Lab 3. Ohm s Law Goals To underst Ohm s law, used to describe behavior of electrical conduction in many materials circuits. To calculate electrical power dissipated as heat. To underst use a rheostat, or variable resistor, in an electrical circuit. To learn how to connect electrical components so that current can flow around circuit, to learn how to use, connect, read s (current reading instruments) voltmeters (voltage reading instruments). To measure observe behavior of voltage across corresponding current through a simple resistor (electronic component) a tungsten-filament light bulb. Introduction One of most basic electrical circuits is a resistor connected to a voltage source, such as a battery or power supply. A quantity called resistance, R, of a component is defined as ratio of potential difference, V, across component to current, I, flowing through component, or R = V I (3.1) When V is expressed in volts I is expressed in amperes (amps), n R is in SI units of ohms (Ω). The power, P (in SI unit of watts), dissipated by that component in form of heat is given by P = I( V ) = I 2 R = ( V )2 R (3.2) The resistance of some materials is constant over a wide range of voltages currents. When a material behaves in this way, it is called ohmic. Electrical components made from ohmic materials are called resistors. By measuring current flowing through a component as a function of voltage across component, one can determine wher ratio V /I is a constant or not. If it is constant, n 27
CHAPTER 3. OHM S LAW 28 component is ohmic constant resistance in ohms can be determined. If voltage to current ratio is not constant, device is not ohmic does not obey Ohm s law. A voltmeter is used to measure A voltmeter voltage measures an electric ispotential used to measure or voltage current. difference Idealbetween voltmeters two s points to which will not it affect is connected. currents Thus or to voltages measure in voltage circuitacross as a measurements particular device are being a circuit, made. one Real wire meters from only approximate voltmeter is this connected ideal. to one end of device a second wire from voltmeter is connected to or end of device. This type of connection is called a parallel Anconnection. measures The voltmeter electrical in Figure current 1 is represented that flows through by a box it. marked To measure with letter current V. flowing through a particular device in a circuit, must be connected in such a way that same current An flows through measures electrical as through current that device. flows through The it. To measure is simply flow current meter flowing for through electrical a particular current, so device wire in a at circuit, one end of device must must be connected be disconnected in such a way that same current flows through as through device. The is simply a flow inserted. The disconnected wire end is now connected to one terminal of a new meter for electrical current, so wire at one end of device must be disconnected wire is connected inserted. between The disconnected second terminal wire end of is now connected to one device terminal to restore of flow of current a through new wire is circuit. connected This between type of connection second terminal is calledof a series connection. device The to inrestore Figure 3.1flow is represented of current by through a box marked circuit. with This type letter of A. connection is called a series connection. The in Figure 1 is represented by a box marked with letter A. Caution: If current flows backwards through, tries to respond by registering Caution: aif negative current current. flows backwards Since through meter needle, can only show positivetries values, to respond this can damage meter. by registering To checka that negative current. is Since connected meter withneedle correct can only polarity, show positive quickly tap knifevalues, switch this (See can Figure damage 3.1) before meter. closing To check it completely. that If meter is connected does try with to deflect in negative correct direction, polarity, exchange quickly tap connections switch (See of Figure two wires 1) before connected closing toit. completely. If meter does try to deflect in negative direction, interchange connections of two wires connected to. Current versus voltage for a 100 Ω (nominal) resistor 2. Current versus voltage for a 100 Ω (nominal value) resistor In this exercise voltage across current through a known resistor are measured as current In this exercise voltage across current through a known resistor are measured as current through through circuit circuit is varied. is varied. The power The power supply supply voltage voltage is kept is constant, kept constant, but but current current flowing inflowing circuit in is controlled circuit is controlled with a variable by a variable resistor, also resistor, called also a rheostat. called a rheostat. (See Figure (See 3.1.) Figure 1.) Figure Figure 3.1. 1. Circuit connections. The The rheostat rheostat has has three three terminals. terminals. Two Two terminals terminals are are on on ends ends of of device device are are fixed, fixed, third is connected to a sliding contact that can be moved from one end of device to third is connected to a sliding contact that can be moved from one end of device to or. The resistance between end terminals has a fixed value, but resistance between or. one of The resistance end terminals between end sliding terminals contact has can a be fixed varied value, from but zero to resistance fixed between value of whole device. 26
CHAPTER 3. OHM S LAW 29 one of end terminals sliding contact can be varied from zero to fixed value of whole device. Preliminary calculations Assuming that power supply voltage is fixed at 5.0 V, calculate following quantities to two significant digits: 1. The current through nominally 100 Ω resistor when resistance of rheostat is a maximum (340 or 360 Ω your rheostat is marked with value to use here), when resistance of rheostat is zero. 2. The maximum power dissipated as heat by 100 Ω resistor. The rated maximum power for this resistor is 0.50 W. If power you calculate exceeds 0.50 W, please ask your TA for help before proceeding! Equipment set-up Caution: If current flows backwards through, tries to respond by registering a negative current. Since meter needle can show only positive values, this can damage meter. The can also be damaged if magnitude of current is much larger than current rating of chosen scale. To check that is connected with correct polarity to a safe current scale, quickly tap knife switch (See Figure 3.1) without closing it completely. If meter tries to deflect in negative direction, exchange connections of two wires connected to. If meter tries to deflect off-scale in correct direction, use a current scale with a higher current rating. If meter passes se two tests, close knife switch completely proceed to make measurements. 1. Turn current knob on power supply to straight-up or 12 o clock position, set power supply voltage to 5.0 V. 2. Build circuit shown in Figure 3.1, leaving switch open, that is, not making electrical contact. Be sure to use an scale with a current rating large enough to measure maximum current you calculated above. By convention s read positive when electrical current flows into positive terminal (red) of meter n flows out of negative terminal (black) of meter. 3. Set rheostat for maximum resistance by moving slide so that current must travel through entire coil. 4. Tap knife switch to make sure that connections are correct. If all is well, n close switch. Both voltmeter should read non-zero values. If measured current is below current rating of a more sensitive scale, open knife switch, move connection to more sensitive scale, tap knife switch closed to test new scale. Use most sensitive current scale that can hle current safely (reading stays on-scale).
CHAPTER 3. OHM S LAW 30 Data collection 1. Make at least ten different measurements of voltage corresponding current by adjusting rheostat between its minimum maximum resistance. To obtain data points at low currents, you can lower voltage supplied to circuit by power supply to some value less than 5 V. Ask your TA for help as necessary. 2. How does current measured by change if is connected between power supply rheostat instead of between rheostat resistor? What if it is connected between power supply switch? Verify your answers experimentally. Data analysis 1. Draw a graph of voltage across nominal 100 Ω resistor as a function of corresponding current flowing through it. 2. Is graph linear? Draw a best fit smooth line through your data points, from your graph find an equation for V as a function of I in SI units. 3. Does resistor exhibit ohmic behavior? Explain your reasoning. If so, what is real value of resistance? How does your value compare to nominal 100 Ω value indicated by color code painted on it? Current versus voltage for an incescent light bulb Equipment set up Caution: Be sure to leave switch open while you construct new circuit. Before closing switch, have your TA check your circuit. 1. Build a circuit analogous to one in Figure 1, but use 22 Ω rheostat instead of 340 or 360 Ω one used above replace 100 Ω resistor with small light bulb. 2. Use highest current scale on to begin with. You can always change to a more sensitive scale if measured current is low enough. 3. Make sure that power supply is still set to 5 volts. Data collection 1. Make at least ten different measurements of voltage corresponding current by adjusting rheostat between its minimum maximum resistance. 2. Does current flow through light bulb even when bulb is not glowing? Be sure to take data over full range of possible values, wher bulb glows or not. Data analysis 1. Make a graph of voltage difference between light bulb terminals as a function of current. What is current flowing through light bulb if voltage across it is zero? Be sure to plot this point on your graph!
CHAPTER 3. OHM S LAW 31 2. Is light bulb ohmic? Explain your reasoning. If so, what is its resistance? If not, what are minimum maximum values of its resistance? 3. What is maximum power dissipated by light bulb? (This power is dissipated primarily in form of heat, but some also appears in form of visible light.) What is power dissipated by bulb when it first begins to glow? Summary Compare contrast electrical behavior of resistor light bulb. Consult a textbook try learn why light bulb exhibits a more complicated behavior than resistor. Explain this in your notes. AA extract information from representation correctly Labs: 1-12 Grading Rubric No Effort Progressing Expectation Exemplary No visible attempt is made to extract information from experimental setup. Information that is extracted contains errors such as labeling quantities incorrectly, mixing up initial final states, choosing a wrong system, etc. Physical quantities have no subscripts (when those are needed). Most of information is extracted correctly, but not all of information. For example physical quantities are represented with numbers re are no units. Or directions are missing. Subscripts for physical quantities are eir missing or inconsistent. All necessary information has been extracted correctly, written in a comprehensible way. Objects, systems, physical quantities, initial final states, etc. are identified correctly units are correct. Physical quantities have consistent informative subscripts. AB construct new representations from previous representations construct a different representation. Representations are attempted, but omits or uses incorrect information (i.e. labels, variables) or representation does not agree with information used. Representations are constructed with all given (or understood) information contain no major flaws. Representations are constructed with all given (or understood) information offer deeper insight due to choices made in how to represent information. Labs: 1-12 AC evaluate consistency of different representations modify m when necessary No representation is made to evaluate consistency. At least one representation is made but re are major discrepancies between constructed representation given experimental setup. There is no attempt to explain consistency. Representations created agree with each or but may have slight discrepancies with given experimental representation. Or re is inadequate explanation of consistency. All representations, both created given, are in agreement with each or explanations of consistency are provided. Labs: 3-8, 10-12
CHAPTER 3. OHM S LAW 32 AD use representations to solve problems Labs: 4, 5, 6-8, 10, 11 No Effort Progressing Expectation Exemplary solve problem. The problem is solved correctly but no representations or than math were used. The problem is solved correctly but re are only two representations: math words explaining solution. The problem is solved correctly with at least three different representations (sketch, physics representation math or sketch, words math, or some or combination) AG Mamatical Labs: 3, 6, 8, 10-12 No representation is constructed. Mamatical representation lacks algebraic part ( student plugged numbers right away) has wrong concepts being applied, signs are incorrect, or progression is unclear. No error is found in reasoning, however y may not have fully completed steps to solve problem or one needs effort to comprehend progression. Mamatical representation contains no errors it is easy to see progression of first step to last step in solving equation. The solver evaluated mamatical representation with comparison to physical reality. AI Graph Labs: 3, 6, 8, 12 No graph is present. A graph is present but axes are not labeled. There is no scale on axes. The data points are connected. The graph is present axes are labeled but axes do not correspond to independent dependent variable or scale is not accurate. The data points are not connected but re is no trend-line. The graph has correctly labeled axes, independent variable is along horizontal axis scale is accurate. The trend-line is correct, with formula clearly indicated. AJ Circuit diagram Labs: 3, 4 No representation is constructed. Components of circuit are missing, or connected incorrectly. Components are not clearly labelled. Diagram is missing key features but contains no errors. It may be difficult to follow electrical pathways, but it can be determined which components are connected with sufficient scrutiny. Circuit diagram contains minimal connecting lines, components are neatly arranged to ensure labels are readily identified to appropriate components. BA identify phenomenon to be investigated No phenomenon is mentioned. The description of phenomenon to be investigated is confusing, or it is not phenomena of interest. The description of phenomenon is vague or incomplete but can be understood in broader context. The phenomenon to be investigated is clearly stated. Labs: 1-3, 5, 7-12
CHAPTER 3. OHM S LAW 33 BE describe what is observed concisely, both in words by means of a picture of experimental setup. Labs: 1-3, 5, 7-12 BF identify shortcomings in an experiment suggest improvements Labs: 1-3, 5, 7, 8, 10-12 No Effort Progressing Expectation Exemplary No description is mentioned. identify any shortcomings of experimental. A description is incomplete. No labeled sketch is present. Or, observations are adjusted to fit expectations. The shortcomings are described vaguely no suggestions for improvements are made. A description is complete, but mixed up with explanations or pattern. OR The sketch is present but relies upon description to underst. Not all aspects of design are considered in terms of shortcomings or improvements, but some have been identified discussed. Clearly describes what happens in experiments both verbally with a sketch. Provides or representations when necessary (tables graphs). All major shortcomings of experiment are identified reasonable suggestions for improvement are made. Justification is provided for certainty of no shortcomings in rare case re are none. BG identify a pattern in data search for a pattern The pattern described is irrelevant or inconsistent with data. The pattern has minor errors or omissions. OR Terms labelled as proportional lack clarityis proportionality linear, quadratic, etc. The patterns represents relevant trend in data. When possible, trend is described in words. Labs: 1-3, 5, 7-9, 11, 12 BI devise an explanation for an observed pattern explain observed pattern. An explanation is vague, not testable, or contradicts pattern. An explanation contradicts previous knowledge or reasoning is flawed. A reasonable explanation is made. It is testable it explains observed pattern. Labs: 1-3, 5, 7-9, 11, 12 GD record represent data in a meaningful way Data are eir absent or incomprehensible. Some important data are absent or incomprehensible. They are not organized in tables or tables are not labeled properly. All important data are present, but recorded in a way that requires some effort to comprehend. The tables are labeled but labels are confusing. All important data are present, organized, recorded clearly. The tables are labeled placed in a logical order. Labs: 1-12 GE analyze data appropriately analyze data. An attempt is made to analyze data, but it is eir seriously flawed or inappropriate. The analysis is appropriate but it contains minor errors or omissions. The analysis is appropriate, complete, correct. Labs: 1-12
CHAPTER 3. OHM S LAW 34 IA conduct a unit analysis to test self-consistency of an equation Labs: 1-6, 8-12 No Effort Progressing Expectation Exemplary No meaningful attempt is made to identify units of each quantity in an equation. An attempt is made to identify units of each quantity, but student does not compare units of each term to test for self-consistency of equation. An attempt is made to check units of each term in equation, but student eir mis-remembered a quantity s unit, /or made an algebraic error in analysis. The student correctly conducts a unit analysis to test selfconsistency of equation. EXIT TICKET: Turn off power to all equipment. Disassemble circuit place small components in plastic tray. Quit any software you have been using. Straighten up your lab station. Put all equipment where it was at start of lab. Report any problems or suggest improvements to your TA. Have TA validate Exit Ticket Complete.