HMWK tomorrow, Lab Friday! Resistance, Voltage, Current

Lab Friday - Bring Lab Journal to class.

Homework due Thursday:   READ p. 400-405; p. 405#1-7.  (#1 is a bit confusing; you are to compare/contrast      Static discharge         vs        electric current   (one is a specific case of the other!))

Notes – Resistance, Voltage, Current

(R) Resistance – opposition to charge movement (e^- movement)

Resistance is dependent on 4 things:

            Material type – conductor vs. insulator – Conductors have less R

            Temperature – Higher temp à higher R    (discuss how digital thermometers use this principle)

            Length of wire – longer wire à higher R – discuss extension cord used on vacuum cleaner – bad, heats                                                                         wire rather than working vacuum

            Thickness of wire – thicker wire à lower R  discuss thick wires better if high current demand, not needed                                                                     for little electronics like cell phone though.

            Unit for resistance: Ohms (  Ω  )  ß greek letter omega

(V) Voltage:  Electric Potential

Voltage is the “Push” that makes charges move.

-related to the concentration of charge on an object.

            Units for voltage:  Volts ( V )

( I )  Current (the I is for intensity): flow of charge.

Current is the quantity of charge that passes a point per second (rate of charge flow)

Units for current:  Amperes ,  or Amps.  ( A ).

Next, we related Newton’s 2^nd law to Ohm’s law:

                                                (using some slight variations….)

F = m • a   Where Force is the push, mass (inertia) is the resistance to acceleration, and acceleration is “movement”

V = R • I   (usually people write it V = I•R)  or your book shows I = V / R.  All the same.

The above relationship is Ohm’s law, a basic electrical relationship.

We tried examples of using Ohm’s law to find unknown values:

Ex:  V = 6V   R = 3Ω.  Find I:    (we found I to be 2A)

Ex: you see that your toaster says “10A”.  Find its resistance.   Now since you know you’re in North America, you can assume you have 120V coming out of the wall outlet.     You should find the answer to be 12 Ω. 

Last, we graphed the effect of resistance on current present in a 10V circuit.  This showed an inverse relationship:  As resistance increases, current decreases, approaching zero.    (the image below is 1V, same idea though)

As resistance decreases, current increases, approaching infinity.  ß this is what happens in a “short circuit”  - resistance is effectively eliminated from a circuit and a huge amount of current is present, causing even good conductors to heat up and create a fire risk.  

We talked about short circuits with batteries in class and what not to do (eliminating resistors and cooking batteries)

Finally, we had a demonstration with series and parallel circuits and 25W (high resistance) and 300W (low resistance) light bulbs.  

Tomorrow we’ll play with circuits more as well as multimeters to test voltage, current, and resistance, and we’ll have a lab on Friday.  Bring your lab journal.