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)
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.
how transformers work
In a transformer, the changing magnetic flux is provided by the alternating current.
• Alternating current changes in both magnitude and direction.
• As the magnitude and direction of the current (moving electric charges) changes, this induces a magnetic field that also changes in magnitude and direction.
ΔΦ
• This results in the magnetic flux (Φ) also changing in time ( ⁄∆𝑡) allowing an emf to be
generated in the secondary coil.
• The proportion of number of turns in the coils is the same as the proportion of potential difference across the coils.
lenz’s law in transformers
The direction of current induced in the secondary coil, opposes the changing flux it experiences (Lenz’s law)
faraday’s law in transformers
If the two coils in a transformer have a common iron core, the flux through both coils will be identical. Thus: Bothcoilshavethe same rate of change of flux
step up transformer
When the number of turns on the secondary coil is GREATER than the primary, the transformer is said to be a STEP-UP transformer. (Vs > Vp)
step down transformer
When the number of turns on the secondary coil is LESS than the primary, the transformer is said to be a STEP-DOWN transformer. (Vs < Vp)
the current in the secondary coil will be large and, to avoid heat losses due to the large current, a thicker wire is used in the secondary coil.
ideal transformers
100% efficiency
For an ideal transformer, input power is equal to output power
factors affecting the operation of a transformer
Seeing as the transformer operates using Faraday’s law, factors that influence its operation can be discussed:
• The rate of change of magnetic flux can be increased by increasing magnetic flux (Φ = 𝐵𝐴𝑐𝑜𝑠𝜃) by:
o Increasing the strength of the magnetic field (magnetic flux density) – B
o by increasing the magnitude of the alternating current.
• By using a soft iron core which is easily magnetised and enhances B.
• The angle of the coil facing the field does not change and is irrelevant.
• The main factor is to increase the number of turns (N) in the secondary coil as emf ∝ N
convert AC to DC
Alternating current can be converted into direct current using a diode, or more specifically a series of diodes arranged in a specific way in what is known as a bridge rectifier. These components are built into electrical devices to solve the problem.
rectification
the conversion of AC to DC.
half wave rectification
a single diode only allows for half wave rectification
diode placed in circuit w/ an AC supply in series, Current only flows in one direction (only positive peaks are picked up). Only half the AC cycle is used )
full wave rectification
achieved using a bridge rectifier which is a series of four diodes connected
results in both halves of the AC being used.
where does a magnetic field exist
around a permanent magnet or a current-carrying conductor
magnetic field lines direction
away from north and into south around a magnet, from south to north within a magnet
determine direction of magnetic field
right hand grip rule
thumb - current
(conventional -flow from positive to negative)
・- out if the oage
x - into the page
fingers- direction of field (N-S)
determine direction of magnetic field
right hand grip rule
thumb - current
(conventional -flow from positive to negative)
・- out if the oage
x - into the page
fingers- direction of field (N-S)
solenoid
multiple looped conductor (coil)
magnetic fields develop around each turn of the wire.
magnetic fields interact w each other to produce a north and south pole
field inside - concentrated and uniform
field outside- weak and divergent
direction of field in solenoid
RIGHT HAND SOLENOID RULE
thumb- north
fingers - current
direction of field in solenoid
RIGHT HAND SOLENOID RULE
thumb- north
fingers - current
motor effect
a current carrying conductor will experience a force if placed in a magnetic field
field strengthened- 2 fields act in same direction
field weakened- 2 fields act in opposite directions
determine direction of Force created by interaction of 2 fields
FBI leFt hand motor rule
F- Thumb- force
B- second finger- field direction
I- middle finger - current
maximum force ( current and field direction at 90 degrees/perpendicular)
factors affecting magnitude of the electromagnetic force
- stronger magnets- stronger field- greater force
-greater current- greater force - I + B are perpendicular- theta is 90 therefore force is maximum
magnetic flux density
B
magnetic field strength
measure of strength + direction of magnetic field at a given point in space
motor set up
coil of wire ( through which current flows) free to rotate in a magnetic field
would rotate ti 189 then flip back
uses split ring commutator to ensure continuous rotation
split ring commutator
used in motor
swaps the negative and positive connections every half turn so that current always flows around in same direction therefore force is in same direction therefore - continuous rotation
structure of dc motor
Cells:
Provides direct-current and determines the polarity of the brushes.
Note you must consider conventional current i.e.
from +, through the circuit, to -.
Coil:
A conductor that carries the current through the magnetic field.
Brush:
Conducts current to and from the split ring commutator while allowing the commutator to turn. The polarity of the brushes is fixed.
Split-ring commutator:
Reverses the direction of the current every half a rotation to ensure the armature continues to rotate in the same direction.
Thus ensures one direction of current in the field.
Armature:
The coils/s, axis and commutator together form the armature of the motor.
factors influencing Φ
B- Stronger magnets - greater magnetic flux density
change Area- greater area enclosed by loop- greater Φ able to go through
change angle of surface- maximum at 0 or 180, 0 at 90 (As the coil becomes more parallel to B, the magnetic flux (𝜙) decreases. )
angle- angle between the magnetic flux density (B) and the normal to the loop of the area (A)
ferromagnetic
can be magnetised but does not hold its magnetism
iron, nickel, cobalt some alloys
