Motors and Generators Flashcards
F=BILsin(theta)?
Force on a current carrying wire
-direction given by right hand palm rule
two parallel current carrying wires?
Current in same direction = attract
Current in opposite direction = repel
- due to magnetic fields around wires given by right hand grip rule
equation: F/L = k[(I1I2)/d]
torque?
turning force = Force x perpendicular distance between line of action of the force and the pivot (T=Fd)
torque on a current carrying coil in a magnetic field is given by: T= BANIcos(theta)
First Hand Investigation to demonstrate the motor effect:
Method:
1) Permanent magnets set up so opposite poles face each other with an electronic balance between them.
2) A wire attached in circuit to a power supply is put across the electronic balance, perpendicular to the magnets.
3) Power supply is switched on running a current through the wire.
Result:
Depending on the direction of the current the balance will either increase or decrease, either meaning the wire is experiencing a force, showing the motor effect.
Galvanometer:
Used to measure the magnitude and direction of small DC currents.
1) Connected in series so that current flows through the coil
2) Coil then experiences a force due to current flowing through a wire (coil) in a magnetic field (permanent radial magnet, ensures torque is constant). The torque produced is directly proportional to the current (T=BANIcos(theta))
3) Spring provides counter-torque so the coil and so the coil and connected pointer come to rest when the torque from the motor effect = the counter-torque
n. b. works in both directions to show direction of current.
Loudspeaker:
To convert electrical signal into sound.
1) AC passes through the voice coil. voice coil sits in a cylindrical magnet.
2) Voice coil then experiences a force due to the current flowing through a wire in a magnetic field (motor effect)
3) Voice coil is connected to the speaker cone so that the speaker cone moves with it, creating pressure waves in the air (sound).
4) Alternating current continually changes direction the voice coil continually pushed out and in. Pitch determined by frequency of current and volume by magnitude.
If youre reading this look a diagrams of AC and DC motors and generators
good work
-all are really similar except ac induction motor (different to normal ac motor)
Lenz’s Law:
Account for it using the conservation of energy:
Lenz’s Law states that an induced EMF(voltage)/current is always in a direction such that its magnetic field opposes the changing field that created it.
Lenz’s Law is a result of conservation of energy. We know that energy cannot be created or
destroyed and as you will soon see we need Lenz’s Law for this to be true for induction.
Suppose we have a changing magnetic field which produces a current, we know this will happen because of induction. But this current generated will also produce its own magnetic field. Now if this magnetic
field from the current is in the same direction as the original changing magnetic field, then an even
bigger magnetic field would be present and this larger magnetic field would in turn induce more
current, which would in turn produce a larger magnetic field and so on. As you can see this would
result in the current becoming larger and larger and larger, and the magnetic field becoming
stronger and stronger and stronger. This would mean that you get all this energy from nothing.
As this cannot happen, the induced current must be in such a direction that the magnetic field
produced by this induced current opposes the original magnetic field. This is what Lenz’s Law states.
Magnetic Flux?
Magnetic Flux Density?
Magnetic Flux = The total amount of magnetic field passing through a given area.
-Unit = Webbers (Wb)
= B x perpendicular area
Magnetic Flux Density = Magnetic field strength per m^2.
- In a diagram indicated by density of field lines.
- Unit = Tesla (T) or Webbers per m^2
Induction Cooktop:
Induction cooktops have a coil below the cooktop. AC power is supplied through this coil which
induced a changing magnetic field (DC power would not work as the magnetic field needs to be
changing). The changing magnetic field from the coil induces eddy currents in the metallic pan/pot.
Due to resistance, the eddy currents create heat which cooks the food. Because the heat is
produced from the resistance from the eddy currents, it works best with metal pots/pans that are
conductors (as larger eddy currents are induced).
Electromagnetic Brakes:
Eddy currents are used in electromagnetic breaking. The electromagnets will create a magnetic field
that cuts the spinning disc. Now because of this relative motion between the magnetic field and the
metal disc, by Lenz’s Law the spinning disc will want to generate a current to produce its own
magnetic field to oppose the magnets magnetic field. And so eddy currents will be created in the
spinning disc which create their own magnetic field that opposes the original changing magnetic
field. The repulsion of these two magnetic fields causes the spinning disc to slow.
perform a first-hand
investigation to demonstrate the
production of an alternating current
basically just set up an AC generator with a ammeter and turn the crank.
gather and analyse information to identify how transmission lines are:
– insulated from supporting structures
– protected from lightning strikes
– insulated from supporting structures
The transmission wires are separated from the tower by an insulator. Ceramic discs are mostly used.
– protected from lightning strikes
Sacrificial wires hang above the transmission lines, if lightning strikes then the current from the lightning will pass through these wire and then down to earth at the next power pole. The actual transmission lines remain unaffected.
Equations of Transformers:
Vp/Vs = np/ns = Is/Ip
V=IR
P=VI (i.e. conservation of energy as when one goes up the other goes down)
P(loss) = (I^2)R
Perform an investigation to model the structure of a transformer to demonstrate how
secondary voltage is produced:
Just make a transformer i.e iron core (square with middle cut out) with primary windings on one end and secondary windings on the other.
When AC flows through the primary coil it creates a changing magnetic flux in the secondary coil, which induces a current via faraday’s law. Voltage depends on input voltage and number of coils.
