Goals. Introduction (4.1) R = V I

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1 Lab 4. Ohm s Law Goals To understand Ohm s law, used to describe behavior of electrical conduction in many materials and circuits. To calculate electrical power dissipated as heat. To understand and use a rheostat, or variable resistor, in an electrical circuit. To learn how to connect electrical components so that current can flow around circuit, and to learn how to use, connect, and read ammeters (current reading instruments) and voltmeters (voltage reading instruments). To measure and observe behavior of voltage across and corresponding current through a simple resistor (electronic component) and 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 (4.1) When V is expressed in volts and 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 (4.2) The resistance of some materials is constant over a wide range of voltages and currents. When a material behaves in this way, it is called ohmic. Electrical components made from ohmic materials are called resistors. 17

2 18 CHAPTER 4. OHM S LAW A voltmeter measures electric potential or voltage difference between two points to which By measuring current flowing through a component as a function of voltage across it is connected. Thus to measure voltage across a particular device in a circuit, one wire from component, voltmeter one is can connected determine to wher one end of ratio device V /I and is a constant a second or wire not. from If it is constant, voltmeter n is component connected isto ohmic or and end constant of device. resistance This intype ohms of can connection be determined. is called Ifa parallel voltage to current ratio connection. is not constant, The voltmeter device in is Figure not ohmic 1 is represented and does not by obey a box Ohm s marked law. with A voltmeter letter V. is used to measure voltage and an ammeter is used to measure current. Ideal voltmeters and ammeters will not An affect ammeter currents measures or voltages electrical in current circuithat as flows measurements through it. To aremeasure being made. current Real meters flowing only through approximate a particular this device ideal. in a circuit, ammeter must be connected in such a way that same current flows through ammeter as through device. The ammeter is simply a flow Anmeter ammeter for measures electrical current, electrical so current wire that one flows end through of device it. To measure must be disconnected current flowing and through ammeter a particular inserted. device The disconnected in a circuit, wire ammeter end is now must connected be connected to one in terminal such a way of that ammeter same and a new wire is connected between second terminal of ammeter and device to current flows through ammeter as through device. The ammeter is simply flow meter for restore flow of current through circuit. This type of connection is called a series connection. electrical current, The ammeter so in wire Figure at one 1 is end represented of device by a must box marked be disconnected with letter and A. ammeter inserted. The disconnected wire end is now connected to one terminal of ammeter and a new wire Caution: is connected If current between flows backwards second terminal through of ammeter, and ammeter device to restore tries to respond flow of current through by registering circuit. This a negative type ofcurrent. connection Since is called meter a series needle connection. can only show The ammeter positive in Figure 4.1 values, is represented this can bydamage a box marked meter. with To check letter A. that ammeter is connected with correct polarity, quickly tap switch (See Figure 1) before closing it completely. If meter does try to deflect in negative direction, interchange connections of two wires connected to ammeter. 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 and current through a known resistor are measured as current In through this exercise circuit voltage is varied. across Theand power supply current voltage through isa kept known constant, resistor but are measured current flowing as incurrent circuit through is controlled circuit with is varied. a variable The resistor, power supply also called voltage a rheostat. is kept (See constant, Figure but 4.1.) current flowing in circuit is controlled by a variable resistor, also called a rheostat. (See Figure 1.) Figure Figure Circuit connections. The rheostat has three terminals. Two terminals are on ends of device and are fixed, and The rheostat third is has connected three terminals. to a sliding Two contact terminals that can arebe onmoved ends from of one device end of anddevice are fixed, to and or. thirdthe is connected resistance to between a sliding contact end terminals that can has bea moved fixed value, frombut one end resistance of device between to or. one of The resistance end terminals between and end sliding terminals contact has can a be fixed varied value, from but zero to resistance fixed between value of whole device. one of end terminals and sliding contact can be varied from zero to fixed value of whole device. 26

3 19 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), and 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 ammeter, ammeter tries to respond by registering a negative current. Since meter needle can show only positive values, this can damage meter. The ammeter can also be damaged if magnitude of current is much larger than current rating of chosen scale. To check that ammeter is connected with correct polarity and to a safe current scale, quickly tap knife switch (See Figure 4.1) without closing it completely. If meter tries to deflect in negative direction, exchange connections of two wires connected to ammeter. 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 and proceed to make measurements. 1. Turn current knob on power supply to straight-up or 12 o clock position, and set power supply voltage to 5.0 V. 2. Build circuit shown in Figure 4.1, leaving switch open, that is, not making electrical contact. Be sure to use an ammeter scale with a current rating large enough to measure maximum current you calculated above. By convention ammeters read positive when electrical current flows into positive terminal (red) of meter and 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 ammeter connections are correct. If all is well, n close switch. Both ammeter and 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, and tap knife switch closed to test new scale. Use most sensitive current scale that can handle current safely (reading stays on-scale).

4 20 CHAPTER 4. OHM S LAW Data collection 1. Make at least ten different measurements of voltage and corresponding current by adjusting rheostat between its minimum and 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 ammeter change if ammeter is connected between power supply and rheostat instead of between rheostat and resistor? What if it is connected between power supply and 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, and 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 incandescent 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 and replace 100 Ω resistor with small light bulb. 2. Use highest current scale on ammeter 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 and corresponding current by adjusting rheostat between its minimum and 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.

5 21 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! 2. Is light bulb ohmic? Explain your reasoning. If so, what is its resistance? If not, what are minimum and 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 and contrast electrical behavior of resistor and light bulb. Consult a textbook and try learn why light bulb exhibits a more complicated behavior than resistor. Explain this in your notes. Before you leave lab please: Turn off power to all equipment. Disassemble circuit and place small components in plastic tray. Straighten up your lab station. Report any problems or suggest improvements to your TA.