Monday March 4, 2013 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?

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)

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

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 )

Practice Problem What is the equivalent resistance of a 100- , a 330-  and a 560-  resistor when these are in a series arrangement? (Draw the circuit and calculate).

Practice Problem What is the equivalent resistance of a 100- , a 330-  and a 560-  resistor when these are in a parallel arrangement? (Draw the circuit and calculate).

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

Circuits Homework problem Due Monday March 11 th.

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.