Electric Current

Dr. Bill Pezzaglia

Updated 2012Oct31

V. Current & Conduction

A. Current

B. Resistance & Ohms Law

C. Electric Power

D. Circuits

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A. Current

1) Current as Flow of Charge

2) Conduction Model

3) Conservation of Charge

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1. Current is the Flow of Charge

a. Definition: the flux of (positive) charge

b. SI Units:Amp=Coulomb/secoften use mA or A.

• 2 mA threshold of feeling• 10 mA pain• 20 mA can’t let go• 100 mA DEAD• 100,000 A Lightning

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2. Conduction Model

In metals, it’s the electrons that really move (smaller, lighter than protons).

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NOTE: Electrons flow in opposite direction of “conventional current”

3. Conservation of Charge

Conservation laws in the universe:• Conservation of momentum• Conservation of Energy• Conservation of Angular momentum

• Conservation of Charge• Current flowing out of a Leyden Jar (capacitor) must match the

loss of charge in jar:

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B. Resistance

1) Ohm’s Law

2) Resistivity

3) Non-Ohmic devices

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1. Ohm’s Laws (1872) [1826?]

• Current through a device is proportional to voltage

• Resistance “R” is in units of “ohms”

• Ohm=Volt/Amp=kgm2/(sC2)

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2. Resistance of a device

• Resistance of a device is proportional to length, inversely proportional to cross section area

• Resistivity of materials divided into classes:• Insulator: High resistance (Rubber)• Semiconductor Medium resistance (e.g.

Carbon)• Conductor Low resistance (metals)• Superconductor Zero resistance!

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The historical definition of the “ohm was a 1 meter column of mercury with cross section area of 1 mm2. The unit system has been changed slightly since that time, such that the column of mercury would only be 0.96 ohms.

3. Non-Ohmic Behaviora)Resistance changes with temperature (and the

temperature changes with current).• Conductors: resistance increases with temperature• Semiconductors: resistance decreases with

temperature

b)Light Bulb: resistance approximately increases linearly with current (more current, higher temperature)

c) Vacuum Tube: Non linear

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3d Diodes

Diode• Has low resistance in one direction• High resistance in other direction

• Behaves like a “one way street”. Current can only flow in direction of arrow.

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C. Electric Power

1) Source of Power (Batteries)

2) Electric Work

3) Joule Heating

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1. The Electric Battery(a) First Battery? 400 AD? The Baghdad Battery is the common name for a number of artifacts probably discovered in the village of Khuyut Rabbou'a (near Baghdad, Iraq) in 1936. These artifacts came to wider attention in 1938, when Wilhelm König, the German director of the National Museum of Iraq, found the objects in the museum's collections, and in 1940 (having returned to Berlin due to illness) published a paper speculating that they may have been galvanic cells, perhaps used for electroplating gold onto silver objects. -wikipedia

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1b. Luigi Galvani (1737-1798)

•1786 first battery cell (two different metals in contact)

•1791 Animalistic nature of electricity (frog legs jump from electric charge)•(Mary Shelly used this idea in her “Frankenstein” book)

•Did he also do work on corrosion? (galvanized nails?)

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http://www.corrosion-doctors.org/Biographies/VoltaBio.htm

1c. Voltic Pile

• 1786 Galvani creates first cell

• 1793 Volta shows cell creates an electric current

• 1800 makes 30 volt “Voltic Pile” from a column of cells

• 1826 Ohm determines voltage is the driver of current

• 1830 Faraday figures out the electrochemical reactions of a battery.

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http://www.corrosion-doctors.org/Biographies/VoltaBio.htm

Alessandro Volta(1745-1827)

2. Electric Work

(a) Battery is like a “pump” that increases energy of charge (current) passing through it

• The “voltage gain” is called EMF (electromotive force), measured in units of “volts”

• Change in potential energy of charge q passing through battery is: U=V q

• Often “EMF” is given the symbol “” or E

• “Cathode” is plus end, “anode” the negative

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2b. Electrical Power

• Power is units of Watts=Joule/Second

• 1841 Joule shows electrical work is equivalent to mechanical work

• Electric Power:

• Hence: Watt=VoltAmpOften we use Kilowatts or Megawatts

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2c. Energy Usage

• Energy or work done is Power x Time

• Hence: Joule=Wattsecond

• However, the PGE uses the weird unit of:Kilowatt-Hour

• 1 kWh=3.6106 Joules

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3. Joule Heating

In a resistor, the electrical energy is converted into heat.

(a) Power lost in Resistor:

(b) You can show that less energy is lost in transmission wires if you use high voltage with low current, rather than low voltage with high current.

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D. Circuits

1) Networks of Resistors

2) House wiring

3) Rectifiers

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1. Networks of Resistors

(a) Resistors in series add (same current)

(b) Resistors in parallel have same voltage, total resistance:

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R2

R2R1

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2. Which circuit will give most light?

Hints:•Batteries in series give most voltage

•Bulbs in parallel will each get same voltage (in series they would share it)

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3. Rectifiers

(a) DC or “Direct” Current is constant

(b) AC or “Alternating” Current oscillates polarity

(c) Rectifiers change AC into DC

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