Electromagnetism Flashcards
Electric field
A force experienced by a charge due to an electric field
Electric fields between parallel plates
- Plates produce uniform electric field throughout the plate
Work in electric fields
w = Fd = Vq = 0.5mv^2
Effect of a charged particle in a Magnetic field
F = qvBsin@
Right Hand Push Rule
Thumb - Direction of conventional current (Proton’s velocity)
Fingers - Direction of magnetic field
Palm - Direction of force
Motor Effect
- When a current-carrying conductor is placed in a magnetic field, it experiences a force
- A moving charge with a constant velocity produces a magnetic field
- Electrons move as a stream in conductor with uniform velocity and produces a magnetic field surrounding the conductor
F = BIlsin@
Magnetic flux
A measure of the total magnetic field that passes through a particular area
- Faraday demonstrated that a change in magnetic field would result in an induced electromotive
% = BA cos@
Lenz’s law
An induced EMF always gives rise to a current whose magnetic field will oppose the original change in flux
Eddy currents
Special type of current that is induced when a metal plate experiences a change in magnetic flux
AC current
- Function by mutual induction where a changing current in one coil causes an induced EMF in the area of another coil
- AC current fed into primary coil. induces current in secondary
- AC current switches periodically to maintain changing magnetic field
Step-up Transformers
Increases secondary voltage compared to primary voltage
Step-down Transformers
Decreases secondary voltage compared to primary voltage
Power Loss in Transmission Lines
The resistance in the transmission lines causes an inefficient transfer of energy when the electrical energy transmitted is eventually transformed into heat energy
- To reduce: current must be reduced before entering the transmission lines
Limitations of Transformers
- Resistive heat production
Iron core experiences change in flux - Eddy currents induced
- Eddy currents in the iron core generates a significant amount of heat energy via resistance. Iron core heats up
- Inefficient transfer of energy (also fire hazard)
Lamination as a solution to limitations of transformers
Iron core laminated by insulation sheets, introducing electrical discontinuity in the core
- Reduces eddy currents formed inside the core
Other solutions to limitations of transformers
- Ferrite core can substitute iron core
}Good mag flux transmitters, but poor electrical conductors - Water & oil as coolants
- Fans to increase air circulation
- Heat sinks to disperse heat elsewhere
Electromagnetic Braking
- Applies the EM induction principle
- Metal wheels pass through magnetic field, eddy currents are produced in wheel
- Eddy currents are induced in a specific way to counteract the motion of the wheel
Advantages of EM Braking
- Little to no noise
- Smooth brake effect
Disadvantages of EM Braking
EM Braking is incapable of holding the transport system after coming to rest. Mechanical braking still required
DC Motors
- Device which converts electrical energy into mechanical energy
- DC current is fed through a coil in a magnetic field, which produces a rotation motion due to the motor effect
- Current changes direction every 180
DC Motor: Armature
The frame around which the coil of wire is around
Has an axis on which it can freely rotate
DC Motor: Coil
One or more turns of wire wound around the armature
DC Motor: Stator
Stationary/permanent/ electromagnet that provides the external, radial magnetic field around the coils
- Radial magnetic field ensures the sides of the coil are always travelling perpendicular to mag field
DC Motor: SPLIT-ring commutator
Device with two metal semi-circular rings that reverses the direction of the current flowing in each coil every 180
DC Motor: Brushes
Conducting contacts that connect the external circuit to the commutator
Torque in DC Motor
t = BIAsin@
Back EMF
The induced EMF produced in the coil of a motor due to its rotation in a magnetic field
EMFnet = V - EMFback
Generators
A device which converts mechanical energy into electrical energy by applying the principle of electromagnetic induction
AC Generator
SLIP Rings
- Two cylindrical metal conductors that rotate freely with the armature
- Constant electrical contact
DC Generator
SPLIT Rings
- Flux-time graph identical to AC
- EMF-time graph always in one direction
