Download science world 1 – chapter 1

science world 1 – chapter 10
how things work
ELECTRIC CIRCUITS
When electrons are forced to travel along metal wires, an
electric current is produced. The flow of electric current is
called electricity. Electrons must move around a pathway
called an electric circuit. For current to flow, it must have a
complete pathway, or circuit, to follow. If there is a break in the
circuit, the electrons stop moving and no current flows.
The cell or battery supplies the energy to push the current
around the circuit. The amount of energy supplied by the cell is
measured in volts. The voltage is a measure of the electrical
potential energy of the cell.
Materials through which electric current can move easily are
called conductors (e.g. metals). Materials through which
electric current cannot easily move are called insulators (e.g.
rubber, plastic, glass, wood and paper).
CIRCUIT DIAGRAMS
Drawing electric circuits can be confusing and takes time. So
electricians have decided on a simple symbol for each of the
parts in a circuit. Just like when drawing scientific diagrams,
circuit diagrams should be drawn in lead pencil and using a
ruler.
Students to copy “Electrical Symbols” from p 192.
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SERIES & PARALLEL CIRCUITS
Two different circuit arrangements can be used to connect a
number of light globes to a single power supply.
In a series circuit, the electric current is made to pass through
every globe, one after the other. In a parallel circuit, the
electric current splits up and portions go through each globe.
(Draw Figures 10 and 12 on p 192 and 193.)
Many circuits are quite complicated and contain some devices
connected in series and some connected in parallel. Such
circuits are called combination circuits.
SIMPLE MACHINES
Simple machines are things such as levers, pulleys, ramps,
screws and gears. Complex machines such as clocks and
bicycles are built up of many simple machines. Machines are
devices that help you do things more easily. They do this in
three different ways.
1.
2.
3.
Machines magnify the force you use (e.g. using a crowbar
to lift a heavy object).
Machines change the direction of the force (e.g. using a
pulley to lift the sails up on a boat).
Machines make things go faster (e.g. using the gears on
your bicycle).
It is impossible to get more energy out of a machine than you
put in. Machines transfer energy from the effort to the load.
Due to friction there is always some energy wasted as it is
converted to heat and sound.
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LEVERS
A crowbar is a type of simple machine called a lever. A lever is
a long bar or stick which moves around a fixed point called the
pivot or fulcrum. The object that you want to move is called
the load and the force you apply to move the load is called the
effort.
A lever makes work easier by reducing the force needed to
move something. However, in order to use less force, the lever
must be moved through a greater distance.
There are many types of levers around you. Pliers have the
pivot between the load and the effort. Wheelbarrows have the
load between the pivot and the effort.
effort
load
load
effort
pivot
pivot
You can get a measure of how useful a simple machine is by
calculating its mechanical advantage using the following
formula:
Mechanical Advantage = Load_
Effort
INCLINED PLANES & WHEELS & AXLES
A ramp is an example of a machine called an inclined plane.
Rolling a barrel up a ramp is much easier than lifting it into
place. Inclined planes have a force advantage – you apply a
small effort over a large distance to lift or move a heavy load
over a small distance.
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A screw is an inclined place in a spiral shape – it is essentially
an inclined plane wrapped around a cylinder. Some objects are
actually a double inclined plane which changes the direction of a
force e.g. axes and wedges.
A steering wheel is an example of a wheel-and-axle. The axle
is the central rod or column and the steering wheel is attached to
this axle. A steering wheel enables a small force to be applied
and the axle will turn. Doorknobs and screwdrivers are also
examples of a wheel-and-axle.
PULLEYS & GEARS
A pulley is a grooved wheel with a rope over it. A single pulley
does not magnify your force, but it does change the direction of
the force. To magnify your force you need to use more than one
pulley. This lets you lift heavy loads by using only a small
effort.
Gear wheels are wheels with teeth on them. The teeth of one
gear fit into the teeth of another. Gears are used to transfer the
force from one wheel to another. In a bicycle, the gear wheel
attached to the pedals is called the driving gear and the gear
wheel on the back wheel is called the driven gear.
Gears can speed things up or slow things down. To speed things
up the driving gear has more teeth (is larger) than the driven
gear (see Figure 53 on p 206). To slow things down the driving
gear has less teeth (is smaller) than the driven gear (see Figure
54 on p 206).
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SPELLING WORDS
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
EASY
current
globe
series
voltage
battery
switch
simple
machine
force
pivot
planes
gears
pulleys
crowbar
effort
wedge
screw
engine
load
effort
HARD
electricity
circuit
conductor
insulator
ammeter
parallel
combination
appliance
complex
mechanical
magnify
direction
advantage
axle
arrangement
leverage
fulcrum
inclined
rotating
bicycle
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