3.7 Resistance

Question 1

Resistance is an electrical property of a?

Answer 1

Conductor

Question 2

A higher resistance requires a?

Answer 2

Higher voltage

Question 3

What are the key factors that determine the electrical resistance of a object?

Answer 3

-length of object
-cross sectional area
-what its made from

Question 4

Resistance increases in direct proportion to the?

Answer 4

Length of a resistor (double length = double resistance)

Question 5

Resistance increases in inverse proportion to the?

Answer 5

Cross sectional area (double area = half the resistance)

Question 6

When comparing different materials for there resistance to electrical current what’s important to remember?

Answer 6

The tests must be done with the same dimensions and environments

Question 7

What is specific resistance also known as?

Answer 7

Resistivity

Question 8

What is specific resistance used for?

Answer 8

To calculate the resistance of conductors of any size

Question 9

What is the equation for calculating a materials resistance no matter its dimensions?

Answer 9

R= ⍴ x length / cross sectional area
(⍴ = specific resistance)

Question 10

What is the SI units for specific resistance?

Answer 10

Ohm metres

Question 11

How can you determine the value of resistance by looking at a resistor?

Answer 11

They have a colour code system indicated on the body of the resistor

Question 12

Can resistors have a tolerance when made?

Answer 12

Yes depending how precise they’re made

Question 13

What happens if a resistors maximum power limit is reached?

Answer 13

The temperature will become too great and risks damage to the circuit or fire

Question 14

What do the first two colour bands on a resistor indicate?

Answer 14

The first values which will then be multiplied by the third band

Question 15

What does the third colour band indicate on a resistor?

Answer 15

What the first and second numbers will be multiplied by

Question 16

What does the fourth colour band on a resistor indicate?

Answer 16

The amount of tolerance in the resistor

Question 17

What is the definition of the tolerance of a resistor?

Answer 17

The extent to which it can differ fro the rated value

Question 18

Why is tolerance added to resistors?

Answer 18

So that sections overlap one another so that no waste is produced

Question 19

What is the electrical energy applied to a resistor converted into?

Answer 19

Heat

Question 20

What must the rate of which energy is converted to heat and dissipated by a resistor be less than?

Answer 20

The wattage rating

Question 21

What does overheating the resistor do to the resistor?

Answer 21

Damages the materials its constructed from which affects the technological and physical properties

Question 22

How can a resistor be designed to stop it from overheating?

Answer 22

Increase it surface area while maintains the cross sectional area

Question 23

What is the power rating of a resistor?

Answer 23

The maximum power (wattage) that can be dissipated without affecting function

Question 24

Alongside the rated value of the resistor what is also stated?

Answer 24

The power rating at ambient temperature

Question 25

What will happen if the resistor is subject to work at an incorrect ambient temperature?

Answer 25

It will fail as it wont be able to dissipate the heat energy fast enough

Question 26

What happens to the current if you have multiple resistors connected in series?

Answer 26

It must flow through them in turn like water in a pipe

Question 27

How do you calculate the combined resistance when you have multiple connected in series?

Answer 27

Just simply add them up

Question 28

When adding resistors in parallel why does the resistance decrease?

Answer 28

Because the area through which the current can flow has increased

Question 29

What is the equation for adding up resistors in parallel?

Answer 29

1/R1+1/R2+1/R3

Question 30

When adding resistors in parallel what’s important to remember?

Answer 30

To have the same bottom number

Question 31

What do variable resistors have?

Answer 31

Adjustable resistance values

Question 32

What is a variable resistor also known as?

Answer 32

An electromechanical transducer

Question 33

How does a variable resistor work?

Answer 33

By sliding a contact over a resistive element

Question 34

What is a variable resistor called when used as a potential divider with three terminals?

Answer 34

Potentiometer

Question 35

What is a variable resistor called when it only has two terminals?

Answer 35

Rheostat

Question 36

How are variable resistors controlled?

Answer 36

Electronically or mechanically

Question 37

What are electronically controlled resistors called?

Answer 37

Digital potentiometers

Question 38

What type of resistor is used in a circuit requiring variation in voltage?

Answer 38

A potentiometer

Question 39

What type of resistor is used in a circuit requiring a variation in current?

Answer 39

Rheostat

Question 40

How is a potentiometer used to control a dc motor?

Answer 40

By varying resistance it changes the speed

Question 41

Where are pre-set potentiometers commonly mounted?

Answer 41

Printed circuit boards

Question 42

Where are pre-set rheostats usually found?

Answer 42

On a printed circuit board

Question 43

When are pre-set potentiometers and rheostats set?

Answer 43

Manufacture or maintenance

Question 44

Increasing the temperature of a conductor increases the?

Answer 44

Rate of thermal agitation and collisions therefore increases resistivity

Question 45

Conductors whose resistance increases in line with temperature are said to have?

Answer 45

A positive temperature coefficient (PTC)

Question 46

What does a positive value in front of the temperature coefficient represent?

Answer 46

The resistance increases with increasing temperatures

Question 47

What does a negative value in front of the temperature coefficient represent?

Answer 47

The resistance decreases with increasing temperatures

Question 48

What is the equation for proportional change in resistance?

Answer 48

Change in temperature x temperature coefficient

Question 49

What happens to a semiconductors resistivity when temperature increases?

Answer 49

It decreases

Question 50

What is a fixed resistor?

Answer 50

A resistor with a fixed electrical resistance

Question 51

What is an axial lead resistor?

Answer 51

A resistor which the connecting leads enter and leave the body (like a tyre on a axial)

Question 52

What is the most common axial lead resistor?

Answer 52

Carbon film

Question 53

What are the two types of thermistors?

Answer 53

-positive temperature coefficient resistors -negative temperature coefficient resistors

Question 54

What are positive temperature coefficient thermistors made from?

Answer 54

Polycrystalline titanate ceramics

Question 55

What are positive temperature coeffcient thermistors used for in the aircraft industry?

Answer 55

Temperature sensors

Question 56

What materials are used to make negative temperature coefficient thermistors?

Answer 56

Semiconductors

Question 57

What happens to electrons in a negative temperature coefficient when temperature rises?

Answer 57

When temperature rises more electrons escape from their orbits which increases conductivity

Question 58

What changes the resistance value in a voltage-dependent resistor?

Answer 58

Voltage

Question 59

What are dedicated tools used to adjust variable resistors called?

Answer 59

Trimmers

Question 60

What does a Wheatstone bridge do?

Answer 60

Compares the voltages in two branches of a circuit and can be used to determine the value of unknown resistance

Question 61

What is a galvanometer?

Answer 61

A very sensitive ammeter that measures both positive and negative currents

Question 62

When using a Wheatstone bridge what does it mean when its said to be balanced?

Answer 62

There is no potential difference between the two points of a circuit