Current Electricity L-1: Exploring Ohm's Law

	Explanation: For any electrical element in a circuit, there is a relationship between the potential difference between the terminals of the element, the current that flows through the element and a property of the element itself called its resistance. The resistance may, or may not, depend on the other two variables. If a graph of the {current, voltage} pairs is linear, then the element is said to obey OHM'S Law and the slope of the linear relationship is called the RESISTANCE. Resistance is measured in ohms.
	Note: This activity specifically draws on the CPU Project simulator for current electricity
	Exploring RESISTANCE: Open the first SIMULATION - Ohm's Law. If the circuit has not already been built, make a series circuit with the following components from the toolkit: A variable voltage supply, a resistor and an ammeter. Switch to the schematic mode (and turn on the voltage pressure option). Choose the arrow to begin the simulation. Use the OPTION CLICK feature to set the voltage to the lowest value [hold down the option key, point to the element and click the mouse]. When the dialog window pops up you can use the slider to alter the voltage. When you click in the DONE box, the voltage will be set (close) to the value you have selected. You can now read both the voltage and the current in the elements. Record at least 6 pairs of {current, voltage} for the element by changing the voltage of the variable voltage supply.
	Current Voltage
	Use your graphing calculator to prepare a plot of {i, V}. Place the current values in a list, L1, and the voltage values in a second list, L2. Plot the data. Use STAT to perform a regression of the data. You may try various functions to see which fits "best." Transfer the function to the y= palette and plot both the data and the best regression together. Returning to the equation which you have transferred to the y= palette, what is the slope of the best-fitting line? Slope = ____________________ Return to you simulation. OPTION CLICK on the resistor. What value do you read from the dialog box? How does this value compare with the value you measured graphically?
	Wires come in all sizes; some are thick, some are thin, some are long and others short. Think about the factors which may influence how a wire can resist the flow of electrical current?
	Listen to your class mates as they discuss their ideas about what may influence the resistance of a wire. Record here some of the ideas that are offered.
	What is different about a wire? In this part of the activity the characteristics of a wire will be determined. Open the simulator. If the circuit has not already been constructed, assemble a series circuit consisting of a variable power supply, a variable resistor and an ammeter. Switch to the schematic mode; turn on the voltage pressure option. OPTION CLICK on the power supply and set the voltage to some convenient value, say 20 volts. Record the voltage here V= _______________ volts What is the resistance of 1 meter of a wire? OPTION CLICK on the variable resistor. Choose the "thin nichrome wire." Set the length to a convenient value and record the data{ length, current, voltage}. OPTION CLICK on the resistor and set the length to a different value. Record the data triplet again. Repeat for at least 6 data sets.
	length Current Voltage
	Listen as the members of your group discuss a way in which you might find the resistance of one meter of the wire. Record what seems to be the best procedure.
	Listen to the other groups' best ideas for determining this quantity. Record here the consensus procedure.
	If you need to make any plots use your graphing calculator and store the data in lists. If you need to fit any functions to the data use the STAT menu. When you have completed the activity record here the value of the resistance per meter for the thin nichrome wire. Thin nichrome _____________________ ohms/meter
	Suppose that you had a THICK nichrome wire. Would this wire have more or less resistance than a thin wire? Record you ideas and reasons here.
	Listen to the ideas of the members of your group. What is the consesus of your group?
	Listen to the ideas of other groups. Is there unanimaty? What precedure can you use to answer the issue?
	How is a THICK wire different ? Return to the simulator. OPTION CLICK on the resistor and choose the option thick nichrome wire. Repeat your previous experiment for 6 different lengths of this wire. What do you find is the resistance of 1 meter of thick nichrome wire? thick nichrome wire ____________________ ohms/meter for comparison thin nichrome wire _____________________ ohms/meter
	length Current Voltage
	Does copper have more or less resistance than nichrome ? Wires can be made of any conductor - aluminum, copper, silver, gold(!), steel. Return to the simulator. OPTION CLICK on the resistor and choose either a thick or thin copper wire. Repeat the previos experiment for 6 different lengths of this wire. What do you find is the resistance of 1 meter of _________ copper wire? __________ copper wire ________________ ohms/meter for comparison __________ nichrome wire ______________ ohms/meter
	length Current Voltage
	What are some of the considerations an electrical engineer must make when she specifies the size and type of wire for a transmission line (those wires that are strung between tall towers)? You might to think about weight, cost, electrical properties, etc .
	Discuss with members of your group what you think might be the relationship between resistance and the variables length , thickness , and kind of wire. After your group has reached a decision, listen to the suggestions of the other group. Record your class relationship here.
	You might have reached a relationship that resembles the following:
	Resistivity is a property of every material. Good conductors have small resistivity while insulators have a large value of resistivity. You have measured the resistance of the wires and inferred the resistance for 1 meter of each wire [you may need to use your graphical data]. In a reference work, find the value of resistivity for copper and for nichrome. Record those values here. resistivity (nichrome) ____________________ (ohm m) resistivity (copper) _____________________ (ohm m)   Use your data for resistance, resistivity and length to solve for the cross-section area of the thick and thin wires. area (thin copper) _________________m2 area (thin nichrome) _______________m2 area (thick copper) ________________m2 area(thick nichrome) ______________m2 Are these results consistent? ........ reasonable?