Elec Flashcards
Define and state two examples of a conductor
A conductor is a material of low resistance that allows electric current to flow through it.
1, Copper
2, Aluminium
3, Brass
Define and state two examples of an insulator
An insulator is a material of high resistance that doesn’t allow a current to pass through it
1, Rubber
2, Plastic
3, Glass
State two main effects of electric current and give two examples of what uses this
- Heating effect: Cookers, water heaters, irons
- Magnetic effect: Bells, relays, motors
- Chemical effect: Primary and secondary cells and electroplating.
An ammeter has …… resistance and must be connected in …….
LOW . SERIES
A voltmeter must have …….. resistance and be connected in ……….
HIGH . PARALLEL
State two possible causes of shorts (or earth faults) in electrical circuits:
- Dampness
- Insulation becoming hard or brittle due to age or heat.
- Accidental damage.
State the instruments used to measure current and continuity:
- Current = Ammeter or fluke
- Continuity = BM80 or Bridg Megger
State Ohm’s Law
Current is directly proportional to applied voltage, inversely proportional to resistance, when the temperature remains constant.
Name two semiconductor materials used in the electronics industry:
- Germanium
- Silicon
When to magnets are placed with similar poles together
Rejection occurs
With two magnets placed with opposite poles together …… occurs
Attraction
Name any two applications of electromagnets:
- Electric bell
- Relay
- Lifting magnet
State what happens when a current carrying conductor is placed in a magnetic field between two magnets. What is one practical application for this action
The two fields will interact and cause a force to be exerted on the conductor. The field is strengthened above the conductor and weakened below, thus tending to move the conductor downwards.
This is the basic application for the electric motor.
Draw
Define magnetic flux and state it’s unit
Magnetic flux is the amount of magnetic field field (or number of lines of force) produced by a magnetic source. The unit of magnetic flux is the Weber.
Name two practical applications where magnets are used
Motors and Generators
Briefly describe the principle of operation of a transformer:
A transformer consists of two electric circuits linked by a ferromagnetic core. One coil is termed the primary winding which is connected to an electrical supply and the secondary winding is usually connected to a load. When an alternating voltage is applied to the primary winding an alternating current will flow in that winding and a magnetic flux will be induced within the iron core of the transformer. This alternating flux will induce an emf then across both coils.
State the advantages of using an auto transformer instead of a double wound transformer:
A saving in cost as less copper is needed
Less volume, hence less weight
Higher efficiency resulting from less I2R losses
A continuously variable output voltage if a sliding contact is used
Name two types of transformer construction:
- Shell
- Core
In a purely resistive ac circuit the current is …….. with the voltage.
In phase
In a purely inductive AC circuit the current …… the voltage by ……. Degrees
the current lags the voltage by 90 degrees
In a purely capacitive circuit the current ….. the voltage by …… degrees.
The current leads the voltage by 90 degrees.
Name three types of power in an AC circuit:
1, True or active power
2, Apparent power
3, Reactive power
What is meant by the ‘rating’ of a transformer?
The rating of a transformer is stated in terms of the volt - amperes it can withstand without overheating. The rating is either I1 x I2 or V2 x I1 where I2 is the full load secondary current.
What is an auto transformer?
An auto transformer is a transformer which has parts of its winding common to the primary and secondary circuits.
State a disadvantage of an auto transformer compared to a double wound transformer:
The primary and secondary windings are not separate so if there is an open circuit fault it will apply to both sides of the circuits.
Name three types of power in an AC circuit:
- True or active power
- Apparent power
- Reactive power
If the length of a piece of wire of constant cross sectional area is halved, the resistance of the wire is ……
Halved
If the cross sectional area of a certain length of cable is doubled the resistance of the cable is……
Halved
State two types of resistor construction and give one practical application of each:
Wire wound = power circuits, motor starters
Metal oxide = Electronic equipment
Carbon = Electronic equipment
Explain with the aid of a sketch what a capacitor is, and state its purpose in an electrical circuit:
A capacitor is an electrical device that is use to store electrical energy. In its simplest form a capacitor consists of 2 plates that are separated by an insulating material known as a dielectric.
State one advantage and one disadvantage of electrolytic capacitors:
Advantage = it can provide a far higher capacitance than any other types of capacitor of its size due to containing oxide film only a few microns thick
Disadvantage = it can only be used in a DC supply
What safety precautions should be taken when a capacitor is disconnected from a supply source
What a capacitor is disconnected from the source it may still be charged and hold this charge for some time. This could be dangerous and give an electric shock if no handled safely. To avoid this the capacitor must be discharged after the source is disconnected as well. This is done by connecting a high value resistor across the capacitor terminals.
Name three factors that affect the resistance of a conductor:
- Temperature of the material.
- Type of the material.
- Cross sectional area of the conductor
- Length of the conductor
State two methods of identifying a resistor:
Colour banding
Letter and digit code
State Kirchhoff’s law:
At any junction in an electric circuit the total current flowing towards that junction is equal to the total current flowing away from the junction.
State Kirchhoff’s voltage law:
In any closed loop in a network, the algebraic sum of the voltage drops taken around the loop is equal to the resultant emf acting in the loop.
Name three factors on which capacitance depends:
- The area of the plate.
- The plate spacing ( dielectric thickness )
- The nature of the dielectric
With reference to the current flowing what is meant by the time constant of a a circuit containing capacitance in series with a resistor?
Time constant is the time taken for the current to reach its final value if the initial rate of increase is maintained. Time constant is equal to C x R.
