Thursday, September 23, 2010

Why There Is No Change in Temperature During Melting? Yong

One experiment has been carried out to investigate the specific heat capacity and latent heat of water during heating process. The students observed that the temperature is kept constant during phase change, whereby the temperature showed 0 celcius for a few minutes until all the ice melted. They thought that something went wrong in their experiment. So they asked me. These are my explanation :

During the phase change of a solution, it is the ice that is undergoing the phase change. When a phase change occurs, as in the melting of a solid, the thermal energy goes into separating the molecules, "breaking the intermolecular attractions", so to speak, (hydrogen bonds, in the case of water), and not into changing the temperature of the solution. That is why the temperature constant at melting as well as boiling. The energy is used to weaken the molecular bond, has to work against the atmospheric pressure and used to separate the intermolecular bond between the solvent.


Wednesday, September 22, 2010

Why Alternating Current Is Used in Transformer, not Direct Current? : Haris Fikri

One student asked why ac is used in trandformer, not direct current? Actually, we can ask how does transformer work? Ron Kurtus explained inFeb 2009 as :

A transformer is an electrical device that is used to change the voltage in alternating current (AC) electrical circuits.

The fact that the potential energy can be readily changed from one voltage to another through the use of a transformer is a major advantage of AC electricity over direct current (DC). AC transformers can change power line high voltages to house current voltage. They also are used to change the voltage from house current to that used by low voltage devices. Often the AC in these small transformers or adapters is also changed to DC.

Basic principles of transformers

A transformer combines several major characteristics of electricity and magnetism to change AC voltages. First of all, you need to know the principles for creating an electromagnet and creating electricity.

Creating an electromagnet

A wire with DC electric current flowing through it has a magnetic field around it. Placing a compass near a wire and observing the needle move when the DC current is turned on can demonstrate this.
By wrapping the wire around a piece of iron, the magnetic field is increased many times due to the realignment of the iron atoms, each which acts as a tiny magnet. The iron core and wire wrapping is called an electromagnet.

Relation to voltage

The greater the current through the wire the greater the strength of the electromagnetic field. Since voltage and current are proportional for a given resistance, according to Ohm's Law V = IR, the strength of the electromagnetic field is proportional to the voltage used. Double the voltage and you double the strength of the electromagnet.

Relation to turns of wire

The greater the number of turns around the iron core the greater the strength of the electromagnet. The strength is approximately proportional to the number of turns. Triple the number of turns and you triple the strength of the electromagnet.(Experiment idea: measure the change of strength of an electromagnet by changing the voltage and/or number of turns.)

Direction of magnetism

The direction of the magnetic field is determined by the direction of the current and the direction of the turns around the iron core. If you change the direction of the current, the north and south poles of the electromagnet will switch.
With DC electricity, you must physically change the wires to change the direction of the current. With AC electricity, the direction changes with each cycle.
Thus, one end of an AC electromagnet is switching from north to south and back again 60 times per second in the U.S. or 50 times per second in some other countries.

Creating electricity

Electricity is created either when a wire is moved through a magnetic field or when a magnetic field is moved past a wire. Moving the magnetic field past the wire can be done by physically moving a magnet past the wire or by somehow changing the amount of the magnetic field.

Transforming the voltage

To transform or change the voltage of AC electricity, you use an AC electromagnet and the principles described above.

AC electromagnet

An AC electromagnet continually changes the direction of its magnetic field. This means the field goes from zero to N to zero to S and so on. If you would put an AC electromagnet near a wire, then the changing magnetic field should create a current in the wire.
Or better yet, why not wrap the wire around the iron core of the electromagnet? This is how a transformer works.


A transformer can be a long piece of iron with wire having with AC current going through it and wrapped around the piece of iron near one end. It also has wire that creates electrical current wrapped around it at the other end. A more common configuration is a square or donut shaped iron core with the wire wrapping on both sides.
Transformer changes voltage
Transformer changes voltage

Output proportional to turns

The strength of the magnetic field is proportion to the input voltage and the number of turns around the core (called the primary coil). By reversing the rule, the output voltage is proportional to the strength of the changing magnetic field and the number of turns (called the secondary coil).
For example, if you wanted to increase your house voltage from 110 volts (110V) to 220V in order to power your electric stove, you could use a transformer with twice the turns in the secondary coil as in the primary coil.