1. Conceptual Physics
Chapter 23:
ELECTRIC CURRENT

2. Electric Potential Unit of measurement: volt, Example:
Twice the charge in same location has twice the electric potential energy but the same electric potential.
3 times the charge in same location has 3 times the electric potential energy but the same electric potential
1 volt 
1 coulomb
1 joule
This is just a quick review from last week.

3. Flow of Charge
Why does heat flow from the end in flame to the end in your hand?
Because there is a temperature difference between the ends.
Only when the ends reach the same temperature (thermal equilibrium) does energy (heat) cease to flow.

4. Flow of Charge
Water flows from high pressure to low pressure until equilibrium is reached – then it ceases to flow.
Or for example, when there is a difference in pressure at two ends of a pipe… water will flow until the pressure equalizes.
Likewise…
When the ends of an electrical conductor are at different electric potentials—when there is a potential difference—charge flows from one end to the other.
When we scuff our shoes on the rug and collect extra electrons, we create a potential difference between our body and say a doorknob. Electricity will flow from me to the doorknob until there is no more potential difference. This happens fast! You get a quick shock… then nothing.
If we want electricity to continue to flow, what can we do?
How would we get water to continually flow over a waterfall and into a pond?

5. Flow of Charge
A continuous flow is possible if the difference in water levels—hence the difference in water pressures—is maintained with the use of a pump.
Use a pump!
Likewise…
To attain a sustained flow of charge in a conductor, some arrangement must be provided to maintain a difference in potential while charge flows from one end to the other. A pump for electric charge! A battery!

6. Electric Current
Electric current is the flow of charged particles through conductors.
Measured in Coulombs per Second or Amperes
Amperes is after Andre Ampere
Recall, 1C = charge on 6.25 billion billion electrons
In metal wires:
Conduction electrons are charge carriers that are free to move throughout atomic lattice.
Protons are bound within the nuclei of atoms.
In fluids:
Positive ions and electrons constitute electric charge flow.

7. Which of these statements is true?
Electric Current
CHECK YOUR NEIGHBOR
Which of these statements is true?
Electric current is a flow of electric charge.
Electric current is stored in batteries.
Both A and B are true.
Neither A nor B are true.
A. Electric current is a flow of electric charge.

8. Which of these statements is true?
Electric Current
CHECK YOUR ANSWER
Which of these statements is true?
A. Electric current is a flow of electric charge.
A. Electric current is a flow of electric charge.
Explanation:
Voltage, not current, is stored in batteries. The voltage will produce a current in a connecting circuit. The battery moves electrons already in the wire, but not necessarily those in the battery.

9. Electric Current Terminology Matters! Charge flows through a circuit;
Voltage is potential difference and so is established across a circuit.
If you forget, think back to the water pipe example…
Water flows THROUGH because of a pressure difference ACROSS the ends of the pipe.
So Voltage produces current if there is a closed circuit.

10. Voltage Sources
Voltage sources provide the electric potential DIFFERENCE that causes current to flow from higher potential to lower potential.
Some voltage sources are:
Generators
Batteries
Unit of Electrical Potential Difference is the Volt (V)
Voltage sources are the pumps. They create the pressure difference that induces flow. (A Voltage source only can provide that potential difference if the circuit is closed)
Generators – An Alternator, for example, does work to pull negative charges away from positive ones. We’ll explore generators more in Ch. 25
Batteries – Convert chemical energy into electrical energy (potential). Separate positive and negative ions.

11. Voltage Sources Electric potential difference (continued)
Example: Water from a higher reservoir to a lower one—flow continues until no difference
No flow of charge occurs when potential difference is zero.
The purpose of this slide is just to point out the EP DIFFERENCE. That a battery (like a water pump) maintains a difference in EP so that current will flow.

12. Voltage Sources
Comparison of a water circuit and an electrical circuit. Introduce RESISTANCE
Current that flows depends not only on voltage but also on the electrical resistance. (in the water example this = the hose itself)

13. Voltage Sources So here is an actual circuit. No switch here though.
In chemical batteries:
* Work by chemical disintegration of zinc or lead in acid.
* Energy stored in chemical bonds is converted to electric potential energy.
The tiny filament in the bulb is the resistance.

14. Electric Resistance Ω How much current flows in a circuit depends on:
Voltage
Electrical resistance (measured in Ohms) Ω
Resistors are circuit elements that regulate current inside electrical devices.
Show omega – symbol for Ohms
Resistors are Circuit elements that regulate current inside electrical devices.
Discuss Mr. Ohm
The stripes on the resistors are color coded to show their resistance in Ohms

15. Electric Resistance Is this slide useful?
Maybe just point out that the two openings provide resistance… keep all the water from exiting all at once.
The larger opening provides LESS resistance than the small one. When we have less resistance we have greater flow (current) for the same pressure difference.

16. Factors Affecting Electric Resistance
Cross Sectional Area: Thin wires provide more resistance than thick wires
Length: Doubling the length, doubles the resistance
Material: Rubber provides much more resistance than copper of the same size
Temperature: The higher the temperature, the more the resistance
1. Inversely proportional to cross-sectional area (think of hose example and this makes sense) (return to previous slide to show this)
2. Directly proportional to length – again hose example works
3. Material: Specifically its conductivity (like trying to make ketchup flow in a hose instead of water!)
4. Temperature

17. Electric Resistance Resistance comes from any load on a circuit.
Like a bulb. The large resistance provided by the filament causes some electrical energy to be converted to heat and light.

18. Electric Resistance Superconductors
Materials with zero electrical resistance to the flow of charge.
Flow of charge is without generation of heat.
Now that we have discussed resistance, the idea of superconductors might be better appreciated.
In a Superconductor – zero electrical resistance. Zero Heat.
[High-temperature superconductors
Refers to ceramic materials that can carry much current at a low voltage.]

19. Ohm’s Law
Ohm’s law gives us the relationship between voltage, current, and resistance.
OHM’S LAW
The current in a circuit varies in direct proportion to the potential difference, or voltage, and inversely with the resistance.

20. Ohm’s Law In Equation Form: voltage Current  resistance
WRITE: I = V/R
Example 1: Keep R constant, 2x Voltage results in 2x Current (WHAT happens to the Current if I keep the same R but I double the V?)
Example 2: 2R and 2V, Current is unchanged. (WHAT happens to the Current if I Double the R and Double the V?)

21. Ohm’s Law CHECK YOUR NEIGHBOR
When you double the voltage in a simple electric circuit, you double the
current.
resistance.
Both A and B.
Neither A nor B.
A. current.

22. A. current. Ohm’s Law CHECK YOUR ANSWER
When you double the voltage in a simple electric circuit, you double the
A. current.
A. current.

23. Ohm’s Law
Think!
How much current is drawn by a lamp that has a resistance of 100 ohms when a voltage of 50 volts is impressed across it?

24. Ohm’s Law
Think!
How much current is drawn by a lamp that has a resistance of 100 ohms when a voltage of 50 volts is impressed across it?

25. Electric Shock Damaging because of current passing through the body.
Caused by voltage
The amount depends on the body’s resistance
Electric potential difference between parts of your body depends on body condition and resistance, which can range from 100 ohms (soaked in salt water) to 500,000 ohms (very dry skin).
In your house, 120V is the standard Potential Difference. This is capable of producing harmful current in your body.
Water can be a good insulator if it is pure (distilled – no ions). BUT normal tap water has many ions which make it a good conductor.
So NEVER use electrical devices near water.
Also, we sweat which produces a layer of salt on our skin. This dissolves when wet, creates ions, decreases resistance.
Current always passes along the path of least resistance that connects the two points at different electric potential. How hurt you are depends on the path the electricity takes. (across heart – very bad)
Electric shock causes our muscles to contract (after all our brains have a nervous system which uses electrical signals). Can cause you to be unable to let go, to be thrown, etc.
It can overheat tissues in the body or to disrupt normal nerve functions. It can upset the nerve center that controls breathing.
Grounding Wire (see extra info):
Third (longest) prong on a plug. (in order that it be the first to connect)
Provides a ground – a way for charge to escape.
Prevents harm to the user if there is a short circuit that might otherwise use person as a path to the ground.

26. Electric Shock
Birds sit along high voltage wires. Why don’t they get shocked?
Every part of the bird’s body is at the same high potential as the wire, and it feels no ill effects.
For the bird to receive a shock, there must be a difference in potential between one part of its body and another part.
Most of the current will then pass along the path of least electric resistance connecting these two points.

27. Direct and Alternating Current
Direct current (dc)
Flows in one direction only.
Electrons always move from the negative terminal toward the positive terminal.
Batteries provide DC current.
Always from neg to pos.

28. Direct and Alternating Current
Alternating Current (ac)
Electrons in the circuit are moved first in one direction and then in the opposite direction, alternating to and fro about fixed positions.
This is accomplished by alternating the polarity of voltage.
Think of the agitator in a washing machine. Electrons are sloshing back and forth. How fast? 60 Hz (60 times per sec) This is the standard in North America. (AC at 60HZ and 120V)
Thank Nikola Tesla for AC! Share some info about him here??

29. Direct and Alternating Current
Power transmission is more efficient at higher voltages.
Europe adopted 220 V as its standard.
U.S. continued with 120 V because so much equipment was already installed.
Why do we use AC? What’s the advantage?
Because it’s easy to step up or down the voltage using transformers. And very high voltage is much better for long-distance electrical transmission because much less energy is lost to heat. It’s more efficient.
So many electrical devices we use though require DC current…. Can we switch from AC to DC? Yes!

30. Direct and Alternating Current
Converting from ac to dc
Household current is ac, but current in laptop is dc.
The converter uses a diode, a tiny electronic device that acts as a one-way valve to allow electron flow in one direction only.
Many devices we power require DC not AC.
Point out the converter on my laptop plug. Or the large wall plug on other converters… bring one?

31. Direct and Alternating Current
Converting from ac to dc
When input to a diode is ac, output is pulsating dc…
Using a diode though, causes pulsating current, not steady DC current (since it’s like a one way valve).
There are ways to fix this though...

32. Direct and Alternating Current
Converting from ac to dc
A capacitor is used in conjunction with the diode (or usually a pair of diodes) to smooth out the current.
Charging and discharging of a capacitor provides continuous and smoother current.
In practice, a pair of diodes is used so there are no gaps in current output.
The capacitor acts like the pool in this picture. You put in water one bucketful at a time… not very steady. But by having a reservoir, the output can be a smooth stream.

33. Speed and Source of Electrons in a Circuit
How fast do electrons move in a circuit?
Let the kids take a guess.
Speed of light? If that were the case, we’d never need to build multi million dollar particle accelerators! Just bend a wire at right angles and electrons would come streaming out!

34. Speed and Source of Electrons in a Circuit
When we flip the light switch on a wall and the circuit,
an electric field is established inside the conductor.
The electrons continue their random motions while simultaneously being nudged by the electric field.
Turns out, the electric field travels at near the speed of light, but the electrons themselves “drift”, slowly making progress in one direction because they are influenced by the electric field.
Speed of electrons in a circuit is about .01 cm per second!
Current is established through the wires at nearly the speed of light.
It is not the electrons that move at this speed.
It is the electric field that can travel through a circuit at nearly the speed of light.
Think of a marching band stretching down along a city street in a parade. Now if you whisper in the ear of the first one in line to start marching, it will take awhile for the ones in the back to realize that the march has started. This is NOT how electricity works in a circuit. Rather, it’s analogous to shouting “FORWARD MARCH!” And all start at once. The field (my shout) travels at near the speed of light. But the electrons do not.

35. Speed and Source of Electrons in a Circuit
Misconceptions about electric current:
“Current is propagated through the conducting wires by electrons bumping into one another.”
NOT true: Electrons that are free to move in a conductor are accelerated by the electric field impressed upon them.
True, they do bump into one another and other atoms, but this slows them down and offers resistance to their motion.
Electrons throughout the entire closed path of a circuit all react simultaneously to the electric field.

36. Speed and Source of Electrons in a Circuit
Misconceptions about electric current:
“Electrical outlets in the walls of the homes are a source of electrons.”
NOT true: The outlets in homes are ac. Electrons make no net migration through a wire in an ac circuit.
When you plug a lamp into an outlet, energy flows from the outlet into the lamp, not electrons. Energy is carried by the pulsating electric field and causes vibratory motion.
Electrical utility companies sell energy. You provide the electrons.
When you are jolted by an AC electric shock, the electrons making up the current in your body originate in your body.
Electrons do not come out of the wire and through your body and into the ground; energy does.
The energy simply causes free electrons in your body to vibrate in unison.
Small vibrations tingle; large vibrations can be fatal.

37. Electric Power
Electric power is the rate at which electric energy is converted into another form.
Power = current  voltage
Units: Watts or Kilowatts
Example: 100-watt lamp draws
0.8 ampere.
SHOW THE FOLLOWING ON THE BOARD:
Think back to when we originally learned about power. We said that: Power = Work/Time
But we know Work is synonymous with a change in Energy (Work/Energy Theorem).
So Power = Energy/Time
But now I’ve told you that Power = current x voltage. Does this conflict with our previous definition? NO!
Current = Charge / Time AND Voltage = Energy / Charge
Multiply the two and you get Energy / Time which is our original definition of Power.
******************
When you receive a power bill, it shows that I used so many kWh (kilowatt hours). Well that’s ENERGY! The electric company sells ENERGY and charges me for how much energy I consume.
CQ: Does a 40W bulb HAVE 40W or USE 40W when lit? (power is energy per time) Answer: USE

38. Electric Power
Think!
How much power is used by a calculator that operates on 8 volts and 0.1 ampere? If it is used for one hour, how much energy does it use?
Power = current × voltage = (0.1 A) × (8 V) = 0.8 W.
Energy = power × time = (0.8 W) × (1 h) = 0.8 watt-hour, or kilowatt-hour.

39. Compact Fluorescent Lamps (CFLs)
Compact fluorescent lamps (CFLs) are a type of fluorescent lamp that fits into a standard lightbulb socket.
For the same wattage, CFLs emit much more light and much less heat than incandescent bulbs.
Incandescent bulbs dissipate most of their energy in the form of heat (90%), not light (10%). So, they are not energy efficient.
Fluorescent lamps, on the other hand, emit much less heat, which is why you can touch them without burning yourself.
CFLs are just compact fluorescent lamps. You can replace a 100W incandescent with a 25W CFL and still have the same light!

40. Light-Emitting Diodes (LEDs)
Another light source even more long-lasting is the light-emitting diode (LED).
Between CFLs and LEDs, common-use incandescent bulbs will soon be history.
LEDs began as simply the little red light on your device to tell you it’s charging. Now you can buy LED bulbs that light up your whole room! They’ve come a long way!

41. Electric Circuits
A circuit is any path along which electrons can flow from the negative terminal to the positive terminal
What is a circuit?
When the switch is open, no complete circuit, no current. Closed – completes the circuit, current flows.

42. Electric Circuits Two common ways to make a circuit: SERIES
Forms a single pathway for electron flow between the terminals of the voltage source
PARALLEL
Forms branches, each of which is a separate path for the flow of electrons
There are two types of circuits. We will be exploring these in depth.

43. Series Circuits

44. Series Circuits Electric current through a single pathway.
Total resistance to current is the sum of individual resistances.
Current is equal to the voltage supplied by the source divided by the total resistance of the circuit.
The total voltage impressed across a series circuit divides among the individual electrical devices in the circuit so that the sum of the “voltage drops” across the resistance of each individual device is equal to the total voltage supplied by the source.
The voltage drop across each device is proportional to its resistance.
If one device fails, current in the entire circuit ceases.
Draw a simple series circuit with three resistors and go through these bullets one at a time.

45. Series Circuits Current (I) is SAME throughout the whole circuit.V
Current (I) is SAME throughout the whole circuit.
Voltage drops across each resistor will SUM to the battery voltage.
Equivalent Resistance – Find by adding up the resistance of every individual resistor.
REQ = R1 + R2 + R3

46. Parallel Circuits

47. Parallel Circuits
Voltage is the same across each device and is equal to the voltage of the battery/source.
The total current in the circuit divides among the parallel branches. The amount of current in each branch is inversely proportional to the resistance of the branch.
The total current in the circuit equals the sum of the currents in its parallel branches.
As the number of parallel branches is increased, the overall resistance of the circuit is decreased.
A break in one path does not interrupt the flow of charge in the other paths.
Draw a simple parallel circuit with three resistors and go through these bullets one at time.

48. Parallel Circuits
As you open more checkout lanes, the traffic in each is decreased. This is how current behaves in a parallel circuit.

49. Parallel Circuits V R1 R2 R3
Voltage (V) is SAME across each resistor, and equal to the battery voltage.
Current (I) through each resistor will SUM to the current through the battery.
Equivalent Resistance:
REQ = R1 + R2 + R3

50. Electric Circuits CHECK YOUR NEIGHBOR
When two identical lamps in a circuit are connected in parallel, the total resistance is
Less than the resistance of either lamp.
the same as the resistance of each lamp.
less than the resistance of each lamp.
None of the above.
A. less than the resistance of either lamp.

51. Electric Circuits CHECK YOUR ANSWER
When two identical lamps in a circuit are connected in parallel, the total resistance is
A. less than the resistance of either lamp.
Explanation: Resistors in parallel are like extra lines at a checkout counter. More lines means less resistance, allowing for more flow.
A. less than the resistance of either lamp.

52. Overloading Parallel Circuits
Homes are wired in parallel.
As more and more devices are connected to a circuit, more current moves through the wires.
There is an amount of current each device can carry before it overheats. When the current is excessive, overheating can result in a fire.
Note: With a battery, you would simply drain the battery very quickly and then no more current would flow. But this can be really dangerous in a house where the supply is “unlimited”

53. Electric Circuits
Safety fuses are wires that melt when the given current is exceeded. In modern buildings, circuit breakers are used.
A circuit breaker is an automatic switch that turns off when the current is excessive
Safety fuses are connected in series along the supply line to prevent overloading in circuits.