EXPERIMENT 1 Ohm’s Law Objectives • Become familiar with the use of a digital voltmeter and a digital ammeter to measure DC voltage and current. • Construct a circuit using resistors, wires and a breadboard from a circuit diagram. • Construct series and parallel circuits. • Test the validity of Ohm’s law • Reduce a complicated resistance circuit to a simple one-resistor equivalent circuit. Note: If you are color blind or suspect that you are, you may find the color codes on the resistors difficult. Please consult your lab instructor for advice or help. Introduction One of the fundamental laws describing how electrical circuits behave is Ohm’s law. According to Ohm’s law, there is a linear relationship between the voltage drop across a circuit element and the current flowing through it. Therefore the resistance R is viewed as a constant independent of the voltage and the current. In equation form, Ohm’s law is: RIV×= (1) Here, V = voltage applied across the circuit and has SI units of volts (V) I = current flowing through the circuit and has SI units of amperes (A) R = resistance of the circuit and has SI units of ohms (Ω) Equation (1) implies that, for a resistor with constant resistance, the current flowing through it is proportional to the voltage across it. If the voltage is held constant, then the current is inversely proportional to the resistance. If the voltage polarity is reversed (that is, if applied voltage is negative instead of positive), the same current flows but in the opposite direction. If Ohm’s law is valid, it can be used to define resistance as: IVR = Where, R is a constant independent of V and I.It is important to understand just what is meant by these quantities. The current (I) is a measure of how many electrons are flowing past a give point during a set amount of time. The current flows because of the electrical potential (V), sometimes referred to as the voltage applied to a circuit. In much the same way that a gravitational potential will cause mass to move, electrical potential will cause electrons to move. If you lift a book and release it from a height (high gravitational potential) it will fall downward (to a lower potential). The electrical potential works in a similar way. If we induce a large negative charge on one end of a wire and a large positive charge on the other, the electrons in the wire will flow because of the charge difference. The resistance of the circuit is just that; it is a measure of how difficult it is for the electrons to flow. This resistance is a property of the circuit itself, and just about any material has a resistance. Materials that have a low resistance are called conductors and materials that have a very high resistance are called insulators. Some materials have a moderate resistance and still allow some current to flow. These are the materials that we use to make resisters like the ones we will use in this experiment. In short, the electrical potential causes the current to flow and the resistance impedes that flow. Two or more resistors can be connected together either in series or in parallel. Resistors in series means that the resistors are connected one after another (figure 1a). Resistors in parallel means resistors are connected side by side, parallel to one another (figure 1b). Table 1 summarizes the equations and rules for two resistors in series and parallel circuits. The voltage from the power supply is Vs, the total current in the circuit is Is and the equivalent (total) resistance of the circuit is R s for the series circuit (and R p for the parallel circuit).Table 1 Equations for two resistors in series and parallel circuits Series Parallel 21VVVs+= 21VVVP== 21IIIs== 21IIIP+= 21RRRs+= 21111RRRp+= or 2121RRRRRp+= When two resistors (R1 and R2) are connected in series, the equivalent resistance Rs is given by21RRRs+= . Thus, the circuit in Figure 1a behaves (draws current from a given applied voltage) as if it contained a single resistor Rs. When two resistors (R1 and R2) are connected in parallel, the equivalent resistance Rp is given by: 21111RRRp+= or 2121RRRRRp+= (2) Therefore, the circuit shown in Figure 1b behaves as if it contained a single resistor having a resistance of Rp. Using these two relationships, a complex circuit can be redrawn as a circuit with a single resistor. You may wish to review the process of finding the equivalent resistance of circuits in your Physics textbook. Prove this relationship before you come to lab. Apparatus THE DC POWER SUPPLY A DC power supply is used to provide varying voltage to a circuit. The power supply used in this lab is shown in Figure 2. The black and red connectors are the negative (-) and positive (+) output terminals respectively. The voltage knob controls the output voltage of the power supply and the current knob sets a limiting current. In this lab, adjust the current control to its maximum setting (all the way clockwise) at all times. Note: Prior to making any change in the circuit, always turn the voltage knob to its minimum setting (all the way counterclockwise) and turn off the power supply! So the next time you turn on the power supply its output will be zero volts.Figure 2 DC Power Supply Figure 3 Digital Multimeter THE DIGITAL MULTIMETER The digital multimeter is shown in Figure 3 above. As its name suggests, a multimeter has multiple functions. It can be used for several different purposes, two of which are a voltage measuring device (a voltmeter) and a current measuring device (an ammeter) will be used in this experiment. To use the multimeter as a voltmeter, the dial selector is set to one of the positions labeled “V”. The probing cables are then connected to the plugs labeled “VΩ” and “COM”. There are two types of “V” settings. The setting with the tilde (~) over it is used for measuring AC voltage. The other type of “V” setting has two lines over the V –one line is solid and the second line is dashed this indicates DC voltage. AC is an abbreviation for alternating current. An AC voltage is a voltage whose magnitude and polarity vary with time. DC is an abbreviation for direct current. A DC voltage is a constant voltage. During this experiment, only the DC setting is used. There are two DC voltage settings on the