Unit 2 Electrical Quantities and Ohm’s Law

Preview Electricity –Standard set of values Values of electrical measurement –Standardized Understood by everyone who uses them Must be the same for everyone

The Coulomb Quantity measurement for electrons –One coulomb contains 6.25 x electrons 6,250,000,000,000,000,000 electrons Charles Augustin de Coulomb –French scientist in 1700s –Experimented with: Electrostatic charges Coulomb’s law of electrostatic charges –Attraction and repulsion of forces

The Ampere Amount of electricity flowing through a circuit –One ampere (A) is defined as one coulomb of electricity flowing past a given point in one second André Ampère –Scientist in 1700s-1800s –Worked in electromagnetism

FIGURE 2–1 One ampere equals one coulomb per second.

FIGURE 2–2 Current in an electric circuit can be compared to flow rate in a water system.

Current Flow Theories Electron flow theory –Current flows from negative to positive Conventional current flow theory –Current flows from positive to negative

FIGURE 2–3 Conventional current flow theory and electron flow theory.

Speed of Current Establish exactly what is being measured –Current: flow of electrons through a conductive substance FIGURE 2–5 Electrons moving from atom to atom.

Speed of Current (cont’d.) Consider whether the circuit is DC, AC, or radio waves –Radio waves move at approximately the speed of light –Velocity of AC through a conductor is less than the speed of light –In a DC circuit, the impulse of electricity can appear to be faster than the speed of light

FIGURE 2–6 When a ball is pushed into one end, another ball is forced out the other end. This basic principle causes the instantaneous effect of electric impulses.

FIGURE 2–7 The impulse of electricity can appear to travel faster than light.

Basic Electric Circuits A complete path must exist before electricity can flow through a circuit –A complete circuit is often referred to as a closed circuit –If the switch is opened, there is no longer a closed loop and no current can flow Often referred to as an incomplete, or open, circuit –A short circuit has very little or no resistance

FIGURE 2–8 Current flows only through a closed circuit.

FIGURE 2–9 A short circuit bypasses the load and permits too much current to flow.

Basic Electric Circuits (cont’d.) Grounded circuits –Occur when a path other than the one intended is established to ground –Grounding conductor Extra conductor Provides the return path and completes the circuit back to the power source Used to help prevent a shock hazard

FIGURE 2–11 The grounding conductor provides a low-resistance path to ground.

The Volt Voltage: potential difference between two points of a conducting wire –Carrying a constant current of one ampere when power dissipated between these points is one watt Also referred to as potential difference or electromotive force (EMF) –Volt: amount of potential necessary to cause one coulomb to produce one joule of work

. FIGURE 2–12 Voltage in an electric circuit can be compared to pressure in a water system.

The Ohm Unit of resistance to current flow –Named after German scientist Georg S. Ohm FIGURE 2–13 A resistor in an electric circuit can be compared to a reducer in a water system

The Watt Amount of power being used in a circuit –Named after English scientist James Watt FIGURE 2–15 Force equals flow rate times pressure.

FIGURE 2–16 Amperes times volts equals watts.

Other Measures of Power FIGURE 2–17 Common power units.

Ohm’s Law It takes one volt to push one ampere through one ohm –In a DC circuit, current is directly proportional to voltage Inversely proportional to resistance Formula: –E (volts) = I (amperage) x R (resistance)

FIGURE 2–18 Chart for finding values of voltage, current, and resistance.

FIGURE 2–19 Using the Ohm’s law chart.

FIGURE 2–20 Formula chart for finding values of voltage, current, resistance, and power.

Metric Prefixes FIGURE 2–24 Standard metric prefixes.

FIGURE 2–25 Standard prefixes of engineering notation.

The SI System Prefixes just discussed –Referred to as metric units of measure Part of the SI (System Internationale) system FIGURE 2–26 SI base and supplementary units.

FIGURE 2–27 Derived SI units.

Summary Coulomb: quantity measurement of electrons Ampere (A): one coulomb per second Letter I: stands for intensity of current flow –Normally used in Ohm’s law formulas Voltage: referred to as electric pressure, potential difference, or electromotive –Represented with an E or a V

Summary (cont’d.) Ohm (Ω): measurement of resistance (R) in an electric circuit Watt (W): measurement of power in an electrical circuit –Represented by a W or a P (power) Electric measurements: generally expressed in engineering notation –Uses steps of 1000

Summary (cont’d.) Before current can flow, there must be a complete circuit A short circuit has little or no resistance