Electronics Lab! Syllabus and Course Overview! oltage Current and Ohm s Law! Kirchoff s Laws! The oltage Divider! Syllabus and Course Overview! n http://www.stlawu.edu/academics/programs/physics!
oltage, Current, Ohm s Law! n Current!!! n oltage! Flow of charge Units: coulomb/sec --> C/s amps, A Electric potential energy/ charge; Units: Joule/C --> olts n Resistance! Opposition to current flow Units: ohms Ω, /A oltage, Work and Energy!!U E = q! Work =!"U E =!q" = "KE High High U E Bucket of water=charge People= battery I Light Bulb Low Low U E
Current: the flow of charge! n While it is the negative charges that move, current is defined to move in the direction of positive charge! + n Current moves from Higher Potential (+) to lower Potential (-)! n Positive charge leaves a region of more positive charge and moves to a region of more negative charge!! n You must have a completed loop for current to flow.! + - oltage: Potential Difference! n The energy per charge required to move an electron from one point to another! + - Work = "U = q" Δ Relative to what??? GROUND U=mgh h ΔU=mgΔh 3
GROUND! oltage= Potential Difference =! Difference between what? n Reference point where voltages are measure from--define as zero. Potentials below this point are negative. Potentials above this point are positive.! n Ground=common=earth! GROUND symbols! n Signal Ground => This is the reference point from which the signal is meaured. It is also called the common point! n Chassis Ground a connection to the box containing the circuit often used to shield from electromagnetic noise. It is also a safety to keep you from being electrocuted by AC! n Earth Ground => literally a connection to the earth which is a huge sink for electrons.! We will not really distinguish between these symbols in lab 4
Ground:! Grounds: application! Function Generator C L R Inductor R L Function Generator Oscilloscope Ch Ch channel Oscilloscope channel 5
Ohm s Law! n Potential difference = current x resistance! n =IR! Current I Slope? olts a b c a) resistor b) rheostat c) potentiometer Power! n Power=current x voltage! P = I = I(IR) = I R = R = R n Units: watts! 6
Kirchoff s Laws!! n KCL: Kirchoff s current Law OR node law! n The sum of currents flowing into a node is zero! I I I -I -I 3 -I 4 =0 I 3 I 4 n A node is a junction of two or more wires a place where the current must split! Kirchoff s Laws!! n KL: Kirchoff s voltage Law! n The sum of the voltages (or potential differences) around a loop is zero! R I R. Pick a direction for the current to flow. Pick a direction to move around the loop 3. If you cross a battery from negative to positive, you get a positive voltage, + 4. If you cross a resistor in the direction of the current, you have a negative IR. + IR + IR + = 0 7
Kirchoff s Laws!! R R + IR + IR = 0 =! IR! IR I I =! /( R + R ) R R! IR! IR = 0 I I = /( R + R ) = IR + IR NOTE: For the left circuit we chose the wrong direction For the current. Therefore, when we solve for the current we get a negative number, telling us the current goes in the opposite direction of the one we picked. oltage Divider! Assume no current flows out So I 3 =0 and I =I =I I R in -IR -IR =0 in I I 3 in -I(R +R )=0 out =IR I= out /R I R out in -( out /R )(R +R )=0 in = out (R +R )/R out / in = R /(R +R ) 8
Ammeter/oltmeter! Read oltage ACROSS an element Two wires in meter A R R Read Current THROUGH The circuit Need to break circuit to put in meter 9