Fundamental Electrical Engineering
Electric Charge (Q)
- Characteristic of subatomic particles that determines their electromagnetic interactions.
- An electron has a -1.602^10-19 Coulomb charge.
- The rate of flow of charged particles is called current.
Current
- Current = (Number of electrons that pass in one second) ∙ (charge/electron)
* -1 ampere = (6.242^1018 e/sec) ∙(-1.602^10-19Coulomb/e).
* Notice that an ampere = Coulomb/second.
- The negative sign indicates that the current inside is actually flowing in the opposite direction of the electron flow.
- I = DQ/DT – the derivative or slope of the charge when plotted against time in seconds.
- Q = ∫ I ∙ DT – the integral or area under the current when plotted against time in seconds.
AC and DC Current
- DC Current has a constant value.
- AC Current has a value that changes sinusoidal.
* Notice that AC current changes in value and direction.
* No net charge is transferred.
Why Does Current Flow?
- A voltage source provides the energy (or work) required to produce a current.
* Volts = joules/Coulomb = DW/DQ
- A source takes charged particles (usually electrons) and raises their potential so they flow out of one terminal into and through a transducer (light bulb or motor) on their way back to the source’s other terminal.
Voltage
- Voltage is a measure of the potential energy that causes a current to flow through a transducer in a circuit.
- Voltage is always measured as a difference with respect to an arbitrary common point called ground.
- Voltage is also known as electromotive force or EMF outside engineering.
What is Ohm’s Law?
Ohm's Law states that, at a constant temperature, the electric current flowing in a conducting material is directly proportional to the applied voltage, and inversely proportional to the Resistance.
Why is Ohms Law important?
Ohm’s Law is the relationship between power, voltage, current, and resistance. These are the very basic electrical units we work with. The principles apply to alternating current (ac), direct current (dc), or radiofrequency (rf).
Ohm’s Law
I = V / R
I = Current (Amperes) (amps)
V = Voltage (Volts)
R = Resistance (ohms)
Characteristics of Ohm’s Law
- Voltage: Difference of potential, electromotive force, ability to do work.
- Unit of measure Volt Symbol V (Current: Flow of electrons Unit of measure Ampere Symbol.
- Resistance: Opposition to current flow Unit of measure Ohm often seen as the Greek letter Omega Symbol R.
Circuit
- Current flows from the higher voltage terminal of the source into the higher voltage terminal of the transducer before returning to the source.
* The source expends energy & the transducer converts it into something useful.
Simple Circuits
- Series circuit
* All in a row.
* 1 path for electricity.
* 1 light goes out and the circuit is broken.
* only one end of each component is connected
* e.g. Christmas tree lights
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| SERIES CIRCUIT |
- Parallel circuit
* Many paths for electricity.
* 1 light goes out and the others stay on.
* Both ends of a component are connected
* e.g. household lighting
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PARALLEL CIRCUIT |
Resistance
- Property of a device that indicates how freely it will allow current to flow given a specific voltage applied. If low in value, the current flows more freely.
- Measured in ohms (Ω = V/A or KΩ = V/mA)
- V = I∙R This is Ohm’s Law
- Obeys the passive sign + -convention V if I > 0 then V > 0
Resistor Colour Code
Conductance (G)
- Conductance is the reciprocal of resistance
- Its unit is the Siemen = amps/volts = 1/Ω
- G = I / V = 1 / R
- An equivalent measure of how freely a current is allowed to flow in a device
Resistivity (ρ)
- Property of a material that indicates how much it will oppose current flow.
- R= (ρ · length) / (cross-sectional area).
- Units are ohms · meters (Ω · m).
- As the wire gets bigger, so does the cross-section making the resistance smaller.
- As the length gets longer, the resistance goes up proportionately.
Conductors
- Materials with electrons that are loosely bound to the nucleus and move easily (usually one electron in the outer shell)
- Their low resistance goes up as the material is heated, due to the vibration of the atoms interfering with the movement of the electrons
- The best conductors are superconductors at temperatures near 0o Kelvin
Semiconductors
- Materials with electrons that are bound more tightly than conductors (usually 4 electrons in the outer shell)
- Impurities are added in controlled amounts to adjust the resistivity down
- Semiconductors become better conductors at higher temperatures because the added energy frees up more electrons (even though the electron flow is impeded by the increased atomic vibration)
Insulators
- Materials that have all 8 electrons in the outer shell tightly bound to the nucleus
- It takes high temperatures or very high electric fields to break the atomic bonds to free up electrons to conduct a current.
- Very high resistivities and resistances.
Resistivity of Materials
- Silver - A conductor - ρ=1.64 ^10-8 ohm-m
- Copper - A conductor - ρ= 1.72^10-8 ohm-m
- Gold – A conductor - ρ=2.45^10-8 ohm-m
- Aluminium - A conductor - ρ= 2.8^10-8 ohm-m
- Silicon - A semiconductor - ρ=6.4 ^10+2 ohm-m
- Glass – An insulator – ρ=1^10+12 ohm-m
Capacitors
- Composed of two conductive plates separated by an insulator (or dielectric).
- Commonly illustrated as two parallel metal plates separated by a distance, d.
* C = ε A/d
* where ε = εr εo
* εr is the relative dielectric constant
* εo is the vacuum permittivity.
Effect of Dimensions
- Capacitance increases with
- Increasing surface area of the plates,
- Decreasing spacing between plates, and
- Increasing the relative dielectric constant of the insulator between the two plates.
Types of Capacitors
- Fixed Capacitors
* Nonpolarized
* May be connected to the circuit with either terminal of the capacitor connected to the high voltage side of the circuit.
- Insulator: Paper, Mica, Ceramic, Polymer
- Electrolytic
* The negative terminal must always be at a lower voltage than the positive terminal
Plates or Electrodes: Aluminium, Tantalum.
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