2A DIRECT CURRENT CIRCUITS AND MEASUREMENTSLaws of ElectricityV = IRP = IVChemistry 331Chapter 2 Electrical Components andCircuits The purpose of this chapter is to discuss basic direct current (dc) circuit components in preparation for the two following chapters that deal with integrated circuits and microcomputers in instruments for chemical analysis. 2A DIRECT CURRENT CIRCUITS AND MEASUREMENTS Some basic direct current circuits and how they are used in makingcurrent, voltage, and resistance measurements will be considered. The general definition of a circuit is a closed path that may be followed by an electric current. A galvanometer is a device with a rotating indicator that will rotate from its equilibrium position when a current passes through it. Agalvanometer has a negligible resistance.Figure 1. Ampermeter An ampermeter (ammeter) is a galvanometer with a calibrated current scale for its indicator and a bypass resistor (called a shunt) for a fixed fraction of the current, shown in Figure 1. Many ammeters have severalselectable shunts which provide their corresponding current meter ranges. Typically, ammeters can be found with calibrated ranges of 1 micro-A for full scale deflection up to 1000 A for full scale deflection, and in multiples of 10 between these extremes. Figure 2. Voltmeter A voltmeter, shown in Figure 2, is just a calibrated galvanometer with aseries resistor so that the total resistance of the path is increased. The galvanometer range is calibrated for the current Ig passing through it. This scale is adjusted to display the potential difference between pointsA and B, (voltage) by substituting Vg values for Ig on the scale where Vg = Ig Rg and Rg is the total resistance of the voltmeter. Voltmeters may have more than one calibrated scale which can be selected by changing the resistance Rg. Current in a circuit is the flow of the positive charge from a high potential (+) to a low potential (-). Meters are labeled to indicate the proper direction of current flow through them. A reverse flow of DC current may destroy a meter. Electrical charge will not move through a conducting path unless there is a potential difference between the ends of the conductors. All materials resist the flow of current through them, requiring work to be done to move the charge through the material. The source of energy ina circuit which provides the energy to move the charge through the circuit can be a battery, photocell, or some other power supply. An electrical circuit is a circuitous path of wire and devices. A schematic drawing of a real circuit utilizes the symbols shown in Figure 3.Figure 3. Circuit Symbols An example, Figure 4, shows a circuit with a DC. power supply in a series with a resistor, a parallel branch with a resistor and voltmeter, and an ammeter. Figure 4. Example of an Electric Circuit. BASIC ELECTRIC CIRCUIT The flashlight is an example of a basic electric circuit. It contains asource of electrical energy (the dry cells in the flashlight), a load (thebulb) that changes the electrical energy into a more useful form ofenergy (light), and a switch to control the energy delivered to the load. A load is any device through which an electrical current flows andwhich changes this electrical energy into a more useful form. Thefollowing are common examples of loads: A light bulb (changes electrical energy to light energy). An electric motor (changes electrical energy into mechanical energy). A speaker in a radio (changes electrical energy into sound). A source is the device that furnishes the electrical energy used by theload. It may be a simple dry cell (as in a flashlight), a storage battery(as in an automobile), or a power supply (such as a battery charger). Aswitch permits control of the electrical device by interrupting thecurrent delivered to the load. Schematic of a Basic Circuit , the FlashlightLaws of Electricity Ohm’s law describes the relationship among potential, resistance and current in a resistive series circuit. In a series circuit, all circuit elements are connected in sequence along a unique path, head to tail, as are the battery and three resistors shown in Figure 2-1. Ohm’s Law may be written as:V = IR Where V is the potential difference in volts between two points in a circuit, R is the resistance between the two points in ohms, and I is the resulting current in amperes. diagrams for determining resistance and voltage in a basic circuit, respectively.Using Ohms's Law, the resistance of a circuit can be determined knowing only the voltage and the current in the circuit. In any equation, if all the variables (parameters) are known except one, that unknown can be found. For example, using Ohm's Law, if current (I) and voltage (E) are known, you can determine resistance (R), the only parameter not known: . 2) A steady increase in resistance, in a circuitwith constant voltage, produces aprogressively (not a straight-line if graphed)weaker current.In simpler terms, Ohm’s Law means: 1) A steady increase in voltage, in a circuit withconstant resistance, produces a constant linearrise in current.TECHNICAL DEFINITION ALERT! Ohm's Law is a formulation of the relationship of voltage, current, and resistance, expressed as: Where: V is the Voltage measured in volts I is the Current measured in amperes R is the resistance measured in Ohms Therefore:Volts = Amps times ResistanceOhms Law is used to calculate a missing value in a circuit.In this simple circuit there is a current of 12 amps (12A) and a resistive load of 1 Ohm (1W). Using the first formula from above we determine the Voltage: V = 12 x 1 : V = 12 Volts (12V)If we knew the battery was suppling 12 volt of pressure (voltage), and there was a resistive load of 1 Ohm placed in series, the current would be: I = 12 / 1 : I = 12 Amps (12A)If we knew the battery was suppling 12V and the current being generated was 12A, then the Resistance would be: R = 12/12 : R = 1W Note: Remember a battery is not measured in amperage as is commonly believed with beginners to electronics. The battery supplies the pressure that creates the flow (current) in a given circuit. The amperage rating on a battery is "How long the battery will last for one hour while driving a circuit of that amperage". It is measured in Amperage-Hours. So a 1000mAh would last for 1 hour in a one amp circuit. (1000mAh is 1A for one hour)An easy way to remember the formulas is by using this diagram.To