2.2 Resistance Flashcards
The potential difference between two points is
the work done, that is the loss of electrical potential energy per unit charge passing through the two points
A voltmeter
measures PD, it must be connected in parallel
Power means
rate of doing work or rate of transfer of energy
Work =
V x I x change in t
Power =
VI
Ohm’s Law
The current through a conductor is proportional to the PD across it. This law applies to metals at constant temperature.
Resistance =
PD across conductor/ current through conductor
Ohm=
VA^-1
Conductance, G =
current through conductor/ PD across conductor
Unit of conductance
Siemens, S
S=
AV^-1
Two examples of non-ohmic conductors
- Filament lamp
- LED
G=
1/R
P=
IV = I^2R = V^2/R
R=
(resistivity x length)/ CS area
Unit of resistivity=
Ohm metre
A wire’s resistance is proportional to
its length
A wire’s resistance is inversely proportional to
its cross sectional area
The conductivity is the
reciprocal of a material’s resistivity
A superconductor is
a material that below a certain temperature loses all its electrical resistance
Uses of superconductors
- Power transmission cables
- MRI scanners
- Electromagnets
There is a limit to the current that a superconducting wire can carry
because it gives rise to a magnetic field, and too great a magnetic field makes a superconductor ‘go normal’ even at very low temperatures
PRACTICAL: Investigation of the I-V relationship for a conductor
Starting from 0 increase the PD in steps then present the data on a graph (I vertical, V horizontal)
PRACTICAL: Determining the resistivity of the metal of a wire
Change the length of the wire using crocodile clips
Measure the resistance with an ohm meter
Measure the diameter with a caliper or a micrometer
Plot a graph (R vertical L horizontal) and calculate using resistivity = [(pi x diameter^2) / 4] x gradient
PRACTICAL: How the resistance of a metal wire depends on temperature
Put a waterproofed coil of wire in some water connected to a multimeter on ohms
Start with ice water, then room temperature, then boiling water etc.
Plot a graph
