The Flow of Electricity

Objectives:

    1.    Define electric current, state its unit, and name two devices used to measure it.

    2.    Define resistance and state its unit.

    3.    State and apply Ohm's Law.

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

    5.    Contrast direct current and alternating current.

Resources:

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

Notes:

The Flow of Electricity:  

Voltage:

Voltage is the measure of energy available to move electrons.   It requires a gradient (more energy on one side than the other) and a conductor.  The flow continues until the circuit reaches a common potential (equal charge) and there is no longer a flow of charge.  

In the picture to the left, the tank on the top represents the electric potential energy (V).  The charge is flowing down a gradient from an area of greater GPE to lower GPE.  This system will eventually run out of energy as its supply in the top tank dwindles.

 

 

(www.saburchill.com/physics/ images/0189.jpg)

In this system a pump has been installed to maintain the gradient.  In this way the system recycles energy with only small losses due to efficiency (WO/WI).  This is how a battery works.

 

 

(www.saburchill.com/physics/ images/0189.jpg)

Electric current

Electric Current is the flow of electrons through a wire or solution.  In a solid the electrons are passed from one positively charged metallic atom to next but in solution the electron is carried by the ions present in the solution.  A solution capable of carrying charge is called an electrolyte.  Electrolyte solutions are found in batteries as well as in all living things.

 

(www.pbs.org/.../edison/sfeature/ images/acdc_all_off.gif)

Resistance:

Is the opposition to the flow of electricity.

The resistance of a wire depends on its length, thickness and temperature

 

(dwp.bigplanet.com/.../nss-folder/ pictures/Resistors.jpg)

Ohmís Law

States that the current is equal to the voltage divided by the resistance

 

Current = voltage / resistance

I = V/R

Amperes = volts / ohms

Now this is why I took physics


A problem on dairy farms is "stay voltage," caused by corroded wiring, poor wiring practicies, or ground currents associated with nearby power lines. These conditions can result in several volts of potential difference between metal objects such as watering bowls, feed troughs, or milking equipment, and the ground. Cows feel slight shocks that make them nervous, resulting in reduced milk output. As a result, farmers can face serious financial losses. Figure 28-69 shows a circuit model for a stray voltage situation; the 1.5 kilo-ohm resister represents the resistance of corroded connections and poor wiring; you can assume the ground has negligible resistance.

(www.owlnet.rice.edu/~ruy/ media/cowphys.jpg)