# Unit D: Electrical Principles & Technology

### Review of the Atom

• protons have a positive charge

• electrons have a negative charge

• neutrons have no charge (neutral)

• atoms may gain or lose electrons and become ionized

• charged particles can either repel or attract each other

### Electrical Charge

• most objects have an equal amount of positive and negative charge so they are neutral overall

## Charges:

• when there are more protons than electrons, the object has a positive static charge

• when there are more electrons than protons, the object has a negative static charge

Bringing a charged object near a neutral object may cause charge separation in the neutral object

### Law of attraction

Opposite charges attract each other, like charges repel each other (No this does not apply to love)

### Electrical Discharge

Build up of charge on one object may cause a transfer of charge to another object the resulting spark is the electrical discharge

• (ex) “shock” from touching someone, lightning, Van de Graaf generator, Tesla coil

• a Van de Graaf generator uses friction from a belt being rubbed to build up a static charge

# Current Electricity

### Electrical Current

• static electricity is not useful since it doesn’t flow continuously

• electric current is the continuous flow of electrons

## Circuit:

a circuit is the continuous path that an electrical current flows through

It is made up of:

1. conductor – wires

2. load – device to convert energy eg) light bulb

3. energy source – battery

## conductors:

Allow electrical charge to move eg) copper wiring

• small devices (cell phones, MP3 players etc) use very little current

• large devices (cars, generating stations) use a lot of current

# Voltage vs. Amperes

## Voltage or potential difference:

a measure of how much electrical energy each charged particle carries

• the unit of voltage is the volt (V)

easiest way to measure volts is with a voltmeter

## Amperes (A):

The rate at which electrical current flows is measured in a unit called

# Electrical Safety

REASON WHY YOU SHOULD'NT FLY A KITE NEAR ELECTRICAL WIRES

### Short Circuits

• a short circuit is an unintended path for electricity

• electric current will always take the shortest path available

• if a power line is down, electricity will not flow because the circuit is open

• if you go too close you will close the circuit and the electricity will flow through you!

# Dangers of Electrical Shock

Both voltage and current can be dangerous!!

• Skin is a bad conductor of electricity therefore a power supply voltage must be higher than 40 volts to “push” a fast flow of charges through a human body and cause electrocution

• However, high voltage is only dangerous if there is a flow of electrons (current) you can get a zap of 10,000 volts from scuffing your feet on a carpet and then touching a door knob but thankfully death doesn’t happen because there is no current!

# Common Voltages !

Voltage (volts)

• human cell 0.08 V

• microphone 0.1 V

• photocell 0.8 V

• electrochemical cell 1.1 V to 2.9 V

• electric eel 650 V

• wall outlets in house 120 V, 240 V

• generators in power stations 550 V

The current rating must be higher than ten milliamps  (10 mA or 0.01 A)

At a current lower than 10 mA, even a high voltage power supply cannot electrocute you

# Electric Current Ratings

Electrical Device

• electronic wrist watch 0.00013 A (0.13 mA or milliamps)

• electronic calculator 0.002 A (2 mA)

• electric clock 0.16 A (160 mA)

• light bulb (100W) 0.833 A

• television (color) 4.1 A

• vacuum cleaner 6.5 A

• oven element 11.4 A

• toaster 13.6 A

• car starter motor (V-8) 500 A

### too much electricity flowing through a person’s body can cause the following:

• pain

• loss of muscle control

• burns

• damage to the heart

• death

# Insulators

• Electricity is more dangerous when current can flow easily, like a copper wire.

• Current does not flow as easily through insulators such as wood, plastic, rubber and air.

• To prevent injury and short circuits many conductors are wrapped in an insulator (Hence why all power cords are coated in plastic)

• Moist materials tend to conduct better than dry materials

• If an insulator is damaged, shock from a short circuit is possible

# Protection from Electricity

Our homes and the devices we use are equipped with safety features to protect us from electric shocks many appliances and devices have three-pronged plugs the third prong connects the device to the ground wire of the building to provide another pathway for electricity just in case of a short circuit

Fuses and circuit breakers interrupt a circuit when too much current flows through it fuses have a wire in them that melts if the current gets too high breakers have a wire that trips a switch if it heats up too much both a blown fuse and a tripped breaker will open the circuit, not allowing electricity to flow

The current in a lightning strike can be 30 000 A (current as low as 0.1 A can be fatal) lightning rods are placed on the top of tall buildings etc and they are connected to the ground by a wire the lightning strike is then carried to the ground to be discharged

# Cells & Batteries

## Battery:

Provides power, contains cells where electrons move via a conductor from one type of metal to another type of metal.

### 4 Parts of a cell:

1. Two electrodes, essentially two pieces of metal, both have to be different metals. Why??

2. A paste that contains ions, the electrodes are submerged in this paste. This paste is called an electrolyte.

3. A conductor which connects the electrode giving up electrons to the electrode that is taking electrons.

4. A porous membrane (sometime called a salt channel)

## Dry Cells:

Electricity producing cells, “dry” because the electrolyte inside is a paste

## Wet Cells:

Electricity producing cells, “wet” cells because the electrolyte (ex H2SO4(aq)) is a liquid.

• Use in batteries for cars.

• Cheaper and easier to make than dry cells but you have to be careful not to spill the electrolyte which is highly corrosive

# RechargeableCells

• The chemical reactions inside a rechargeable cell are reversible, these are called secondary cells

• Can be used to store energy from an outside source (solar panels)

# Electrochemistry

## Electrochemistry:

The study of chemical reactions that involve electricity

## electrolysis:

An industrial process that is used to separate useful elements

• (ex) hydrogen and oxygen gases for fuel for the space shuttle

## electroplating:

Uses current to deposit atoms of a metal onto an electrode

• (ex) silver plating

## electrorefining:

Process that removes impurities from a metal

# Controlling the Flow of Electrical Current

## Insulators:

Electrons in insulators are tightly bound to the positive nucleus of their atoms and they cannot flow

• Glass

• Rubber

## Conductors:

Electrons are not tightly bound and are free to move (with some resistance)

• Metals

## Superconductors:

Superconductors are perfect conductors

• The electrons have no resistance to flow

• The temperature must be very low (close to absolute zero) for superconductivity

VARIOUS RESISTORS, THE DIFFERENT COLOUR BANDS CORRELATES TO VARIOUS RESISTANCE AMOUNTS

## Resistance:

Is a measure of how difficult it is for electrons to flow through a substance

• Resistance is measured in Ohms (Ω)

• The resistor gains energy from each electron that passes through it

• This energy can be released as heat or light

(ex) in a space heater or light bulb, liquids can also be good resistors

• Adding resistors to a series line consecutively increases the total resistance of that line...

(ex) Total (Ω) = Resistor1 (Ω) + Resistor 2 (Ω) + Resistor 3 (Ω)....

(ex) Total 150 Ω = R1 (50 Ω) + R2 (75 Ω) + R3 (25 Ω)

## Resistors:

Resistors allow only a certain amount of electric current to pass

## Switches:

Used to control the flow of electricity through a circuit

• When the switch is on, two conductors are pressed together, closing the circuit and making electricity flow

• switches are enclosed in an insulating case for protection

## Variable Resistors:

Allow the gradual adjustment of electric current

THE 'OLDSCHOOL' METHOD OF DRAWING A VARIABLE RESISTOR

• also called rheostats

• made of one single curved resistor

• a dial changes the amount of the resistor that is used

## Transistors:

A switch that is controlled by an electric input, however it does not have any moving parts

# Modelling & Measuring Electricity

### Modelling Electricity

Because electricity “flows”, we can use water to be a model for how electricity behaves

A waterfall is a good model for voltage a

• A change in elevation allows the water to flow because of gravitational potential energy in a circuit, a change in potential difference from a battery allows the electrons to flow

• The higher the potential difference (waterfall) the higher the voltage (energy)

A pipe is a good model for resistance and current

• (Ex) If you use a pipe to drain a pool, a longer, thinner pipe will have the most resistance and will drain it much slower (lower current)

• If you use a short, wide pipe, it will have less resistance and will drain faster (higher current)

# Ohm’s Law

Georg Ohm found a relationship between voltage (V), current (I) and resistance (R):

1. Resistance in a conductor is constant

2. Current is directly proportional to voltage

Thus...Increasing the voltage in a conductor will increase the current since resistance is constant

where:   V = voltage in Volts (V)

I = current in Amperes (A)

R = resistance in Ohm’s (Ω)

V = IR

### Example 1

Calculate the voltage if the current is 0.5 A and the resistance is 2.0 Ω.

I = 0.5 A  , R = 2.0 Ω, V = ?

V = IR

V = (0.5 A)(2.0 Ω)

V = 1.0 V

### Example 2

Calculate the current if the voltage is 1.2 V and the resistance is 0.6 Ω.

V = 1.2 V, R = 0.6 Ω, I = ?

V = IR or... I = V/R

I = 1.2V/0.6Ω

I = 2.0 A

### Example 3

Calculate the resistance if the current is 1.1 A and the voltage is 2.2 V.

V = 2.2 V, I = 1.1 A, R = ?

V = IR or... R = V/I

R = 2.2V/1.1 A

R = 2.0 Ω

REVIEW QUESTIONS

# Test Meters

A TYPICAL MULTIMETER

All test meters use a small amount of current

## Test Meters:

Measures the following

• voltmeter - potential difference between two points in a circuit (voltage)

• ammeter - electrical current which is the rate of flow of electricity

• galvanometers - measures small amounts of currents

• ohmmeters - measures resistance

• multimeters - all in one measuring tool, measures voltage, current or resistance in a circuit

# Analyzing & Building Electrical Circuits

### Circuit Drawings

Engineers and scientists use special symbols to draw, plan and analyze electrical circuits

## Circuits have 4 basic parts:

1. Source – provides energy and a supply of electrons

2. Conductor – path for current

3. Switch – controls current flow

4. Load – converts electrical energy into another form of energy

• you can draw simple circuits using the common circuit symbols

### Examples

Draw each of the following circuits:

1.    3 cell battery, 1 motor, 1 switch

2.    2 cell battery, 1 resistor, 2 lamps, 1 switch

3.    3 cell battery, 3 lamps, 1 ammeter, 1 switch

4.    4 cell battery, 1 fuse, 3 lamps, 1 motor, 1 rheostat

5.    3 cell battery, 2 lamps, 2 switches that must control the lamps from either end of a room (both on/off function)

# Series Circuits

A SERIES CIRCUIT - CLOCKWISE WE HAVE THE BATTERY, A LAMP, ANOTHER LAMP, A AMMETER, AND A SWITCH.

## Series circuit:

A circuit which has all loads in one single loop

• electrons have only one path the follow

• an interruption results in no flow of current

• adding loads increases the resistance in the circuit

# Parallel Circuits

A PARALLEL CIRCUIT - TOP TO BOTTOM WE HAVE A BATTERY, A LAMP, AND ANOTHER LAMP.

## parallel circuit:

A circuit which has more than one pathway for the electricity to flow through

• loads in separate loops will not interfere with each other

• loads in separate loops reduces the resistance in the circuit

# Applications of Circuits

CHRISTMAS LIGHTS THAT USE SERIES LIGHTING - YOU GET WHAT YOU PAY FOR

• House wiring uses parallel circuits…you don’t want one light bulb burning out to shut down all power!

• Houses also have a circuit in series so that you can turn off all power at once if needed

• Christmas lights use parallel wiring so the bulbs will stay lit even if one burns out

• Microcircuits (microelectronic circuits) are extremely small circuits that are made up of transistors and resistors