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# Electricity - PowerPoint PPT Presentation

Electricity. M.D. Electricity Did you know?. 7 percent of power generated at large central stations is lost during transmission to the user over high-voltage lines! 1 lightning bolt has enough power to service 200,000 homes! An electric eel can produce a voltage of up to 650 Volts!!

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### Electricity

M.D.

ElectricityDid you know?

• 7 percent of power generated at large central stations is lost during transmission to the user over high-voltage lines!

• 1 lightning bolt has enough power to service 200,000 homes!

• An electric eel can produce a voltage of up to 650 Volts!!

• 20 mA of current running through your body

This is the work done per unit charge to transfer a charge from one point to another. (Also Voltage)

i.e V = W

Q

Unit: Volt (V) or JC-1

Volt = The P.d between two points is one volt if one joule of work is done bringing one coulomb from one point to another.

Potential at a point = This is the p.d between a point and the Earth, where the Earth is at zero potential.

Discovered independently in 1745 by von Kliest and van Musschenbroek using the Leyden Jar while studying electrostatics.

http://micro.magnet.fsu.edu/electromag/java/lightning/

• This is the ratio of the charge on a conductor to its p.d

i.e. C = Q

V

• Unit: Farad (F) or C V-1

• Capacitor: This stores charge

• Parallel Plate Capacitor:

• C = A

• d

• A = area of overlap of plates

• d = distance between plates

•  = permittivity of dielectric (insulator between plates)

EnergyStored in a Charged Capacitor

To charge a capacitor one plate is connected to + terminal and the other to – terminal and the power supply is turned on.

An equal – charge builds up on one plate and a + charge on the other.

This charge remains even when disconnected from the power supply.

It can be discharged by connecting it to a conductor.

W = ½ CV 2

http://lectureonline.cl.msu.edu/~mmp/kap23/RC/app.htm

http://www.thephysicsteacher.ie/lcphysicscapacitance.html

Allow a.c. to flow but block d.c.

Tune in radio stations (variable capacitor)

Smooth out variations in d.c.

Camera flash

Filtering: allow certain frequencies of an alternating signal to pass but block others

This is the flow of electric charge.

{In a metal conductor it is the flow of electrons}

Size of current in a conductor is the amount of charge passing any point of that conductor per second.

I = Q t

Unit: Amp (A) or C s -1

e-

_

+

e-

• Electrons flow from – to +

• Conventional current flows from + to – i.e. flow of positive charge. (This the defined direction of an electric current).

• d.c. = direct current flows in one direction caused by a power supply.

• a.c. = alternating current is when the current reverses direction every so often e.g. mains is 100 times per sec.

• Current is the same at every point in a series circuit.

• Sum of current flowing into a junction equals sum of current flowing out of junction

• Ammeter = used to measure current and is always connected in series in the circuit.

• {Galvanometer = sensitive ammeter/microammeter}

• Current is the same at every point in a series circuit.

• Sum of current flowing into a junction equals sum of current flowing out of junction

• Ammeter = used to measure current and is always connected in series in the circuit.

• Galvanometer = sensitive ammeter/microammeter

This can also be said to be the energy lost by 1 coulomb as it moves between 2 points in a circuit.

i.e. V = W

Q

Note:W = VQDivide both sides by t (time)

W = VQ

t t

P = VI

(P = W and I = Q) t t

Voltages in series:

V = V1 + V2 + V3

Voltages in parallel:

V1 = V2 = V3

• Voltmeter is used to measure voltage and is always connected in parallel with the part of the circuit to be measured.

Electromotive Force(e.m.f.)

E.M.F. (E): a voltage applied to a circuit.

Unit: Volt

• Electric cell: device that converts chemical energy into electrical energy and is a source of E.M.F.

• Sources:

• Simple Cell

• Primary Cell

• Secondary Cell

• Thermocouple

• Mains

Copper and zinc plates are electrodes

Dilute sulfuric acid and copper sulfate is the electrolyte

Plates chemically react with the acid leaving the plates charged

Copper electrode is a positive anode

Zinc electrode is a negative cathode

This simple cell can’t be recharged as the chemicals are used up as a current flows

e.m.f. ≈ 1 V

This type of cell can’t be recharged.

Also known as a dry cell because the electrolyte is generally a chemical paste.

This is a cell that can be recharged.

Also known as an accumulator.

E.g. Car battery is a lead-acid accumulator.

Resistance (R )

This is the ratio of the p.d. across a conductor to the current flowing through it.

i.e. R = V I

Unit: ohm ()

http://micro.magnet.fsu.edu/electromag/java/filamentresistance/

This states that for certain conductors (mainly metals) the current flowing through them is directly proportional to the p.d. across them at a constant temperature.

i.e. V = IR

http://micro.magnet.fsu.edu/electromag/java/ohmslaw/

+

_

+

Bulb

_

Resistors in Series and Parallel

In series the total resistance is:

R = R1 + R2 + R3

R1

R2

R3

R1

R2

R3

• In parallel the total resistance is:

• 1 = 1 + 1 + 1

• RR1R2R3

• http://lectureonline.cl.msu.edu/~mmp/kap20/RR506a.htm

Resistance depends on;

Temperature

Material of conductor

Length

Cross-sectional area

• Temperature

• The resistance of a metallic conductor increases as the temperature increases. e.g. Copper.

• The resistance of a semiconductor/insulator decreases as the temperature increases. E.g. Thermistor.

• Length:

• Resistance of a uniform conductor is directly proportional to its length.

• i.e. R L

• Cross-sectional area:

• Resistance of a uniform conductor is inversely proportional to its cross-sectional area.

• i.e. R 1

• A

Factors affecting Resistance of a conductor

Material:

The material also affects the resistance of a conductor by a fixed amount for different materials. This is known as resistivity ().

R = L = constant of proportionality

AUnit: ohm meter (m)

 = Rd2(For a wire with circular cross-sectional area)4L

Uses:

Temperature control

Fail-safe device (automatic switch circuit off)

Measure an unknown resistance

R1 = R3 (When it’s balanced)

R2 R4

Metre Bridge:

R1 = (|AB|)

R2 |BC|

http://www.magnet.fsu.edu/education/tutorials/java/wheatstonebridge/index.html

http://www.electronics2000.co.uk/calc/calcwstn.htm

r1

r2

r4

r3

B

A

C

I

D

This is connected directly across the voltage and divides voltage into the ratio of the resistances.

E.g A rheostat (variable resistor, two fixed resistors.

The greater voltage is across the greater resistor.

The sum of the voltages is the voltage supply.

If one of the resistances is extremely large then the voltage across it is almost the same as the voltage supply.

Heat

Chemical

Magnetic

Joule’s Law:

States that the rate at which heat produced in a conductor is directly proportional to the square of the current provided its resistance is constant i.e. P = I 2R

• In order to prevent power lines from overheating, electricity is transmitted at a very high voltage (EHT: Extra High Tension).

• From Joule’s law the larger the current the more heat produced hence a transformer is used to increase voltage and lower current

• i.e. (P = VI).

Electrolysis = the chemical effect of an electric current.

Voltameter = electrodes, electrolyte and container.

Inactive electrodes = electrodes that don’t take part in the chemical reaction e.g. platinum in H2SO4

Active electrodes = electrodes that take part in the chemical reaction e.g. copper in CuSO4

Ion = an atom or molecule that has lost or gained 1 or more electrons.

Charge carriers = In an electrolyte the charge carriers are + and – ions carriers.

• Uses:

• Electroplating to make metal look better, prevent corrosion

• Purifying metals

• Making electrolytic capacitors

Relationship between V and Ifor conductors

Metallic conductor:

Negative electrons are the charge carriers

I

I

V

V

I

V

• Filament bulb:

• Negative electrons are the

• charge carriers

• Semiconductor:

• Negative electrons and positive

• holes are the charge carriers

Relationship between V and Ifor conductors

Active electrodes:

Positive and negative ions are the charge carriers

I

I

I

I

V

V

V

V

• Inactive(Inert) electrodes:

• Positive and negative ions are the charge

• carriers

• Gas:

• Positive and negative ions and electrons are

• the charge carriers

• Vacuum:

• Electrons are the charge carriers

Electricity entering the home is supplied at 230V a.c.

2 wires enter the house from the mains: Live + neutral and pass through the meter box.

These 2 wires pass into a distribution box with fuses.

Radial circuit: for appliances that take a large current. Each circuit has their own live + neutral wires and fuse e.g. cooker, electric shower.

• Ring circuit: for connections to

• sockets. Live terminals are connected

• together as are the neutral terminals.

• Lights: connected in parallel and a

• number of them are connected to

• the same fuse.

Safety in house circuits:

Switch: should always be connected in the live wire.

Fuse: piece of wire that will melt when a current of a certain size passes though it. Connected to the live wire.

• Safety in house circuits

• MCBs: miniature circuit breakers are found in the distribution box. They are bimetallic strips(for small currents) and electromagnets (for large currents). Can be reset when the switch trips, faster than fuse.

• RCDs: residual current devices protect sockets and people against electrocution by detecting a difference between current in live and neutral wire (30mA).

Safety in house circuits:

Bonding: All metal taps, pipes, water tanks etc are connected to the earth

Earthing: Earth wire prevents electrocution from touching metal parts of appliances by providing a path of least resistance when faults occur.

Kilowatt-hour (kW h)

This is the amount of energy used by a 1000 W appliance in one hour.

The E.S.B charge bills based on the no. of units (kW h) used in the home.

Slide 2: Lightning Bolt Image

http://www.msha.gov/Accident_Prevention/Tips/lightning.htm

Electric eel image ~ Amy Lebeau

www.nfpa.org/riskwatch/teach_eslp_pkk_04.html

Slide 3:Animation ~ Irina Nelson and Johnny Erickson

www.slcc.edu/schools/hum_sci/physics/tutor/2220/e_potential

Slide 4: None

Slide 5: First capacitor image

www.mainlinegroup.co.uk/jacksonbrothers/5250.htm

Slide 6: Capacitor image ~ Christopher Borg

http://qarnita.tripod.com/comp.htm

Slide 7: Bulb and battery animation ~ David Chase Edventures.com

http://discover.edventures.com/functions/termlib.php?action=&termid=153&alpha=c&searchstring=

Electric Motor animation ~ UK Motion Gallery

www.bbc.co.uk/science/robots/techlab/v_rollerbots.shtml

Slide 8: None

Slide 9: None

Slide 10:None

Slide 11: Voltages in series image ~ Andrew Turner Primary School Science

Slide 12: Voltages in series and parallel image ~ Graham Knot

http://ourworld.compuserve.com/homepages/g_knott/elect27.htm

Slide 13: Lemon battery image and video link ~ Carol and Wayne Campbell

Slide 14: None

Slide 15: Battery image ~ EDF Energy

www.edfenergy.com/powerup/keystage3/in/page2.html

Slide 16: Lead-acid battery image ~ EUROBAT The Association of European Storage Battery Manufacturers.

www.mpoweruk.com/cell_construction.htm

Slide 2: Resistors image

www.sffej.net/educational/resistor_Colour.htm

Resistor colour codes

Slide 3:George Ohm image~ www.past.dk/artefacts/photos/53/photo-1113908435-89551-5995.tkl?o

Slide 4: None (Note: Use P, for previous and N, for next on key board to go back and forth between photos if no remote control available. Both circuits are connected to a 12V power supply and can be compared in terms of how bright the 3 bulbs are)

Slide 5: None

Slide 6: Temperature and resistance animation ~ Science Joy Wagon (www.sciencejoywagon.com)

www.regentsprep.org/Regents/physics/phys03/bresit/default.htm

Slide 7: Cross sectional area and resistance animation ~ Science Joy Wagon (www.sciencejoywagon.com)

www.regentsprep.org/Regents/physics/phys03/bresit/default.htm

Slide 8: Resistors image

Slide 9: Sir Charles Wheatstone image ~ from the BT Connected Earth Collection.

See www.connected-earth.com

Slide 10: Sunset Power Lines

www.tonyboon.co.uk/imgs/pages/powerlines.htm

Slide 11: Hoffman Voltameter image

www.dalefield.com/earth/hydrogen1.html

Slide 12: Electroplating image ~ www.finishing.com/faqs/howworks.html

Slide 13: None

Slide 14: None

Slide 15: Circuit Breaker image ~ Edfenergy

www.edfenergy.com/powerup/keystage3/in/page2.html

Slide 16: Circuit Breaker image ~ Edfenergy as above

Light Circuit image ~ www.buzzybee.org/diy/projects/electrical/lighting/wiring.html

Slide 17: None

Slide 18: None

Slide 19: None

Slide 20: None