Peter's Physics Pages
Physics for Industrial Design with Peter Eyland
Note: The entire Physics for Industrial Design course (totalling 21 Lectures) is made up of the Physics for Building Management Course (14 Lectures) and the Physics for Industrial Design Course (additional 7 Lectures).
Two types of electric charge are needed are needed to explain electric attraction and repulsion.
Conductors have lots of electric charges that are relatively free to move around within the material.
Insulators, or Dielectrics, do not have many charges that move easily and so you can have an imbalance of charge at different places in the material.
Coulomb's Law: The size of the electric force between two charged particles is directly proportional to the product of their charges and inversely proportional to the square of their separation and acts along the line joining them.
Electric charges have fields around them.
Electric field lines start on positive charges and end on negative charges.
Electric Field Strength is in N.C-1
E is a vector and V is a scalar.
Equipotentials are surfaces of equal potential at right angles to field lines.
For an isolated charge in air:
For charged parallel plates in air:
Compasses are magnetised suspended needles.
Bar magnets have North and South Poles.
Like Poles repel and unlike Poles attract.
The Earth acts as if it has a big magnet inside it.
field lines start on positives and end on negatives
field lines are closed loops: out from N and into S
a tangent to a field line gives the direction of the force on a positive charge
a tangent to a field line gives the direction a compass needle will point
charges can exist singley as plus or minus
magnetic poles always exist in pairs
An electric current has a magnetic field associated with it.
Winding a wire into a helix forms the coiled tube called a solenoid.
An electromagnet is a solenoid with an Iron core.
Electromagnets are used for bells, relays and circuit breakers etc.
The force on a current in a magnetic field can be used to make motors, meters and loudspeakers etc.
: magnetic induction is measured by the force per unit charge and the speed of the charge at right angles to the field lines.
The magnetic force (Lorentz force) is: F = q.v x B
size: where q is the angle between v and B
direction: right hand screw rule rotating from v towards B.
Charges moving in magnetic fields tend to move in circular arcs, spirals and helices.
The force on a current element in a magnetic field is given by: dF= I.dl x B
The Hall effect is produced by a magnetic force pushing the charge carriers sideways, giving a potential difference VH = d.vD.B
The sign of the Hall potential difference indicates whether the (majority) carriers are positive or negative.
The Hall probe measures both the size and direction of magnetic induction.
The Biot Savart definition is:
Ampere's rule is:
Radially from a straight current:
At the centre of a circular current:
In a "short wide" solenoid:
In a "long narrow" solenoid:
The force/length between two long straight current carrying wires is:
The Magnetic Dipole Moment gives the turning effect of an external magnetic field on a current loop.
The Magnetisation of a material is the net magnetic moment per volume.
Ferromagnetism comes from the large increase in magnetisation when domains are aligned.
Hysteresis occurs because the domain re-organisation lags behind the changes in the applied field.
Paramagnetic materials are weakly attracted to a magnet.
Diamagnetism materials are weakly repelled from a magnet.
Temperature decreases magnetisation.
Magnetic Flux in T.m2:
Faraday's and Lenz's laws:
Alternating Currents are produced by rotating a conducting coil in a magnetic field.
Eddy currents deposit energy in large conductors when the magnetic flux through them changes.
Energy losses are minimised by transporting electricity at high potential then using transformers to reduce the potential and increase current at the load.
Transformers can be used to match load resistance so that there is maximum power transfer.
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