Electrical principles Flashcards
What is the definition of electrical current?
Rate of movement of charge
What is the definition of emf?
Force required to cause current to flow in a circuit
What is the unit of current?
Amps (A)
What is the unit of p.d?
Volts (V)
What are the two main effects of electrical current and give 3 examples of each?
Heating effect- cookers, irons, kettles
Magnetic effect - motors, generators, transformers
What is the difference between a.c and d.c?
D.c flows in one direction around a circuit
a.c constantly changes direction in a circuit
What is the unit to measure resistance?
Ohms
What is the unit to measure e.m.f?
Volts (V)
What is a conductor? and give two examples
A material with low resistance that allows electric current to flow e.g copper, aluminum
What is an insulator? and give two examples
A material with high resistance that doesn’t allow current to flow e.g plastic, rubber
What is a semi conductor? and give two examples
Material with electrical conductivity between and insulator and a conductor e.g silicon, germanium
What instrument is used to measure current?
Ammeter
What instrument is used to measure potential difference?
Voltmeter
What instrument is used to measure resistance?
Ohmmeter
What instrument is used to measure e.m.f?
Voltmeter
Complete sentence…
An ammeter has a … resistance and must be connected in … series with the load.
Low resistance
in series with load
Complete sentence…
A Voltmeter has a … resistance and must be connected in … series with the load.
High resistance
in parallel with load
What instrument is used to measure insulation resistance?
MIT 420
What instrument is used to measure power?
Wattmeter
What instrument is used to measure rotational speed?
Tachometer
What instrument is used to measure continuity?
MIT 420
State ohms law
V=IR
State 3 causes of short circuits
Dampness
break down of insulation
Accidental damage
What is the function of a fuse in an electrical circuit?
Used to prevent overloading of electrical circuits
State 4 types of resistor construction and give one practical application of each
Surface mount technology - electronic equipment
Wire wound resistance - power circuits
Metal oxide resistors - electronic equipment
Carbon resistors - electronic equipment
State 2 methods of resistor identification
Colour coding
Letter and digit code
What are 4 factors that can effect the resistance of a conductor?
Length of conductor
Cross sectional area of the conductor
Type of material
Temperature of material
State Kirchhoffs current law
At any junction in an electrical circuit the total current flowing towards that junction is equal to the total current flowing away from junction
State kirchhoffs voltage law
In any closed loop in a network the algebraic sum of voltage drops taken around the loop is equal to the resultant e.m.f acting in the loop
What is the main use of a potentiometer?
Provides an adjustable voltage divider circuit, can obtain various voltages from a fixed p.d.
What is the main use of a rheostat?
Used to control current flow
How is a capacitor charged?
Capacitor effectively blocks direct current and stores it
How is a capacitor discharged?
The cell is removed from the circuit and all the current stored in the capacitor is released
What safety precautions should be taken when disconnecting a capacitor from a supply?
Ensure capacitors discharged before handling to avoid electric shock
What is meant by the time constant of a circuit?
Name 5 types of capacitor commonly used
Variable air capacitors
Mica capacitors
Paper capacitors
Ceramic capacitors
Plastic capacitors
What is an advantage and disadvantage of electrolytic capacitors?
Adv larger capacitance than other types of similar dimensions
Disadv must be connected with correct polarity
What 3 factors does capacitance depend on?
Nature of dielectrics
Area of the plate
Distance between the plates
What is the definition of a magnetic field?
Area around a magnet and electric current
What is the definition of magnetic flux? What is the symbol?
Amount of magnetic field produced by a magnetic source
Unit Wb
What is the definition of magnetic flux density? What is the unit and symbol?
Amount of flux passing through a defined area that is perpendicular to the direction of flux.
Symbol B Unit Tesla T
What are 3 applications of electromagnetism?
Electric bell
Relay
Lifting magnets
What happens when a current carrying conductor is placed in a magnetic field between two magnets? and state one application
Two fields interact and cause a force exerted on conductor
e.g electric motor
What 4 factors does the force on a current carrying conductor in a magnetic field depend on?
Flux density
Strength of current
Length of conductor perpendicular to magnetic field
Direction of field and current
What is electromagnetic induction?
When a conductor moves across a magnetic field an e.m.f. is induced. If the conductor is part of a closed circuit the a current is induced.
What are 3 practical application of electromagnetic induction?
What is Faradays law?
An induced e.m.f. is set up whenever the magnetic field changes
The magnitude of the induced e.m.f is proportional to the rate of change of the magnetic flux linking the circuit
What is Lenz’s law?
The direction of the induced e.m.f is always such that it tends to set up a current opposing motion or the change of flux responsible for inducing that e.m.f.
What is the principle of a generator?
What is self-inductance and symbol?
When the emf is induced in the same circuit as that which the current is changing
Symbol L
What is mutual inductance and symbol?
The emf is induced in a circuit by a change of flux due to a changing current in an adjacent circuit
Symbol M
What is the unit of inductance?
Henry (H)
What are 4 factors that affect the value of inductance of an inductor?
Number of turns of wire
Cross sectional area of coil of wire
The presence of magnetic core
The way the turns are arranged
What are 2 advantages of a.c compared to d.c?
Easier and cheaper to generate
More conveniently distributed
What is frequency and units?
Number of cycles completed in 1 minute
Units - Hz
What a.c value is used when choosing insulating material?
Peak values
What a.c value is used when choosing fuses?
R.m.s values
What is the principle of operation of a transformer?
Uses mutual induction, two electrical circuits linked by a common ferromagnetic core.
Primary winding is connected to the supply of electricity and the secondary winding which is connected to a load
What is meant by the rating of a transformer?
Is the amount of volt-amperes that it can transform without overheating.
How is cooling achieved in a small transformer?
Air
How is cooling achieved in a large transformer?
Oil
What is an auto transformer?
Part of its winding is common to the primary and secondary circuits.
What are 3 advantages of an auto transformer over a double wound transformer?
Saving cost as less copper is needed
Less volume so less weight
Higher efficiency
What is one disadvantage of an auto transformer over a double wound transformer?
If an open circuit occurs in the secondary winding the full primary voltage appears across the secondary
What does impedance mean when referring to an a.c. circuit?
The ratio of the applied voltage to the current
What is series resonance and state a practical example?
When the applied voltage and current are in phase
E.g radio receiver.
