electrodynamics Flashcards
how does split ring commutator work
This component allows the arm of the coil on a particular side in the magnetic field, to always have current flowing in the same direction and hence to always experience a force in the same direction. This allows for continuous rotation of the coil. there will be many coils of conductor within the motor to increase the force it generates. These coils are all at different angles to each other ensuring that the motor maintains continuous and smooth torque during operation.
In other words: it reverses or alternates the direction of current in the coil of the motor every 180° (or half revolution).
In order to work, it must have a split in the ring, so that the current has to pass through the coil. The split in the ring also allows for no current to flow through the coil when it is in its vertical position in the field. With no current flowing in the coil momentarily, there is also no force acting on the coil. The momentum of the coil ensures that it continues to rotate momentarily. Once past vertical, current is restored, and a force is exerted once again.
motor
converts electrical energy into mechanical energy
electromagnetic induction
Moving a wire in a magnetic field or moving a permanent magnet into and out of a small coil causes an emf (electromagnetic force) to be induced. emf is only induced when there is relative motion between the conductor and the magnetic field.
the direction of the induced emf opposes the change
producing it
magnetic flux linkage
the product of the number of turns on the coil and the flux through the coil (NФ)
Magnetic flux (Φ)
measurement of the total magnetic field which passes through a given area.product of the magnetic flux density (B) and the perpendicular area (A) that
the field penetrates.
faraday’s law
the emf induced is directly proportional to the rate of change of magnetic flux (flux linkage)
faradays law explained
The greater the rate of change in flux, the greater the induced emf.
All factors inherent in magnetic flux can affect the induced emf, but emf is directly proportional to the rate of change of magnetic flux, and hence, the speed at which the flux changes, is also a factor for consideration.
Finally, an induced emf causes a current to flow in a conducting coil. Induced emf is directly proportional to induced current according to Ohm’s law: V = IR.
lenz’s law
the induced current flows in a direction so as to set up a magnetic field to oppose the change in magnetic flux
lenz’s law explained
According to Lenz’s law, magnetic flux through a coil must remain constant. To achieve this, the induced emf in a coil will generate a current that will have its own magnetic field. (direction- use right hand solenoid rule). This magnetic field will oppose the magnetic field of the magnet, and change to that magnetic field, which is inducing the current. will oppose the magnetic field of the magnet, and change to that magnetic field, which is inducing the current.
flemmings right hand rule
find induced current ( faraday’s law)
electrical generator
diode
a component that only allows current to flow in one direction
generator
converts mechanical energy into electrical energy.
A practical AC generator produces alternating current in its coil and transfers AC to the external circuit.
structure of ac generator
Turbine : Spins the coil in the field – usually due to steam, wind, hydro etc.
Coil/Armature: Rotated in the field. The coil cuts the magnetic field of the magnets as it rotates and thus induced current flows. It carries the induced current through the magnetic field.
Brush: Conducts current to and from the slip rings while allowing the slip rings to turn. Each brush is always associated with the same slip ring, therefore, the polarity of the brushes alternates.
Slip Rings: To transfer the alternating induced current in the coil to the external circuit by maintaining the coil’s connection to the external circuit.
sinusoidal nature of flux + ac generator rotation
rate of change of change of flux determined by gradient of that line
rotation- diff gradients - max + min
As the coil turns, there is a rate of change of flux in the coil – i.e.: the magnetic flux changes over time as the coil is moved in the field
gradient = 0 - 0 emf
gradient = negative + steepest produce positive max emf
gradient = positive + steepest produce negative max emf
(because of -sign of lenz’s law)
emf
Emf is only induced in a situation where the flux is changing in time (rate of change of flux). This is achieved by moving the coil in most cases.
• Hence:
o Emf is a maximum when the arms of the coil are moving perpendicularly across magnetic
field lines (maximum rate of change of flux linkage).
o Emf is a minimum when the arms of the coil are moving parallel to the magnetic field lines
and not actually cutting across them (minimum rate of change of flux linkage).
o Emf is positive when the rate of change of flux linkage (gradient of the magnetic flux line)
is negative (and vice versa).
mutual induction
allows an alternating current in one coil (called the primary (1°) coil) to induce an alternating current in a neighbouring coil (the secondary (2°) coil).
The magnetic field set up around the primary coil varies as the alternating current varies. This creates a changing magnetic flux which is received by the secondary coil. An emf is induced in the secondary coil as a result (Faraday’s Law). Thus, an induced current flows in the secondary coil to set up a magnetic field which opposes the change in flux it experiences (Lenz’s Law) as shown below.
only work on alternating current and will not work on direct current. This is because direct current gives a steady, unchanging magnetic field so NO emf would be induced in the secondary coil.
mutual induction in wireless charging
the charger has an AC input, producing a field that is received by a coil in the target device, such as a cell phone or electric toothbrush, and which induces a current for chrging in that device.
transformers
devices that transfer electric energy from one alternating-current circuit called the primary (1°) circuit to another circuit called the secondary (2°) circuit, either increasing (stepping up) or reducing (stepping down) the potential difference.
used extensively in electricity networks, as well as for many devices in our homes. For instance, charging devices for laptops and other electronics, use transformers to step down the potential difference to meet the requirement of the device. Transformers are also used to step up potential difference so that electricity can travel further with less energy loss.
