January 30, 2008 Introducing Current and Direct Current Circuits

Current Current is defined as the flow of positive charge. I = Q/t I: current in Amperes or Amps (A) Q: charge in Coulombs (C) t: time in seconds

In a normal electrical circuit, it is the electrons that carry the charge. So if the electrons move this way, which way does the current move? Charge carriers e-e- I

Circuit components Cell Battery

Circuit components Light bulb Wire Switch

Circuit components V Voltmeter  Ohmmeter  Ammeter

Sample problem Draw a single loop circuit that contains a cell, a light bulb, and a switch. Name the components bulb cell switch

Sample problem Now put a voltmeter in the circuit so it reads the potential difference across the light bulb. bulb cell switch V

Series arrangement of components Series components are put together so that all the current must go through each one Three bulbs in series all have the same current. I

Parallel arrangement of components Parallel components are put together so that the current divides, and each component gets only a fraction of it. Three bulbs in parallel I 1/3 I I

Minilab #1 Draw a circuit containing one cell, one bulb, and a switch. Wire this on your circuit board. Measure the voltage across the cell and across the bulb. What do you observe?

January 31, 2008 Resistance, Resistivity, and Ohm’s Law

Minilab #2 Draw a circuit containing two cells in series, one bulb, and a switch. Wire this on your circuit board. What do you observe happens to the bulb? Measure the voltage across the battery and across the bulb. What do you observe?

Minilab #3 Draw a circuit containing two cells in series, two bulbs in series, and a switch. Wire this on your circuit board. What do you observe happens to the bulbs when you unscrew one of them? Measure the voltage across the battery and across each bulb. What do you observe?

Minilab #4 Draw a circuit containing two cells in series, two bulbs in parallel, and a switch. Wire this on your circuit board. What do you observe happens to the bulbs when you unscrew one bulb? Measure the voltage across the battery and across each bulb. What do you observe?

General rules for voltage and current… How does the voltage and the current from a cell or battery get dispersed in a circuit… when there is one component? when there are two components in series? when there are two components in parallel?

Resistors Resistors are devices put in circuits to reduce the current flow. Resistors are built to provide a measured amount of “resistance” to electrical flow, and thus reduce the current.

Circuit components Resistor

Resitance, R Resistance depends on resistivity and on geometry of the resistor. R =  L/A  : resistivity (  m) L: length of resistor (m) A: cross sectional area of resistor (m2) Unit of resistance: Ohms (  )

Ohm’s Law Resistance in a component in a circuit causes potential to drop according to the equation:  V = IR  V: potential drop (Volts) I: current (Amperes) R: resistance (Ohms)

Friday, February 1, 2008 Power in Electrical Circuits

Ohmmeter Measures Resistance. Placed across resistor when no current is flowing. 

MiniLab #5 Set up your digital multi-meter to measure resistance. Measure the resistance of the each light bulb on your board. Record the results. Wire the three bulbs together in series, and draw this arrangement. Measure the resistance of all three bulbs together in the series circuit. How does this compare to the resistance of the individual bulbs? Wire the three bulbs together in parallel, and draw this arrangement. Measure the resistance of the parallel arrangement. How does this compare to the resistance of the individual bulbs?

MiniLab #6 Measure the resistance of the different resistors you have been given. Make a table and record the color of the first three bands (ignore the gold band) and the resistance associated with the band color. See if you can figure out the code.

Resistor codes Resistor color codes are read as follows: resistors/resistor.htm resistors/resistor.htm It is helpful to know this code, but you will not be required to memorize it.

Monday, February 4, 2008 Power and Equivalent Resistance

Power in General P = W/t P =  E/  t Units Watts Joules/second

Power in Electrical Circuits P = I  V P: power (W) I: current (A)  V: potential difference (V) P = I 2 R P = (  V) 2 /R

Resistors in circuits Resistors can be placed in circuits in a variety of arrangements in order to control the current. Arranging resistors in series increases the resistance and causes the current to be reduced. Arranging resistors in parallel reduces the resistance and causes the current to increase. The overall resistance of a specific grouping of resistors is referred to as the equivalent resistance.

Resistors in series R1R1 R2R2 R3R3 R eq = R 1 + R 2 + R 3 R eq =  R i

Resistors in parallel R1R1 R2R2 R3R3 1/R eq = 1/R 1 + 1/R 2 + 1/R 3 1/R eq =  1/R i )

MiniLab #7 What is the equivalent resistance of a 100- , a 330-  and a 560-  resistor when these are in a series arrangement? (Draw, build a circuit, measure, and calculate. Compare measured and calculated values).

Minilab #8 What is the equivalent resistance of a 100- , a 330-  and a 560-  resistor when these are in a parallel arrangement? (Draw, build a circuit, measure, and calculate. Compare measured and calculated values.)

Tuesday, February 5, 2008 Combination Circuits Kirchoff’s Rules

Minilab #9 Draw and build an arrangement of resistance that uses both parallel and series arrangements for 5 or 6 resistors in your kit. Calculate and then measure the equivalent resistance. Compare the values.

Kirchoff’s 1st Rule Kirchoff’s 1 st rule is also called the “junction rule”. The sum of the currents entering a junction equals the sum of the currents leaving the junction. This rule is based upon conservation of charge.

Sample problem Find the current I 4 (magnitude and direction). 4.0 A 3.0 A 1.5 A I4I4

Kirchoff’s 2nd Rule Kirchoff’s 2 nd rule is also referred to as the “loop rule”. The net change in electrical potential in going around one complete loop in a circuit is equal to zero. This rule is based upon conservation of energy.

Sample problem Use the loop rule to determine the potential drop across the light bulb. 1.5 V 9.0 V V 2.0 V V 3.0 V

Minilab #10 Draw and build a double loop circuit and verify Kirchoff’s rules for that double loop circuit.

Ohm’s Law Lab Using the resistors provided, design an experiment to create a graph such that the terminal voltage of the cell will appear as the slope of a best fit line. You must use 8 unique resistance values in your experiment.