HCI Practical Electricity Notes

Hwa Chong Institution Sec 4 Physics 1 Name: _________________________________________ Class: ___________ Date: ______________ Sec 4 Physics (SSMT / IP) Topic 20: Practical Electricity I can… By the end of this topic, you should be able to:  describe the use of the heating effect of electricity in appliances such as kettles, ovens and heaters  recall the relationships P = VI, E = VIt  apply the relationships for electrical power and energy to new situations or to solve related problems  calculate the cost of using electrical appliances where energy unit is the kW h  compare the use non-renewable and renewable energy sources such as fossil fuels, nuclear energy, solar energy, wind energy and hydroelectric generation to generate electricity in terms of energy conversion efficiency, cost per kW h produced and environmental impact  state the hazards of (i) damaged insulation (ii) overheating of cables (iii) damp conditions  explain the use of fuses and circuit breakers in electrical circuits and of fuse ratings  explain the need for earthing metal cases and for double insulation  state the meaning of the terms: live, neutral and earth  describe how to wire a mains plug  explain why switches (placed after fuses), fuses and circuit breakers are wired into the live conductor 20.1 JOULE HEATING We all know that when an electric current flows in through a resistor, some thermal energy is given out. A common would be the filament shown on the right. When a current flows in the high resistance element, the electrons collide frequently with the ions of the element. Such collisions increase the average kinetic energy of the ions in the filament and give rise to a heating effect. This effect is called Joule heating , also known as ohmic heating or resistive heating. (a) The passage above mentioned a “high resistance element” , how high a resistance should it be? Will it work with insulators? (b) Name some other uses for Joule heating. Check your understanding

Hwa Chong Institution Sec 4 Physics 2 Examples of Joules heating 20.2 Equations for Joule heating In this section, we are going to derive a few equations that can guide us in solving problems related to electrical energy. Let’s take V as the potential difference across the heating element, I as the curren t flowing through it, and R as the resistance of the heating element. Derive the equations , 2 2 and VP VI P I R P R   using the fact that WorkPower time and W qV . WP t Vq t   Since qI t , P IV And V IR , 2P I R And VI R , 2VP R

Hwa Chong Institution Sec 4 Physics 3 Table of equations for Joule Heating Power Energy P IV W IVt 2P I R 2W I Rt 2VP R 2VtW R In solving physics problems related to Joule heating, it is not uncommon to relate it to other forms of energy such as kinetic energy 21 2KE mv  , potential energy  GPE mgh , thermal