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Electricity Electricity W Richards Worthing High

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Electricity Electricity W Richards Worthing High Powered By Docstoc
					Electricity
       W Richards


   Worthing High School
                 Basic ideas…
Electric current is when electrons start to flow around a
circuit. We use an _________ to measure it and it is
measured in ____.


Potential difference (also called _______) is how big the
push on the electrons is. We use a ________ to measure
it and it is measured in ______.


Resistance is anything that resists an electric current. It
is measured in _____.”


 (Words: volts, amps, ohms, voltage, ammeter, voltmeter)
More basic ideas…
    If a battery is
    added the current
    will ________
    because there is a
    greater _____ on
    the electrons



    If a bulb is added
    the current will
    _______ because
    there is greater
    ________ in the
    circuit
    Current in a series circuit
If the current                        The
here is 2                             current
amps…                                 here will
                                      be…

 The current                          And the
 here will                            current
 be…                                  here will
                                      be…




     In other words, the current in a series
        circuit is THE SAME at any point
   Current in a parallel circuit
A PARALLEL circuit is one where the current has a “choice
of routes”

                             Here comes the current…



                              Half of the current
                              will go down here
                              (assuming the bulbs
                              are the same)…


                              And the rest will
                              go down here…
  Current in a parallel circuit
If the
current
here is 6                And the
amps                     current here
                         will be…

The current
here will be…


                       The current
The current            here will be…
here will be…
    Voltage in a series circuit
If the voltage       V
across the
battery is 6V…


…and these
bulbs are all
identical…


…what will the   V         V
voltage across
each bulb be?                     2V
   Voltage in a series circuit
If the voltage       V
across the
battery is 6V…




…what will the
voltage across   V
two bulbs be?
                                 4V
Voltage in a parallel circuit
                  If the voltage across
                  the batteries is 4V…



                What is the
                voltage here?             4V
   V


             And here?
                                          4V
   V
                       Summary
In a SERIES circuit:


         Current is THE SAME at any point
     Voltage SPLITS UP over each component


In a PARALLEL circuit:


      Current SPLITS UP down each “strand”
    Voltage is THE SAME across each”strand”
An example question:


          6V
                         A3
3A
               A1

     V1


           A2

     V2             V3
    Advantages of parallel circuits…

There are two main reasons why parallel circuits are used
  more commonly than series circuits:


1) Extra appliances (like bulbs) can be added without
   affecting the output of the others


2) If one appliance breaks it won’t affect the others
   either
         Resistance
    Resistance is anything that will
  RESIST a current. It is measured
   in Ohms, a unit named after me.


                                           Georg Simon Ohm
                                              1789-1854
The resistance of a component can be
calculated using Ohm’s Law:

                                              V
Resistance    =   Voltage (in V)
   (in )         Current (in A)
                                       I          R
          An example question:
                                       Ammeter
                                       reads 2A
                              A




      V
                1) What is the resistance across
                   this bulb?
                2) Assuming all the bulbs are the
Voltmeter
                   same what is the total resistance
reads 10V
                   in this circuit?
       Current-voltage graphs

   I                                        I


                      2. Bulb
              V                                    V
                       I



1. Resistor                             3. Diode
                                V




          Explain the shape of each graph
     Three simple components:

1) Diode – only lets current flow in one direction


2) Light dependant resistor – resistance DECREASES when
   light intensity INCREASES


3) Thermistor – resistance DECREASES when temperature
   INCREASES
Wiring a plug
                     DC and AC      V

DC stands for “Direct
Current” – the current only
flows in one direction:
                                            Time



                                 1/50th s

AC stands for “Alternating                  240V
Current” – the current
changes direction 50 times
every second (frequency =                     T
50Hz)

                                    V
                        Fuses

Fuses are _______ devices. If
there is a fault in an appliance
which causes the ____ and
neutral (or earth) wire to cross
then a ______ current will flow
through the _____ and cause it
to _____. This will break the
_______ and protect the
appliance and user from further
_____.

 Words – large, harm, safety, melt, live, circuit, fuse
              Circuit breakers




If the current becomes too high the __________ is
activated. This will ______ the iron and the contact will
be _______. This will break the circuit.
Circuit breakers have two main advantages over fuses:
they work ______ and can easily be ______.

Words – electromagnet, broken, attract, reset, quicker
                   Earth wires
Earth wires are always used if an appliance has a _____
case. If there is a _____ in the appliance, causing the live
wire to ______ the case, the current “_______” down the
earth wire and the ______ blows.




       Words – fuse, fault, metal, surges, touch
              Power and fuses
Power is “the rate of doing work”.
The amount of power being used in
an electrical circuit is given by:             P

   Power = voltage x current
    in W     in V      in A               V         I


Using this equation we can work out the fuse rating for any
appliance. For example, a 3kW (3000W) fire plugged into a
240V supply would need a current of _______ A, so a
_______ amp fuse would be used (fuse values are usually 3,
5 or 13A).
               Power and fuses
Copy and complete the following table:

 Appliance   Power rating   Voltage (V)    Current     Fuse needed
                 (W)                      needed (A)   (3, 5 or 13A)
 Toaster         720           240

   Fire         2000           240

 Hairdryer       300           240

  Hoover        1000           240

 Computer        100           240

  Stereo         80            240
                  Charge (Q)
As we said, electricity is when electrons move around a
circuit and carry energy with them. Each electron has a
negative CHARGE. Charge is measured in Coulombs (C).
We can work out how much charge flows in a circuit using
the equation:


      Charge = current x time
                                               Q
       (in C)    (in A)   (in s)


                                         I         T
                Example questions
      Charge (C)             Current (A)              Time (s)
                                  5                      2
          0.4                     1
          20                     0.5
          50                                            250
                                  3                      60

1) A circuit is switched on for 30s with a current of 3A. How much
   charge flowed?
2) During electrolysis 6A was passed through some copper chloride
   and a charge of 1200C flowed. How long was the experiment on
   for?
3) A bed lamp is switched on for 10 minutes. It works on a current of
   0.5A. How much charge flowed?
            Energy and charge
The amount of energy that flows in a circuit will depend on
the amount of charge carried by the electrons and the
voltage pushing the charge around:


 Energy transferred = charge x voltage
       (in J)          (in C)   (in V)
                                                E


                                           V         Q
             Example questions
1) In a radio circuit a voltage of 6V is applied and a charge
   of 100C flows. How much energy has been transferred?
2) In this circuit the radio drew a current of 0.5A. How
   long was it on for?
3) A motor operates at 6V and draws a current of 3A. The
   motor is used for 5 minutes. Calculate: a) The motor’s
   resistance, b) the charge flowing through it, c) the
   energy supplied to it
4) A lamp is attached to a 12V circuit and a charge of
   1200C flows through it. If the lamp is on for 10 minutes
   calculate a) the current, b) the resistance, c) the energy
   supplied to the bulb.
Comparing magnets and solenoids

Magnet:
             N         S




Solenoid:
                    Electromagnets
  The strength of an electromagnet can be increased by
    doing three things:


  1) Increasing the voltage


  2) Increasing the number of coils


  3) Inserting an iron core

    (The poles can be reversed by
reversing the direction of the current)
 Electromagnetic
     induction

The direction of the induced
  current is reversed if…
1) The magnet is moved in the
   opposite direction
2) The other pole is inserted
   first
 Electromagnetic
     induction

The size of the induced current
  can be increased by:
1) Increasing the speed of
   movement
2) Increasing the magnet
   strength
3) Increasing the number of
   turns on the coil
                   AC Generator




Induced current can be increased in 4 ways:
1) Increasing the speed of movement
2) Increasing the magnetic field strength
3) Increasing the number of turns on the coil
4) Increasing the are of the coil
                The National Grid
Electricity reaches our homes from power stations through the
National Grid:




                   Step up                 Step down
Power station                                               Homes
                 transformer              transformer

If electricity companies transmitted electricity at 240 volts through
overhead power lines there would be too much ______ loss by the time
electricity reaches our homes. This is because the current is ___. To
overcome this they use devices called transformers to “step up” the
voltage onto the power lines. They then “____ ____” the voltage at
the end of the power lines before it reaches our homes. This way the
voltage is _____ and the current and power loss are both ____.

                Words – step down, high, power, low, high
                   Transformers
Transformers are used to _____ __ or step down
_______. They only work on AC because an ________
current in the primary coil causes a constantly alternating
_______ ______. This will “_____” an alternating
current in the secondary coil.

Words – alternating, magnetic field, induce, step up, voltage

We can work out how much a transformer will step up or
step down a voltage:

Voltage across primary (Vp)        No. of turns on primary (Np)
Voltage across secondary (Vs)      No. of turns on secondary (Ns)
             Static electricity
Static electricity is when charge “build up” on an object
and doesn’t move, e.g. rubbing a rod:
Two different
rods will
attract each
other if they
have different
charge:



Two rods
made of the
same material
will repel each
other due to
having the
same charge:
Van de Graff generators
              A charge builds up on
              the dome due to
              electrons being “______
              off” by the belt. If a
              big enough ______ is
              built up then the voltage
              becomes high enough to
              _____ the air molecules
              and the electrons “___”
              down to Earth – this is
              an electric _________.
              Words – charge, jump,
              current, rubbed, ionise
Use of static 1 - Photocopiers
                          Photocopiers use static
                          electricity. They work by: 1)
                          Copying an ______ of the page
                          onto a ______ plate, 2) Light
                          then causes the charge to ____
                          away, leaving an “electrostatic
                          impression” of the page, 3) The
                          charges left on the plate
                          ______ small drops of black
                          powder, 4) The powder is
                          transferred from the plate onto
                          the _____, 5) The paper is
                          _____ to “fix” the powder.

 Words – heated, leak, paper, image, charged, attract
 Use of static 2 - Printers




Inkjet printers work by spraying charged drops of ink
onto a page. The droplets can be directed using two
oppositely charged plates. The voltage on these plates
can be easily swapped or varied. The inkjet cartridge
can also moved across the page by the printer
            Dangers of static
During refuelling the fuel gains electrons from the pipe,
making the pipe positive and the fuel negative. The
resulting voltage may cause a spark – bad news!




Solution: Either earth the fuel tank with a copper rod or
 connect the tanker to the plane by a copper conductor.
                 Electrolysis
Electrolysis is used to extract a HIGHLY REACTIVE metal.

                                When we electrolysed
                                  copper chloride the
                                 negative chloride ions
                                 moved to the positive
                              electrode and the positive
                               copper ions moved to the
                                  negative electrode –
                               OPPOSITES ATTRACT!!!

                                    = chloride ion

                                    = copper ion
        Some example questions
1) A current of 2A flows through some copper chloride for 1 minute
   and 0.01g of copper is deposited at the negative electrode.
   a) How much would be deposited if the current was increased to
   6A?
   b) How much would be deposited if the current was kept at 6A and
   the experiment was left for another minute?
   c) How much charge flowed in question (b) above?
2) A current of 0.05A flows through some copper chloride for 500
    seconds and 0.05g of chlorine is released at the positive
    electrode.
   a) How much would be deposited if the current was increased to
   0.1A?
   b) How much would be deposited if the current was kept at 0.1A
   and the experiment was left for 250 seconds instead?
   c) How much charge flowed in question (b) above?

				
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