Evan's Space

Wonders of Physics


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Processes for Transfer of Thermal Energy

Three processes of thermal transfer: conduction, convection and radiation.

Conduction vs Convection:

Similarity:

  • both requires a medium for thermal transfer to take place.

Differences:

  • convection cannot take place in solid but conduction can.
  • conduction is due to vibration and collision of molecules while convection is due to fluid density changes.

Radiation vs Conduction & Convection

  • Radiation can take place in vacuum but both conduction and convection require a medium.

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Marshmallow Hulk in Vacuum Jar

When the pump is switched on and the air in the jar is gradually removed, the pressure in the jar decreases. There will be fewer air molecules per unit volume in the far. Hence rate of collision of the air molecules with one another and with the wall and hulk will be reduced. As pressure P = F/A, the force acting per unit area decreases, the pressure decreases.

In the marshmallow, there are pockets of air at normal atmospheric pressure initially. As the pressure in the jar decreases, the pockets of air in the marshmallow expands due to this pressure difference. Hence the hulk expands and its volume increases.


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Steel is used to make permanet magnet

Steel is a hard magnetic material. When magnetized, it is not as strong as soft iron. But steel retains its magnetism, hence it is used to make permanent magnets e.g compass need, speaker, fridge door.

Iron is a soft magnetic material. It can be easily and strongly magnetised. But it loses its magnetism easily. Hence it is used to make temporary magnet e.g. Electromagnet.

Do remember that compass is like a freely suspended magnet. It is made of steel!