Electeical Principles Flashcards
Define electric current
The rate of movement of electric charge
Define EMF
The force that causes current to flow. Measure in Volts
Define potential difference
The change in electrical potential between two points in a circuit.
Define resistance
The oposition to current flow that a resistor exerts in a circuit.
Define Absolute potential
The voltage at a point in the circuit with reference to earth.
Define a conductor
A material with low resistance that allows current to flow
Define an Insulator
A material with high resistance that doesn’t allow a current to flow.
Define a semi conductor
A material that is neither a good or bad conductor or insulator but can be made to exhibit properties of either
Name 5 conductors
Copper, aluminium, brass, platinum, carbon
Name 5 insulators
Plastic, rubber, glass, air, paper, Cork
Name two semiconductors
Silicon and Germanium
Define an Ammeter
An instrument that has low resistance and is used to measure current. Placed in series with the load.
Define a voltmeter
An instrument that has high resistance and is used to measure potential difference. Placed in parallel with the load.
Define an Ohmmeter
An instrument used to measure resistance. Placed in parallel with the resistor once it has been removed from the circuit.
Define a Wattmeter
An instrument that is used to measure power.
What are MIT 420s or Bridge Meggers used for?
Measuring continuity and insulation resistance.
What are Tachometers / Stroboscopes used for?
To measure rotational speed.
What is a multimeter? State two examples.
A universal instrument used to measure voltage, current and resistance. E.g. an Avometer or Fluke
What is a Cathodic Ray Oscilloscope used for?
To observe voltage wave forms
Define Ohms Law
The current (I) flowing through a circuit is directly proportional to the applied voltage (V) and inversely proportional to the resistance (R), provided the temperature remains constant.
I =V/R
Define Electrical Power
Power is what the equipment consumes when it is in use. (Meausred in Watts)
What two things must be considered when deciding insulation material?
The maximum temperature rating of the material.
The maximum voltage present
What are three causes of short circuits?
Dampness.
Insulation becoming hard or brittle with age or heat.
Accidental damage.
What is the purpose of a fuse?
To prevent overloading of electrical circuits.
Name four factors that affect resistance.
Length
Type of material
Material cross section
Temperature of material
Name four types of resistors
Surface Mount Technology (SMT)
Wire wound resistors.
Metal oxide resistors.
Carbon resistors.
State Kirchoffs Current Law
The amount of current flowing towards a junction or node in a circuit is equal to the amount flowing away.
State Kirchoffs Voltage Law
The sum of the voltage drops around a closed loop in a circuit is equal to the emf’s acting on the loop.
What is internal resistance?
The resistance within a cell, which is the opposition of the material in a cell to the current flow.
What is a potentiometer?
A type of variable resistor consisting of three terminals and is used for voltage control.
What is a Rheostat?
A type of variable resistor using two terminals to control current.
Give an application of potentiometers and rheostats.
The volume control of a radio of TV.
What is a capacitor
A device that is used to temporarily store electrical energy or charge.
What are the factors that affect Capacitance?
Type of dielectric material.
Distance between plates.
Area of plate.
Define Time Constant
The Time the voltage would take to reach its maximum if it continued to rise at its initial rate.
Calculated t = CR (seconds)
Name at least 3 capacitors and their applications.
Variable air capacitors
Mica Capacitors
Paper Capacitors
Ceramic Capacitors
Plastic capacitors
Titanium Oxide capacitors.
Electrolyte capacitors.
Supercapacitors.
How are capacitors discharged?
Connected to a load. Current and voltage both decrease over time.
How are capacitors charged?
Current decreases over time and voltage increases. Electrons build up a negative charge on one plate, creating a potential difference.
Define Magneto Motive Force (mmf)
The force that causes a magnetic field to be set up around a circuit. Measured in Amps
Define Magnetic Flux
The amount of magnetic field created by a magnetic source. Measured in Webers (Wb)
Define Magnetic field
The area around a bar magnet where the magnetic force can be detected.
Define Magnetic Flux Density (B)
The amount of magnetic Flux passing through a defined area. Measured in Tesla (T)
Define Magnetic field strength (H)
The amount of mmf per unit length of a magnetic circuit.
What is the Motor effect
When a current carrying conductor is placed in a magnetic field it experiences a force. The direction of the force can be determined using flemmings left hand rule.
How does a motor work
When a current carrying conductor is placed in a magnetic field, it experiences a force. In a motor are loops of wire in a magnetic field. When current flows through the loops, they experience forces that cause the loop to rotate, creating torque.
What four factors affect the force on a current carrying conductor?
The Flux Density
Strength of current
Length of conductor
Directions of the field and current.
Describe flemmings left hand rule
Shows acting forces in a motor:
Thumb = Motion
First finger = field
Second finger = current
What are Faradays 2 laws on electromagnetic Induction?
An induced emf is set up whenever the magnetic field linking that circuit changes.
The magnitude of the induced emf in any circuit is proportional to the change of the magnetic Flux linking the circuit.
What is Lenz’s law?
The direction of an induced emf is always such that it tends to set up a current opposing the motion, or the change of Flux responsible for indilucing that emf.
Describe how an AC generator works
When a conductor is passed through a magnetic field an emf is induced into it. In an AC generator coils of wire are turned in a magnetic field by a prime mover. As the coils cut the Flux an AC emf is induced which is transmitted to the load via a commutator and brushes. The commutator converts the AC to DC.
What is flemmings right hand rule?
Generator rule:
First finger: field
Second finger: emf
Thumb - motion
Define Self inductance (L)
Is when an emf is induced into a coil caused by the current changing in it.
Define mutual inductance (M)
When an emf is induced into a coil caused by the current changing in an adjacent coil.
What are the factors that affect Inductance?
Number of turns in a wire
The cross-sectional area of the coil
The presence of a magnetic core
The way the turns are arranged.
Define Periodic Time (T)
The time for one cycle of an ac waveform.
Define RMS value
The value of the AC waveform that would give the same heating effect as the equivalent DC.
Define frequency
The number of complete ac cycles each second. Hertz (Hz)
Define power rating
The maximum apparent power the transformer can transform without overheating
Name two types of transformer core
Shell and core
Describe the waveform in a purely resistive AC circuit
In phase with the voltage
Describe the ac waveform in a pure inductive circuit
The current lags the voltage by 90 degrees
Describe the waveform in a pure capacitive circuit
The current leads the voltage by 90 degrees.
Define inductive reactance
The opposition to current flow that an inductor exerts in an AC circuit.
Define impedance (Z)
The total opposition to current flow that is excerted by the combined components in an ac circuit.
Define Series Resonance
When the supply voltage and current are in phase.
