1. Charge Flow, Voltage (again), Resistance, AC/DC, e- in a circuit
Electric Current
Chapter 34:
Charge Flow, Voltage (again), Resistance, AC/DC, e- in a circuit

2. Flow of Charge
When ends of a conductor are at different electric potentials, there is a potential difference between the ends (terminals).
When there is a potential difference, or difference in potential voltage, across ends of a conductor then charge will flow.
The flow of charge continues until both ends reach a common electric potential.

3. Electric Current
Electric current is the flow of electric charge in electrons.
In solid conductors, e- carry the charge through the circuit because they are free to move. p+ stay put.
Electric current is measured in amperes.
Ampere (A) = flow of 1 coulomb (C) of charge per second (s).
The conductor has a net charge of ZERO.

4. Voltage Source
Is something that produces a potential difference across a conductor.Potential difference IS voltage, & that CAUSES current.
Amnt of current that flows in a circuit depends upon voltage provided by the source. Dry cells, wet cells, & generators are capable of maintaining steady flow that allows charges to move.
Remember current flows through a circuit, while a potential difference occurs across a circuit.

6. Electric Resistance
Electrical resistance of a conductor is a measure of its opposition to the flow of electric current.
The resistance of a wire depends upon the conductivity of the material, the thickness and length of the material.

8. Resistance & Ohms Thick wires have less resistance than thin wires.
Long wires have more resistance than short wires.
Also, increased temperature means increased resistance.
Superconductivity – little to no resistance at very low or very high temps.
Resistance is measured in units called ohms.

9. Ohm’s Law
Ohm discovered that the current in a circuit is directly proportional to voltage and inversely proportional to resistance, or:
Current (amps) = Voltage (V) / Resistance (Ω)
I = V / Ω
1 ampere = 1 volt / 1 ohm
How much current is drawn by a lamp that has R of 100 Ω when voltage of 50 V is impressed across it?
I = V / R ; I = 50 V / 100 Ω; I = 0.5 A

10. Ohm’s Law & Electric Shock
When a bird perches with both feet on one wire, no electric potential is created, therefore it is not shocked.
When shocked, damage to the body is caused by current passing through the body.
The current depends upon the voltage applied and the resistance of the human body.
Resistance of the human body ranges from 100 ohms (soaked in salt water) to ohms (very dry).
Less resistance means more current, meaning more damage; hence why its easy to get shocked when wet.

11. Why is a third ground prong required on some electronics
Why is a third ground prong required on some electronics? Robert Hughes, former Prof Engineer, ( )
Its the ground terminal, intended to be continuous from the switch to the device, to insure that the equipment part you are likely to touch is always at ground potential.
Ground potential is the safest, potential of a rod driven into earth, or a cold water pipe. You are not likely to be hurt by electricity if you touch something that is grounded to the earth.
The device’s neutral conductor is also directly connected to ground potential by a grounding jumper wire.
This is so any short circuit between the frame of a toaster, mixer, heater, etc., and the hot conductor in the electrical system creates an immediate short circuit, which trips the circuit protective device, immediately denergizing the circuit.
This eliminates the possibility of a person being shocked or electrocuted by the voltage that would otherwise be on the chassis of the equipment.
Think 120v, 240, 277, or 480 volts, very painful & deadly.

12. Direct Current & Alternating Current
Electric current can be direct current (flowing on one direction) or alternating current (flowing in two directions.)
In the U.S., alternating current is more popular because it is able to transmit over great distance without losing as much energy to heat.
Tesla (AC) vs Edison (DC). Tesla won (we use AC, but he personally lost).

13. Converting DC to AC
Diodes are used, allowing e- to only flow in one direction. LEDs. Capacitators are used, storing current for short amts of time to even out impulses.
Step up transformers are used, with separate coils of wires that use magnetic fields to increase current
Can convert DC to AC, AC to DC & DC to a different DC.
Step down transformers are used to reduce electric line voltage to a safe level before entering buildings.

15. Source of Electrons
The source of electrons in a circuit is the conducting material itself.
Energy flows into the circuit.
The energy creates an electric field, begins vibrating electrons within the conductor.
In DC circuits, e- drift slowly.
In AC circuits, e- move back & forth.
Is EM radiation emitted by phones an issue? Pg 692

16. Speed of Electrons in a Circuit
At room temperature, electrons in a metal wire move at a few million kilometers per hour due to thermal motion.
When a battery or generator is attached to the wire, an electric field is generated.
The electric field can travel through a circuit at almost the speed of light.
The electrons continue their random motion, but now are moved along by the electric field.

17. Electric Power A charge moving in a circuit expends energy.
The rate at which electric energy is converted into another form of energy (kinetic/mechanical, heat, light, sound, etc) is called electric power.
Electric power is equal to the product of current and voltage or,
Electric power (P) = current (I) x voltage (V) or,
1 watt = 1 ampere x 1 volt, or
I = P / V

18. Answer: B Assessment Questions
Electric charge will flow in an electric circuit when
electrical resistance is low enough.
a potential difference exists.
the circuit is grounded.
electrical devices in the circuit are not defective.
Answer: B

19. Assessment Questions
The electric current in a copper wire is normally composed of
electrons.
protons.
ions.
amperes.
Answer: A

20. Assessment Questions
Which statement is correct?
Voltage flows in a circuit.
Charge flows in a circuit.
A battery is the source of electrons in a circuit.
A generator is the source of electrons in a circuit.
Answer: B

21. Assessment Questions
Which type of copper wire would you expect to have the least electric resistance?
a thick long wire
a thick short wire
a thin long wire
a thin short wire
Answer: B

22. Assessment Questions
When you double the voltage in a simple electric circuit, you double the
current.
resistance.
ohms.
resistors.
Answer: A

23. Assessment Questions
To receive an electric shock there must be
current in one direction.
moisture in an electrical device being used.
high voltage and low body resistance.
a difference in potential across part or all of your body.

24. Assessment Questions
The difference between DC and AC in electrical circuits is that in DC
charges flow steadily in one direction only.
charges flow in one direction.
charges steadily flow to and fro.
charges flow to and fro.
Answer: B

25. Assessment Questions
To convert AC to a fairly steady DC, which devices are used?
diodes and batteries
capacitors and diodes
capacitors and batteries
resistors and batteries
Answer: B

26. Assessment Questions
What is it that travels at about the speed of light in an electric circuit?
charges
current
electric field
voltage
Answer: C

27. Assessment Questions
When you buy a water pipe in a hardware store, the water isn’t included. When you buy copper wire, electrons
must be supplied by you, just as water must be supplied for a water pipe.
are already in the wire.
may fall out, which is why wires are insulated.
enter it from the electric outlet.
Answer: B

28. Assessment Questions
If you double both the current and the voltage in a circuit, (P = I V) the power
remains unchanged if resistance remains constant.
halves.
doubles.
quadruples.
Answer: D