1. Chapter 34
Electric Current

2. 34.1 Flow of Charge
Potential difference – difference in potential (voltage) across the ends of a conductor
Conductor – a substance through which electricity can flow

3. Flow of Charge
Water analogy: Water flows from high pressure (higher position) to low pressure (lower position) until water pressure equalizes
To keep water flowing, keep pumping water back to high pressure region (higher level)

4. 34.2 Electric Current Electric current – the flow of electric charge
Measured in AMPERES
= 1 coulomb per second

5. I ~ V We use the symbol I for current.
Electric Current
We use the symbol I for current.
I ~ V
Potential Difference can cause flow of charge

6. 34.3 Voltage Sources
Voltage source – an “electrical pump” that can provide a sustained potential difference
Can be dry cells, wet cells or generators

7. 34.3 Voltage Sources
Electromotive force – (EMF) the potential difference (voltage) that provides the “electrical pressure” to move electrons between the terminals in a circuit

8. Voltage Sources
Voltage is impressed across a circuit, but charge flows through the circuit
Water Analogy (again):
Water flows from high pressure to low pressure, but only water flows, not the pressure.

9. 34.4 Electrical Resistance
Electrical resistance (R) – the resistance of a material to the flow of an electric current through it
Measured in OHMS
Named after Georg Simon Ohm

10. Electrical Resistance
Depends on:
conductivity of the wire material
thickness of the wire
length of the wire
I ~ 1/R

11. 34.5 Ohm’s Law
voltage
Current = resistance
volt
Units: ampere = ohm

12. 34.5 Ohm’s Law Another way to say this law: V = voltage I = current
R = resistance
V = I R

13. 34.6 Ohm’s Law and Electric Shock
What causes electric shock
in the human body?
Current flowing through the body
0.07 A can disrupt heart rhythm and kill

14. 34.6 Ohm’s Law and Electric Shock
What effects current?
voltage and resistance
The voltage of the current’s source and the resistance of your skin

15. 34.6 Ohm’s Law and Electric Shock
The resistance of your skin can range from 100 ohms to about 500,000 ohms
100 ohms if you’re soaked in salt water
500,000 ohms if your skin is very dry

16. 34.6 Ohm’s Law and Electric Shock
Why is it dangerous to work with electrical equipment near water?

17. 34.6 Ohm’s Law and Electric Shock
In order for a shock to occur, there must be a difference in electrical potential between one body part and another.

18. 34.6 Ohm’s Law and Electric Shock
This is why a bird can stand on a live electrical wire and not get electrocuted.
(Notice both feet are touching the wire at ALL times!!)

19. 34.7 Direct Current and Alternating Current
Direct current – (DC) a flow of charge (electric current) that is always in one direction
EX: battery-operated

20. 34.7 Direct Current and Alternating Current
Alternating current (AC) – electric current that rapidly reverses direction
EX: electricity in our homes
Rates:
60 hertz (in North America)
50 hertz (most everywhere else)

21. DC vs AC
current
current AC DC
time
time

22. 34.8 Converting AC to DC
Diode – a tiny electronic device that acts as a one-way valve to allow electron flow only in one direction

23. 34.9 Electron Speeds in Circuits
Electric Fields propagate through a conductor at near the speed of light.
However, electrons are another matter:
in direct current they drift through the circuit very slowly (3 hours for a meter of wire)
in alternating current there is no net movement at all – back and forth only

24. 34.10 Source of Electrons
Electric power sources provide ENERGY, not electrons!
Energy is carried by the electric field, causing the electrons within an appliance to vibrate back and forth (AC) or drift in one direction (DC).
Your don’t get electric shock from electrons entering your body, but from the electric field making your own electrons move.

25. 34.11 Electric Power ampere · volt = C/s · J/C = J/s = watt
current · voltage
ampere · volt =
C/s · J/C =
J/s =
watt

26. Chapter 34 Vocabulary Alternating current Ampere Diode Direct current
Electrical resistance
Electric current
Electric power
Electromotive forces
Ohm
Ohm’s law
Potential difference
Voltage source