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:

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)

• Sometimes called potential difference (electrical PE)

• Measured in volts (V) (1Joule/coulomb)

• The higher the voltage, the more energy each electron carries

• In order for there to be voltage there must be a continuous replacement of charge. (battery, power plant) 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.

• Is measured according to how many electrons pass a given point each second.

• The symbol for current is I

• The unit of measurement is the amperes (A) or amp (1coloumb/second or 6.24 x 106 electrons)

• The net charge on the wire carrying the current is zero. (www.pbs.org/.../edison/sfeature/ images/acdc_all_off.gif)

Resistance:

Is the opposition to the flow of electricity.

• The symbol is R

• The unit of resistance is the ohm (W)

• Good wires have low resistance

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

• Thinner wires have greater resistance than thick wires

• Longer wires have greater resistance than shorter wires

• Hotter wires have greater resistance due to the increased KE (friction) within the wire.

• Superconductors often are kept at very low temperatures where the resistance becomes near zero (dwp.bigplanet.com/.../nss-folder/ pictures/Resistors.jpg)

# Ohm’s 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.

• (a) The resistance from the cow's mouth to hoof is approximately 350 ohms. How much current will flow through the cow in the situation shown? (It only takes about 1 mA to affect milk production.)
• In an effort to diagnose the problem the farmer moves the cow aside and connects a multimeter between the watering bowl and the ground. What will it read if it's set to measure (b) voltage and (c) current?

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