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1 Introduction to Electricity John R. Ebden Seneca College.

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1 1 Introduction to Electricity John R. Ebden Seneca College

2 Introduction to Electricity2 What is electricity? X Static electricity X Current X Voltage X Resistance X AC X DC ?

3 Introduction to Electricity3 Atoms and electrons XElectronics is based on controlling the movement of electrons

4 Introduction to Electricity4 Static electricity XRubbing objects can cause electrons to be added or removed from the object. XThe object then becomes charged because it no longer has the same number of electrons as protons. + - + + Unlike charges attractLike charges repel

5 Introduction to Electricity5 Static electricity XWhen a plexiglass rod is rubbed with a silk cloth, electrons move from the rod to the cloth, leaving the rod positively charged, and the cloth negatively charged.

6 Introduction to Electricity6 Static electricity XIf the now positively charged rod is placed near your hair, it will cause the electrons in your hair to move towards the end of the hair. XYour hair will become attracted to the positive charged rod making it stand up.

7 Introduction to Electricity7 Static electricity XStatic electricity occurs when electric charges build up on an object, but the electric charges cannot move around. He is charged up !!He just lost his charge !!

8 Introduction to Electricity8 Static electricity XStatic electricity is a big problem for companies that deal with non-conducting materials like plastics. XThe high voltages can destroy sensitive electronic components XAt companies like Celestica personnel must wear ESD coats, work on ESD surfaced tables, and wear grounded wrist straps

9 Introduction to Electricity9 Current electricity XThere is another type of electricity, current electricity, which consists of a continuous flow of electrons. XIt requires a source of electrons and a pathway to carry or conduct them. XThe source of electrons might be a battery or a generator. G

10 Introduction to Electricity10 Current electricity XCurrent electricity must flow along a pathway, usually metal, called a conductor. XCopper, aluminium, and gold are common conductors. XSome materials prevent the flow of electrons and are called insulators.

11 Introduction to Electricity11 Current electricity XA bulb or an electric appliance may be part of that pathway. Electrons are returned to the battery or generator along the pathway completing the electric circuit.

12 Introduction to Electricity12 Voltage XCurrent electricity flows because there is a difference in electrical pressure or potential between two points along a conductor. XThis potential difference is called voltage.

13 Introduction to Electricity13 Direct current (DC) and alternating current (AC) XElectric circuits can be connected to either a direct current (DC) or an alternating current (AC) electricity supply.

14 Introduction to Electricity14 Direct current (DC) XThe stream of electrons flow in only one direction around the circuit, from the negative terminal to the positive terminal

15 Introduction to Electricity15 Direct current (DC) XIn the early days, the concept of electron flow was not fully understood so scientists randomly decided that current in a conductor flowed from the positive terminal and into the negative terminal. XIt is still convention today to show current flowing in this direction (ie. opposite to electron flow).

16 Introduction to Electricity16 Alternating current (AC) XThe electrons move backwards and forwards around an electric circuit in this case

17 Introduction to Electricity17 How electricity is made XAC (most common) is made using a generator. XDC is made by: X batteries (chemical reaction) X power supplies (rectification of AC)

18 Introduction to Electricity18 DC Sources - Batteries

19 Introduction to Electricity19 DC Sources – Power Supplies

20 Introduction to Electricity20 Making AC X AC Electricity can be made or generated by moving a wire (conductor) through a magnetic field.

21 Introduction to Electricity21 Magnetism X A bar magnet has a north and south pole. It is placed under a sheet of paper and iron filings are sprinkled over the top of the paper. XThese lines of filings show the magnetic field around the magnet.

22 Introduction to Electricity22 Making AC X If a coil of wire is moved within a magnetic field electricity is produced in the coil. XAs loop is “cutting” more lines of magnetic field when it is horizontal, the current is a maximum.

23 Introduction to Electricity23 Making AC X When loop is vertical no magnetic field lines are being “cut”, and current is zero.

24 Introduction to Electricity24 Making AC X When loop has rotated 180 o it is flat again. XCurrent is now maximum but in the opposite direction.

25 Introduction to Electricity25 Making AC XThe current produced changes direction every half turn (180 degrees ). This is called alternating current or AC. XThe generators at large power stations produce nearly all the electricity we use in this way.

26 Introduction to Electricity26 Making AC XThe voltage in the loop varies with angle as shown.

27 Introduction to Electricity27 Making AC X1 rev = 1 cycle = 360 degrees Xdomestic supply = 60 cycles/sec = 60 Hz X  loop rotates at 60 rev/sec = 3600 rpm

28 Introduction to Electricity28 Making AC XIn many parts of Europe the frequency employed is 50 Hertz (cycles/sec)

29 Introduction to Electricity29 Making AC X3 phase current is used to supply commercial buildings as it is more efficient. XProduced by 3 loops rather 1

30 Introduction to Electricity30 Current XCan compare water flowing through a pipe with current in a conductor “flow of electrons”

31 Introduction to Electricity31 Current XA flow meter measures the water flow in litres/sec XAn ammeter measures the current in A (1 A = 6.24  10 18 electrons/sec)

32 Introduction to Electricity32 Current 1 Coulomb (C) = charge carried by 6.24  10 18 electrons  1 Ampere = 1Coulomb/second (A = C.sec -1 )

33 Introduction to Electricity33 Current XThe movement of charge is called electric current. XThe more electrons per second that pass through a circuit, the greater the current. XCurrent is the rate of flow of charge. I = Q/t

34 Introduction to Electricity34 Voltage XA pump can be used to cause the water to flow in the pipe. XThe force that makes the water flow is called pressure (measured in Pa or psi)

35 Introduction to Electricity35 Voltage XA generator is an electrical “pump” XThe electrical pressure is called electrical potential or voltage (measured in V)

36 Introduction to Electricity36 Voltage Force between charges is: F = kQ 1 Q 2 /r 2 To separate unlike charges (or bring like charges together) work is done W = Force x distance W =  Fdx W =  Fdx

37 Introduction to Electricity37 Voltage XWhen two objects have a difference in charges, we say they have a potential difference or voltage between them. XThe unit of voltage is the volt. XThunderclouds have hundreds of millions of volts between them.

38 Introduction to Electricity38 Voltage

39 Introduction to Electricity39 Voltage XA difference in potential energy is defined as voltage. XThe voltage between two points is one volt if it requires one joule of energy to move one coulomb of charge from one point to another. XV = Work/Charge or V = W/Q XVolt = Joule/Coulomb XVoltage is defined between points.

40 Introduction to Electricity40 Ammeters and voltmeters XAmmeters are connected into the circuit. The circuit must be “broken” to put one in. XVoltmeters are connected across components to measure voltage. No need to break the circuit Note: voltage does not flow, current does

41 Introduction to Electricity41 Measuring voltage XVoltmeter is placed in parallel XNo need to change circuit

42 Introduction to Electricity42 Measuring current XAmmeter is placed in series XWill need to “break” the circuit

43 Introduction to Electricity43 Resistance XWhen water flows in a pipe, its flow is impeded by factors such as the size of the pipe, roughness of the pipe, etc. X Anything that impedes or resists current flow in a circuit is called resistance.

44 Introduction to Electricity44 Resistance XElectrical resistance is measured with an ohmmeter. Units are ohms (  ) XThe power must be off before making any measurements.

45 Introduction to Electricity45 Ohm’s Law XRelates current, voltage and resistance: I = V/R or, V = IR or, R = V/I I = V/R or, V = IR or, R = V/I

46 Introduction to Electricity46 Example - What is the current? Lamp has resistance of 96  and battery is 12 V I = 12V/96  = 0.125A = 125x10 -3 A = 125mA

47 Introduction to Electricity47 Meters - Analogue (VOM)

48 Introduction to Electricity48 Meters - Digital (DMM)

49 Introduction to Electricity49 Meters - Oscilloscope X Used for analysing waveforms

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