Magnetism Flashcards
Magnet - properties
a body with magnetic properties:
1) exerts forces on other magnets that cannot be explained
2) has two poles pointing in opposite directions
3) like poles repels and opposite attract
Magnetic moment
MIU or mu or m or μ
describes the strength of a magnet
Magnetic field
A model claiming that magnets creates a magnetic field around themselves and exert forces by this field on one another. Happens over long distances as well as short.
Field lines
Visualization of structure, direction and strength of a magnetic field. The density of field lines describes the density of the magnetic field.
Nuclear magnetic resonance imaging (MRI)
Exploits the fact that elementary particles acts as tiny compasses/has magnetic properties, and lines up with field lines.
A magnetized body
The body attains a non-zero net magnetic moment
Comparison of electric charges and magnetic poles
Same:
like repel
opposite attract
Difference:
the opposite electric charges can be separated, the magnetic poles cannot be separated. - Like a worm when cut in two.
the structure of electric and magnetic fields will be different because of this fact. Magnetic field lines are closed loops (field lines end at the same pole they originate from). Electric field lines start at a positive charge and ends at a negative charge.
Magnetic flux density
symbol: B, unit: T, tesla
a physical quantity characterizing the strength of a magnetic field (e.g: density of the lines) and the forces acting on a magnet in an external magnetic field ( another magnets field)
Magnetic flux density in a homogenous magnetic field
direction and magnitude of magnetic flux density is the same everywhere
Interaction between a magnet and an external magnetic field
m*B
Strength of the interaction depends on
1) The magnetic moment of the magnet
2) The magnetic flux density
The forces acting on the magnet rotates it to align it with the field lines.
Advantageous state of magnet in external magnetic force
At lower energy -> when the magnetic moment is aligned with the field lines.
Magnetic effect of electric current
Electric current flowing through a conductor induces a magnetic field. The magnetic flux density is proportional to the electric current.
Electromagnet
A coil with electric current passing through it. The coil is a conductor wound up, to increase the flux density and create a more concentrated field. An electromagnet creates a strong homogenous magnetic field.
Electromagnetic induction
Because moving charges creates a magnetic field, moving magnets creates magnetic fields too. (Faraday observations).
Electromagnetic induction occurs when a magnetic field that changes over time and an electric field is created.
Lenz’s law
The induced voltage and current will act against the phenomenon creating it. Thus, electric energy can be created without investing work.
What happens with the voltage when the change in the magnetic field is fast and big?
Greater induced voltage
Self-induction
Induction of voltage in the same coil in which the current causes a change in the magnetic field. Occurs every time a circuit is turned on or off. It delays the change of current flowing through the circuit.
LC-circuit/Resonant circuit
Consistant of: coil and capacitor. (No ohmic resistor)
A charged capacitor may arise a voltage oscillation in the circuit.
Half period of oscillation in a LC circuit
Initially: max voltage, max charge, no current, no magnetic field
Discharging: current flows from + to - plate, which induces a magnetic field (B) in the coil. A gradual build-up of B and current proceeds, it is gradual because Lenz’s law hinders changes (Delay of changes). Capacitor becomes empty, voltage is zero. B and current is max.
Re-charging: B and current decreases (gradually according to Lenz’s law). Capacitor is recharged with opposite polarity.
Finally: max voltage, max charge, no current, no magnetic field
-> The change happens periodically
Electromagnetic oscillations
charge, voltage, electric flux density and energy of the capacitor changes periodically. the electric and the magnetic changes are coupled and are therefore called electromagnetic oscillations.
(Booklet) A magnet is placed in an external magnetic field. By what factor would the strength of the interaction between them increase if both the magnetic flux density and the magnetic moment is doubled.
Strength of the interaction would increase by 4, because of the formula m*B
(Booklet) What is the phenomenon “electromagnetic induction”?
Creating an electric field with changing magnetic field.
(Booklet) Self-induction is…
when a voltage is induced in a coil because of the changing current flowing through it.
(Booklet) An ideal resonant circuit is composed of…
Coil and capacitor
(PP2016)
Compare electric and magnetic interactions. Which statement is correct?
A: Like electric charges attract but like magnetic poles repel
B: While electric charges may exert either attractive and repulsive forces, magnetic forces may only exert attractive
C:Electric charges can be isolated from each other but magnetic poles cannot.
D: Magnetic poles can be isolated from each other but electric charges cannot.
C
