Magnetic Fields Flashcards
Define a Magnetic Field
A field surrounding a permanent magnet or a current-carrying conductor in which magnetic objects experience a force
Describe Magnetic Field lines
Arrows point from north to south
Equally spaced and parallel magnetic field lines represent a uniform field - strength of the magnetic field does not vary
Field is stronger when the magnetic field lines are closer
Like poles repel and unlike poles attract
What happens when a wire carries a current
A magnetic field is created around the wire, the field is created by the electrons moving within the wire.
Any charged particle that moves creates a magnetic field in the space around it
What are magnetic field lines for a current-carrying wire
Concentric circles centred on the wire and perpendicular to it - right hand grip rule can be used to determine the direction of magnetic field
Thumb points in the direction of the conventional current
Magnetic field lines in a coil
North and south poles at the opposite faces
Refer to text book to check descriptions
What is the strength of magnets and magnetic fields measured in
Tesla (T)
What happens when a current-carrying conductor is placed in an external magnetic field
Two fields interact just like the fields of two permanent magnets, they experience equal and opposite forces
What happens when a current-carrying wire is placed between the poles of a magnet
The direction of the force experienced by a current-carrying conductor placed perpendicular to the external magnetic field can be determined using Fleming’s Left Hand rule
Flemings Left Hand Rule
Fuck - Force
Man - Magnetic Field
City - Current
What is the magnitude of the force experienced by a wire in an external magnetic field dependent on
Force is directly proportional to current, length, sin theta and strength of the magnetic field
F = BILsin(theta)
What is Magnetic Flux Density
Strength of a magnetic field
Measured in Tesla
Is magnetic flux density a vector or scalar
Vector
Motion of a charged particle in a magnetic field
Circular
Straight line once out of the field
Why does a current-carrying wire experience a force in a magnetic field
Each electron in the wire experiences a tiny force
What is the force on a charged particle in a magnetic field
Perpendicular to its velocity, therefore, has no effect on velocity
F = BQV
What can = force on an electron in a magnetic field
mv^2 / r
What is radius of a charged particles circular path proportional to
velocity and mass
What is radius of a charged particles circular path inversely proportional to
Magnetic fields, charge
How to induce an emf
A coil, magnet and sensitive voltmeter are needed.
Attach the voltmeter to a coil.
When the magnet is pushed towards the coil, an emf is induced across the ends of the coil
When the magnet is pulled away, a reverse emf is induced.
Repeatedly pushing and pulling will induce an alternating current.
Faster the magnet is moved, the larger the induced emf.
Explain EM Induction
Energy is always conserved.
Some of the work done to move the magnet is transferred into electrical energy. The motion of the coil relative to the magnetic field makes the electrons move because they experience a magnetic force Bqv.
The moving electrons constitute an electrical current within the coil, so the process has produced electrical energy
Define Magnetic Flux
Product of the component of the magnetic flux density perpendicular to the area and the cross-sectional area
Magnetic Flux = BAcos(theta)
Units = Wb
What is Magnetic Flux linkage
Product of the number of turns in the coil and the magnetic flux
When is an emf induced in a circuit
When there is a change in the magnetic flux linking the circuit
What is Faradays Law
Magnitude of the induced emf is directly proportional to the rate of change of magnetic flux linkage
The constant of proportionality is -1
What happens when a magnet is moved towards a coil with no voltmeter connected across it
If the magnet is moved towards the coil, the induced current is such that the end facing the magnet is north pole - work needs to be done in order to push the magnet towards the coil.
If the end was a south pole, then principal of conservation of energy would be violated, as the attraction between the coil and the magnet creating electrical energy out of nowhere
Work done is equal to the electrical energy produced in the coil
What happens when a magnet is moved away from a coil with no voltmeter connected across it
The motion of the magnet must once again be opposed so that you do work. The end the magnet is moving away from becomes the south pole
What is Lenz’s Law
Direction of the induced emf or current is always such as to oppose the change producing it
What is Lenz’s Law
The direction of the induced emf or current is always such as to oppose the change producing it
Explain how an AC generator works using Faraday’s Laws
Has a flux linkage
As the coil rotates at a steady frequency, the flux linkage changes with time t. This variation is sinusoidal and is caused by the changing cos(theta)
For a given generator, induced emf is directly proportional to rate of change in cos(theta)
What does a simple transformer consist of
A laminated iron core, a primary (input) coil and a secondary (output) coil
How do transformers work
An alternating current is supplied to the primary could
This produces a varying magnetic flux in the soft iron core
The secondary coil is linked by this changing flux
The iron core ensures that all the magnetic flux created by the primary coil links the secondary could and none is lost
A varying emf is produced across the ends of the secondary coil
Describe a step-up transformer
Has more turns on the secondary than on the primary coil
Describe a step-down transformer
Fewer turns on the secondary coil than on the primary coil
How can transformers be made efficient
Low-resistance windings help reduce power losses due to the heating effect of the current
Making a laminated core with layers of iron separated by an insulator helps to minimise currents induced in the core itself (eddy currents) - minimises loss due to heating
What is a velocity selector
Device that uses both electric and magnetic fields to select charged particles of specific velocity
Two parallel horizontal plates connected to a power supply produce a uniform electric field of field strength E between the plates.
A uniform magnetic field of flux density B is also applied perpendicular to the electric field.
Charged particles travelling at different speeds that are to be sorted enter through a narrow slit Y.
Electric and Magnetic Field Lines deflect them in opposite directions - only for particles with a specific speed v will these deflections cancel so that they travel in a straight line and emerge from a narrow slit Z
For an undeflected particle - Electric force = Magnetic force
EQ = BQv
v = E / B
