Chapter 25 Flashcards
What did Faraday Discover?
Faraday discovered that there is a current when you push a magnet into a coil, but holding it there had no effect
- this caused the needle on a compass to deflect in the opposite direction
When is there current through a coil of wire?
There is current through a coil of wire, if and only if the magnetic field passing through the coil is CHANGING
what is an electromagnetic induction?
creating and electric current by changing a magnetic field
what is an induced current?
the current in a circuit due to a changing magnetic field is called and induced current
what is an induced emf?
the circuit acts as if an emf were connected to it for a short time; this is called and induced emf
what is magnetic flux?
the induced emf is caused by a change in quantity called the magnetic flux
Φb = B*A = BAcosθ
What is Lenz’s Law?
The induced current creates its own induced magnetic field, to determine the direction of the induced current we use Lenz’s Law
THE DIRECTION OF THE INDUCED URRENT IS SUCH THAT THE INDUCED MAGNETIC FIELD OPPOSES THE CHANGE IN FLUX
What is Lenz’s Law: The magnet is at rest?
- creates a flux downward through the coil
2. there is no change in flux, therefore there is NO current
What is Lenz’s Law: The magnet is moving down?
- magnet moving downward increases the downward flux through the coil
- loop generates upward pointing field to oppose flux
- counter-clockwise current is needed to induce upward magnetic field
What is Lenz’s Law: The magnet moving up?
- magnet moving up decreases the downward flux through the coil
- loop generates downward pointing field to oppose flux
- clockwise current is needed to induce downward magnetic field
What is Faraday’s Law?
- An emf is induced in a conducting loop if the magnetic flux through the loop changes. If the flux changes by ΔΦ during the time interval Δt, the magnitude of the emf is:
V = ΔΦ/Δt
What is an Electric Motor?
Electricity + Magnetic Field = Makes things turn
What is a Generator?
Turning Loop + Magnetic Field = Electricity
What is Motional Emf?
- A bar moving in a magnetic field, causes charges to separate and accumulate at each end of the bar producing an induced Electric field
- The separation continues until equilibrium is reached
qvB = Eq or ΔV = EL = vBL or I =ΔV/R = vBL/R
What is Magnetic Drag?
The current carrying bar in a magnetic field experiences a force, Magnetic Drag: will cause the wire to slow down and stop unless and equal and opposite force pulls the bar
F = vL^2*B^2/R
What is Mutual Induction?
Inductance between the two separate nearby coils is called mutual induction
What is Self Induction?
Self-induction occurs when the changing flux from the circuit arises from the circuit itself
What is Gradual Increase?
If the current in the solenoid is increasing then so is the flux inside the solenoid. The induced field will oppose the increase in flux
What is Gradual Decrease?
If the current in the solenoid is decreasing then so is the flux inside the solenoid. The induced field will oppose the decrease in flux
What is an Inductor?
A device designed to produce a specified magnetic field
Eg: Ideal Solenoid - is the most basic inductor
B = UoIN/L
Φ = NBA = N^2UoAɪ/L
V = ΔΦ/Δt = N^2Uo*A/L(Δɪ/Δt)
What is Inductance (L)?
measures how much a solenoid opposes change in a current
V = -L(Δɪ/Δt)
SI Unit: Henry (H); 1H = 1Tm^2 = 1Vs/A
The Inductance of an Ideal Solenoid is?
L = UoN^2A/L
RL Circuits: Increasing Current?
Current gradually increases towards its maximum V/R value
I(t) = V/R(1 - e^-t/τ) or τ = L/R
RL Circuits: Decreasing Circuits?
Current stops flowing but the inductor opposes the change in current
I(t) = V/R(e^-t/τ)
Block on a spring and LC Circuit formulas respectively:
f = (1/2π)√k/m and (1/2π)√1/LC
