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Volume B Chapter 18




        Electricity
What we will Learn…
• What is a current?
• How to draw electric circuit diagrams
• What is potential difference, voltage and E.M.F?
• What is resistance?
• S/E: Circuits arranged in series versus parallel
• S/E: Fixed resistors versus variable resistors
• Effects of electric current
What is Electricity???




               Definition:

Electricity is a form of energy that can
be easily changed to other forms.
Where does Electricity come from?
Mainly 2 sources:
1) Power Stations
   - Supply a lot of electricity
   - Used in many electrical
   appliances
2)    Electric Cells (batteries)
     - Supply a little electricity
     - Portable
     - Safe
How does an Electrical
   Appliance Work?
• To make an electrical appliance work,
  electricity must flow through it.
• The flow of electricity is called an electric
  current.
• The path along which the electric current
  moves is called the electric circuit.
What is an Electric
          Current???
                electric
                  cell            connecting
                                     wire

                filament



                       flow of electrons




Definition:

An electric current is the rate of flow of electric
charges in a circuit.
Electric Charges
• Electric charges are made up of positive
  charges (protons) and negative charges
  (electrons).

• When these charges flow in a circuit, a
  current is produced.
How does electricity flow?
• The battery in a circuit gives energy to
  the electrons and pushes them around
  a circuit, from the negative terminal of
  the cell, round the circuit and back to
  the positive terminal of the cell.
How to Measure Current?
• The SI unit for electric current is ampere (A).
• Smaller currents are measured in
  milliamperes (mA).

                 1 A = 1,000 mA

                 1 mA = 0.001 A



• Different electrical components and
  appliances require different sizes of current
  to turn them on.
Instrument to Measure Current
• An ammeter is an instrument used for
  measuring electric current.
Ammeter
• It must be connected in series in the circuit.

• Positive side of ammeter must be connected
  nearest to the positive terminal of the battery
  (electric cell), and vice versa.
Electric Circuits
• Electric circuits are                   connecting wires
  made up of electrical
  components                                electric cell




• These components
  must be joined
  together without any
  gap in between to
                             light bulb             circuit board
  form a closed circuit.
                           Note:
                           Components refer to the
                           light bulb, wires, battery
Electric Circuits
 • Incomplete circuits are called open circuits.

                   connecting wire is             no source of
                       missing                  electrical energy




Both the circuits in the diagram are incomplete, hence they are known
as “open circuits”.
An electric current flows
       only when there is:
• a source of electrical energy and

• a closed circuit
                                      connecting wires

                                        electric cell




                         light bulb             circuit board
How to draw Circuit Diagrams
  Component            Symbol   Component           Symbol

                          +                                  +


    An electric cell                Battery



                                                   Switch (open)


   Light bulb (lamp)                Switch         Switch (closed)




   Connecting wires             Connecting wires
     (not joined)                   (joined)




Symbols are used to represent the various electrical
components in circuits.
Examples of circuit diagrams:
Switches
A switch is used to open or close a circuit.




                 Main switch used
                 in buildings




Switches used on circuit
Circuit diagrams for open
   and closed circuits




Open circuit.   Close circuit.
Bulb does not   Bulb will light
light up when   up when the
the switch is   switch is
Series and Parallel
There are 2 ways in which an
  electric circuit can be
  arranged:

1. Series
2. Parallel
Series Circuit
• A series circuit connects the components one
  after the other
• A single loop is formed
• A break in any part of a series circuit stops the
  flow of current in the whole circuit.
Parallel Circuit
• A parallel circuit divides into two or more branches.
• The current divides and flows through each parallel
  branch.
• If a component breaks or is removed, the other
  components remain on.
Which of the following is a
series circuit?
Which is a parallel circuit?




 Series Circuit Parallel Cir
Draw the circuit diagram for
the following set up and state
whether it is a series or
parallel circuit.
Draw the circuit diagram for
the following set up and state
whether it is a series or
parallel circuit.
Which is brighter?
Which of the following is a
series circuit?
Which is a parallel circuit?




 Series Circuit     Parallel Circ
Voltage
• An electric cell gives energy to the electrons and
  pushes them round a circuit. Voltage is a measure
  of how much energy the electrons receive.

• Different voltages are supplied by different cells and
  batteries.


                                              1.5 V Dry Cell




  12 V Car Battery                          9 V Dry Cell
How to Measure
        Voltage?
• The SI unit for voltage is volt
  (V).
• A voltmeter is an instrument
  used for measuring voltages.
Voltmeter
• Voltmeters must be connected
  in parallel to the circuit.
• The positive side of voltmeter
  is connected to the positive
  terminal of the cell, and vice
  versa.
Recall: Ammeter!!!
• It must be connected in series in the
  circuit.

• Positive side of ammeter must be
  connected nearest to the positive
  terminal of the battery (electric
  cell), and vice versa.
Electromotive Force
        (e.m.f)
• Electromotive force is the
  same as voltage.

• E.m.f refers to the amount of
  energy supplied by the electric
  source (eg. battery) to each unit
  of electric charge.
Electromotive Force
       (e.m.f)
Potential Difference
       (P.d.)
Potential Difference
•
                      (p.d)
    Remember Diffusion?
• High  Low
                                     electric
• Similarly, electric charges          cell            connecting
                                                          wire
  will flow from a point of
                                     filament
  higher potential (energy) to
  lower potential
                                            flow of electrons




•This difference in electric potential between 2 points in a
circuit is known as the potential difference. It is the same
as VOLTAGE also.
Resistance
• When an electric current flows through a
  circuit, there will be some resistance that
  opposes it. (similar to friction)

• It can be measured by dividing voltage by the
  current.

                     V         R = Resistance
               R=              V = Voltage
                      I        I = Current
Resistance
• Good conductors of electricity have LOW
  RESISTANCE. (Eg. Metal objects)
  – Electricity is able to flow through them
    very easily

• Poor conductors of electricity have HIGH
  RESISTANCE. (Eg. Wood, cloth)
  – Electricity is not able to flow through
    them easily
Resistance
• The SI unit for resistance is ohm ()
• Different electrical components have different
  resistance
• For example, nichrome wires have a higher
  resistance than copper wires.


          So should we use
          nichrome or copper
          to make wires?
Example 1
• An electric rice cooker operates at 240 V and
  uses a current of 8 A. What is the resistance of
  the rice cooker?
                                  240
 Voltage (V) = 240             R=
                                   8
 V
 Current (I) = 8 A              = 30 

                           V
                      R=
                           I
Resistors
• An electrical component that is specially made to have a
  certain resistance is called a resistor.
• They can be connected in a circuit to resist the current
  flow.
Fixed Resistors and
         Variable Resistors
• Fixed resistors have only one resistance value
• Variable resistors can be adjusted to change
  the resistance.



     fixed resistor symbol          variable resistor symbol



   •Variable resistors are useful
   in light dimmers and other
   electric appliances
S/E: Resistors
• Resistors can be connected in
  series or parallel
                                    extra resistor in
                      Resistors connected
                                    series results in
                                     dimmer bulb

                            in SERIES


                                    extra resistor in
                       Resistors connected
                                    parallel results
                                    in brighter bulb
    single resistor
                            in PARALLEL
S/E: Resistors in Series
• When resistors are
  connected in series, the
  resistance will add up and
  increase.
• R = R 1 + R 2 + R 3…
                                      2       3




           The total resistance for this circuit is:

                         2+3=5
S/E: Resistors in Parallel
• When resistors are
  connected in parallel, the
  resistance will decrease
• This is because they will
  provide alternate routes for
  the current to flow.
S/E: Resistors in Parallel

   1   1    1    1
     =    +    +
   R   R1   R2   R3


What is the resistance of the             6
circuit?


           1                               6
   1   1
     =   +
   R   6   6

   1   2          1                      The final resistance
     =          =               R = 3
   R   6          3                      is smaller
What we will Learn…
• What is a current?
• How to draw electric circuit
  diagrams
• What is potential difference, voltage
  and E.M.F?
• What is resistance?
• S/E: Circuits arranged in series versus
  parallel
Heating Effect of Electric Current
• When an electric current flows through a wire, the wire heats up.
  Electrical energy has been converted into heat energy.

• The greater the resistance of the wire, the greater the amount of
  heat produced. This heating effect is used in common electrical
  appliances.




        Iron                          Kettle               Hair Dryer
A kettle uses both copper and nichrome wires.
Copper has low resistance while nichrome has high resistance.

Which material, copper or nichrome, should be used for the heating
element, and for the external wire?




                                               Use copper wire
                                               for the external
                                               wire as it has low
                                               resistance &
                                               produces less heat


 Use nichrome wire for the heating
 element as it has high resistance&
 produces a lot of heat
Heating Effect of Electric Current

        ARGON (inert/unreactive gas)
         filament wire produces
         heat and light



      In a light bulb, the heated filament which
      is also a resistance wire, becomes so
      hot that light is also emitted.
Chemical Effect of Electric Current
What is ELECTROLYSIS?
Definition:
• Electrolysis is the chemical change that occurs when an electric
  current passes through solutions or molten compounds.
Electrolysis
There are 2 uses of
  electrolysis:

1) Electroplating
2) Extraction of Metals
1. Electroplating
• In electroplating, a key is covered with a thin
  layer of copper when electricity is passed
  through the solution.
                                         -       +


   key             copper    copper on
                   wire      the key

                  copper
                  sulphate
                  solution
          start                     after a few minutes
Uses of Electroplating




Metal objects can be plated with a
thin layer of another metal.
2. Extraction of metals
• Some metals (eg. sodium, aluminium) are obtained
  by electrolysis.

• To extract the metal
   – heat the solid compound of the metal until it melts
   – pass an electric current through the molten
     compound
Magnetic Effect of a Current
• A straight wire is placed near a compass. When an
  electric current flows through the wire, the compass
  needle is deflected. This shows that an electric
  current has a magnetic effect.




            current
            in wire

                             compass needle is
                                deflected
Electromagnet
• An electromagnet is a magnet that is made by using electricity
• It consist of a wire coiled around a metal rod (eg. Iron)




                           coil of wire
                                          iron core




       to battery



                                          compass needle is strongly
                                            attracted to iron core
Electromagnet
The magnetic effect of the electromagnet can be
    increased by:

1.   increasing the current (by using more batteries)

2.   increasing the number of turns of wire in the coil

3.   Using an iron rod
Differences between an electromagnet and a
                   magnet:
      Electromagnet                     Magnet


                              A permanent magnet which
 A temporary magnet which
                              retains magnetism until it is
  can be turned on and off
                               purposely demagnetised
    using electric current.
                                        (spoilt).



Magnet can be made stronger   Magnet remains the same
         or weaker                    strength
Uses of Electromagnets
•   Cranes that lift iron/steel
•   Iron/steel separators
•   Electric bells
•   Magnetically levitated trains
•   Electric motors
                                                 crane




         Electric bell
                                    Electric motor in fans

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Ch 18 electricity

  • 1. Volume B Chapter 18 Electricity
  • 2. What we will Learn… • What is a current? • How to draw electric circuit diagrams • What is potential difference, voltage and E.M.F? • What is resistance? • S/E: Circuits arranged in series versus parallel • S/E: Fixed resistors versus variable resistors • Effects of electric current
  • 3. What is Electricity??? Definition: Electricity is a form of energy that can be easily changed to other forms.
  • 4. Where does Electricity come from? Mainly 2 sources: 1) Power Stations - Supply a lot of electricity - Used in many electrical appliances 2) Electric Cells (batteries) - Supply a little electricity - Portable - Safe
  • 5.
  • 6. How does an Electrical Appliance Work? • To make an electrical appliance work, electricity must flow through it. • The flow of electricity is called an electric current. • The path along which the electric current moves is called the electric circuit.
  • 7. What is an Electric Current??? electric cell connecting wire filament flow of electrons Definition: An electric current is the rate of flow of electric charges in a circuit.
  • 8. Electric Charges • Electric charges are made up of positive charges (protons) and negative charges (electrons). • When these charges flow in a circuit, a current is produced.
  • 9. How does electricity flow? • The battery in a circuit gives energy to the electrons and pushes them around a circuit, from the negative terminal of the cell, round the circuit and back to the positive terminal of the cell.
  • 10. How to Measure Current? • The SI unit for electric current is ampere (A). • Smaller currents are measured in milliamperes (mA). 1 A = 1,000 mA 1 mA = 0.001 A • Different electrical components and appliances require different sizes of current to turn them on.
  • 11. Instrument to Measure Current • An ammeter is an instrument used for measuring electric current.
  • 12. Ammeter • It must be connected in series in the circuit. • Positive side of ammeter must be connected nearest to the positive terminal of the battery (electric cell), and vice versa.
  • 13.
  • 14. Electric Circuits • Electric circuits are connecting wires made up of electrical components electric cell • These components must be joined together without any gap in between to light bulb circuit board form a closed circuit. Note: Components refer to the light bulb, wires, battery
  • 15. Electric Circuits • Incomplete circuits are called open circuits. connecting wire is no source of missing electrical energy Both the circuits in the diagram are incomplete, hence they are known as “open circuits”.
  • 16. An electric current flows only when there is: • a source of electrical energy and • a closed circuit connecting wires electric cell light bulb circuit board
  • 17. How to draw Circuit Diagrams Component Symbol Component Symbol + + An electric cell Battery Switch (open) Light bulb (lamp) Switch Switch (closed) Connecting wires Connecting wires (not joined) (joined) Symbols are used to represent the various electrical components in circuits.
  • 18.
  • 19.
  • 20. Examples of circuit diagrams:
  • 21. Switches A switch is used to open or close a circuit. Main switch used in buildings Switches used on circuit
  • 22. Circuit diagrams for open and closed circuits Open circuit. Close circuit. Bulb does not Bulb will light light up when up when the the switch is switch is
  • 23. Series and Parallel There are 2 ways in which an electric circuit can be arranged: 1. Series 2. Parallel
  • 24. Series Circuit • A series circuit connects the components one after the other • A single loop is formed • A break in any part of a series circuit stops the flow of current in the whole circuit.
  • 25. Parallel Circuit • A parallel circuit divides into two or more branches. • The current divides and flows through each parallel branch. • If a component breaks or is removed, the other components remain on.
  • 26. Which of the following is a series circuit? Which is a parallel circuit? Series Circuit Parallel Cir
  • 27. Draw the circuit diagram for the following set up and state whether it is a series or parallel circuit.
  • 28. Draw the circuit diagram for the following set up and state whether it is a series or parallel circuit.
  • 30. Which of the following is a series circuit? Which is a parallel circuit? Series Circuit Parallel Circ
  • 31.
  • 32. Voltage • An electric cell gives energy to the electrons and pushes them round a circuit. Voltage is a measure of how much energy the electrons receive. • Different voltages are supplied by different cells and batteries. 1.5 V Dry Cell 12 V Car Battery 9 V Dry Cell
  • 33. How to Measure Voltage? • The SI unit for voltage is volt (V). • A voltmeter is an instrument used for measuring voltages.
  • 34. Voltmeter • Voltmeters must be connected in parallel to the circuit. • The positive side of voltmeter is connected to the positive terminal of the cell, and vice versa.
  • 35. Recall: Ammeter!!! • It must be connected in series in the circuit. • Positive side of ammeter must be connected nearest to the positive terminal of the battery (electric cell), and vice versa.
  • 36. Electromotive Force (e.m.f) • Electromotive force is the same as voltage. • E.m.f refers to the amount of energy supplied by the electric source (eg. battery) to each unit of electric charge.
  • 39. Potential Difference • (p.d) Remember Diffusion? • High  Low electric • Similarly, electric charges cell connecting wire will flow from a point of filament higher potential (energy) to lower potential flow of electrons •This difference in electric potential between 2 points in a circuit is known as the potential difference. It is the same as VOLTAGE also.
  • 40.
  • 41. Resistance • When an electric current flows through a circuit, there will be some resistance that opposes it. (similar to friction) • It can be measured by dividing voltage by the current. V R = Resistance R= V = Voltage I I = Current
  • 42. Resistance • Good conductors of electricity have LOW RESISTANCE. (Eg. Metal objects) – Electricity is able to flow through them very easily • Poor conductors of electricity have HIGH RESISTANCE. (Eg. Wood, cloth) – Electricity is not able to flow through them easily
  • 43. Resistance • The SI unit for resistance is ohm () • Different electrical components have different resistance • For example, nichrome wires have a higher resistance than copper wires. So should we use nichrome or copper to make wires?
  • 44. Example 1 • An electric rice cooker operates at 240 V and uses a current of 8 A. What is the resistance of the rice cooker? 240 Voltage (V) = 240 R= 8 V Current (I) = 8 A = 30  V R= I
  • 45. Resistors • An electrical component that is specially made to have a certain resistance is called a resistor. • They can be connected in a circuit to resist the current flow.
  • 46. Fixed Resistors and Variable Resistors • Fixed resistors have only one resistance value • Variable resistors can be adjusted to change the resistance. fixed resistor symbol variable resistor symbol •Variable resistors are useful in light dimmers and other electric appliances
  • 47. S/E: Resistors • Resistors can be connected in series or parallel extra resistor in Resistors connected series results in dimmer bulb in SERIES extra resistor in Resistors connected parallel results in brighter bulb single resistor in PARALLEL
  • 48. S/E: Resistors in Series • When resistors are connected in series, the resistance will add up and increase. • R = R 1 + R 2 + R 3… 2 3 The total resistance for this circuit is: 2+3=5
  • 49. S/E: Resistors in Parallel • When resistors are connected in parallel, the resistance will decrease • This is because they will provide alternate routes for the current to flow.
  • 50. S/E: Resistors in Parallel 1 1 1 1 = + + R R1 R2 R3 What is the resistance of the 6 circuit? 1 6 1 1 = + R 6 6 1 2 1 The final resistance = = R = 3 R 6 3 is smaller
  • 51. What we will Learn… • What is a current? • How to draw electric circuit diagrams • What is potential difference, voltage and E.M.F? • What is resistance? • S/E: Circuits arranged in series versus parallel
  • 52.
  • 53. Heating Effect of Electric Current • When an electric current flows through a wire, the wire heats up. Electrical energy has been converted into heat energy. • The greater the resistance of the wire, the greater the amount of heat produced. This heating effect is used in common electrical appliances. Iron Kettle Hair Dryer
  • 54. A kettle uses both copper and nichrome wires. Copper has low resistance while nichrome has high resistance. Which material, copper or nichrome, should be used for the heating element, and for the external wire? Use copper wire for the external wire as it has low resistance & produces less heat Use nichrome wire for the heating element as it has high resistance& produces a lot of heat
  • 55. Heating Effect of Electric Current ARGON (inert/unreactive gas) filament wire produces heat and light In a light bulb, the heated filament which is also a resistance wire, becomes so hot that light is also emitted.
  • 56. Chemical Effect of Electric Current What is ELECTROLYSIS? Definition: • Electrolysis is the chemical change that occurs when an electric current passes through solutions or molten compounds.
  • 57. Electrolysis There are 2 uses of electrolysis: 1) Electroplating 2) Extraction of Metals
  • 58. 1. Electroplating • In electroplating, a key is covered with a thin layer of copper when electricity is passed through the solution. - + key copper copper on wire the key copper sulphate solution start after a few minutes
  • 59. Uses of Electroplating Metal objects can be plated with a thin layer of another metal.
  • 60. 2. Extraction of metals • Some metals (eg. sodium, aluminium) are obtained by electrolysis. • To extract the metal – heat the solid compound of the metal until it melts – pass an electric current through the molten compound
  • 61. Magnetic Effect of a Current • A straight wire is placed near a compass. When an electric current flows through the wire, the compass needle is deflected. This shows that an electric current has a magnetic effect. current in wire compass needle is deflected
  • 62. Electromagnet • An electromagnet is a magnet that is made by using electricity • It consist of a wire coiled around a metal rod (eg. Iron) coil of wire iron core to battery compass needle is strongly attracted to iron core
  • 63. Electromagnet The magnetic effect of the electromagnet can be increased by: 1. increasing the current (by using more batteries) 2. increasing the number of turns of wire in the coil 3. Using an iron rod
  • 64. Differences between an electromagnet and a magnet: Electromagnet Magnet A permanent magnet which A temporary magnet which retains magnetism until it is can be turned on and off purposely demagnetised using electric current. (spoilt). Magnet can be made stronger Magnet remains the same or weaker strength
  • 65. Uses of Electromagnets • Cranes that lift iron/steel • Iron/steel separators • Electric bells • Magnetically levitated trains • Electric motors crane Electric bell Electric motor in fans