3.7 Resistance Flashcards
Resistance is an electrical property of a?
Conductor
A higher resistance requires a?
Higher voltage
What are the key factors that determine the electrical resistance of a object?
-length of object
-cross sectional area
-what its made from
Resistance increases in direct proportion to the?
Length of a resistor (double length = double resistance)
Resistance increases in inverse proportion to the?
Cross sectional area (double area = half the resistance)
When comparing different materials for there resistance to electrical current what’s important to remember?
The tests must be done with the same dimensions and environments
What is specific resistance also known as?
Resistivity
What is specific resistance used for?
To calculate the resistance of conductors of any size
What is the equation for calculating a materials resistance no matter its dimensions?
R= ⍴ x length / cross sectional area
(⍴ = specific resistance)
What is the SI units for specific resistance?
Ohm metres
How can you determine the value of resistance by looking at a resistor?
They have a colour code system indicated on the body of the resistor
Can resistors have a tolerance when made?
Yes depending how precise they’re made
What happens if a resistors maximum power limit is reached?
The temperature will become too great and risks damage to the circuit or fire
What do the first two colour bands on a resistor indicate?
The first values which will then be multiplied by the third band
What does the third colour band indicate on a resistor?
What the first and second numbers will be multiplied by
What does the fourth colour band on a resistor indicate?
The amount of tolerance in the resistor
What is the definition of the tolerance of a resistor?
The extent to which it can differ fro the rated value
Why is tolerance added to resistors?
So that sections overlap one another so that no waste is produced
What is the electrical energy applied to a resistor converted into?
Heat
What must the rate of which energy is converted to heat and dissipated by a resistor be less than?
The wattage rating
What does overheating the resistor do to the resistor?
Damages the materials its constructed from which affects the technological and physical properties
How can a resistor be designed to stop it from overheating?
Increase it surface area while maintains the cross sectional area
What is the power rating of a resistor?
The maximum power (wattage) that can be dissipated without affecting function
Alongside the rated value of the resistor what is also stated?
The power rating at ambient temperature
What will happen if the resistor is subject to work at an incorrect ambient temperature?
It will fail as it wont be able to dissipate the heat energy fast enough
What happens to the current if you have multiple resistors connected in series?
It must flow through them in turn like water in a pipe
How do you calculate the combined resistance when you have multiple connected in series?
Just simply add them up
When adding resistors in parallel why does the resistance decrease?
Because the area through which the current can flow has increased
What is the equation for adding up resistors in parallel?
1/R1+1/R2+1/R3
When adding resistors in parallel what’s important to remember?
To have the same bottom number
What do variable resistors have?
Adjustable resistance values
What is a variable resistor also known as?
An electromechanical transducer
How does a variable resistor work?
By sliding a contact over a resistive element
What is a variable resistor called when used as a potential divider with three terminals?
Potentiometer
What is a variable resistor called when it only has two terminals?
Rheostat
How are variable resistors controlled?
Electronically or mechanically
What are electronically controlled resistors called?
Digital potentiometers
What type of resistor is used in a circuit requiring variation in voltage?
A potentiometer
What type of resistor is used in a circuit requiring a variation in current?
Rheostat
How is a potentiometer used to control a dc motor?
By varying resistance it changes the speed
Where are pre-set potentiometers commonly mounted?
Printed circuit boards
Where are pre-set rheostats usually found?
On a printed circuit board
When are pre-set potentiometers and rheostats set?
Manufacture or maintenance
Increasing the temperature of a conductor increases the?
Rate of thermal agitation and collisions therefore increases resistivity
Conductors whose resistance increases in line with temperature are said to have?
A positive temperature coefficient (PTC)
What does a positive value in front of the temperature coefficient represent?
The resistance increases with increasing temperatures
What does a negative value in front of the temperature coefficient represent?
The resistance decreases with increasing temperatures
What is the equation for proportional change in resistance?
Change in temperature x temperature coefficient
What happens to a semiconductors resistivity when temperature increases?
It decreases
What is a fixed resistor?
A resistor with a fixed electrical resistance
What is an axial lead resistor?
A resistor which the connecting leads enter and leave the body (like a tyre on a axial)
What is the most common axial lead resistor?
Carbon film
What are the two types of thermistors?
-positive temperature coefficient resistors
-negative temperature coefficient resistors
What are positive temperature coefficient thermistors made from?
Polycrystalline titanate ceramics
What are positive temperature coeffcient thermistors used for in the aircraft industry?
Temperature sensors
What materials are used to make negative temperature coefficient thermistors?
Semiconductors
What happens to electrons in a negative temperature coefficient when temperature rises?
When temperature rises more electrons escape from their orbits which increases conductivity
What changes the resistance value in a voltage-dependent resistor?
Voltage
What are dedicated tools used to adjust variable resistors called?
Trimmers
What does a Wheatstone bridge do?
Compares the voltages in two branches of a circuit and can be used to determine the value of unknown resistance
What is a galvanometer?
A very sensitive ammeter that measures both positive and negative currents
When using a Wheatstone bridge what does it mean when its said to be balanced?
There is no potential difference between the two points of a circuit
