The Flow of Electricity

Objectives:

    1.    Give examples of voltage sources that can maintain a potential difference in a circuit..

    2.    Distinguish between AC and DC.

    3.    Describe the structure and uses of dry cells and wet cells.

Resources:

    http://www.sasked.gov.sk.ca/docs/physics/u3b33phy.html

Notes:

Key Terms:

    voltage source    alternating current    direct current    dry cell    wet cell    thermocouple

Review:

Electric current

• The symbol for current is I

• The unit of measurement is the ampere (A) or amp

Resistance:

• The symbol is R

• The unit of resistance is the ohm (W)

Voltage

• The symbol is V

• The unit of voltage is the volt

Ohm’s Law

• Current = voltage / resistance

• I = V/R

• Amperes = volts / ohms

Everybody knows that in order for a electrical device to work it needs a power source with enough energy to power the device.  More exactly there needs to be enough potential difference (voltage) in the source to power the device.  One type of voltage source is the electrochemical cell.

Electrochemical Cell:

·        Provides a steady electric current from chemical energy.

·        3 types

Dry Cell:

(naio.kcc.hawaii.edu/chemistry/ everyday_battery_fig1.gif)

The dry cell consists of a zinc can, electrolyte paste, and a carbon rod in the center.

•  A chemical reaction takes place between the zinc can and the paste.

• Electrons flow from the negative (zinc) terminal to the positive (carbon) terminal

• Electrons are then pumped back to the negative terminal

• The difference in the amount of electrons at the terminals creates electron pressure or what is called an electrochemical gradient.

• A series of dry cells connected to one another is called a battery.

 Wet Cell:

Also called a voltaic cell and is another type of electrochemical cell.

  Consists of an electrolyte solution, a zinc rod, and a copper rod,

• The zinc is broken down to release electrons in the electrolyte solution (H₂SO₄)

• The excess electrons then flow from the negative terminal (zinc) to the positive terminal (copper) and then back through the electrolyte solution to the zinc terminal.

• Again this is driven by electron pressure

• Commonly used as car batteries

 Thermocouple:

Is a device that changes heat energy into electrical energy.

Consists of a heat source,         a copper wire,              an iron wire

and a cold source.

•  Generates electricity as a result of temperature differences

• The greater the temperature difference, the greater the electrical current.

• Used as thermostats in cars, heaters and ovens.

Current direction:

• When current flows in one direction it is called DC (direct current)

• Examples are dry cells, wet cells, and thermocouples.  

(www.sunblock99.org.uk/sb99/people/ DMackay/new6_mov.gif)

• When current flows in both directions it is called AC (alternating current)

• Nearly all industry runs on AC (An example is the current in your home)

•    110-120V home & 220-240V industry, foreign countries and large applications (oven, air conditioning)

• Made in large electric generators

•    Generators in America alternate charge at 60Hz (cycles/sec)

(www.sunblock99.org.uk/sb99/people/ DMackay/rot1_mov.gif)

• Electrons are pumped back and forth without a net change in position
• Can be converted back to DC using a device known as a diode which acts as a one-way valve allowing the electrons to flow in only one direction