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 energies  Q mc  ,  vQ ml , or  fQ ml and so on. Remember that energy can change from one form to another (though not indiscriminately). (c) A toaster is designed to operate with a voltage of 120 V, and a clothes dryer is designed to operate at 240 V. Based solely on this information, which appliance uses more power? (d) When an incandescent light bulb is turned on, a constant voltage is applied across the tungsten filament, when then becomes white hot. The temperature coefficient of resistivity for tungsten is a positive number. What happens to the power delivered to the bulb as the filament heats up? (e) Calculate the resistance of a 40-W automobile headlight designed for 12 V.   . 22 12 3640 VR P    (f) Lightning bolt is a spectacular example of electric current in a natural phenomenon. There is much variability to lightning bolts, but a typical event might transfer 10 9 J of energy across a potential difference of 75 10 V during a time interval of about 0.2 s. Use this information to estimate, (i) the total amount of charge transferred between the cloud and ground; (ii) the current in the lightning bolt; (iii) the average power delivered over the 0.2 s. (i) 9 7 10 20 C5 10 Wq V    (ii) . 20 100 A02 qI t   (iii) . 9 910 5 10 W02 energyP time    Check your understanding

Hwa Chong Institution Sec 4 Physics 4 20.3 A new unit kWh In this section, we shall discuss a new unit of energy called kilowatt hour, or kWh. This physical unit is used by electric companies to charge users of electricity. How many J are there in 1 kWh?     . 6 J1 kWh 1 1000 1 3600 ss 3 6 10 J    Below is a sample domestic electric and water bill. You should note that the price of electricity varies according to its usage, whether it is domestic, non-domestic, high tension, extra high tension etc. In Singapore, the price of electricity is reviewed every quarter of a year. The price of electricity for 1 Jan to 31 Mar is 25.65 cents per kWh. 0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 Jul-09 Nov-10 Apr-12 Aug-13 Dec-14 Electric tariffs (in cts) Months

Hwa Chong Institution Sec 4 Physics 5 Example 1 Air conditioners are one of the most energy consuming devices a normal person can use. Consider a typical Mitsubishi Electric Starmex System 3 inverter, at full load, it consumes 2.18 kWh per hour. How much would it cost a family which uses this aircon at full load for 6 hours per day for a period of 30 days? Use the latest available electric tariff of Singapore to solve the problem. Solution:     .. $. Price 2 18 6 30 0 2565 100 65   Clearly, we cannot apply the normal rule for significant figures here because the electric company will not follow physics. Example 2 Find the electrical energy stored in the iPhone battery given 3.7 V and 1400 mAh. . 33 7 1400 10 3600 19 kJ Energy VIt        Fossil fuels vs renewable sources of energy (We had a more detailed discussion in the topic of energy, Sec 3) “Continued research has made renewable energy more affordable today than 25 years ago. The cost of wind energy has declined from 40 cents per kilowatt -hour to less than 5 cents. The cost of electricity from the sun, through photovoltaics (literally meaning "light-electricity") has dropped from more than $1/kilowatt-hour in 1980 to nearly 20cents/kilowatt -hour today. And ethanol fuel costs have plummeted from $4 per gallon in the early 1980s to $1.20 today. But there are also drawbacks to renewable energy development. For example, solar thermal energy involving the collection of solar rays through collectors (often times huge mirrors) needs large tracts of land as a collection site. This impacts the natural habitat, meaning the plants and animals that live there. The environment is also impacted when the buildings, roads, transmission lines and transformers are built. The fluid most often used with solar thermal electric generation is very toxic and spills can happen.

Hwa Chong Institution Sec 4 Physics 6 Solar or PV cells use the same technologies as the production of silicon chips for computers. The manufacturing process uses toxic chemicals. To xic chemicals are also used in making batteries to store solar electricity through the night and on cloudy days. Manufacturing this equipment has environmental impacts. Also, even if we wanted to switch to solar energy right away, we still have a big problem. All the solar production facilities in the entire world only make enough solar cells to produce about 350 megawatts, about enough for a city of 300,000 people. That’ s a drop in the bucket compared to our needs. California alone needs about 55,000 megawatts of electricity on a sunny, hot summer day. And the cost of producing that much electricity would be about four times more expensive than a regular natural gas-fired power plant. ” http:/