Electrical

Question 1

What are the 3 main effects of current?

Answer 1

1. Magnetic (relays,motors,generators)
2. Heating (Cookers,water heater,electric fence)
3. Chemical (Primary & secondary cells & electroplating)

Question 2

Whats the difference between AC & DC?

Answer 2

• DC -current flow in 1 direction in a constant magnitude
• AC - current changes in 1 direction rising to maximum back to 0 then to maximum in other direction

Question 3

Charge

What is current?

Answer 3

Flow rate of charge

Question 4

What is Ohms law?

Answer 4

Current flowing through a conductor is directly proportional to the applied Voltage through it & inversely proportional to the Resistance providing temperature is constant

Question 5

What does a fuse do?

Answer 5

Prevents overcurrent in a circuit,if the current exceeds the fuse melts breaking the circuit

Question 6

Insulation resistance decreases due to…

Answer 6

• Dampness
• Brittleness with age
• Accidental damage

Question 7

If volatge is the same across all resistors in parallel,what is the current?

Answer 7

Current is different

Question 8

In a series circuit the current is the same through each component,what is the voltage?

Answer 8

The voltage is different

Question 9

Name 2 types of resistors & their uses

Answer 9

1. Surface mount technology (SMT) used in electronics
2. Wire wound used in motor starters & power circuits

Question 10

Name 2 ways to identify resistors

Answer 10

1. Colour coding
2. Alpha numerical coding

Question 11

R=pL/A

What 4 things affect resistance

Answer 11

1. Type of material
2. Length of conductor
3. Cross-sectional area of conductor
4. Temp of material

Question 12

Name hazards with Capacitors

Answer 12

A charged capacitior can cause electric shock, need to discharge across a resistive load to make safe.

Question 13

What does a capacitor consist of?

Answer 13

2 plates seperated by insulating material known as dialectric.

Question 14

What is Kirchoffs current law?

Answer 14

Current going into a junction is always the same a current out

Question 15

What is Kirchoffs Voltage law?

Answer 15

In any closed loop the amount of voltage drop = the total voltage

Question 16

Series circuit/variable resistor

Describe a Rheostat

Answer 16

• Variable resistor with 2 terminals,1 fixed &1 sliding
• Controls current flow by changing resistance
• Connected in series
• Slider to left=max current
• Slider to right=min current

Question 17

Voltage

Describe a Potentiometer

Answer 17

• Variable resistor ,3 terminals
• Provides an adjustable voltage divider circuit,produces a variable p.d from fixed e.m.f
• Slider left=V increase
• Slider right=V decrease

dimmer switch,tv controls

Question 18

What is Absolute potential?

Answer 18

Voltage with respect to earth

Question 19

Name 3 things that determine capacitance

Answer 19

1. Area of plate
2. Thickness of dialectric
3. Dialectric material

Question 20

In paralell circuits what is the same over each capacitor?

Answer 20

Voltage

Question 21

In series circuits what is the same over each capacitor

Q

Answer 21

Charge

Question 22

In a capacitor how long does it take for voltage to reach steady state?

Answer 22

5T (5xtime)

Question 23

Name 2 types of capacitors

Answer 23

1. Fixed
2. Variable

Question 24

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Disadvantage of electrolytic capacitors?

Answer 24

Used in DC circuits

Question 25

Advantage of electrolytic capacitors

Answer 25

Larger capacitance

Question 26

How to charge a capacitor

Answer 26

* Turn on switch,emf applied to capacitor * Electrons build up on 1 plate with neg charge,positive on other * Surplus of positive ions on one plate * No electron go through plate * Flow continues till capacitor is charged

Question 27

What is Time constant?

Answer 27

Time it takes for capacitor to reach max charge if inital charge is maintained

Question 28

Describe a magnetic field

Answer 28

Area around a magnet where magnetic force can be detected

Question 29

What is Flux density measured in?

Answer 29

Tesla (T)

Question 30

Describe magnetic flux

Answer 30

Amount of magnetic field produced by a magnetic source

Question 31

In relation to magnetism, what is Reluctance?

Answer 31

Magnetic resistance to flux

Question 32

# Magnetism Name 4 things that affect Force

Answer 32

1. Flux density 2. Current strength 3. Length of conductor 4. Direction of field + current

Question 33

What happens when a current carrying conductor is placed in a magnetic field?

Answer 33

Field strong on one side of conductor, weak on other, the conductor moves outward | Application DC Motor , used in generators

Question 34

Explain Self-Inductance

Answer 34

When EMF is induced in the same circuit in which current is charging

Question 35

Explain Mutual-inductance

Answer 35

When EMF is induced in a circuit by a change in flux due to current charging in an adjacent circuit

Question 36

Explain the principle of a generator

Answer 36

* Conductors form a circuit that move through mag field * EMF induced in conductor * Generators convert mechanical energy to electrical energy

Question 37

Define electromagnetic induction

Answer 37

When a conductor is moved across a field,cutting the lines of flux it produces **EMF & current** in a conductor

Question 38

# Back What is Lenz's law?

Answer 38

The changing current in a conductor induces a back EMF across it to oppose the motion which produces it

Question 39

Name 3 factors that affect inductance

Answer 39

1. Number of turns of wire 2. Cross-sectional area of coil 3. Presence of mag core

Question 40

In safety, **insulation** uses

Answer 40

Peak value

Question 41

In safety, **fuses** use

Answer 41

RMS value

Question 42

State 2 advantages to AC

Answer 42

* Easier to distribute * Cheaper to generate

Question 43

What is the power rating of a transformer?

Answer 43

The Volt-Amperes that it can transform without overheating

Question 44

What does Back EMF do?

Answer 44

Tries to stop changes in current, when current at max there are no changes so it drops to 0

Question 45

What is the steady state current?

Answer 45

Max

Question 46

What is the steady state Voltage?

Answer 46

0

Question 47

What is inductive reactance?

Answer 47

Opposition to changing current flow

Question 48

Name 2 conditions of resonance

Answer 48

1. Power factor = 1 2. Phase angle = 0

Question 49

Nmae 2 practical applications of resonance

Answer 49

1. Radar recievers 2. Radio recievers

Question 50

In resonance name 3 powers

Answer 50

1. Apparent 2. True 3. Reactive

Question 51

How does a capacitor discharge?

Answer 51

The negative electrons on one plate flow towards the positive plate,acting as a voltage source in the circuit.When the p.d between both plates is 0 the flow stops

Question 52

What is periodic time?

Answer 52

Time it takes to complete one cycle

Question 53

Define frequency of a waveform

Answer 53

Number of cycles completed in 1 second

Question 54

# CCL In an AC inductive circuit current?

Answer 54

Lags V by 90°

Question 55

In a capactive circuit current?

Answer 55

Leads V by 90°