Elec

Question 1

Define and state two examples of a conductor

Answer 1

A conductor is a material of low resistance that allows electric current to flow through it.
1, Copper
2, Aluminium
3, Brass

Question 2

Define and state two examples of an insulator

Answer 2

An insulator is a material of high resistance that doesn’t allow a current to pass through it
1, Rubber
2, Plastic
3, Glass

Question 3

State two main effects of electric current and give two examples of what uses this

Answer 3

1. Heating effect: Cookers, water heaters, irons
2. Magnetic effect: Bells, relays, motors
3. Chemical effect: Primary and secondary cells and electroplating.

Question 4

An ammeter has …… resistance and must be connected in …….

Answer 4

LOW . SERIES

Question 5

A voltmeter must have …….. resistance and be connected in ……….

Answer 5

HIGH . PARALLEL

Question 6

State two possible causes of shorts (or earth faults) in electrical circuits:

Answer 6

1. Dampness
2. Insulation becoming hard or brittle due to age or heat.
3. Accidental damage.

Question 7

State the instruments used to measure current and continuity:

Answer 7

1. Current = Ammeter or fluke
2. Continuity = BM80 or Bridg Megger

Question 8

State Ohm’s Law

Answer 8

Current is directly proportional to applied voltage, inversely proportional to resistance, when the temperature remains constant.

Question 9

Name two semiconductor materials used in the electronics industry:

Answer 9

1. Germanium
2. Silicon

Question 10

When to magnets are placed with similar poles together

Answer 10

Rejection occurs

Question 11

With two magnets placed with opposite poles together …… occurs

Answer 11

Attraction

Question 12

Name any two applications of electromagnets:

Answer 12

1. Electric bell
2. Relay
3. Lifting magnet

Question 13

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

Answer 13

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

Question 14

Define magnetic flux and state it’s unit

Answer 14

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.

Question 15

Name two practical applications where magnets are used

Answer 15

Motors and Generators

Question 16

Briefly describe the principle of operation of a transformer:

Answer 16

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.

Question 17

State the advantages of using an auto transformer instead of a double wound transformer:

Answer 17

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

Question 18

Name two types of transformer construction:

Answer 18

1. Shell
2. Core

Question 19

In a purely resistive ac circuit the current is …….. with the voltage.

Answer 19

In phase

Question 20

In a purely inductive AC circuit the current …… the voltage by ……. Degrees

Answer 20

the current lags the voltage by 90 degrees

Question 21

In a purely capacitive circuit the current ….. the voltage by …… degrees.

Answer 21

The current leads the voltage by 90 degrees.

Question 22

Name three types of power in an AC circuit:

Answer 22

1, True or active power
2, Apparent power
3, Reactive power

Question 23

What is meant by the ‘rating’ of a transformer?

Answer 23

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.

Question 24

What is an auto transformer?

Answer 24

An auto transformer is a transformer which has parts of its winding common to the primary and secondary circuits.

Question 25

State a disadvantage of an auto transformer compared to a double wound transformer:

Answer 25

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.

Question 26

Name three types of power in an AC circuit:

Answer 26

1. True or active power 2. Apparent power 3. Reactive power

Question 27

If the length of a piece of wire of constant cross sectional area is halved, the resistance of the wire is ……

Answer 27

Halved

Question 28

If the cross sectional area of a certain length of cable is doubled the resistance of the cable is……

Answer 28

Halved

Question 29

State two types of resistor construction and give one practical application of each:

Answer 29

Wire wound = power circuits, motor starters Metal oxide = Electronic equipment Carbon = Electronic equipment

Question 30

Explain with the aid of a sketch what a capacitor is, and state its purpose in an electrical circuit:

Answer 30

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.

Question 31

State one advantage and one disadvantage of electrolytic capacitors:

Answer 31

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

Question 32

What safety precautions should be taken when a capacitor is disconnected from a supply source

Answer 32

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.

Question 33

Name three factors that affect the resistance of a conductor:

Answer 33

1. Temperature of the material. 2. Type of the material. 3. Cross sectional area of the conductor 4. Length of the conductor

Question 34

State two methods of identifying a resistor:

Answer 34

Colour banding Letter and digit code

Question 35

State Kirchhoff’s law:

Answer 35

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.

Question 36

State Kirchhoff’s voltage law:

Answer 36

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.

Question 37

Name three factors on which capacitance depends:

Answer 37

1. The area of the plate. 2. The plate spacing ( dielectric thickness ) 3. The nature of the dielectric

Question 38

With reference to the current flowing what is meant by the time constant of a a circuit containing capacitance in series with a resistor?

Answer 38

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.