## DC Circuits Week 3

## Week 3 DC Circuits

See my graphical lesson plan also https://www.learn.org.au/ueenee-course-pages/ueeneee104a-dc-circuits/dc-circuits-week-3/lesson-plan-week-3-dc/

**From my lesson plan and some but not all including:**

T6 EMF sources energy sources and conversion electrical energy encompassing:

- basic principles of producing a emf from the interaction of a moving conductor in a magnetic field.
- basic principles of producing an emf from the heating of one junction of a thermocouple.
- basic principles of producing a emf by the application of sun light falling on the surface of photovoltaic cells
- basic principles of generating a emf when a mechanical force is applied to a crystal (piezo electric effect)
- principles of producing a electrical current from primary, secondary and fuel cells
- input, output, efficiency or losses of electrical systems and machines
- effect of losses in electrical wiring and machines
- principle of conservation of energy

T7 Resistors encompassing:

- features of fixed and variable resistor types and typical applications
- identification of fixed and variable resistors
- various types of fixed resistors used in the Electro technology Industry. e.g. wire-wound, carbon film, tapped resistors.
- various types of variable resistors used in the Electro technology Industry e.g. adjustable resistors: potentiometer and rheostat; light dependent resistor (LDR); voltage dependent resistor (VDR) and temperature dependent resistor (NTC, PTC).
- characteristics of temperature, voltage and light dependent resistors and typical applications of each
- power ratings of a resistor.
- power loss (heat) occurring in a conductor.
- resistance of a colour coded resistor from colour code tables and confirm the value by measurement.
- measurement of resistance of a range of variable’ resistors under varying conditions of light, voltage, temperature conditions.
- specifying a resistor for a particular application.

T8 Series circuits encompassing:

- circuit diagram of a single-source d.c. ‘series’ circuit.
- Identification of the major components of a ‘series’ circuit: power supply; loads; connecting leads and switch
- applications where ‘series’ circuits are used in the Electro technology industry.
- characteristics of a ‘series’ circuit – connection of loads, current path, voltage drops, power dissipation and affects of an open circuit in a ‘series’ circuit.
- the voltage, current, resistances or power dissipated from measured or given values of any two of these quantities
- relationship between voltage drops and resistance in a simple voltage divider network.
- setting up and connecting a single-source series dc circuit
- measurement of resistance, voltage and current values in a single source series circuit
- effect of an open-circuit on a series connected circuit

T12 Effects of meters in a circuit encompassing:

- selecting an appropriate meter in terms of units to be measured, range, loading effect and accuracy for a given application.
- measuring resistance using direct, volt-ammeter and bridge methods.
- instruments used in the field to measure voltage, current, resistance and insulation resistance and the typical circumstances in which they are used.
- hazards involved in using electrical instruments and the safety control measures that should be taken.
- operating characteristics of analogue and digital meters.
- correct techniques to read the scale of an analogue meters and how to reduce the ‘parallax’ error.
- types of voltmeters used in the Electrotechnology industry – bench type, clamp meter, Multimeter, etc.
- purpose and characteristics (internal resistance, range, loading effect and accuracy) of a voltmeter.
- types of voltage indicator testers. e.g. LED, neon, solenoid, volt-stick, series tester, etc. and explain the purpose of each voltage indicator tester.
- operation of various voltage indicator testers.
- advantages and disadvantages of each voltage indicator tester.
- various types of ammeters used in the Electrotechnology industry – bench, clamp meter, multimeter, etc.
- purpose of an ammeter and the correct connection (series) of an ammeter into a circuit.
- reasons why the internal resistance of an ammeter must be extremely low and the dangers and consequences of connecting an ammeter in parallel and/or wrong polarity.
- selecting an appropriate meter in terms of units to be measured, range, loading effect and accuracy for a given application
- connecting an analogue/digital voltmeter into a circuit ensuring the polarities are correct and take various voltage readings.
- loading effect of various voltmeters when measuring voltage across various loads.
- using voltage indicator testers to detect the presence of various voltage levels.
- connecting analogue/digital ammeter into a circuit ensuring the polarities are correct and take various current readings.

Reviewing the DC Lab:

Exactly what and why and how everything happened in the lab from the previous week. Please watch the video for full explanation. Topic 12 meters, especially current meter internal resistance is reviewed here. We had discussed the problems with high internal resistance in current meters once before, and also the effect of low voltmeter resistance. The lab is a vital introduction to Kirchhoff’s voltage and current laws and more involved series circuits.

DC Lab 1 from week 2 (video of how it should be done)

Power in DC circuits powerpoint

## Review of previous work

## VOLTAGE, CURRENT, AND RESISTANCE Chapter 2 from Floyd 8th ed.

**Ampere: The Unit of Current (from Floyd p38)**

Current is measured in a unit called the ampere or amp for short, symbolized by A.

One ampere (1 A) is the amount of current that exists when a number of electrons having a total charge of one coulomb (1 C) move through a given cross-sectional area in one second (1 s).

See Figure 18. Remember, one coulomb is the charge carried by 6.25 X 1018 electrons.

EXAMPLE 3

Ten coulombs of charge flow past a given point in a wire in 2 s. What is the current in amperes?

I =Q/t =1 0C/2s = 5A

If there are 8 A of direct current through the filament of a light bulb, how many coulombs have moved through the filament in 1.5 s?

Q= I times t……. so 8 times 1.5 is equal to 12 C of charge.

**Ohm: The Unit of Resistance**

Resistance, R, is expressed in the unit of ohms, which is symbolized by the Greek letter omega (n).

One ohm (1 n ) of resistance exists when there is one ampere (1 A) of current in a material with one volt (1 V) applied across the material.

Conductance T h e reciprocal of resistance is conductance, symbolized by G. It is a measure of the ease with which current is established.

The unit of conductance is the siemens, symbolized by S. For example, the conductance of a 22 kohm resistor is

G = 1/22 kohm = 45.5uS

Occasionally, the obsolete unit of mho is still used for conductance

The material in Chapter 2 about resistance colour codes etc, we will start to do in week 3.

**Voltage**

Voltage is defined as energy per unit of charge and is expressed as

V=W/Q

where V is voltage in volts (V), W is energy in joules (1), and Q is charge in coulombs (C). As a simple analogy, you can think of voltage as corresponding to the pressure difference created by a pump that causes water to flow through a pipe in a closed water system.

## OHM’S LAW, ENERGY, AND POWER Floyd Chapter 3

You should have studied now about Joules and energy and power. We will start to use the unit of Watts in week 3.

Energy is the ability to do work, and power is the rate at which energy is used.

In other words, power, P. is a certain amount of energy. W, used in a certain length of time (t), expressed as follows:

P =W/ t

where P is power in watts (W), W is energy in joules (J), and t is time in seconds (s). Note that an italic W is used to represent energy in the form of work and a nonitalic W is used for watts, the unit of power. The joule is the SI unit for energy.

Energy in joules divided by time in seconds gives power in watts. For example, if 50 J of energy are used in 2 s, the power is 50 J/2 s = 25 W. By definition,

**One watt is the amount of power when one joule of energy is used in one second.**

Thus, the number of joules used in One second is always equal to the number of watts. For example, if 75 J are used in 1 s, the power is

P=W/t = 75J/1s =75Watts = 75 W

Chapter 3 is a huge chapter and is very important, giving us the equations for power and the facts about resistors and power ratings.

We will do some questions from Chapter 3 in class. You should have done them all for homework by now, as they were set last week and mentioned in week 1 also.

# Series Circuits Floyd Chapter 4

A very major chapter in your DC study. This puts together all that you have done so far, and more. We will go through this in class in week 3.

Pictures from Floyd Chap 4.

If you have been slow to borrow the book or buy the book, do not delay, **you NEED this book.**

Can you convert this board layout to a circuit diagram? Find it in your Floyd textbook and give it a go. This is an essential skill for a technician and you will be examined on this in upcoming quizzes.

This is the last page of the week 2 practical. It’s also featured in the video, but the 68 ohm resistor graph is not shown there. Take careful note of how the mA values were obtained and plotted on the graph. This is examinable of course. It’s considered essential knowledge and skill.

This was a quick wrap up of the first part of the W3 theory class. Power had just been introduced into your calculations and you are advised to check Chapter 4 of your Floyd textbook to find examples like this one.

As I indicated in class, this circuit is a test of where we are at now in the unit of study. If you can struggle through this question and get the correct results and understand what you are doing, WOW, I am impressed because you are really doing quite well. We did spend some time on this in class and I went through how all the results were gained with question and answer from the class.

**If you have not bought iCircuit as yet, you should soon consider doing so.** Here I demonstrated how we can use it to solve Kirchhoff’s Voltage Law in a simple series circuit. The voltage sources are aiding in this version of my circuit, but if you recall… we had one battery reversed and ended up with zero current flow. I had a handout for this style of study for KVL and passed it out in class. I reproduce it here as a thumbnail and give the link to the activity. I did ask you all to do this in iCircuit and test the outcome against the theory.

A Voltage divider introduced and the ratio method of voltage determination.

Three methods used to find voltages at points V1, V2 and V3.

This is really well covered in your Floyd textbook.

Make sure you do not just sleep with the textbook under your pillow…. reading the book and learning from the examples would be beneficial to your study.

Previous week 3 stuff here: