9. Basic Priciples of Electricity (2)

8/3/2019 9. Basic Priciples of Electricity (2)

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Basic electricity principles

WP3


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Energy principles

A balloon moves forward as air

blows out at the back end.

An example of transformationof pressure into movement.


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Hot air ballon

Here heat from combustionis transferred into mechanical

energy, lifting the balloon into

the sky.


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Electrical charge

Comb

Hair

By combing electrons move from the hair to the comb.

The hair then is erected as their will be a lot of positive charges

Left on the surface of the hair. These charges are repelling each other

Causing the hair to erect.

+

+

+

+

---

-- +

++

-


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Electrical current

----

- --

--

-

--

-

---

Few electrons, low

current flow

--

--

--

Many electrons, high

current flow1 Ampere is approximately

67 000 000 000 000 electrons

per second


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Resistans

-

-

--

When electrons collide heat is generated

and the electrons dont flow so fast.

The current is reduced due to resistance.

There is low resistance in metallic materials like copper, while

high resistance in materials like stone or glass. Copper can be used

As a conductor while glass can be used as an electrically insulating

Material.

Heat


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Conductor area

-

-

--

Heat

-

There will be less electrical flow in a thinconductor than in a wide conductor


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Battery

+

-

-

There is a surplus of electrons causing them to flow from

the negative pole to the positive just like water flow from theupper vessel to the lower


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Potential energy transformed into

mechanical energy

The rotor rotates as the water

turns it around. This is the

Principles of hydro power.


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Electricity production from hydro

power

The rotor can drive a

generator, and then

electrical power

is produced

Generator


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Hydro power

The electrical power produced is proportional to the water heightAnd the water flow through the turbine.

Turbine

Generator

Water height


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Closed loop with a load

To get electrons to flow you need a closed loop in an

electrical conductor. If we want to turn the electron flow into

something useful there must also be a load in the circuit. This

can be a lamp, that is a resistor producing light, and not only

heat, when current is flowing through.

ResistorLamp


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Similarity between a resistor and a

valve in a pipe

ResistorValve

Pump

You can compare Voltage, Current and resistance with Pressure,

water flow in the pipe and pressure drop in a valve.

If the resistance is half, the current will be twice as high:

Voltage = Current * Resistance or U = I * R


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Energy versus power

20oC 25oC

Now Two hours later

30oC20oC

Small

radiator

Large

radiator

With the large radiator you have higher power, that is

gives more Energy per time period (like per hour)

Power = 0.5 kW

Power = 1.0 kW

Energy = 0.5 kW * 2 hours = 1 kWh



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Energy versus power

20oC 25oC

Now One respectivelyTwo hours later

25oC20oC

Small

radiator

Large

radiator

With the large radiator we heat the room to 25 o C in one

hour compared to two hours with the smaller radiator

Power = 0.5 kW

Power = 1.0 kW




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Power generation

In a battery we produce directed current, DC.

In a generator we produce Alternating current,AC.

An example of a simple generator is the Dynamowhich produces the light for your bike.

To produce electric power with a generator weneed to produce a magnetic field.

A magnetic body always have a North and a Southpole, and a magnetic field between these.

An example is Earth.


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Magnetic field

N

S

N

S


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A magnetic field can induce an

electric current

when a conductor is moving through the field. This is calledinduction. The direction of the current is switching with

the direction of the conductor movement.

N S

e-


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Current in opposite direction when

conductor moves opposite direction

N S

e-


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Explanation

The electrical field in the conductor tries to

stop the movement of the conductor in the

magnetic field. When current passes the conductor, a

magnetic field is produced around it


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Current - field interactions

Field counteract

Fields enhance each other


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A coil becomes a magnet

Current is flowing through

the conductor

A magnetic field is produced turning the

coil into a magnet


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The field is enhanced by putting

a metal bar in the coil

N S


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A simple generator

.

Current in

through paper

Current out

through paper

A coil

S

N


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One period- the sinus curve

.

.

.A B

A

B

Max

..B A.

B

A

Max but

other

direction

..A B

One period

Current


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A motor is a generator but the

oppositeIn the generator the coil ( the rotor) is turned in the magnetic

field by a mechanical force and a current is generated.

In the motor a current is generating an alternating magnetic

field which is turning the coil, or the rotor as it is called.


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The transformer

When you move a conductor in a magnetic field an electricfield is created in the conductor. This is called EMF,

Electro Motoric Force.

The size of this depend on:

- The length of the conductor in the magnetic field

- How fast the conductor is moving in the field- How strong the magnetic field is

The first factor, the length of the conductor, is used in the

Transformer. High Voltages can be transformed to lowVoltages or the opposite.


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Transformer

Principally the transformer is two coils formed around

an iron frame, with different number of windings.

U2=Voltage =50V

N2=Windings =50 turnsU1=Voltage =100VN1=Windings =100 turns

N1 is twice as long as N2, and thus the voltage is twice as high.

The transformer is only possible to use for AC.


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Transformer

If the current is increased, the magnetic

flow is also increasing.

When the current flows in oppositedirection also the magnetic flow changes

direction.

The power is the same in both windings.


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Transformer power same at both

windings

U2=Voltage =50V

N2=Windings =50 turns

I2=current= 1 A

P2= power = U2*I2=50

U1=Voltage =100VN1=Windings =100 turns

I1=current= 0.5 A

P1=power= U1*I1= 50W


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Circuits in series

U = 20 V = 5+5+5+5 V

I = same in all lamps

U=5V U=5V U=5V U=5V


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Circuits in series

The serial circuits are comparable to if we have a water

pipe with several valves in series. If you first close one

vale a little, the flow goes down. If you then close the

second valve in series, the flow goes down even more. We

have the same with the resistances in the lamps


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Parallel circuits

It is more common to connect equipments and apparatus in

parallel. Then the current will vary in parallel lines.

Even here we can compare with water in pipes, but here

we have connected two pipes from one pump, in parallel.

If we then close one valve, the flow goes down in this line,

but it goes up in the other one! You can test it yourself

easily.


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Parallel circuits

20V

I= 2A

I1=0.5A

I1=0.5A

I1=0.5A

I1=0.5A


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The FuseThe purpose of the fuse is to protect expensive equipment

from to high currants.

TV Stereo PCFuse 10A

I=2A I=4A I=5A

Max current would be 2+4+5= 11 A if all apparatus are

in operation simultaneous. Then the fuse will break the

circuit. Otherwise there might be a fire instead, as the

conductors only are designed for 10 A.

If there is a short cut in some apparatus, the fuse will also

break the current, to save the rest and avoid fire.


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Measurements

A

Current is measured in

series

V R

Voltage and resistance are measured

in parallel


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Experiment for transformation of

mechanical energy into heat

Bend a spike up and down and sense the temperature change


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Heat to electricity

MagnetsWinding

Metal strip takingcurrent from the winding

that is turning around

When the candles are burning the heat turns the wheel around.

Then the winding is moving around in the magnetic field

between the magnets, and electricity is generated.


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Wind to electricityMagnetsWinding

Metal strip taking

current from the winding

that is turning around

The blow turns the wheel around.

Then the winding is moving around in the magnetic field

between the magnets, and electricity is generated.

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9. Basic Priciples of Electricity (2)

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