magnets Flashcards
magnetic field
any region in space where magnetic forces can be felt. the direction of the magnetic field at a point is the direction of the force on a north pole if it were placed at that point
magnetic field line
a line drawn so that the tangent to it at any point shows the direction of the magnetic field at that point
force on current carrying conductor in a magnetic field
will always experience a force unless the conductor is parallel to the magnetic field
direction of the force is always
-perpendicular to current
-perpendicular to magnetic field
magnetic flux density (B)
F = ILB
B at a point in a magnetic field is a vector whose magnitude is equal to the force that would be experienced by a conductor of length 1m carrying a current of 1A at right angles to the field at that point and whose direction is the direction of the force on a north pole placed at that point
unit: Tesla (T)
force on a moving charge in a magnetic field
F = qvB
q = charge
v = velocity
the ampere
the constant current which, if maintained in two straight parallel conductors of infinite length, of negligible cross section and placed 1m apart in a vacuum, would produce a force on each conductor of 2*10^-7 newtons per metre of length
the coulomb
the amount of charge that passes any point in a circuit when a current of 1A flows for 1s
electromagnetic induction
whenever the magnetic field passing through a coil changes an emf is induced in the coil. if the circuit is closed an induced current will flow
magnetic flux (Φ)
Φ = BA
a = area
b = mag. flux density
scalar
unit: weber (Wb)
faraday’s law
states that the size of the induced emf is directly proportional to the rate of change of the flux
faraday’s law equation
E = (final flux - initial flux)/time taken
E = NdΦ/dt
N = no. of coils
Lenz’s law
states that the direction if an induced current is always such as to oppose the change producing it
e.g. if north pole brought towards coil current will flow anticlockwise (north pole) to repel magnet
law follows from the principle of the conservation of energy
generators
device that converts mechanical energy into electrical energy e.g. elec. power stations
rotate a coil in a magnetic field which induces emf and current
mutual induction
where changing the magnetic field in one coil causes an induced emf to appear in a nearby coil.
size of the emf can be increased by
-having coils near each other
-winding coils together on soft iron core
-increasing no. of coils
transformers
device used to change the value of an alternating voltage
-consists of 2 coils, primary and secondary, connected by a soft iron core
step down transformer
if the no. of primary turns Np > than no. of secondary turns Ns then output voltage Vo < input voltage Vi
step up transformer
if Np < Ns then Vo > Vi
transformer eqns
Vi/Vo = Np/Ns
ViIp = VoIs
uses of transformers
- power stations step up/down voltage leaving station and entering houses
- computers, tvs, radios etc. use transformers as different parts of the appliance need high or low voltages
self induction
this occurs whenever the current passing through a coil changes. when the current changes the magnetic field changes causing an emf to be induced to oppose the changing current (back emf).
a coil which has the property of self induction is often called an inductor.
alternating current
this is where the flow of current is constantly changing direction
e.g. 50Hz, each electron goes a full cycle in 1/50 of a second
rms voltage
Vrms = Vo/root 2
where Vo = max/peak voltage
alternating voltage
in a 50Hz system the voltage has a full cycle between +/- 325V in 20ms.
the electrical energy per sec on average is equivalent to 230V d.c. supply (root mean square value)
a.c. across pure resistor
if an alternating current is applied across a pure resistor (no induction or capacitance) then:
instantaneous current = voltage at that instant/ resistance
i.e. I = V/R
