Evan's Space

Wonders of Physics

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Unsual Behaviour of Less Dense Balloon in Air or Water

ballon movement in denser air or water

Answer: Option A

Of course if a heavy ball is suspended from the car we all know the ball will move in opposite direction of a decelerating car due to inertia. We are all familiar to this where inertia is applied to a body which is denser than the surrounding medium (air or liquid) which is less dense. That’s why this balloon’s behaviour surprises us!

Let’s assume the water molecules are initially moving at constant speed with the tank before the deceleration.  Due to inertia, when the tank decelerates, the water molecules continue its state of motion forward. Hence the water molecules gush to the right side of the tank, displacing (pushing) the balloon to the left. Hence the balloon moves to the left!

Refer to this Youtube video by Smarter Everyday and you can see the similar experiment of helium balloon (less denser) in the air (denser) of a car.


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Electromagnetism Videos

Simple Electromagnet
When current flows through a coil, magnetic field is generated and there will be a north and south poles on the coil.
To increase the strength of the magnetic field:
– increase the size of the current by increasing the emf of the battery
– increase the number of turns in the solenoid
– insert a soft iron core

Demonstration of Right-Hand Grip Rule (RHGR) to determine magnetic field line direction

The video below shows that as the current flows through the wire, there is a magnetic field around the wire. The compass will follow / indicate the direction of the magnetic field line. Use you RHGR to confirm the direction.

Rod Between a U-shaped Magnet
As current flows through the rod, due to the combined magnetic field of both the around the current-carrying rod and magnets, there is a force acting on the rod in the direction from a region of stronger magnetic field towards a region of weaker magnetic field around the rod.

To know the direction of the force on the rod easily, you can apply your Fleming’s Left Hand Rule.



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Processes for Transfer of Thermal Energy

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

Conduction vs Convection:


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


  • 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.