Resistance Flashcards
Describe voltage, resistance and current in a series circuit.
Voltage=V1+V2, Resistance=R1+R2 and Current is the same throughout.
Describe voltage, resistance and current in a parallel circuit.
Voltage is the same on each branch, 1/Resistance= 1/R1+1/R2 and Current= I1+I2.
What is the definition of resistance?
The potential difference across the circuit divided by the resulting current through the circuit (R=V/I).
What is the potential difference?
The amount of electrical energy to other forms when one coulomb of charge flows between two points.
What is a superconductor?
A conductor with zero resistance at very low temperatures.
What is the superconducting temperature?
The temperature at which a material, when being cooled, suddenly loses all its electrical resistance, and becomes a superconductor.
What causes resistance?
Collisions between free electrons and the positive ions of the metal lattice.
What are some uses for superconductors?
Used in nuclear fusion, particle accelerators and MRI scanners.
What is a high temperature superconductor?
A superconductor for which the transition temperature is above the boiling point for liquid nitrogen (77K)
What are the factors of resistance of a wire?
- The length- if doubled, resistance doubled
- The area- if doubled, resistance halved
- The material of the wire
What is Ohms law?
The current flowing through a metal wire at a constant temperature is proportional to the potential difference across it.
If resistance increases, what happens to drift velocity?
It decreases.
How does resistivity change with different materials?
Conductors have small values of resistivity whereas insulators have high values of resistivity.
What are the advantages to using superconductors?
There is no resistance so no heat loss, which means that large currents can be maintained from 0 volts.
What are the disadvantages to using superconductors?
Many technical difficulties of achieving and sustaining low temperatures and materials being hard to manufacture and made into wire.
What is power and its unit of measurement?
The energy transferred per second (P=W/t) measured in Watts (Js-1).
How do we cool superconductors?
Using liquid nitrogen or helium.
What is the gradient of a current over voltage graph for a metallic conductor at constant temperature?
A straight line through the origin (Ohms law is obeyed).
What is the gradient of a current over voltage graph for a lamp filament?
For low voltages, the graph is straight but the gradient decreases smoothly as the pd increases.
What happens to a metal when temperature increases?
The electrons travel faster between collisions and the metal ions vibrate more, which leads to a decreased current for the same voltage and a higher resistance.
What are the three equations for power?
P= IV
P= I^2R
P= V^2/R
What are the units of resistivity?
Ohm meters (Ωm).
What is resistivity?
The resistance of the wire of the material of unit length.
