Chapter 15 Flashcards
Define Faraday’s Law
Faraday’s law of electromagnetic induction states that the induced emf in a circuit is proportional to the rate of change of total magnetic flux through the circuit
What is the equation from Faraday’s law?
V=-d(phi)/dtwhere phi is the flux linkagealso emf, E=-Nd(phi)/dtwhere N is number of turns and phi is the flux density
Define Lenz’s law
The direction of the induced current in a circuit is always such as to oppose the change that causes it
Define magnetic flux
A term that refers to the lines of force. The lines of force are known as lines of flux, so we get the term flux density.
How do you calculate magnetic flux?
phi=BA where B is magnetic flux density and A is area swept out
How do you calculate flux linkage?
Phi=BAN=phiN where phi is magnetic flux and N is number of turns
How do you calculate the induced voltage when a rod of length L moves at 90deg. to magnetic field lines at speed v?
V=BLv
How are magnetic fields produced?
By current-carrying conductors or permanent magnets
What is a magnetic domain?
A region in a magnet with uniform magnetisation
How do you create a magnet?
Melt a ferrous metal slightly and tap it so all magnetic moments of the atoms align
How do you demagnatise a magnet?
Hit it really hard or heat and cool it quickly
How do you draw a wire, current going into the page?
Circle with a cross in it
What are the rules of drawing field lines?
Arrows go from north to southCan’t cross (vector quantity so just get resultant)Stronger fields when field lines are closer together
How do you tell the poles on a soleroid?
Clockwise current is north; anticlockwise current is south
What are the units of flux and flux density?
Flux: weberFlux density: Testler
How are current and flux related?
As current changes, so does flux as current-turns drive flux
How are flux and induced emf related?
emf is the gradient of the flux graph
What is the transformer equation?
Vs/Vp = Ns/Np = Ip/Is
How are the voltage in the primary coil of a transformer and voltage in the secondary coil related?
Antiphase relationship
How are current and voltage related?
Voltage is differential of current
How does a transformer work?
Alternating current in the primary coilProduces a changing magnetic field in the secondary coilMutual induction:Alternating pd across primary coilProduces an alternating fluxIron core increases flux and guides it to the secondary coilSecondary coil experiences changing flux which induces an emf
How are the currents related in the coils of a transformer?
Antiphase relationship
What are the 2 equations for permeance?
Capital lamda, ^, permeance = flux/current turns = phi/NI^ = permeability*area/length =muA/L
What is the left hand motor rule?
First finger field; second finger conventional current; thumb force
What is a catapult force?
The force experienced by a wire when placed in a magnetic field. Initially, field lines are distorted, but wire moves to make them straighter and shorter
How do you calculate a catapult force?
F=BIL where B is magnetic field strength, I is current and L is length of conductor
What is the right hand generator rule?
First finger field; second finger conventional current, thumb motion
What is back emf?
The emf due to the generator effect
How is the power drawn split in a motor (equation)?
VI=E(back)I + I^2R where VI is power drawn, E(back)I is power to move current against back emf and I^2R is power lost to thermal energy
What is the equation for calculating the emf at a specific rotation (angle from horizontal to perpendicular to coil)?
E=NBAwsin(wt)where N is no. turns, B is flux density, A is area, w is actually omega is is angular frequency and t is time.
At what rotation is the maximum emf achieved?
When coil is parallel to field lines
How is angular frequency calculated?
w=d(theta)/dt or w=2(pi)/T=2(pi)f where T is time period and f is frequency
How do you calculate the maximum emf in a coil?
V=2(pi)f*peak flux linkage
