Sunday, July 19, 2009

Temperature scale

Flash animation: Calibrating thermometer

Saturday, July 4, 2009

Energy Losses in a Transformer

The energy loss of a transformer.
There are 4 main causes for energy loss in a transformer;
i. Heating effect in the coils. ( Your hand phone adapter become hot while charging !! )
Equation of heat energy , E = I x I x R x t
I = current
R = resistance
t = time

ii. Heating effect of eddy currents induced in the coil core.
The changing magnetic field causes the electrons in the soft iron core to circulate around as eddy currents. These eddy currents produce heating in the core and causes further loss of power.

iii. Hysteresis
Energy loss due to magnetization and demagnetization of the iron core of the transformer by alternating current. This transforms some energy into heat in the core.
Heat!?? energy loss again!

iv. Magnetic flux leakage
Flux leakage happens when some of the magnetic flux generated by the primary coil do not pass through the secondary coil.

How to improve the efficiency of a transformer?
To improved the efficiency of a transformer;
i. Thick copper wire of low resistance is used.
ii. The iron core is made of thin laminated sheets insulated from each other.
iii. Used soft iron core, which is able to be magnetization and demagnetization easily.
iv. Secondary coil is wound over the primary coil.

Solve problems involving transformers.
  • Efficiency of a transformer = ( useful output power/Input power ) x 100 %
  • Power = Current x voltage

Thursday, July 2, 2009

Simple Transformers

What you should know:

1. Describe the structure and the operating principle of a simple transformer.

A transformer consists of a primary coil, a secondary coil and an iron core to concentrate the magnetic field lines and link the coils.

A transformer works on the principle of electromagnetic induction. When current flows in the primary coil, a magnetic field is produced. as the current in the primary circuit varies, the strength of the magnetic field also varies.

This alternating magnetic filed line cut through the secondary coil and the induce an e.m.f. and current at the same frequency as in the primary coil.

The soft iron core is used to concentrates the magnetic field lines.

2. compare and contrast a step-up transformer and a step-down transformer

  • A step-uptransformer increases the output voltage of the power supply.
  • A step-down transformer decreases the output voltage of the power supply.
3. Application of transformer in daily life.

i. In mobile phone charger to converts the 240 V main supply to a lower voltage suitable for the appliance.

ii. Transmitting electricity from power plant to housing areas.

Mini quizzes

The input voltage for transformer P, Q, R and S is 240 V.

Table above shows the characteristics of the transformers.
a. What are the main components of a transformer?
b. What is the function of the iron core of a transformer?
c. Which transformer is a step-up transformer?
d. Which transformer is a step-down transformer?
e. Which transformer is a step-up transformer?
f. Which transformer has the same output voltage with its input voltage?
g.Which transformer is has the highest output voltage?
h. Would a transformer work with direct current?