1

Electricity

Basics

2

Electron

• The smallest particle caring electrical charge.

• Electrical charge is measured in Coulombs

eschelectronsCoulomb arg1024.61 18

video

3

Electrical current• Electrical current is flow of electrical charges

(by water analogy)

• Electrical current is measured in amperes

one ampere is an electrical current that one Coulomb of electric charge is transferred in one second

• Electrical current is denoted with letter “I”

ondperCoulombAmpere sec11

4

Materials and electricity

• Conductive materials (conductors):MetalsNon metals – Carbon, electrolytes etc.

• Non conductive materials (insulators):PlasticsGlassCeramicetc.

5

A circuit

• Circuit is a continuous electrical path

• Closed circuit – circuit is unbroken and electricity flows

• Open circuit – circuit is interrupted, preventing electricity to flow

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The task

Our main task is to control the electrical flow for useful purposes.

• To predict the flow of electrons through electrical circuits so to organize consumers and sources of electricity in a properly working circuits

• To prevent electrical system from failure • To predict size of the sources of electricity• To prevent people from accidents • Etc.

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Useful purposes on boats (consumers)

Continuous Long duration Sort duration

•Standby power taken by VHF or the SSB•Refrigerator •Freezer •Instruments:

boat speed

wind speed

fuel available

tachometer

•Navigation lights•Depth finder•Marine charging•Autopilot•Navigation: Radar, GPS•Cabin lights•Water maker•Air conditioning•Entertainment

•Engine starter•Pumps •Trim and tilt system•Electric winches and capstan• Bow thrusters•Microwave •Washing machine

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Factors controls current (I)in a circuit are:

• The electrical driving forces, or voltage (V)

(current increases when voltage increases)

• The resistance to flow of the circuit materials (R)

(current decreases when resistance increases)

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Ohm’s Law

whereI – Current measured in amperes [A]V – Voltage measured in volts [V]R – Resistance measured in ohms []

(Works only when electric current flaws)

R

VI

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Schematic of a circuit

V=12v R=2

62

12

R

VI

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Ohm’s Law

• Ohm’s Law can be rearranged:

RIV

I

VR

R

VI

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Loads in series• Resistive loads in series

act like one continuous load of a total resistance equal to the sum on the individual resistance

R 2

R 3

V

IR1

.321etcRRRR

.321 etcRRR

VI

R1=2R2=5R3=3

I=?Example: V=12V

V1

13

Answer

AV

I 2.110

12

352

12

14

Loads in parallel

The same voltage acts across each of the loads. The currents through the loads are:

11 R

VI

22 R

VI

33 R

VI

R1 R3V

I

1I 2I 3I

2R

Total current is:321 IIII

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Continue from the previous slide

• In other words:

321321

111

RRRV

R

V

R

V

R

VI

RV

R

VI

1

321

1111

RRRR

Remember Ohm’ low?

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Example:

V=12 V

R1=2

R2=5

R3=3

R=? I=?

R1 R3V

I

1I 2I 3I

2R

033.13

1

5

1

2

11

R 97.0

033.1

1R

AR

VI 4.12

97.0

12

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Loads in series/parallel

53.03

1

5

1111

3232

RRR

R1

R3

V

I

1I

2I 3I

2R

1.87553.0

132R

V=12 V; R1=2R=?

R2=5I=?

R3=3 I1,I2,I3=?

Solution:

875.3875.12321 RRR

AR

VI 1.3

875.3

12 AII 1.31

VRIV 2.621.3111

VRIV 8.5875.11.3323232

V1

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Monitoring electrical systems

• THE MULTIMETER (VOM) The multimeter may also known as

the Volt-Ohm-Milliammeter or VOM, or the DMM (Digital Multi- Meter). Because other functions may be included; thus the name multi-function meter or more simply; multimeter

The multimeter is an instrument used to measure:

Voltage in volts, Current in amperes, Resistance in ohms

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ANALOG MULTIMETERS

The analogue multimeter is easily recognised by:

1. A needle-like pointer

2. The semicircular scale

both of which appear on the front panel.

The pointer travels the scale and indicates the measurement on the scale directly beneath it. The scales are related to the settings of the function and range switches.

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DIGITAL MULTIMETERS

The digital multimeter is easily identified by:1. Numerical readout panel2. Absence of a scale

The measurement appears as an ordinary number within the panel so that anyone who can read numbers can read the indicated value.

Reading errors (long associated with analogue meters) are mostly eliminated by digital multimeters.

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OPERATING CONTROLS

The multimeter has two main operating controls:

1. The function switch

2. The range switch....

These are sometimes combined into one dual-purpose switch

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Measuring current

• The multimeter is connected to the circuit in series

• The function switch should be on correct function (amps)

• The range has to be chosen on expected range of the value

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Measuring resistance

• A resistor or other load measured must be isolated from the circuit

• The multimeter is connected to both ends of the item

• The function switch should be on correct function (Ohms)

• The range has to be chosen on expected range of the value

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Measuring voltage

• The multimeter is connected to the circuit in paralel

• The function switch should be on correct function (voltage)

• The range has to be chosen on expected range of the value

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Sources in series

Therefore:

RV

I

1V

2V

3VExample:

321 VVVV

R

VVVI 321

V1=1.5VR=4

V2=1.5V

V3=3.0V I=?

AI 5.14

0.35.15.1

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Sources in series

• Caution!

Batteries work in series must have the same capacity and the same state of charge. Otherwise the battery with the greatest capacity may drive the voltage of the weakest battery negative, which will destroy the weaker battery

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Sources in parallel

• Provided batteries are the same voltage. The net result is a single battery of capacity equal to the sum of the individual capacities.

RV

I

1V 2V

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Sources in parallel

• If they are not of the same voltage, the higher voltage battery will discharge into the lower voltage battery, possibly overcharging and destroying it

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Energy

• Energy is the ability to do work

Energy can be neither created nor destroyed.

Energy can be only transformed form one form to another.

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Power

• Power is the rate of doing work

• Work is done when a force applied

on a body causes it to move.

In electrical circuit the power can be easily calculated:

IVW

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Electrical measurements and units:

• Voltage is measured in Volts (V)

• Current is measured in Amps (A)

• Power is measured in Watts (W)

I

WV

V

WI

IVW

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Example:

• What is the power of a light bulb that draws 1.25 amps at 12 Volts?

WattsIVW _1525.112

• How many amps would a 20Watt lamp draw in a 12 Volt circuit?

AmperV

WI 67.1

12

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Resolve the problem:

• What is the resistance of a light bulb with a label:

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12V 15W