Friday, March 06, 2015

Heating Effect of Electric Current

How Electric Current is Converted Into Heat?

When an electric current moves thru a conductor, there is a considerable friction between the moving electrons and the molecules of the conductor. The energy that is being produced as a result of this process is called "heat".

Practical Application

The heat that is being produced by the current can be used as heating appliances such as electric heater, soldering iron, kettle and the like. The major factor that affects the design of these equipments includes the material of the conductor or the resistive element. The objective is to produce the highest possible heat energy with minimal amount of electric current. Large amount of electric current posed a big safety risk considering the fact that most of these equipments are used n households where majority of the users are non-electrical professionals.

Figure 1. Nichrome Alloy
Nichrome Alloy- this material has a resistance of more than 50 times that of copper and around 70 times compare to aluminum. These means that it will produced at least 50 times more heat compare to copper and aluminum.

Essential Conversion Factor

1 Kwh = 36 x 10 exp(5) Joules
1 calorie = 4.186 Joules
1 kWh = 860 kcal

Example:

An electric kettle with nichrome as a heating element is required to raise the temperature of 1 kg of water from 10 degree C to 100 degree C for a period of 10 minutes. Solve for the required value of the resistance of nichrome if the kettle is to be supplied by a 240 Volts supply. Assume efficiency of the kettle to be 80%.

Solution:

specific heat of water = 1.0 kcal/kg.C

Energy received by water = mass x sp.heat x  (T2-T1)

                                            = (1 kg) x (1 kca/kg.C) x (100-10) C

                                            = 90 kcal x (1 kWh / 860 kcal)

                                           = 0.105 kWh

  • Electrical energy supplied to the kettle = 0.105 / 0.8 (at 80% kettle efficiency) = 0.131 kWh
  • Power = Energy/ time
  • Power supplied to the kettle in 10 minutes = 0.131 kWh / (10/60) .... 1hr = 60 mins
  • Power =  .786 kW = 786 watts

In electric circuit calculation at 240 V supply:

  • P= V (squared) /R   or 
  • R = V*V /  P = 240 (squared) / 786
 R = 74 ohms ----> answer

Summary:

  • The current that passes thru a conductor will produce heat.
  • Nichrome is one of the best heating elements available by several manufacturers
  • The heating element should produce high amount of heat for a minimal amount of current due to safety reasons
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