Magnetic Fields (Unit 4) Flashcards
Definition of the Tesla
1 Tesla is defined as the magnetic field which, acting on a wire carrying 1 A and placed perpendicular to the magnetic field lines, would exert a magnetic force of 1 N on each 1m length of the wire
Use Flemings Left Hand rule
First finger: magnetic Field (points from North to South)
SeCond finger: conventional Current (points from + to -)
ThuMb : Motion (direction of force)
Path of charged particle in a uniform magnetic field
Path is circular because,
Magnitude of force is constant but direction always points towards the centre of a circle
Magnetic force on a charged particle
There is no magnetic force on a charged particle if it is
- stationary
- moving ALONG a magnetic field line
Operation of a cyclotron
Two hollow D-shaped electrodes (Dees)
Uniform magnetic field applied perpendicularly to plane of Dees
Charged particles introduced into Dees near centre of cyclotron
Particles move in circular path due to the magnetic field
Alternating voltage applied across gap between Dees
As charged particle emerges from Dee it is accelerated across gap
Voltage reverses while charged particle moves in circular path through the other Dee.
Particle is again accelerated across gap.
The time taken for particle to move along a circular path through one of the Dees does not depend upon its speed (this time remains constant as particle speeds up)
Magnetic flux, units
Wb (Webers) OR Tm2 (Tesla metres squared) VECTOR
include symbol
Magnetic flux density, B, definition
the number of lines of magnetic flux passing perpendicularly through a cross-sectional area of 1m2.
Magnetic flux density units
T (Tesla) or Wbm-2 (Weber’s per metres squared) VECTOR
Magnetic flux linkage, definition
the product of the number of turns in a coil and the magnetic flux passing perpendicularly through the coil
(include symbol)
Magnetic flux linkage units
Wb turns (Weber turns) VECTOR
Faraday’s Law
the magnitude of the induced emf is equal to the rate of change of flux linkage
Induced emf units
V (Volts) or Wb s-1 (Weber per second) SCALAR
Lenz’s Law
the direction of the induced emf is always such as to oppose the change creating it
emf induced in a rotating coil
when coil rotates in a uniform magnetic field, an emf is induced because there is a constantly changing flux linkage through the coil.
Magnitude of induced emf
the gradient of a graph of flux linkage against time gives the magnitude of the induced emf
Relationship between coil position, flux linkage and induced emf
See sheet
Flux linkage and induced emf graph in a rotating coil
See sheet
Moving straight conductor in a uniform magnetic field
See sheet
Explanation of how a transformer works
- a changing current in primary coil
- produces a changing flux in the transformer core
- producing a changing flux linkage in the secondary coil
- which from Faraday’s Law
- induces an emf in the secondary coil
Operation of a transformer
Transformers only work with alternating current. Direct current does not produce a changing magnetic field in primary coil.
Step up transformer
Increases voltage (decreases current) more turns on secondary than on primary
Step down transformer
Decreases voltage (increases current) less turns on secondary than on primary
Causes of transformer inefficiencies
See table on sheet
Transmission of electrical power at high voltages
Using high voltages reduces current, and hence power loss through heating ( P = I2R). Resistance of wire is unchanged.
