Unit 2 - Resistance Flashcards
What is potential difference
The energy transfered from electrical potential energy to another form of energy for each unit of charge that passes
(Its converted into other forms of energy because its the voltage that goes through a component, for each unit of charge that passes)
What is potential difference messured in
Volts (V)
What is electro motive force
energy that changes to electrical potential energy per charge passing through the source.
what is ohms law
that the potential difference across a component is directly proportional to the current through it. Providing that the temperature stays the same (V=IR)
What is resistance
A measure of how difficult it is for current to flow (measured in ohms)
What does higher resistance mean for the potenial difference
higher resistance means that a larger potental difference is required to produce a current.
Why does resistance occur
Because of the collisions between the free electrons and the ions.
How does resistance rise approximately linearly with temperature
if you increase the temperature of a conductor, the ions move around more energetically so more energy is transfered from electrons to ions. So it is harder for electrons to flow, so resistance rises.
What happens when free electrons and ions collide in a metal
When they collide, it increases the kinetic energy (in vibrations) of the ions, so it increases the temperature of the metal.
What is meant by ohmic and non-ohmic
Ohms law states that R is constant for all values of V and I, conductors that have this property are ohmic, conductors which don’t are non-ohmic
What is resistivity
A measurement of how much a material resists the flow of current
Unit of resistivity?
Ohm meter
What is I-V characteristics
The plot of current against voltage is called an I-V characteristic
Describe I-V graph for a filament lamp
The line is curved, this is because the resistance increases at higher points of voltage. As potental difference is applied to the lamp, a current flows, causing the lamp to light, this current flow means that some heat energy is dissipated in the filament, causing the filament to heat up, so resistance increases. Doesn’t obey ohms law because temperature doesn’t stay the same.
Describe I-V graph for a resistor/metal wire at constant temperature
Current is directly proportional to the potential difference, line is straight, and so resistance is constant as current increases. Therefore it is an ohmic conductor because the temperature is constant.
What is superconductivity?
When a material conducts with zero resistance, because it has been cooled to below a certain temperature.
What is transition temperature?
The temperature below which a superconductor must be cooled, in which suddenly the resistance drops to zero and the material becomes a superconductor
How is superconductivity used in maglev trains
using superconductors to produce a strong magnetic field which makes the train levitate off the track, by doing this there is no resistance between the train and the track, and so the train can reach really high speeds.
How is superconductivity used in MRI scanners
MRI scanners are used for medical diagnosis, using a superconductor, a strong magnetic field aligns hydrogen atoms in the body. Radio waves are absorbed and emitted by these hydrogen atoms and an image of the body can be produced.
How is superconductivity used in Particle Accelerators
Superconductors produce a magnetic field which keep the particles in a beam and steer them aroind the circular detector.
Name 3 uses of Superconductors
-Maglev Trains
-MRI Scanners
-Particle Accelerators
Equation for work done (and units)
Work done(J) = voltage(v) x charge(c)
Alternate unit for voltage rather than V
JC⁻¹
What is equation for energy
E = power x time
E = Voltage x Current x time
(Since power = IV
Define power
Rate of doing work (Watts or sometimes Js⁻¹)
Why is superconductivity achieved?
When it is cooled to a certain temperature, the atoms/ions in the metal vibrate less, so less collisions. And as temperature decreases electrons can move around closer to each other and sometimes form pairs called cooper pairs.
What is the transition temperature usually for metals
A few degrees above absolute zero (absolute zero is in kelvin, equals to -273 celcius)
What is high temperature superconductors
Certain materials with transition temperatures above 77 Kelvin (-196 Celsius)
