Current flow in a circuit (purely resistive and restive-capacitive - Lab Report Example

June 2, CURRENT FLOW IN A CIRCUIT (PURELY RESISTIVE AND RESTIVE-CAPACITIVE) Flow of electric charge in a wire or a circuit takes place when there is potential difference across the circuit and it is referred to as electric current. Presence of moving electrons in electrical circuit result to the flow of current in the circuit. The current may cause effects such as heating or even induce magnetic fields preferably used for generators, motors and inductors. Current generally flows from the positive terminal towards the negative terminal of battery and there is voltage drop across the electrical elements in the circuit due to resistance, reactance or inductance. Current can be flow of negative or positive charges or both (James, Jack and Noble). The conventional current direction of flow is the direction of flow of the positive charges, but the positive charges are immobile. Electrons carry negative charge and in direction opposite to that of electric or conventional current. For instance, if two metal plates or any other conductors are connected together by a wire e.g. copper wire, electrons flow from the conductor with higher potential toward the conductor with lower potential due to the potential difference between them. The electrons flowing between the two is referred as electricity and decreases with decrease in the electrical potential. Therefore, it is necessary to maintain the electrical potential difference by using devices like battery, solar cells, or electrical generators and is connected to loads like motors, light bulbs, heaters etc. Since these devices are loads, they slow down the flow of electrons due to the electrical resistance they offer (James, Jack and Noble). Two main types of current are DC and AC (Direct Current and Alternating Current respectively). For direct current, electrons flow in only in one direction while for alternating current there is frequent reversal of the current flow in cycles and this is the current obtained from an ordinary Outlets in our homes and can be sent over long distances through electrical cables without resistance problems encountered with direct current transmission. There is no individual electron which flows that far for the case of AC current due to the frequent cycling of 60Hz in US or 50Hz in Europe but for DC current electron makes a whole trip round the circuit. It is necessary to control current flowing in a circuit either by controlling the amount of voltage applied or the resistance in the circuit. This is done to avoid the undesirable results like damaging electrical components or valuables at home if for instance the optimal operating conditions are exceeded. Ammeter is used to measure the electrical current flowing through a circuit while voltmeter measures the electrical potential difference. Ammeter is placed in series and voltmeter in parallel with other components in the circuit. The ammeter will only measure the current in the branch it is placed but not current flowing in other branches of the circuit. Ammeters have relatively low resistance while voltmeters have high resistance. Power sources like a battery, generators are needed to keep current flowing in a circuit (Voltage-force which pushes the electrons) and also the circuit must be complete for the electrons to return to their source implying that conductors are necessary for current to flow. Insulators like glass or plastic are needed to avoid current leakage or short circuiting which may result to cut off supply of power to various electrical devices or even damage them. The relationship between current, voltage and resistance in a circuit is clearly defined by Ohm’s law which states that “electrical current is proportional to the voltage and inversely proportional to the resistance between any two points on a conductor provided the temperature of the conductor does not change” (James, Jack and Noble). I=V/R where: I=current (Amperes), V=Voltage(volts), R=Resistance (Ohms).Therefore when current increases, voltage in the circuit increases as long as the resistance remains constant. The law holds both for direct and alternating current. Resistive circuit contains only resistors either in series or parallel arrangement and an electromotive force (emf) which is usually a battery (DC source) or an AC power supply. However, electrical circuit can also be composed of resistors and capacitors either in series or parallel arrangement. In the case of parallel circuits’ loads are parallel to each other, there are two or more electricity flow paths while in series circuit the loads are in a row and only single path is there for electricity to flow (James, Jack and Noble). There is a certain pattern of relationship between parallel and series circuits. For instance in series circuit current is same whereas in parallel circuits voltage is the same.in addition, individual voltages are added in a series circuit but in parallel circuits there is addition of individual currents. “The power dissipated in a resistor is given by P=RI2” (James, Jack and Noble). Series circuit I1=I2=I3=…… VT=V1+V2+V3+…… RT=R1+R2+R3+……. Parallel circuit V1=V2=V3=……. IT=I1+I2+I3+…… GT=G1+G2+G3+……. A simple R-C circuit contains a resistor, a capacitor and a source as shown below. From the figure, the capacitor does not charge when the toggle switch is open because it is not connected to the electromotive force. “ So there will be no charge on the capacitor unless previously charged and hence the difference in potential between the capacitor plates will be zero if the switch is toggled to connect the capacitor to the electromotive force the capacitor charges till the potential difference between the capacitor plates is equal to the electromotive force” (James, Jack and Noble). The current flow through the circuit reduces to zero when Vc=emf. Alternately toggling the switch to the other position, e.m.fs is bypassed and the two capacitors plates are connected. Therefore the capacitor discharges. When the capacitor is fully charged, Q=CV where Q is charge stored in the capacitor, C is capacitance of the capacitor and V is the electromotive force that is charging the capacitor (James, Jack and Noble). The value of Q is zero when capacitor is not charged or fully discharged. Presence of resistance in the circuit slows down current flow. As a result, it requires time to charge to charge the capacitor. Charging a fully discharged capacitor is well described exponentially by the equation, Qt=CV (1-V-t/RC) and the graph of Qt versus time would be depicted as shown in graph (a). (a) Graph of qt against time (b) Similarly, discharging a fully charged capacitor would be described by the following exponential equation. Qt=CV-t/RC and the graph of Qt versus time would look as shown in (b) above. A simple series RC and purely resistive circuits are as shown below: The voltage and current are in phase for a purely resistive circuit as shown above. For the parallel circuit shown below, current splits into two on reaching point A and hence resistors R1 and R2 don’t have the same current flowing them (James, Jack and Noble). Therefore applying junction rule at node A, I=I1+I2and the voltage through resistor R1 and R2 is I1R1and I2R2 respectively which is same. V=I1R1=I2R2 (James, Jack and Noble). Works Cited James, Bennett, et al. Electric current flow in excitable cells. London: Clarendon Press, 1983. Read More