علوم بالانجليزية صف سادس الوحدة الثانية

Heat:
It is the energy that always transfer from the hot body to the cold one.
- The feeling by the hotness of a hot cup due to the transfer of heat from the hot cup to the hand.
- The felling by the coldness of a piece of ice due to the transfer of heat from the hand to the piece of ice.

Sources of heat are:
1- Sun is the main source of heat on the earth.
2- Friction
3- Hammering
4- Burning
5- Passing the electric current through a wire.

Transfer of heat through different media

Conduction (through solids):
Transfer of heat by conduction:
It is the transfer of heat through solids from the hot parts to the cold parts without transfer of molecules of the material.

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2- convection (through liquids and gases):
Heat transfers through liquids and gases from the parts of high temperature to the parts of law temperature by the movement of liquids or gases and this transfer is called "transfer of heat by convection"

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Radiation (through Gases "Air " and space):
Heat of the sun transfers to us through the space by "Radiation"

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The Heat conductors and the Heat Insulators

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First: The effect of heat changes on solids:
Solids expand by heat and contract by cooling

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Applications:
1- Pouring hot water on a metallic cover of jar it is difficult to open it.
- Because heat makes the cover to expand and to be wide, so we remove it easily.

2- Electric cables and telephone wires are not stretched tightly.
- In order not to be cut down because of its contraction in the cold winter.

3- Gaps "small spaces" are left between railway tracks
- Because these gaps allow the railway tracks to expand in summer and contract in winter, so these makes tracks don't bend upwards when they expand, so trains run safely.

4- Spaces are left in the frame work of the bridges.
- To allow them to exapnd in summer and contract in wine so, protecting the bridge from collapsing.

Second: The effect of heat changes on liquids:

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- Liquids expand by heat contract by cooling
Thermometers:
- The basic idea of thermometers depends on expansion of liquids by heat and contraction by cooling.

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Third: The effect of heat changes on gases

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- Gases expand by heat and contract by cooling:

Applications:
- The car tyres (Or balloons) may burst in summer (hot days)
- Because air inside them expands by the action and the heat of the sun.

The electric current
It is a flow of electrical charges that pass from one end of a wire to another.

Soruce of electric current
Electric current can be obtained from: 1- Dry cells
They produce electric currents by changing the chemical energy into electric energy.

Electric generators:
- They produce electric current by changing the kinetic energy into electric energy.
- They can be operated by fuel or dams (like the high dams)

The electric circuit:
It is the closed path through which the electric charges transfer to form a complete cycle.

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Uses of the dry cell:
Dry cells are used in operating the following:
a- Torches.
b- Portable radios.
c- Wall clocks.
d- Children's toys.

What happens if the dry cell is used for a long time?
The chemical reaction in the cell gets weaker and so the electric current gets weaker till the reaction stops and no electric current is produced.

What happens if the dry cell is left without being used for a long time?
a- The dry cell loses its moisture.
b- The paste cannot conduct electricity so the chemical reaction stops and no electric current is generated.