Unit 2.2 - Resistance Flashcards
What does the flow of charge allow us to get out?
Work
What do potential differences cause charges to do?
Flow from a higher to a lower potential
Current
How quickly charges flow
Potential difference
The difference in energy that a coulomb of charge loses between 2 points
Unit of potential difference
Volt
What does 1 volt represent?
1 Joule per coulomb, so the work done per unit charge
Equation involving potential difference
W = QV
What does W represent and what’s its unit in W = QV?
Work/energy
Joules
What does Q represent and what’s its unit in W = QV?
Charge
Coulomb
What does V represent and what’s its unit in W = QV?
Potential difference
Volts
What’s the difference between a battery and a cell?
Battery - Includes cells
Cell - 1 positive and 1 negative (long and short line)
What does the long line represent in a cell?
Positive charge
What does the short line represent in a cell?
Negative charge
What does a resistor do?
Limits/regulates the flow of electrical current in an electric circuit
What does a battery do in terms of energy in a circuit?
Increases the energy
What does a resistor do in terms of energy in a circuit?
Releases energy
What does a steeper “height” mean in terms of potential energy?
Bigger potential energy difference
What does a higher potential difference mean in terms of current?
Higher current
Why does a higher potential difference lead to a higher current?
More kinetic energy is delivered per unit charge to overcome resistance
What does closing a switch do in terms of potential difference?
Brings up the ‘dips’
No potential difference
How can we make a circuit have no potential difference?
Close the switch
What does resistance do and to what?
Slows down a current
What is Ohm’s law?
The ratio of the potential difference to the current is constant if the temperature, pressure, material, e.t.c remain constant
Equation involving resistance
R = V/I
What’s the unit of resistance?
Ohms
Worded equation involving resistance
Resistance = Potential difference/current
What do we mean when we say “Ohmic behaviour”?
That the resistance remains the same
What do we mean when we say “Ohmic behaviour”?
That the resistance remains the same
In a potential difference, what type of energy is it transferred from and to?
Electrical to other forms
What would a non-linear part of an I-V graph represent?
Hooke’s Law being disobeyed due to the temperature of the wire increasing
Describe exactly what resistance IS
-when a current flows through a metal, the free electrons drift in 1 direction
-as the electrons travel through the conductor, they will collide with the positive metal ions in the lattice positions
-these collisions cause electrical resistance, as some of the electrons kinetic energy is lost to the ion during the collision
-this causes the vibrational energy of the ion to increase and the temperature of the metal to increase
What determines the resistance of a conductor?
The frequency of the collisions between electron and the metal ions
What does a higher drift velocity mean in terms of resistance and why?
Higher drift velocity = electrons collide with ions more times each second and therefore lose more kinetic energy = higher resistance
What would be the difference between the current-voltage graph of a metal wire and the filament of a lamp?
metal wire - straight line through the origin
filament of a lamp - curved
Why would the metal wire produce a straight line on a current-voltage graph?
resistance (V/I) is constant as temperature is constant throughout
Why would the filament of a lamp produce a curved line on a current-voltage graph?
initially, resistance (V/I) is constant as temperature is constant
then, temperature increases, so resistance increases
Which factors effect the resistance of a wire?
Temperature
Length
Cross-sectional area
Material
Derive the equation for power in electrical circuits
Equation for potential difference
V = W/Q
Equation for current
I = Q/t
Multiplied together
V x I = W/G x Q/t = W/t (q’s cancel out)
W unit = J
t unit = s
(Power = energy transferred per second, so it works)
P = IV
Worded equation for power in an electrical ciruit
Power (W) = current (A) x potential difference (V)
Equations for power in an electrical circuit
P = IV = I^2R = V^2/R
Why is V^2/R a valid equation for calculating the power of an electrical circuit?
P = IV
And I = V/R
Combined =
P = V^2/R
Why is I^2R a valid equation for cal cutting the power of an electrical circuit?
P = IV
V = IR
Combined =
P = I^2R
What do all metals do to electricity?
Conduct it
Is resistance unique to a metal?
Yes
Which property is resistance related to?
Resistivity
What’s the only property that can change the resistivity of a material?
Temperature
Resistivity
The property of a material related to its resistance
It’s equivalent to the resistance in 1m of a wire of a certain material with a cross-sectional area of 1m^2
What is the resistivity of a material constant for?
At a specific temperature
Resistivity equation
R = ρl
—
A
What does ρ represent in the equation R = ρl
—
A?
Resistivity
What does R represent in the equation R = ρl
—
A?
Resistance
What does l represent in the equation R = ρl
—
A?
Length
What does A represent in the equation R = ρl
—
A?
Cross-sectional area
Unit of Resistivity + explanation
Ωm
Ohm x metre^2
———————. = ohm x metre
Metre
Resistance unit
Ω
What is Resistivity increased by?
Even small amounts of impurity
What could alloys have in terms of resistivity?
Far greater resistivity than any of its constituents
Alloy example
Constantan
If ρ and A are constant, what happens to R and l?
They’re directly proportional, so doubling one doubles the other
What happens to many metals at certain temperatures?
They become superconducting
Superconductivity
Where the electrical resistance of a material falls to zero
What has to be done to a wire for its resistance to be zero?
Has to be cooled to a low enough temperature
Superconducting transition temperature symbol
TT (imagine the second T is small)
Superconducting transition temperature
The temperature at which a material becomes a superconductor and has zero resistance
What is the superconductivity transition temperature typically very close to for most metals?
Absolute zero (0K)
If the superconducting transition temperature of a material is above 196°C, what can be done and why?
Liquid nitrogen can be used to cool the material down enough for superconducting
If a superconducting transition temperature is above WHICH temperature can liquid nitrogen be used to cool the material enough for superconducting?
196°C
What’s the significance of 196°C?
The boiling point of nitrogen
What can be used to cool down a material and keep it below its superconducting transition temperature?
Liquid nitrogen
What is liquid nitrogen used to do?
Keep a cool down a material and keep it below its superconductivity transition temperature
When is reaching superconductivity transition temperatures useful?
Where very high currents are required —> without superconducting wires, very high currents will cause too much power to be dissipated as heat due to the resistance in the wire
What would happen without superconducting wires where very high currents are required?
Very high currents will cause too much power to be dissipated as heat due to the resistance in the wire
Draw and label a graph comparing a non-superconducting metal and a superconductor
(See notes)
What can superconducting materials resist?
A magnetic field from their interiors
What does a magnet moving around a conductor usually cause and how is this different in a superconductor?
A magnet moving around a conductor induces a current to flow in the conductor - this is the basis of an electrical generator
However, in a superconductor, the induced current exactly opposes the magnetic field causing it, which repels the magnet = strong diamagnetism (“The Meissen effect”)
What’s the basis of an electrical generator?
A magnet moving around a conductor inducing a current to flow in the conductor
What’s the name for, in a superconductor, when an induced current (caused by a magnet moving around a conductor) exactly opposes a magnetic field causing it and repels the magnet?
Strong diamagnetism (“The Meissen effect”)
Strong diamagnetism (“The Meisser effect”)
In a superconductor, the induced current from a magnet moving around a conductor exactly opposes the magnetic field causing it, which repels the magnet
How strong is strong diamagnetism?
So strong that a magnet can be made to levitate above a piece of superconducting material
What allows a magnet to be made to levitate above a piece of superconducting material?
Strong diamagnetism (“The Meisser effect”)
What was a significant discovery made with a ceramic in terms of superconductivity?
A ceramic was discovered to superconduct at 30K (ceramics aren’t usually conductors!)
What was the superconductivity transition temperature that was first discovered to be higher than nitrogen’s?
92K, whilst Nitrogen’s is 77K
What’s good about using liquid nitrogen?
Cheap and easily found
What’s the record for the highest superconductivity transition temperature?
138K
Does the entire wire have to be kept under the transition temperature to maintain superconductivity?
No - superconductivity can be maintained even if only parts of the superconductor are kept below the transition temperature - the rest can be at room temperature
What’s a useful fact related to how superconductivity can be maintained?
Only parts of the superconducting wire have to be cooled below the transition temperature to maintain superconductivity - the rest can be at room temperature
Give 5 uses for superconductors
MagLev trains
Electricity production
Particle accelerators
Magnetic resonance imaging (MRI)
Electric motors
How do MagLev trains work?
Cause magnetic levitation, as superconductors more or less eliminate friction between the train and the track
What was so good about MagLev trains and why was this possible?
The lack of friction between the train and the track enabled the train to be able to reach record breaking speeds
Which use of superconductors has factors against it and why?
MagLev trains
Political and environmental factors due to them having biological effect such as affecting the migration paths of animals
Why are superconductors used in energy production?
Generators from superconducting wires are much more efficient than copper
Why are superconductors used in electricity storage?
Once a current is flowing in a superconductor, it doesn’t lose energy, so it can be stored until its needed
What happens once a current is slowing in a superconductor?
It doesn’t lose energy, so it can be stored until its needed
What does MRI stand for?
Magnetic Resonance Imaging
What does MRI allow doctors to do?
Look into the body without cutting flesh
How does MRI work?
Strong magnetic field from superconductors into the body
Hydrogen atoms within molecules gain energy
Released
Displayed graphically
How do superconductors work in particle accelerators? Why are these important?
Superconducting magnetic accelerate protons to a speed nearly that of light - could lead to a theory that explains everything
How can superconductors be used in electric motors?
Superconducting motors are much more efficient than usual motors
What would all of our uses of superconductors usually require? Why?
Very high currents to produce strong magnetic fields
What’s the issue with using ordinary wires at high currents as opposed to superconducting ones?
Heating effects of such high currents would melt the wires and would effect the sensitive measuring equipment used
In which region of the resistance-temperature graph is a potential difference of 0V required to maintain a current?
The whole flat line where the resistance is zero
What does it mean if there’s no resistance in terms of electrons?
There’s NO collisions between electrons and ions
Is V=IR a statement of Ohm’s law? Explain
It is, as potential difference is directly proportional to the current, so long as it’s stated that the resistance is constant
What temperature do we estimate as room temperature?
290K
Describe what resistance is in terms of electrons
When a current flows through a metal, the free electrons drift in one direction
As the electrons travel through the conductor, they will collide with the positive metal ions in their lattice positions
These collisions cause electrical resistance, as some of the electrons kinetic energy is lost to the ion during the collision
This causes the vibrations, energy of the ion to increase and the temperature of the metal to increase
At higher temperatures, the electrons collide more times each second with the ions, which lowers the drift velocity, therefore lowering the current
If p and l are constant, what happens to R and A?
They’re inversely proportional - doubling A halves R
What happens to the resistance of a wire if the diameter increases?
Remember that R and A are inversely proportional, and A= pi x (d/2)^2
So, doubling d will increase the area by 2^2= 4
Sp, R decreases by a factor of 4
Ohm’s law definition
The potential difference across a conductor is directly proportional to the current through the conductor at constant temperature
What is power?
Rate of energy transfer
What is rate of energy transfer?
Power
Define resistance
The ratio of potential difference to current
Describe the resistance of a conductor that obeys Ohm’s law
Constant
What do batteries do to charges?
Give them electrical potential energy
Can we always use the potential difference dividing up method?
only if the values for resistance are constant
