Monday March 4, 2013

Introducing CurrentandDirect 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-

I

Circuit components

Cell

Battery

Circuit components

Light bulb

Wire

Switch

Circuit components

V Voltmeter

Ohmmeter

Ammeter

Sample problemDraw a single loop circuit that contains

a cell, a light bulb, and a switch. Name the components

bulb

cell

switch

Sample problemNow 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

1/3 I

1/3 I 1/3 I

1/3 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 #3Draw 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/tP = E/tUnits Watts Joules/second

Power in Electrical Circuits

P = I V P: power (W) I: current (A) V: potential difference (V)

P = I2RP = (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

R1 R2 R3

Req = R1 + R2 + R3

Req = Ri

Resistors in parallelR1

R2

R3

1/Req = 1/R1 + 1/R2 + 1/R3

1/Req = 1/Ri )

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 1st 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 I4 (magnitude and direction).

4.0 A

3.0 A

1.5 A

I4

Kirchoff’s 2nd Rule

Kirchoff’s 2nd 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 problemUse 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 11th.

Ohm’s Law LabUsing 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.