI protects people
Turns off the circuit, like a switch
A switch is a control element, not a protection element
Replaced once it is blown
With the same amperage rating!
Ground Fault Circuit Interrupter measures circuit current supply and circuit current return, and if there is a difference (even a small one) it means that there is some other path for current to flow, perhaps through a person to earth ground.

12. Circuit Breakers & Fuses
Too many appliances on at the same time in a home cause the overall resistance of the circuit to be lowered.
Result is a circuit carrying more electrical current than is safe it is Overloaded. Fires can result when this happens.
Circuit Breakers act as a switch and open the circuit.
Fuses – blow to open the circuit

13. Resistor Symbol One of the most common circuit elements
Used to limit or restrict current
Resistance amount indicated with a color code
A circle or band around the resistor
The physical size indicates the power rating
Not the resistance value!
The smaller the resistance value, the larger the amount of current
A two lead device with no polarity.

14. FYI : Resistors information
THE END

15. Voltmeter Symbol (Must be wired in Parallel)
Measures across a device
Red lead to positive, black to negative
Measures a voltage difference
Has a high amount of internal resistance
So no current flows from the circuit into the meter
Does not change circuit voltage, current, or resistance
Voltage range is usually a meter setting
Selector switch or button
Some meters have a auto range function

16. Ammeter Symbol Must be wired in Series Measures amperage
Amp meter
Must be placed in the circuit
Current must flow into (through) the meter to be measured
An ammeter has low (zero) internal resistance
So it doesn’t reduce circuit current
Leads must be changed from volts/ohms
Dangerous to the meter!

17. An Electrical Circuit Must have a voltage source Must have a load
A resistor or other device to limit current
Can be a circuit component, like a light
Must have a complete path from one side of the voltage source to the other
Usually a wire
Should have a protection device and a control device
Usually a fuse or circuit breaker and a switch

18. A Simple Series Circuit Series – only 1 path
Battery
Resistor
(Load)
Current Flow + -
The dark lines are wires
Negative and positive voltages are relative to ground.
When ground is placed at the most negative point, then every other voltage relative to ground will be positive.
Voltage is measured across the battery or load, current flows through devices (inside wires).
Ideally, this circuit should have a control device (switch) and a protection device (fuse)
All of the voltage is dropped across the resistor
Current flows from negative to positive

19. Open switch, no current flows
Resistance is infinite ( electrons do not flow )
Voltage is dropped across the switch
Voltage is always used up in a circuit. In other words, all of the voltage is always used. The component with the higher resistance uses more of the available voltage. When the resistance is infinite (as in this circuit with an open switch) the switch drops (uses) all of the voltage, but no current is produced because the resistance is infinite.

20. Closed switch – current flows
Current flows from negative to positive
Here the amount of current is determined by the amount of voltage and the resistance of the light bulb.

21. Parallel Circuit More than 1 path

22. OHM’s LAW – I = find this formula on your chartV R
OHM’s LAW – I = find this formula on your chart
An electric circuit must have a complete path from one side of a power supply (+V) to the other (-V)
Voltage is caused by a build-up charge
Voltage across a resistor creates current
Voltage, current, and resistance follow Ohm’s Law
Current is a flow of charge ( AMPS )
ampere = volts / ohms
OR it could be 1 amp = 1 Coulomb / sec

23. The Ohm’s Law V
I = V / R
R = V / I
V = I x R R I

24. Circuit Calculations R = VS = Vs = 20 V R = 250 Ω
I = ? A ( Solve for I )
I = V / R V / 250Ω = .08A
VS equals source voltage

25. Circuit Calculations R = VS = Vs = 24 V I = 37 mA Convert to Amps
Re-arrange formula to solve for R.
R = V / I V / .037 A = 648 Ω
VS equals source voltage

26. Circuit Calculations R = VS = I = 5.9 mA R = 2.2 kΩ What is V?
V = I x R = A x 2200 Ω = 13 V
VS equals source voltage

27. A Series Circuit R1 R2 VS R3 RT = R1 + R2 + R3 VS = V1 + V2 + V3
(Resistance Adds)
1. Formula for total resistance -
A series circuit has one path for current flow. The same current goes through all the circuit components.
2. Formula for total voltage across -
VS = V1 + V2 + V3

28. A Parallel Circuit The voltage across each branch is the same
The formula for Voltage: VS = V1 = V2 = V3
Hint: use the 1/x button (also shown as the x-1 button on some calculators) to solve for RT.
The formula for Resistance:

29. PhET Ohm’s Law Simulation
What you need to keep in mind when using the simulator.
How do changes in the variables affect the voltage?

30. Series-Parallel Circuit
Work the Problem to Solve for Current
1st  Solve for Load 1 (4 Ω) and Load 2 (4 Ω) ( What formula? )
2nd Add Load 3 (2 Ω) to answer in 1st step ( What Formula ?)
3rd  solve for current ( V = 12 Volts ) (What Formula ? )

31. Electrical Power Formula
P = VI P = Watts ( always write this out )
V = Volts ( V )
I = Amps ( A )

32. Let’s Practice some Math!!