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					                             4. ELECTRICITY AND MAGNETS
                                            CONCEPT MAPS

ELECTRICITY
                                            is obtained from

                                                                                                           has
   is a flow of                     is stopped by                      INSULATORS


                                    is carried by
ELECTRIC CURRENT                                            CONDUCTORS                                POWER


  flows around                                                          have parts which are           indicates



ELECTRIC CIRCUIT                                           have parts which are                     VOLTAGE


             has components                                                                               have a




                     WIRE                BULB              SWITCH            CELLS                  several make



 represented by                           represented in                      used in               BATTERY


CIRCUIT DIAGRAM                     can be represented by               TORCH LIGHT



                                 MAGNET                     attracts              MAGNETIC MATERIALS
                  has end


                                                              used in                   effects felt in area called
NORTH POLE                   SOUTH POLE

   repels         attracts      repels              MAGNETIC COMPAS                      MAGNETIC FIELD


NORTH POLE                   SOUTH POLE


    points                     points                  indicates direction                        has a


  NORTH                        SOUTH                           has directions                    EARTH
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4.1 INTRODUCTION

Aims:
       To bring to students' minds some of the ways in which modern life depends on electricity.
       To warn students of the danger of main electricity.
       To introduce students to the idea of magnets and encourage them to look for/play with magnets.
       To provide advance information to help students organise ideas their about the topics covered in the
        chapter.

Activities:
     Discuss with students some of the ways in which modern life depends on electricity.
     Look at a wide range of everyday electrical items, especially those using batteries.
     Encourage students to look for, make and play with magnets.

Answers:
    Q1. Torches, radios, cassette and CD players, pocket calculators, watches and clocks, automatic
      cameras, some children's toys, remote controls for TVs etc, cars and trucks (for starting and lights etc).
    Q2. A magnet is a piece of metal that pulls certain other small metal objects towards itself and clings on
      to them.
    Q3. Electric lights, air conditioners, fans, heaters of many kinds, hair dryers, radios, televisions, videos,
      record/CD/cassette players, irons, cookers, refrigerators and freezers, washing machines (for clothing),
      dish-washers, computers. The big difference between the electricity from a battery and mains electricity
      is that mains electricity is MUCH more powerful (it has a much higher voltage) - mains electricity can
      kill.


4.2 THE ELECTRIC CIRCUIT

Aims:
    To introduce the idea of a complete electric circuit and an electric current flowing around it.
    To show students where contacts have to be made on bulbs and batteries, and how two batteries (cells)
      are connected together (in series).
    To establish that (i) a bulb lights up when an electric current flows through it, and (ii) an electric current
      can flow around a circuit only if the circuit is complete, (iii) the current leaves one terminal of the
      battery and returns to the opposite terminal.

Activities:
     Connecting up bulbs and batteries with wires (each group needs one bulb, 2 batteries and 2 wires).

Answers:
    Q1. One must touch the metal tip at the bottom of the bulb, and the other must touch any part of the
      metal side below the glass bulb. It make no difference which wire touches which place.
    Q2. (i) The top of one battery must touch the bottom of the other. (ii) One wire must touch the top of
      one battery, and the other wire must touch the bottom of the other battery.
    Q3. An electric circuit is a path which electricity flows around. A complete, unbroken circuit is needed
      before electricity can flow.
    Q4. (i) Only B lights up (it does not matter which wire is connected to which terminal on the bulb). (ii)
      A will not light because there is no wire going to the metal at the bottom tip of the bulb. C and D will
      not light because the circuits are broken - they are not complete. In C the wire stops short of the battery,
      and in D the two batteries are not touching.


4.3 CONDUCTORS AND INSULATORS

Aims:
    To introduce students to the ideas of electrical conductors and insulators.

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       To encourage students to think about the materials that some common objects are made of, and help
        them to classify materials as conductors or insulators.
       To establish as a generalisation that metals and graphite are electrical conductors and most other
        materials are insulators.

Activities:
     Testing different materials (using the simple circuit shown) and classifying them as conductors or
        insulators.

Answers:
    Q1. The blade (but not the handle) of the bush knife, the food tin, the iron from the roof, the cooking pot
      and bucket if they are made of metal (otherwise not), the gold ring.
    Q2. An insulator is something that does not allow electricity to pass through it. Most materials except
      metals and graphite are insulators.
    Q3. The plastic is there as an insulator. It keeps the electric current in the wire and does not allow it to
      flow out into other conductors. In the home, the plastic covering on the wire also protects us from
      dangerous electric shocks.


4.4 MAKING GOOD CONNECTIONS

Aims:
    To make students aware that the success of most electrical devices depends on good electrical
      connections, and to show them how these can be made with wires, terminals and batteries.

Activities:
     Stripping wires and making various electrical connections.
     Examining the battery clips in as many different devices as possible. Students could be encouraged to
        make battery clips for themselves or for the school if necessary.


4.5 LIGHT BULBS

Aims:
    To inform students about the structure of a simple bulb and bulb holder, and to help them understand
      how they work.
    To encourage students to observe and experiment with bulbs and bulb holders.

Activities:
     Handling and observing different kinds of simple filament bulbs and bulb holders as available.
     Observing what happens if we try to use a bulb without the glass.

Answers:
    Q1. Check the drawing with the diagram at the top of page 42.
    Q2. The filament is made of the metal tungsten. It glows white hot and gives out light when an electric
      current flows through it.
    Q3. If the base of the bulb holder was made of a conductor, the electricity would take a short cut
      through the base and not flow through the filament at all! Electricity seems to be very sensible - it
      always takes a short cut if it can find one!


4.6 CELLS AND BATTERIES

Aims:
    To enable students to distinguish between the terms cell and battery.
    To inform students about the structure of a simple dry (Leclanché) cell.

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       To inform students about the voltages of common cells and batteries and that this relates to the "power"
        they produce.
       To make students aware that the voltage produced by cells connected + to (or top to bottom) is equal to
        the sum of the voltage of each cell.

Activities:
     Examining as many different kinds of cells and batteries as may be available; include a car battery if
        possible.
     Taking apart an old dry cell. (If the top is crushed slightly between two stones, the metal and plastic
        covers can easily be levered off and the graphite rod removed. The electrolyte is harmless, but very
        dirty! The zinc cases and graphite rods should be washed and kept for other activities).

Answers:
    Q1. (i) The positive terminal is made of graphite and the negative terminal is made of zinc metal. (ii)
      The positive terminal is at the top of the battery (the graphite rod); the terminal is at the bottom (the
      zinc metal case). (iii) 1.5 volts.
    Q2. The voltages add together when the cells are connected together + to (or top to bottom).
    Q3. To get 4.5 volts, we need 3 cells (1.5 × 3 = 4.5). To get 12 volts, we need 8 cells (1.5 × 8 = 12).


4.7 SWITCHES AND CIRCUIT DIAGRAMS

Aims:
    To help students understand how a switch works.
    To show students how to draw and interpret simple circuit diagrams (six given symbols only).

Activities:
     Making simple circuits with switches; making and testing simple switches; taking apart and examining
        any switches available.
     Drawing and interpreting circuit diagrams.

Answers:
    Q1. This is almost the same as Q3 in Section 4.5 (page 42). If the base of the switch was a conductor the
      electricity would flow straight through it. The switch would be on all the time.
    Q2.




                          Section 4.2 (page 4-2)                           Section 4.3 (page 4-3)

       Q3.     Circuit A: 2 cells, switch, bulb, 3 wires
                Circuit B: 4 cells, switch, bulb, variable resistor, 4 wires

       Q4.     (i) Circuit A: 3 volts (2 × 1.5) Circuit B: 6 volts (4 × 1.5)
                (ii) Circuit A: the switch       Circuit B: the variable resistor


4.8 TORCHES

Aims:
    To help students understand exactly how a torch works.




                                                         24
Activities:
     Taking apart and examining a torch; as a project, students could be encouraged to make their own
        torches using discarded materials to make the case and the switch.

Answers:
    Q1. (i) the spring, (ii) the terminal at the bottom of the bulb, (iii) a circular metal plate, (iv) one end
      connects with the circular metal plate, the other with the spring.
    Q2. Bottom terminal of bulb support wire filament support wire metal side of bulb.
    Q3. The curved reflector reflects the light from the bulb into a strong beam.


4.9 MAGNETS AND MAGNETIC MATERIALS

Aims:
    To introduce students to (permanent) magnets and to making magnets by stroking.
    To establish that magnetic materials are materials which are attracted by a magnet, and that iron (and
      steel), nickel and cobalt are the only magnetic materials.

Activities:
     Playing with magnets; making magnets by stroking; testing various small objects/materials to find out
        which materials are attracted.

Answers:
    Q1. Steel.
    Q2. Attract means to pull towards without physical contact.
    Q3. It is a common mistake to think that all metals (which are conductors) are magnetic, but that is not
      true. Only iron, cobalt and nickel are magnetic. Graphite, which is a conductor (but not a metal), is not
      magnetic either.


4.10 MAGNETIC POLES

Aims:
    To help students learn that a freely suspended magnet will swing so that one end points north.
    To help students understand, and correctly use, the terms north and south poles as applied to magnets.
    To help students generalise correctly about attraction and repulsion between north and south magnetic
      poles and make sure they understand, and correctly use, those terms.
    To give students practise in using the above ideas in solving simple problems.

Activities:
     Suspending bar magnets in paper slings and floating magnetised needles on corks.
     Hanging strings of pins or paper clips from different parts of magnets.
     Bringing together pairs of like and unlike magnetic poles.

Answers:
    Q1. (i) The magnet would be attracted to the iron or steel and would be deflected away from pointing
      north. (ii) A twist in the string might cause the magnet to spin round or it might twist the magnet away
      from pointing north.
    Q2. Repel is the opposite of attract. It means to push away (usually without physical contact).
    Q3. (i) If you have only the one bar of metal, the easiest way to find out if it is a magnet would be to try
      picking up small iron or steel objects such as pins or paper clips. (ii) To find which is the north pole,
      you would have to suspend it so it could swing freely and see which end points to the north. (iii) If you
      have a magnet with the north pole marked, it is very easy to test an unknown bar. Try bringing first one
      pole of the magnet, then the other, towards the same end of the unknown bar! (a) If there is no
      attraction or repulsion, the bar is not made of a magnetic material at all. (b) If the bar is attracted by
      both poles of the magnet, then the bar is made of a magnetic material, but it is not a magnet. (c) If one

                                                         25
        end is attracted and one end is repelled, then the bar is a magnet, and (d) the end repelled by the north
        pole is the north pole.


4.11 THE MAGNETIC COMPASS

Aims:
    To help students learn some basic principles of the construction and use of the magnetic compass.
    To remind students about, and give them practise in using, cardinal points and degrees for stating
      geographical directions.
    To introduce students to the idea of magnetic variation.
    To inform students about the composition, and the historical use, of lodestone.

Activities:
     Playing with magnetic compasses; using compasses to find north and other geographical directions;
        using compasses to take bearings. (If the magnetic variation for the region is known, students could be
        shown how to correct bearings for variation).
     Studying any available maps or globes on which magnetic north or magnetic variation are marked.

Answers:
    Q1. (i) Taking a bearing means using a magnetic compass to find the exact direction of a distant object.
      (ii) Any compass bearing should be corrected for variation because the compass needle does not point
      exactly north. A small angle has to be added or subtracted depending on the magnetic variation in a
      particular region.
    Q2. (i) 45o (ii) 180 o (iii) 225 o (iv) 315 o
    Q3. 135 o or South-east
    Q4. Iron and steel are not used to make the case of a compass because these metals would attract the
      compass needle and might deflect it from pointing north.


4.12 MAGNETIC FIELDS

Aims:
       To help students understand the idea of a magnetic field.
       To introduce students to plotting magnetic fields with plotting compasses and with iron filings.
       To show students the magnetic field of a bar magnet and between pairs of bar magnets.
       To reinforce students' ideas about (i) attraction and repulsion between magnetic poles, (ii) magnetic
        variation.
       To introduce students to the idea that the earth has a magnetic field, and to make them aware (without
        any details) of alternative explanatory models.
       To give students an opportunity to use their knowledge of magnetism to predict a magnetic field.

Activities:
     Plotting magnetic fields with plotting compasses and with iron filings (or black, magnetic sand).
     Looking at any available maps or globes on which magnetic north and variation are marked.

Answers:
    Q1. A magnetic field is the area in which the force of a magnet can be felt. Magnetic fields can be
      plotted using small compasses (plotting compases) or iron filings.
    Q2. The earth's magnetic field is caused by streams of tiny electrical particles from the sun as they shoot
      past the earth.
    Q3. In a horse-shoe magnet, the north and south poles are bent round so they are close together. They
      are unlike poles so many lines of force joining them will show the attraction between them.




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