Transcript Ch. 1.3
Ch. 1.3 Voltage in Electrical Systems Gravitational and Electrical Force Gravitational force and electrical force are two universal forces in nature. These forces are similar because they both exert a force on other objects. Gravitational forces exert forces due to the masses of the objects and electrical forces exert forces due to electrical charges. Gravitational Force Isaac Newton studied the effects of the gravity force between two objects. Newton observed that the distance between the objects affects the gravitational force. This study is resulted in Newton’s universal law of gravitation, which states: Every object in the universe attracts every other object with a force that is directly proportional to the mass of each body and that is inversely proportional to the square of the distance between them. Electric Charge The property of an object that causes electrical force is called electric charge. The word electrical comes from the Greek elektron, which means amber. Amber was the substance the Greeks used to study electrical forces. There are two different types of electrical charge, positive and negative charges. Charges that are alike, such as two positive charges will repel, or push away from, each other. Unlike charges will attract each other. Electric Charge and the Atom The origin of electric charge is found in the atom. The nucleus contains most of the mass in the protons and neutrons. Neutrons have no charge so we say they are neutral. The protons have a positive charge. A cloud of electrons surrounds the nucleus. Electric Charge and the Atom-1 Electrons have a negative charge. The charge of an electron is exactly equal, but opposite of the charge of the proton. A normal atom has no net charge. The net charge is the difference between the positive and negative charges. Electric Charge and the Atom-2 If the object has more positive than negative charges, then the net charge is positive. If the object has more negative than positive charges, then the net charge is negative. If the charges are equal, then the object has no net charge and is neutral. Electric Charge and the Atom-3 When you comb your hair, friction causes electrons to transfer from your hair to the comb. This transfer results in an imbalance of charges and the comb becomes negatively charged while the hair becomes positively charged. The attraction between opposite charges causes your hair to be attracted to the comb. Electric Charge and the Atom-4 The hair and the comb together is considered an isolated system. In an isolated system, particles and charge cannot enter or leave the system so charge is conserved. The principle of conservation of charge states: The net electric charge in an isolated system never changes. Electrical Force Charles Coulomb discovered a relationship between force, charge and distance similar to the universal law of gravitation. Coulombs Law states that the electrical force between two charged bodies is directly proportional to the charge on each body and inversely proportional to the square of the distance between them. Electrical Force - 1 The SI unit for charge is the coulomb (C). The charge on one electron or proton is 1.60 x 10-19 C. This is called the elementary charge. Electric Fields There is an area around any charged object where the effects of the charge can be observed or felt. This area is called an electric field. When representing electric fields in diagrams electric field lines are used. Examples of electric field lines are shown on page 54 of your text in Figure 1.33. Electric Potential When you hold an object above the ground, we say it has potential energy, because if we drop the object it will accelerate to the ground below. In the same way, if you move a charge against an electric field, the electric field creates an electric potential difference. Electric Potential - 1 The unit used to measure electric potential difference is the volt. Electric potential difference is sometimes simply called electric potential or voltage. Voltage is the prime mover in electrical systems. Electric Potential - 2 Electrons flow through a closed circuit like water flows through a hose. The flow of electric charges is called an electric current. In a fluid system, a pump maintains the flow of fluid. In an electrical system, current flow can be maintained with a battery or voltage source such as an electrical outlet. Components of Electrical Systems Electrical systems usually contain four major components: - voltage source - conductors - at least one load - one or more control elements A load is a device that uses the electricity, such as an appliance or machine. A conductor is a material that allows electrons to pass easily. Metals, such as copper, are good conductors of electricity. Components of Electrical Systems-1 The control element is used to turn the current on and off. A switch is the most common control element. An electrical circuit is a closed path through which current flows. Direct and Alternating Current There are two types of current used in electrical circuits. On type is direct current or DC and the other is alternating current or AC. In DC, the current flows in only one direction. In AC the current reverses direction many times each second. The most common DC voltage source is a battery and the most common AC voltage source is an electric generator. DC and Batteries The voltage output of a battery is measured between two terminals called electrodes. The positive electrode is called an anode and the negative electrode is called a cathode. When the batteries are connected in a circuit, electrons move in the circuit as if they were flowing out of the cathode and into the anode. DC and Batteries - 1 If more voltage is needed, batteries can be added together in series. Adding the batteries in series means that the positive terminal of one battery is connected to the negative terminal of another battery. AC Circuits In AC circuits, the current reverses direction many times per second. The majority of AC circuits involve current that alternate or cycle at a rate of 60 times per second. The cycling rate is called the frequency, which is measured in hertz (Hz). For this reason, a cycling rate of 60 times per second is called 60 Hz alternating current. The main source of 60 Hz alternating current is an electric power plant.