Section 2: Resistance Flashcards
Energy transferred (work done)
W=VQ We know that Q=IT Sub W=VIT W=Pt
Volt
The number of joules per coulomb of charge
Resistance
R=V/I units = ohm
Ohm’s Law
The current flowing through a metal wire at a constant temperature is directly proportional to the p.d. Across it (only if R is constant)
Resistance of a wire
Length = 2xl, 2xR
Area = 2xA, 1/2xR
Material
Resistivity
It’s a property
Resistance of a wire of the material of unit length
Remains constant for a specific material regardless of changing length or area R=pl/A
Calculating Resistivity
1) measure length and resistance
2) diameter using a micrometer
3) Resistivity = pi r squared etc.
Variation of resistance with temperature
Temp increase, free electrons have more collisions w/ ions and random vibration energy transferred therefore higher resistance
Superconductivity
Metals cooled to v. Low temps and their resistance disappears
Cooled using liquid helium or nitrogen
Transition (critical) temp
The temperature at which a material, when cooked, loses all its electrical resistance and becomes superconducting
High temperature superconductors
Don’t necessarily need to be as cold as others
Example: ceramic materials
In electromagnets
Superconducting wire, no loss of current and can carry larger currents
(Also generators and motors)
In particle accelerators
Particles need a strong magnetic field to go around corners
Superconducting magnets can do this
In MRI machines
Help produce images of soft tissue without being invasive
Has been used since the 1970s
Electrical power
The energy transferred per second
