EMF, Terminal Voltage, and Internal Resistance Mary Grace DC. Odiamar De La Salle University 2401 Taft Avenue, Malate, Manila, Philippines [email protected]Abstract – This paper is generally about the inspection and examination of voltage sources of direct-current circuits, known otherwise as seats of emf or emf devices. The experiment aimed to illustrate the difference between emf and terminal voltage, to show how an ideal battery is different from a real one with an internal resistance, to measure the electromotive force of a battery, to calculate the internal resistance of a real battery and to show the variants of a battery’s terminal voltage with its current output through experimentation. In this experiment, the internal resistance of Daniell cell (wet cell) was computed by measuring the voltage across the circuit where the values of resistances R were given; current I and internal resistance r were then computed through Ohm’s Law and through the equation of terminal voltage of a real battery, respectively. On the other hand, the carbon battery’s internal resistance was computed by measuring the voltage across the circuit where the values of current were given; internal resistance r was also computed through the equation of terminal voltage of a real battery. The acquired results were very likely to show how a battery’s voltage is related to current and resistance, as well as how to calculate a battery’s internal resistance. Keywords – Emf, Terminal Voltage, Internal Resistance, Battery, Daniell Cell, Dry Cell, Voltmeter, Ammeter, Rheostat I. INTRODUCTION Electric circuits are essential means to transport electric potential energy from one region to another. When charged particles move within a circuit, energy is transported from a source such as a battery to an instrument where the said energy is stored or transformed into other form/s. Generally, today’s modern world is heavily dependent on electric circuits. They have been used for years because of their effectiveness and practicality – they typically allow energy to be conveyed without moving any parts. One should note that electric circuits are at the heart of every electronic device. Every electric circuit consists of components, and these often include sources, resistors, and other circuit elements interconnected in a network [1]. One element to be noted in this paper is the source of the circuit, more often known as the battery. In a battery, a chemical reaction occurs when a load completes the circuit between the terminals. The chemical reaction then transfers electrons from one terminal to another terminal. The electric potential difference is caused by the positive and negative charges on the battery terminals. The maximum potential difference of a battery is called the electromotive force or emf, designated by Ɛ [2]. II. THEORETICAL BACKGROUND A. Seats of EMF and EMF When a potential difference or voltage is applied across a circuit, current will be allowed to flow. These devices that are responsible for the flow of charge in circuits are called seats of emf, emf devices or simply voltage sources. The emf is not really a force but is the work per unit charge that an emf device does in moving positive charges from lower potential terminal (-) to higher potential terminal (+). Its unit is joule per coulomb or volts. A voltage source is labeled with its emf value, Ɛ, which is equivalent to Ɛ = [3]. B. Emf, Terminal Voltage, and Internal Resistance A real battery is made of matter; therefore, there is resistance r to the flow of charge inside the battery. This resistance inside the battery is called internal resistance r. An ideal battery with zero internal resistance has a terminal voltage equal to its emf, no matter how much current is drawn from them. However, a real battery in a circuit where current is drawn from has a terminal voltage which is not equivalent to its emf [4]. Its terminal voltage is given by b = ε – . In this equation we notice that the terminal voltage drops as we draw more current from the battery because of the internal resistance. In rearranging the equation, we can get a battery’s internal resistance by r = C. Daniell Cell and Dry Cell John Frederick Daniell, a chemistry professor in London, has developed an emf device that supplied constant electric current. The Daniell cell is a type of wet cell and has also been called as a gravity cell and crowfoot cell. In the early 1800s, the cell became a popular power supply in the laboratories and in the telecommunications industry. The cell is made of copper and zinc electrodes and an electrolyte solution. This battery converts chemical energy to electrical energy whenever a load is present. Nowadays, better batteries with low internal resistances have been developed. Cars consume lead-acid batteries, which is also a wet cell. This battery comprises of lead and lead oxide electrodes and an electrolyte solution. In contrast to the batteries of the earlier centuries, modern batteries have been constructed to be very portable and
About inspection and examination of voltage sources of direct-current circuits, known otherwise as seats of emf or emf devices.
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EMF, Terminal Voltage, and Internal Resistance Mary Grace DC. Odiamar