Module 4 - Section 1 - Resistance And Resistivity Flashcards
What happens if you place a potential difference over a electrical component ?
IF you place a pd across an electrical component then a CURRENT will flow and this is because the pd works on the charge carriers and this means energy is transferred to them and this means that they gain kinetic energy and this means that they get accelerated.
Then how much current flows depends on the amount of resistance that the component has
What is resistance ?
You can think of a components resistance as a measure of how difficult it is to get current though it
However the mathematical term for resistance is:
R = V / I
R=resistance , V=pd, I=current
What is resistance measured in ?
How can a components resistance be 1ohm ?
Resistance is measured in Ohms ( omega symbol )
The resistance of a component is 1ohm - if a pd of 1V makes a current of 1A flow through the component - because if it doesn’t then that means that the resistance of the component is higher than 1ohm
What are the 3 things that resistance depends on ?
Let’s think about a simple wire first:
- Length (L) - the longer the wire the more difficult it is for current to flow through
- Area (A) - The wider the wire, the easier it is for the electrons to flow through
- Resistivity (p) - This depends on the material the wire is made from as the structure of the material may make it easy or difficult for electrons to pass through - in general the resistivity depends on environmental factors such as temperature
What is resistivity ? And whats it measured in ?
The resistivity of a material is defined as the resistance of a 1m length with a 1m*2 cross-sectional area
p = RA / L
p=resistivity , R=resistance, A=cross-sectional area, L=length
Resistivity is measured in ohm metres (omega m)
What are the typical values for resistivity of conductors ?
For e.g. the resistivity of copper is (at 25C) is 1.72x10*-8
If it’s really really small like e.g. 1x10-7 then check if you converted the area into m2
What are the conductors called that follow ohms law ?
And what is Ohms law ?
Ohmic conductors
IF the temperature is constant, the current through a ohmic conductor is directly proportional to the pd across it
Whats constant in ohmic conductors ?
In ohmic conductors - R is constant
Explain the practical on how to find the resistivity of a wire ?
1) Before you start the experiment, you need to find the cross-sectional area of the wire and you do this by using the formula - pie r*2 - use a micrometer to measure the diameter of the wire and measure this in 3 different places along the wire and use the mean average of these results then half it to get the radius and then plot into the formula
BEFORE YOU FIND THE RESISTIVITY, YOU HAVE TO FIND THE RESISTANCE OF THE WIRE
2) The test wire should be clamped to the ruler and it should be at the start of the ruler where it reads zero
3) Then attach the flying lead to the test wire - the lead is just a wire which has a crocodile clip connected to the end so that you could clip the wire onto any point on the test wire and adjust the length of the test wire
4) Record the length of the test wire that’s CONNECTED to the circuit, then record the voltmeter and ammeter reading
5) Use your readings from the ammeter and the voltmeter and find out the wires resistance using R = V / I
6) Repeat the recordings for different lengths of the wire by moving the flying lead (croc clip) down or up the test wire
7) Plot your results on a graph of resistance against length and draw the line of best fit
8) The gradient of the line of best fit is equal to R/L - so multiply the gradient by the cross sectional area of the wire to find the resistivity of the wire material
REMEMBER KEEP THE TEMP CONSTANT AS THE RESISTIVITY OF THE MATERIAL DEPENDS ON ENVIRONMENTAL FACTORS SUCH AS TEMP - keep it constant by for e.g. keeping the current low so that the current doesn’t heat up the wire
What is an IV characteristic ?
An IV characteristic refers to a graph which shows how the current flowing through a component changes as the pd is increased - the gradient of an IV graph is the resistance
What do IV graphs do ?
They show you how the resistance varies
How can you measure the IV characteristic of a component ?
First you make a test circuit that consists of: a battery, ammeter, voltmeter over the component you are measuring the IV characteristic of and a variable resistor and of course the component
Then You use the variable resistor to alter the potential difference across the component and the current flowing through it and then record the V and I
Then keep on altering the resistor and gather the readings from the ammeter and voltmeter
Then plot a graph of current against pd - and this is the IV characteristic of your component and you can use it to see how the resistance changes
Whats the IV characteristic for a metallic conductor ?
It’s a straight line because metallic conductors are ohmic conductors and therefore if the temp is constant then the current will be directly proportional to the pd therefore the resistance is also constant and doesn’t change
Whats the IV characteristic for a filament lamp ?
The IV characteristic for a filament lamp is curved - it starts steep but gets shallower as the pd increases - this is because when the pd increases - the current increases and when the current increases the temperature of the filament also increases so therefore RESISTANCE increases
Why does the resistivity of a metal increase with temperature ?
So Charge is carried through metal wire by free electrons in a lattice of positive ions this is because the electrons are delocalised in the outer shell of the metal ( metallic bonding ).
Therefore when the temperature increases - the positive ions in the lattice gain more kinetic energy due to the heat and therefore they vibrate more - so therefore the electrons collide with the positive metal ions more often and loose energy to other forms and therefore the resistance increases with temp
The resistivity of a metal increases as the …
Temperature increases
Why are semi conductors used for sensors ?
They are used for sensors because semiconductors have a higher resistivity than metals as they have fewer charge carriers available - however when you supply energy to some types of semiconductors by for example increasing their temp - more charge carriers get released therefore the resistivity of the charge carriers decrease as there is more charge carriers so more charge can flow so their is a greater current so smaller resistance
This means that they are great sensors for detecting a change in environment
What are the 3 semi-conductors that u need to know ?
Thermistors, LDRs, diodes
What are semiconductors used for ?
Sensors
What does the resistance of the thermistor depend on ?
It depends on the temperature
What is a thermistor and how and why does it work ?
A thermistor is a resistor with a resistance that depends on the temperature
When the temperature decreases the resistance goes up - when the temperature increases the resistance down
This is because the thermistor is a semiconductor therefore when the temp increases more charge carriers ( electrons ) get enough energy to break free from their atoms therefore when the temp increases - the amount of charge carriers increases and therefore more charge can flow therefore higher current - therefore less resistance and vice versa
Whats the IV characteristic of a thermistor ?
SO when the temp increases - resistance decreases
So when you increase the current through a thermistor you increase its temp - this means that the resistance decreases
So you get a IV graph of the line starting of shallow and then getting steeper
What is an LDR and how and why does it work ?
An LDR is a resistor with a resistance that depends on the intensity of light
In a LDR the greater the intensity of the light shining on the LDR - the lower its resistance and vice versa
The reason for this is similar to the thermistor - when the light shines on the LDR it proved the energy that releases more electrons - meaning more charge carriers - meaning a lower resistance
What is a diode and what does it do ? Forward bias - threshold voltage etc
A diode is a resistor with a resistance that depends on the direction that the current flows - diodes and LEDs only let current flow in one direction only
Forward bias is the direction in which the current is allowed to flow - its the discretion where the triangle points
Most diodes have a threshold voltage of about 0.6V in the forward direction before they will conduct and let current through
In the reverse bias - the resistance is very high and the current that flows is very small
