Circuit board physics

Circuit Board: Series/Parallel Circuit Lab PURPOSE: The purpose of this experiment is to learn 1) to construct direct current electrical circuits; 2) to represent electrical circuits and 3) to measure currents and voltages. Basics a direct current (DC) circuit is a circuit in which the current flowing through each element of the circuit is constant in time. Its elements are batteries and resistors. The simplest DC circuit comprises a single (ideal) battery and a resistor. It is represented schematically in this way: The current through the circuit is related to the voltage across the battery by Ohm’s law V=RI. A battery is called either a source of electromotive force or, more commonly, a source of emf. (The phrase electromotive force is an unfortunate historical term, describing not a force but rather a potential difference in volts.) The emf of a battery is the maximum possible voltage that the battery can provide between its terminals. The symbol for emf is the greek letter  (epsilon). Because a real battery is made of matter, there is resistance to the flow of charge within the battery. This resistance is called internal resistance r. For an idealized battery with zero internal resistance, the potential difference across the battery (called its terminal voltage) equals its emf. However, for a real battery, the terminal voltage is not equal to the emf for a battery in a circuit in which there is a current. A real battery is represented schematically as an ideal battery with an internal resistance in series: I= ϵ r+R A simple way to evaluate the internal resistance r of a battery when r is relatively large, is to first measure the battery’s emf with the circuit open, and then close the circuit and measure the voltage drop V=RI across the resistor R. The internal resistance will be ϵ−Δ V r= R (1) ΔV The current through the circuit is therefore The resistance of commercial resistors is color coded on the resistors themselves according to the following scheme For example a resistor with color code brown black brown gold will have as first digit 1, as second digit 0, followed by 1 zero and will have a tolerance of ± 5%. Its resistance will therefore be 100  ± 5% When two resistors are in series, the same current flows through both of them. The total (or equivalent) resistance is therefore the sum of the resistances of the two resistors: When two resistors are in parallel, the voltage drop is the same across both resistors. The inverse of the total (or equivalent) resistance is therefore the sum of the inverses of the resistances of the two resistors: Experimental procedure Part I – Using Ohm’s Law 1. Get a resistor with any color code and record its resistance in the data table. Choose it so that the expected current is between 100 and 200 mA. 2. Place the resistor into a pair of springs. 3. Use just one battery. 4. Set your multimeter on the 2 V or setting. -Make sure the black wire of the meter is inserted into the COM port at all times. -Make sure the red wire is inserted into the VΩ port at the moment. 5. Measure the voltage ACROSS the battery and record it. 6. Attach 1 wire from the positive terminal of the battery to one of the resistor springs. 7. Attach another wire from the negative end of the battery to the other resistor spring. 8. Measure the voltage ACROSS the 2 resistor springs and record this value. 9. Set your multimeter on the 200mA or μA DCA setting. (If this is a milliamp setting, all values MUST be divided by 1000 to get amps) -Make sure your red wire is inserted into the mA port at the moment. 9. Remove the wire going from the negative terminal of the battery to a resistor spring. 10. Measure and record the current between these 2 points where the wire was previously. Part II – Two Resistors in Series 1. Get two coded resistors and record their value in the data table II. Again, choose them so that the expected total current is between 100 and 200 mA. 2. Place one resistor in one set of springs. Leave the next set OPEN. Then place the 2nd resistor in the 3rd set of springs at the bottom of the board. Place a wire between the sets to connect the 2 resistors. 3. Measure and record the TOTAL VOLTAGE across ALL the resistors. You do this by touching one multimeter terminal to the very 1st spring the resistors are in and the other terminal to the very last. 4. Measure and record the VOLTAGE DROPS across each resistor by only touching the springs each resistor is in. 5. Measure and record the TOTAL CURRENT by removing the wire going from the negative terminal of the battery to the last resistor. Part III – Two Resistors in Parallel 1.