What is magnetic flux?
Magnetic flux is the product of the area and the component of the magnetic flux density perpendicular to that area
What is one weber?
One weber is defined as the quantity of magnetic flux passing through an area of one square metre when the flux density normal to the plane of the area is one tesla
Is magnetic flux scalar or vector?
Scalar
Can magnetic flux be negative?
Yes because B can point in the negative direction
Can magnetic flux be zero even though B is non-zero?
Yes because when B // A, magnetic flux = 0
What is the formula for magnetic flux?
ϕ = B⊥A
What is magnetic flux linkage?
It is the product of the magnetic flux and the number of turns in the coil
What is the formula for magnetic flux linkage?
Φ = Nϕ = NBA
N = no. of turns
A⊥B
What does Faraday’s law state for a coil?
It states that the magnitude of induced emf of a coil is directly proportional to the rate of change of magnetic flux linkage through the coil
What is the rough meaning of induced emf?
Generation of a potential difference in a coil due to the changes in the magnetic flux through it
What changes magnetic flux linkage?
- N: number of turns
- B: magnetic flux density
- A: area
- Angle
What happens when magnetic flux linkage changes?
Emf is created and current flows
What is the equation for Faraday’s law?
EMF = - d/dx Φ (magnetic flux linkage)
= - d/dx (NBAcos(ωt))
What does Faraday’s law state for conductor?
Magnitude of induced emf in a straight conductor is direcly proportional to the rate of cut of magnetic flux by the conductor
What is Lenz’s law?
The direction of induced emf hence the direction of induced current produces effects to oppose the change causing it
Why is Lenz’s law a law of conservation of energy?
Work is done by external force against the repulsive force produced by the induced current. The work done is converted to electrical energy which is manifested as induced current
Why is energy dissipated in the resistor when a magnet is pushed into the solenoid?
A moving magnet gives rise to an increase of magnetic flux in the solenoid leading to an induced emf hence induced current in the solenoid. Induced current creates flux in the solenoid with a direction that opposes the increasing flux due to the motion of the magnet. Work done by external force is needed to push the magnet into the solenoid at constant speed. By conservation of energy, this work done is converted to thermal energy in the resistor
What is the working principle of a metal detector?
- transmitter coil in a metal detector has a varying electric current flowing through it which generates an magnetic field (outer coil)
- magnetic field generated by the transmitter coil induces a magnetic field in nearby metal targets, the induced magntic field is picked up by the receiver coil
Why when the switch is closed, the current doesn’t immediately change from zero to its maximum value?
- As the current increases with time, the magnetic flux through the loop due to this current also increases, the increasing magnetic flux induces an emf in the circuit that opposes the change in magnetic flux
- By Lenz’s law, the induced emf produces a current that is opposite of the current of the circuit
- The net potential difference across the resistor is emf of the battery minus the opposing induced emf
- As the magnitude of current increases, the rate of increase lessens and hence the induced emf decreases, this opposing emf results in a gradual increase in current
- this effect is called self-induction
What is the working principle behind an induction cooker?
- When high frequency alternating current passing through the coil, high frequency alermating magntic field will be produced and pierce through the frypan
- emf and hence eddy current will be generated in the pan, heating it up
What are eddy currents?
induced current circulating in solid conductors when there is a cutting of magnetic flux
What is the effect of eddy current?
eddy currents can generate significant amounts of heat and cause power loss
How to reduce the effects of eddy currents?
effective lamination which limits the amount of eddy current generated
