Bilayer Organic Solar Cell in MATLAB

Bilayer Organic Solar Cell in MATLAB Chapter 3 Modelling and Simulation 3.1 Introduction This thesis is based on simulation of design characteristic of bilayer organic solar cell in MATLAB so it is very essential to be familiar with modelling and simulation. This chapter explains about modelling and simulation, characteristics of simulation, mathematical modelling (analytical and numerical both) and its properties, electrical modelling, work done in the field of modelling and simulation of OSC and finally small introduction of MATLAB which shows it’s features because of which this simulation work is in MATLAB. 3.2 Modelling and Simulation Modelling and simulation [1-4] is obtaining related data about how something will act without really trying it in real life. MS is using models either statically or over time, to build up data as a basis for making technical decisions. The terms modelling and simulation are often used interchangeably. Simulation skill is the tool set of engineers of each and every application domains and included in the knowledge body of engineering management. Modelling and simulation is a regulation on its own. With the addition of dynamic factor, simulation systems develop their functionality and allow to calculate predictions, estimates, optimization and what-if analyses. The meaningful abstraction of reality, follow-on in the proper necessity of a conceptualization and fundamental assumptions and constraints, is known as modelling. Simulation is execution of a model over time. Conceptualization is targeted by modelling, means modelling belongs to abstraction level and implementation is targeted by simulation, means simulation belongs to implementation level. Conceptualization (modelling) and implementation (simulation)– are the two activities that are jointly dependent, but can nevertheless be conducted by separate individuals. Modelling and simulation has helped to reduce expenses, enhance the feature of products and systems, and document. 3.2.1 Features of Simulation Interest in simulation applications are increasing gradually because of the following reasons- Use of simulation is cheaper and safer as compared to conduction of experiment. As compared to the conventional experiments, simulations can be more realistic because it permits free formation of surroundings parameters that are obtained in the active application area of the final product. As compared to real time, execution of simulation is faster because of this quality it can be used in if-then-else analysis of unlike alternatives, in particular when the essential information to initialize the simulation can simply be founded from functioning data. Tool box of conventional decision support system is being added a decision support simulation system with the use of simulation. Set up of a coherent synthetic environment is permitted by simulation which allows addition of simulated systems in the premature analysis phase through mixed virtual systems with virtual check surrounding to first prototypical elements for concluded system. If managed perfectly, the surrounding can be migrated from the growth and test domain to the domain of training and learning in resulting life cycle phases for the systems. 3.2.2 Steps for Modelling For modelling four basic steps are as follows †¢ Step 1: Monitor – In the first step conceptual model of ground profile and job objectives are developed. †¢ Step 2: Measure – In the second step theoretical model is developed which is used to explain the main processes running in the problem. †¢ Step 3: Describe – In the third step mathematical explanation of these processes are developed and to get a perfect solution verification is also done. †¢ Step 4: Verify – In the fourth step under the light of experimental physical reality, results of mathematical expression is interpretated. Confirm the suggestion, get additional measurements, enhance the complexity or precision of the mathematical result, or modify your conceptual understanding until you have complete understanding of the physical actuality. 3.3 Mathematical Modelling Fig 3.1, shows the simplest explanation of modelling – the method through which we can take out a complex physical actuality from a suitable mathematical reality on which designing of system is based. Development of suitable mathematical expression is done in numerical modelling. Mathematical modelling is a group of mathematical expressions that show the variation of a system from one state to another state (differential equations) and dependence of one variable to the other variable (state equations). The use of mathematical words to describe the performance of a system is mathematical modelling. Performance of photovoltaic system [5-7] is also illustrated by mathematical modelling. Number of different parameters (like – series and shunt resistance, ideality factor, reverse saturation current, open circuit voltage, short circuit current, fill factor, photo-generated current, efficiency) of photovoltaic system can be calculated by mathematical modelling. Fig. 3.1: Simple definition of modelling. 3.3.1 Properties of Mathematical Modelling We prefer mathematical modelling because of the following reasons With the help of mathematical model we can understand and investigate the meaning of equations and useful relations. It becomes very simple to make a educational environment in which preliminary person can be interactively occupied in guided inquiry and hands on actions with the help of mathematical modelling software (like – Stella II, Excel, online JAVA, MATLAB). Mathematical model is build up after the development of conceptual model of physical system. It is used to calculate approximately the quantitative presentation of the system. In order to spot a model’s strengths and weaknesses, quantitative outcomes obtained from mathematical modelling can be compared with observational information. The most important element of the resultant â€Å"complete model† of a system is mathematical model. Complete model is an assembly of theoretical, physical, numerical, visualization and statistical sub-models. 3.4 Types of Mathematical Modelling These can also be divided into either numerical models and analytical models. 3.4.1 Numerical Modelling – It is one of the type of mathematical modelling in which numerical time stepping method is used to obtain model response over time. Results are presented in the form of graph or table. In this thesis numerical modelling is used to analysis the design characteristic of Bilayer Organics Solar Cell. 3.4.2 Analytical Modelling – Modelling having a closed form results is called analytical modelling. In closed form results, mathematical analytic functions are used to present the response to the equations that describe variation in a system. 3.5 Electrical Modelling In this section, the electrical model for bilayer organic solar cell is described. One of the important characteristics of organic materials is their extremely small mobility, which makes modelling of their electrical properties difficult. Another problem in the electrical modelling of organic thin film devices (e. g. planar organic solar cells) was the lack of unique and precise electrical parameters for very thin layers of materials and occasionally lack of any information. Here with the aid of a self consistent loop between the Poisson equation and continuity equations for electrons and holes, the I-V curve of the device is calculated. It is assumed that the electrical current is due to the drift-diffusion transport of carrier. Consequently, in order to model the drift diffusion equations, a self consistent loop between the solutions of Poissons equation and two separate continuity equations for electrons and holes is needed. The design of the loop should be in a way such that the solution of each equation can be used as the initial conditions for the others, to generate a self correcting mechanism. The model that is used is based on the following assumptions: The generated excitons are separated right after absorption and the numbers of the generated electron-hole pairs are directly imported into the continuity equations as the generation rate . The transport properties of the organic materials can be totally modelled by mobility, DOS, bimolecular recombination term and doping levels. The connections between different layers follow the physical rules of hetero-junction connections between conventional semiconductors interfaces. The other two equations, which are solved in a closed loop with the mentioned Poisson equation, are two separate continuity equations one for the electrons and one for the holes. The flowchart of the electrical model using the mentioned equations is shown in Fig. 3.2. Fig. 3.2 : Flowchart of electrical model. 3.6 Work Done in Modelling and Simulation of OSC Pettersson et al (1999)[8] have reported a model based on the experimental short circuit light generated current action spectrum of poly(3-(4’-(1†,4†,7†-trioxaoctyl)phenyl)thiophene) (PEOPT)/C60 fullerene hetero-junction photovoltaic devices. This modelling was completely based on the assumption that generation process of photocurrent is the result of creation, diffusion and dissociation of excitons. Using complex refractive indices and layer thickness, internal optical electric field was computed. We got values for exciton diffusion length of 4.7 and 7.7 nm for PEOPT C60 respectively. Computed photocurrent and electric field distribution were used to study the effect of geometrical architecture with respect to the efficiency of device. Cheknane et al (2007)[9] has reported a photovoltaic cell in which photo-active layer of MDMO-PPV and PCBM material is sandwiched between ITO and Al electrodes, there is an additional interfacial layer of PEDOT/PSS on the top of ITO. Comparision between V-I characteristics of device with and without extra interfacial layer is done and modelled by electrical equivalent circuit. Simulation results show that V-I characteristics of bulk hetero-junction solar cell is affected by extra interfacial layer of PEDOT/PSS. Hwang et al (2007)[10] has reported drift-diffusion time dependent model of OSC based on blends of P3HT and red polyfluorene copolymer. In this model electron trapping and field dependent charge separation is used to investigate the device physics. This model is used to reproduce practical light-generated current transients observed in response to variable intensity step function excited light. Vervisch et al (2011)[11] has reported OSCs simulation using finite element method. Using finite difference time domain process, optical modelling is done and electrical characteristics is obtained by solving Poisson’s and continuity equations. Simulation results show the effect of physical parameters like exciton lifetime on OSC performance. Casalegno et al (2013)[12] has reported numerical approaches that give valuable information of microscopic processes underlying generation of photo-current in OSC. Here 3D master equation approach is used in which equations explaining particle dynamics rely on mean field guess and result is obtained numerically. Reliability of this method is tested against Kinetic Monte Carlo simulation method. V-I curve shows that the result of this method is very close to the exact result. Because of the adoption of mean field approximation for electrostatic interactions, we get biggest deviation in current densities. Strong energy disorder can also affect response quality. Simulation results show that master equation approach is faster than Kinetic Monte Carlo approach. Foster et al (2013)[13] presented a drift-diffusion model to obtain V-I curves and equivalent circuit parameters of bilayer organic solar cell. Minority carrier densities are neglected and final equations are solved with internal boundary condition on material interface and ohmic boundary condition on contacts. From the solution of this model V-I curves are calculated. 3.7 Introduction to MATLAB MATLAB [13] is a high performance language for technical computing. It integrates calculation, visualization and programming in a simple to use surroundings where troubles and solutions are presented in well-known mathematical notation. MATLAB can solve technical computing troubles faster than conventional programming language (like – Forton, C, C++). Typical uses include – Financial modeling and investigation Computational biology Math and computation algorithm development Data acquisition modeling Simulation and prototyping data study Exploration and visualization Graphics application development for scientific and engineering field Graphical user interface building Matrix laboratory is the full form of MATLAB. Basic data element in MATLAB is an array which does not need dimensioning. With the help of MATLAB number of technical computing troubles mainly those with vector and matrix formulations can be solved in a fraction of time. Basically it was written to give simple access to matrix software. For advance science, mathematics, engineering field and high productivity industrial research, progress and study MATLAB is very important instruction tool. Comprehensive collection of MATLAB functions are toolbox. Toolboxes of MATLAB permit us to study and apply specific technology. Toolboxes are available in different areas like – neural network, communication, signal processing, fuzzy logic, simulation, control system and many others. Differential equations are solved very easily in MATLAB [14-17]. We can also do modeling and simulation of solar cell using MATLAB [18,19]. 3.8 Conclusions This chapter explains about modelling and simulation. Presentation of physical configuration or activities of device by conceptual mathematical model that approximates this behavior, is called modeling. Model may either be closed form equation or arrangement of simultaneous equations that are numerically solved. Analytical and numerical both type of analysis can be used in modeling. Simulation is process of imitating the physical system behavior by considering the characteristic of an analogous but different system without resorting direct practical experimentation. For simulation we are using MATLAB which is a high performance technical computing language. We get that MATLAB integrates calculation, programming and visualization in a simple to use surroundings where mathematical expressions are used to express troubles and solutions. Because of all these qualities of MATLAB a system of number of numerical equations used for electrical modelling of bilayer organic solar cell are solved easily and in better way as compared to other programming languages. 3.9 References [1] B. P. Zeigler, Wiley, New York, (1976). [2] A. M. Law and W.D. Kelton, 2nd ed., McGraw-Hill,  New York, (1991). [3] F. Haddix, Paper 01F-SIW-098, Proceedings of the Simulation Interoperability Workshop, Fall (2001). [4] A. Crespo-Mà ¡rquez, R. R. Usano and R. D. Aznar, Proceedings of International System Dynamics Conference, Cancun, Mexico, The System Dynamics Society, (1993), 58. [5] J. S. Kumari and C. S. Babu, International Journal of Electrical and Computer Engineering (IJECE), 2(1), (2012), 26-34. [6] P. Sudeepika, G.Md. G. Khan, International Journal of Advanced Research in Electrical,Electronics and Instrumentation Engineering, 3(3), (2014), 7823-7829. [7] M. Abdulkadir, A. S. Samosir, A. H. M. Yatim, International Journal of Power Electronics and Drive System (IJPEDS), 3(2), (2013), 185-192. [7] L. A. A. Pettersson, L. S. Roman, and O. Ingana, Journal of Applied Physics, 86, (1999), 487-496. [8] A. Cheknane, T. Aernouts, M. M. Boudia, ICRESD-07, (2007), 83 – 90. [9] I. Hwang, C. R. M. Neill, and N. C. Greenham, Journal of Applied Physics, 106, (2009), 094506:1-10. [10] W. Vervisch, S. Biondo, G. Rivià ¨re, D. Duchà ©, L. Escoubas, P. Torchio, J. J. Simon, and J. L. Rouzo, Applied Physics Letters, 98, (2011), 253306:1-3. [11] M. Casalegno, A. Bernardi, G. Raos, J. Chem. Phys., 139(2), (2013). [12] J. M. Foster, J. Kirkpatrick, and G. Richardson, Journal of Applied Physics, 114, (2013), 104501:1-15. [13] A. Knight, CRC Press LLC, (2000). [14] R. K. Maddalli , Indian Journal of Computer Science and Engineering, 3(3), (2012), 406-10. [15] Z. M. Kazimovich and S. Guvercin, International Journal of Computer Applications, 41(8), (2012), 1-5. [16] A. B. Kisabo, A. C. Osheku, A. M. Adetoro, A. Lanre and A. Funmilayo, International Journal of Scientific and Engineering Research, 3(8), (2012), 1-7. [17] V. Nehra, I.J. Intelligent Systems and Applications, 05, (2014), 1-24. [18] S. Nema, R. K. Nema, and G. Agnihotri, International Journal of Energy and Environment, 1(3), (2010), 487500. [19] M. Edouard, D. Njomo, International Journal of Emerging Technology and Advanced Engineering, 3(9), (2013), 24-32.

Impact of Party Drugs on the Youth Culture Essay

Adolescence refers to the age group from 14 to 26 years. Ten percent of this age group use party drugs (Bennett, 2003). According to Arnett (2004) , this period of development is distinguished by five characteristics: identity exploration, instability, self-focus, a feeling of in-between and possibilities (Arnett, 2004, pg. 14). It is against these five characteristics that the impact of party drugs on the youth culture will be assessed. This essay will explore how the characteristics of adolescence place teenagers at risk from drug experimentation and how the perception of policy makers will influence the community’s response to the problem. The exploration of identity involves having a range of experiences that provides the adolescent with the means to assess the possibilities for the purpose of formulating a distinctive self-image. To do this requires that the teenager have a range of experiences that seem distinct from those experienced through their parents. This journey of exploration results in an introverted focus on self and a sense of becoming, of being caught in the middle. In the individualised cultures of western societies, this transition involves a separation from parents and the construction of an independent self-sufficient identity (Arnett, 2004). The instability can often manifest in ‘risky behaviours’. Although adolescence is a time for the construction of a unique self identity, it is also a time when a sense of belonging is engendered through common cultural construction. One subset of this cultural construction is the rave party scene that is a global phenomenon of the youth subculture (Shapiro, 1999). A rave party is often a large gathering of young people in an atmosphere where there is music and laser lights. This sub-culture is linked to the drug culture through party drugs such as ecstasy and ketamine. The effect of these drugs is to create a sense of wellbeing and a feeling of lightness. The choice to take drugs is an individual one and is part of the desire to escape from the pressures of adolescence. The rave party by its very nature is essentially a form of escape. What then are adolescences escaping from? The very nature of adolescence: the sense of being without identity, the lack of a mental framework to manage the adult world and the pressures of identity construction, create tensions in the person. This escapism is not confined to adolescences as many adults abuse alcohol and amphetamines as a means of escaping the pressures of the adult world. For many users, taking the drug is part of the risk behaviour of adolescence that has the pay-off of feelings of well-being. They do not se themselves as drug users as they do not view their use of party drugs as being a problem. This normalises drug use and makes it difficult for agencies to intervene (Duff, 2003). It is wrong however to assume that all users are escaping from something. One of the features of adolescence is the search for identity and self meaning. This search behaviour creates a heightened sense of curiosity in adolescents as they seek to make sense of self. This curiosity can evolve unconsciously to drug abuse through prolonged use as a result of the uplifting effects of the first experience. The need to fund the regular purchase of the drug can lead into dealing. Bad experiences often will not cause a rejection of the drug as these pale against the many pleasures that the individual has experienced. The chain of events can have dire consequences for the individual as a health problem becomes a criminal problem. The long term destruction that criminalisation of drugs causes to young people is good reason to see drug abuse as a health problem. Within party drug users there are the same segments that are feature of any drug user cross section. Some users abuse the drug and are at risk from overdose and dehydration. Others are more controlled and cautious in their use. This group is at risk from being unable to identify the ingredients of the drug that they are purchasing. One of the key problems with party drugs is the inability for the buyer to know the ingredients contained in the drug that they are purchasing (VAAD, 2003). Users of party drugs will tend to repeat their use on a regular basis. According to Baxter (2003) users are concerned that there is no means of determining the purity and reducing the level of risk. Males tend to use party drugs more frequently than females. This tends to indicate that there is greater gender difference where males are less risk averse. This trend is declining as more and more females are using party drugs. This trend reflects the emancipation of women and a stronger sense of independence amongst adolescent women. The traditional delineation of the rite of passage for the male and the female have become blurred in modern society. Women will often use drugs to challenge the traditional perceptions of their role in society. Within the community, there are people that see drug abuse as being criminal while others see it as a social and health problem. The criminal perspective adopts a punishment solution with rehabilitation. Such a position can have a long term effect on the individual due to problems faced with travel and employment. Community response is to increase policing and to use strategies such as sniffer dogs in public and undercover police at venues to catch the suppliers and users. Police raids will be conducted on rave parties. Such an approach marginalises the rave culture and runs the risk of impairing the social development of the individual. The perspective that sees the issue as a health issue seeks to develop preventative programmes that educate people. Greater understanding is sought for the motivation of young people for taking drugs through social research. Strategies for assisting at risk people are developed that provides for free, readily available access to health care and treatment. The health professional will often argue for the decriminalisation of the drug so that it can be obtained in a controlled manner and the purity of the product guaranteed. When considered against the characteristics of adolescence as provided by Arnett (2004), this approach appears to be the least detrimental to the social and personal development of the adolescent. Government programs, such as Ravesafe, adopt this approach. One of the prime reasons that this approach should prevail is that party drugs are seen by the user as being catalysts for self reflection and construction of identity. Their sense of self and social relationships results in a positive self-image which may be in contradiction to how they feel when in the adult world. The party drug tends to alleviate insecurity and doubt. Research has shown that there might be some overflow from the atmosphere of the rave party to real life. The need for early intervention arises from the research findings that drug abuse is often a precursor for youth suicide, crime and metal illness (VAAD, 2003). Interventionist strategies will view drug use as a problem which immediately places the interventionist in opposition to the youth culture. The clash between the pleasures of the drug and the potential harm of the drug creates a generational clash (Duff, 2003). To deal with the issue it is necessary to accept the reality of use and work on fostering environments that support safe use. If this is provided then it will reduce the ‘trial and error’ approach that is a feature of the risk behaviour of adolescents. VAAD (2003) found that the problem can be best solved through drug education that must resist seeing the youth culture as a homogeneous group. Adolescents involved in the rave scene come from a wide variety of backgrounds (Shapiro, 1999). Because there is little understanding of the attitudes of youth subgroups within the rave scene it is necessary for more research to be done (Baxter, 2003). In this way the content and approach of the drug education programme can directly appeal to the target segment. This will help in a more receptive response from the target group. In conclusion, the developmental characteristics of adolescence creates a natural disposition towards drug experimentation amongst some segments of teenagers. Party drugs are often seen as being part of the scene that assist in the integration of the individual with both self and the group that they belong to. Drug use can be used by the female gender to reconstruct identity. Given these features, the perspective that drug use should be a criminal offence resulting in punishment and long term consequences for the individual seems self defeating. The view that drug abuse has the potential to be a health problem will result in a community response that is more supportive to the individual. The safety of the drug can be established and the social stigma placed on adolescents can be removed. Social policy needs to consider the developmental characteristics of adolescence in its construction.

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Asking and Giving Directions in English

These dialogues focus on asking for and giving directions. Practice these  English  dialogues that give directions to different locations in a city. Once you feel comfortable with the vocabulary, ask for directions in your city with a partner or a classmate. Pretend you are  traveling in your city. Key Grammar Points to Remember Imperative form: You should use the imperative form when providing directions. The imperative form is comprised of only the verb without any subject, and it tells someone directly what to do. Here are some examples of the imperative from the dialogue. Take the blue line.Continue going straight.Change to the grayline. While you would not use the imperative form in normal polite speech as it is considered too abrupt, it is appropriate when providing asked-for guidance. Asking questions using how:  How combines with many  adjectives  to ask information about details. Here are some common  questions with how: How long?  Used to ask about the length of timeHow much or many?  Used to ask about price and quantityHow often?  Used to ask about repetition Key Vocabulary Words and Phrases Related to Directions There are a few important grammar and vocabulary points to remember when asking for and giving directions.   Take a right/leftGot itI understandDo you understand?Go straightOppositeTake the first / second / third /  rightGo right / left / straight at the light / corner / stop sign  Continue straight onTurn right / left at the light / corner / stop sign  Get on the bus / subway at 12th Ave. / Whitman Street / Yellow Lane  Follow the signs for the museum / exhibition center / exit   Common Questions When Asking for Directions Is it far? / Is it close?  How far is it? / How close is it?Could you please give me directions?Where is the nearest bank / supermarket / gas station?Where can I find a bookstore / restaurant / bus stop / restroom?Is the museum / bank / department store  near here? Practice Dialogue: Taking the Subway John: Linda, do you know how to get to Samsons and Co.? Ive never been there before.Linda: Are you driving or taking the subway? John: The subway.Linda: Take the blue line from 14th Ave. and change to the gray line at Andrew Square. Get off at 83rd street. John: Just a moment, let me write this down.Linda: Take the blue line from 14th Ave.  and change to the gray line at Andrew Square. Get off at 83rd street. Got it? John: Yes, thanks. Now, once I get to Andrew Square, how do I proceed?Linda: Once you are on 83rd street, go straight, past the bank. Take the second left and continue going straight. Its across the street from Jacks Bar. John: Can you repeat that?Linda: Once you are on 83rd street, go straight, past the bank. Take the second left and continue going straight. Its across the street from Jacks Bar. John: Thanks, Linda. How long does it take to get there?Linda: It takes about a half-hour. When is your meeting? John: Its at 10 a.m. Ill leave at 9:30.Linda: Thats a busy time. You should leave at 9. John: OK. Thanks, Linda.Linda: Not at all. Practice Dialogue: Taking Directions Over the Telephone Doug: Hello, this is Doug.Susan: Hi Doug. This is Susan. Doug: Hi Susan. How are you?Susan: Im fine. I have a question. Do you have a moment? Doug: Certainly, how can I help you?Susan: Im driving to the conference center later today. Could you give me directions? Doug: Sure. Are you leaving home?Susan: Yes. Doug: OK, take a left onto Bethany street and drive to the freeway entrance. Take the freeway toward Portland.Susan: How far is it to the conference center from my home? Doug: Its about 20 miles. Continue on the freeway to exit 23. Take the exit and turn right onto Broadway at the stoplight.Susan: Let me repeat that. Take the freeway to exit 23 and turn right onto Broadway. Doug: Thats right. Continue on Broadway for about two miles and then turn left onto 16th Ave.Susan: OK. Doug: On 16th Ave., take the second right into the conference center.Susan: Oh thats easy. Doug: Yes, its very easy to get to.Susan: How long does it take to get there? Doug: If theres no traffic, about 25 minutes. In heavy traffic, it takes about 45 minutes.Susan: Im leaving at 10  in the morning, so the traffic shouldnt be so bad. Doug: Yes, thats right. Can I help you with anything else?Susan: No thats it. Thanks for your help. Doug: OK. Enjoy the conference.Susan: Thanks, Doug. Bye.   Practice Dialogue: Directions to the Museum (On the street corner) Tourist:  Excuse me, can you help me? Im lost!Person:  Certainly, where would you like to go? Tourist:  Id like to go to the museum, but I cant find it. Is it far?Person:  No, not really. Its about a five-minute walk. Tourist:  Maybe I should call a taxi.Person:  No, Its very easy. Really. (pointing) I can give you directions. Tourist:  Thank you. Thats very kind of you.Person:  Not at all. Now, go along this street to the traffic lights. Do you see them? Tourist:  Yes, I can see them.Person:  Right, at the traffic lights, turn left into Queen Mary Ave. Tourist:  Queen Mary Ave.Person:  Right. Go straight. Take the second left and enter Museum Drive. Tourist:  OK. Queen Mary Ave., straight on and then the third left, Museum Drive.Person:  No, its the second left. Tourist:  Ah, right. The second street on my left.Person:  Right. Just follow Museum Drive and the museum is at the end of the road. Tourist:  Great. Thanks again for your help.Person:  Not at all. Practice Dialogue: Directions to a Supermarket Tom:  Could you go  to the supermarket and buy some food? Theres nothing to eat in the house!Helen:  Sure, but I dont know the way. Weve just moved in. Tom:  Ill give you directions. Not to worry.Helen:  Thanks.   Tom:  At the end of the street, take a right. Then drive two miles to White Ave. After that, its another mile to...Helen:  Let me write this down. I wont remember it! Tom:  OK. First, take a right at the end of the street.Helen:  Got it. Tom:  Next, drive two miles to White Ave.Helen:  Two miles to White Ave. After that? Tom:  Take a left onto 14th Street.Helen: Left onto 14th Street.   Tom:  The supermarket is on the left, next to the bank.Helen:  How far is it after I turn on to 14th Street? Tom:  Its not far, maybe about 200 yards.Helen:  OK. Great. Is there anything special you want? Tom:  No, just the usual. Well, if you could get some beer that would be great!Helen:  OK, just this once!