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Chapter 1 – Section 3. Voltage in Electrical Systems. Objectives. Explain the similarities and differences between Newton’s law of universal gravitation and Coulomb’s law. Describe the force between like and unlike electric charges. Describe how to create an electric field.
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Chapter 1 – Section 3 Voltage in Electrical Systems
Objectives • Explain the similarities and differences between Newton’s law of universal gravitation and Coulomb’s law. • Describe the force between like and unlike electric charges. • Describe how to create an electric field. • Define electric potential difference (voltage). • Differentiate between AC and DC current. • Identify common sources of DC current. • Describe how to connect DC voltage sources so that voltages will add.
Gravitational Force • Newton’s Law of Universal Gravitation states that every object in the universe is attracted to every other object • The force felt is proportional to the product of the mass of each object. • It is inversely proportional to the square of the distance between them.
Gravitational Force Formula FG = G m1m2 / r2 Where FG = force of gravity G = universal gravitational constant = 6.67x10-11 N m2/ kg2 m1 and m2 = the masses of the two objects in kilograms r = the center to center distance between them in meters
Example – Force of the Earth on the Moon • Earth’s mass ME = 6 x 1024 kg • Moon’s mass Mm = 7.4 x 1022 kg • Average separation distance = 3.9 x 108m • Calculate the force of the Earth on the Moon.
Electric Charge • The origin of electric charge is found in the atom. • The nucleus of the atom contains positively charged protons (p+) and neutral neutrons (n0). • The nucleus is surrounded by negatively charged electrons (e-).
Electric Force • Electric charge can be positive or negative. • Unlike charges attract, like charges repel. • 18th century French scientist Charles Coulomb did experiments with charged spheres (Saturday nights were pretty dull in the 18th century). • He discovered the relationship between charge, force and distance.
Coulomb’s Law • The force felt is proportional to the product of the charge on each body. • It is inversely proportional to the square of the distance between them.
Coulomb’s Law Formula Fe = K q1q2 / r2 Where Fe = electrostatic force K = electrostatic constant = 9 x 109 Nm2/ c2 q1 and q2 = the charges of the two objects, measured in coulombs (c) r = the center to center distance between them in meters
Gravitational & Electrical Fields • The gravitational field is an expression of the force felt per unit of mass in the vicinity of another mass. • G = F / m, where G = gravitational field strength, F = force & m = a small test mass. • The units for G are N / kg which also equals m / s2. • On Earth, G = 9.8 N/ kg = g = 9.8 m / s2 .
Electrical Fields • The electric field is an expression of the force felt per unit of charge in the vicinity of another charge. • E = F / q , where E = electric field strength, F = force and q = a small test charge. • The units for E are N / c.
Fields – cont. • Both G & E are vectors. Neither depends upon the size of the test mass or test charge. • G always points in the direction of the mass causing the field. • E points in the direction that a positive test charge would move.
Electric Potential Difference • Imagine two flat metal plates, one charged positively, the other negatively. • Between the plates is a uniform electric field. • In this field, we place a positively charged particle. The particle will cling to the negatively charged plate. • Now, we move the particle from point A to point B, some distance, d, towards the positively charged plate.
Electric Potential Difference - cont • If released, the particle will accelerate back towards the negative plate, i.e point A. • There exists, therefore, an electric potential difference between points A & B. • The magnitude of this potential difference, or voltage, depends upon the magnitude of the electric field and the distance between the two points.
Potential difference formula • DVAB = Ed where, • DVAB = the potential difference between A & B, measured in volts. • E = the electric field, measured in Newtons/ coulomb (N/c) . • d = distance, measured in meters (m).
Electric Current • If the two charged plates are connected by a wire, the charge will flow between them until there is no potential difference. • The flow of electric charge is called current. • The current can be maintained by a potential difference source, or voltage source, such as a battery.
Prime movers • In a fluid system, pressure difference is the prime mover. It is what causes fluids to flow. • In an electrical system, potential difference, or voltage, is the prime mover. It is what causes charges to flow.
Components of Electrical Systems • Electrical systems usually contain four major components: • At least one voltage source such as a battery or generator; • Conductors, such as wires or connections on printed circuit boards; • At least one load, and • One or more control elements, such as a switch.
Electric components - cont • The load is usually an appliance or machine – motors, lights, heaters, TV sets, computers, air conditioners, etc. • Conductors are materials through which charge can easily flow, usually metal. • Control elements may be simple switches, variable controls, diodes, transistors, etc.
AC/DC • An Australian rock band formed in 1973 by brothers Malcolm and Angus Young, who are considered pioneers of heavy metal. They developed the idea for the band's name after their older sister, Margaret, saw the initials "AC/DC" on a sewing machine. • "AC/DC" is an abbreviation for "alternating current/direct current" electricity. They felt the name symbolized the band's raw energy and power-driven performances. The band is colloquially known as "Acca Dacca" in Australia. • DC – direct current; the charges flow in only one direction. Batteries are a common source of DC. • AC – alternating current; the charges reverse direction on a regular basis. In North America, common household AC current is 60 cycles per second, or Hertz (Hz). This means the current reverses 60 times each second. European current is usually 50 Hz. AC is produced by an alternator or generator.
Batteries • Typical sources of DC voltage • Cell – a single unit that holds chemicals that react to separate electrons and positive ions causing a potential difference. • Battery – a collection of cells. • Anode – the positive electrode of a battery • Cathode – the negative electrode. • Batteries can be connected, positive to negative, in order to add their voltages together.
Circuit diagrams • Circuit diagrams are used to design, build, diagnose and repair circuits. They are like a road map to show the path of the current. • Circuit diagrams use symbols to show how a circuit is constructed. • Some common symbols used in circuit diagrams are shown below.
Summary • Newton’s law of gravitation & Coulomb’s law are both inverse square laws, the magnitude of the forces decreasing with the square of the distance. • Atoms are composed of p+, n0, & e- . • Electron flow is called current • Like charges repel, unlike attract. • Electric fields exist around charges. The magnitude depends upon the size of the charge and the distance from it.
More summary • Potential difference, voltage, between two points in a uniform electric field is the product of the field strength and the distance. • Voltage is the prime mover in electrical systems. • A battery is a DC voltage source. It can maintain current in a circuit. • Batteries or cells can be connected in series to increase voltage.
