6.3 Electromagnetism Flashcards
What is a magnetic field
A field surrounding a permanent or a current-carrying conductor in which magnetic objects experience a force
How are magnetic fields created
A moving charge
What does the arrow on a magnetic field line tell you
The direction in which a free North Pole would move - the arrow points from north to south
What does it mean if magnetic field lines are parallel and equally spaced
Represents a uniform field
How can you tell where the field is the strongest
When the magnetic field lines are closer
What are the rules of repulsion and attraction of magnetic poles
Like poles repel and unlike poles attract
What is the right hand grip rule
A way to determine the direction of the magnetic field
What does the thumb represent in the right hand grip rule
The thumb point in the direction of the conventional current
What do the fingers represent in the right hand grip rule
The direction of the field is given by the direction in which the fingers curl around the wire
How does the magnetic field pattern of the solenoid change if the direction of the current is reversed
The direction of the field is reversed
What happens when you place a current-carrying conductor in an external magnetic field
The two fields interact just like in the fields of two permanent magnets- they experience equal and opposite forces
What is Flemings left hand rule
If fingers are positioned perpendicular to eachother, the thuMb will represent Motion, the First finger will represent Field and the seCond finger represents Current
What does the magnitude of the force experienced by a wire in an external magnetic field depend on
Current, Length of wire in the field, sin θ (θ= angle between field and current direction), strength of field
When’s the force on a current carrying wire in an external maximum and when is it zero
The force is maximum when the wire is perpendicular to the field and zero when parallel to the field
State the equation for the force on a current carrying wire in an external field
F=BILsinθ (force= magnetic flux density x current x length of wire x sin(angle between field and current direction))
State the equation for the force on a current carrying wire perpendicular to the field
F=BIL
What are the units of magnetic flux density
Tesla (T)
How is magnetic flux density defined
The strength of the field
What are the SI units for magnetic flux density
Nm^-1A^-1
Is magnetic flux density a vector or scalar quantity
Vector
What happens when a charged particle moves in a magnetic field
The particle will experience a force
How can you determine the direction of the force on a charged particle moving in a magnetic field
Fleming’s left hand rule
why does the force on the beam of electrons in an electron gun with a visible screen is directed downwards
As the electrons enter the field, they experience a downwards force which always remains perpendicular to its velocity
Describe the path of a beam of electrons travelling perpendicular to a magnetic field
Electrons travel in a circular path in the region of the field then keep moving in a straight line once out of the field
why does the speed of the electrons remains unchanged when travelling perpendicular to a field
Because the force has no component in the direction of the motion
State the equation for the force on a charged particle travelling perpendicular to a magnetic field
F=BQV (force= magnetic flux density x charge x speed of charged particle)
What’s the expression for the radius of the circular path taken by charged particles moving perpendicular to a field
r=mv/BQ (radius= mass x velocity/ magnetic flux density x charge)
do faster moving particles move in bigger or smaller circles
faster moving particles travel in bigger circles (r α v)
do more massive particles move in bigger circles
more massive particles move in bigger circles (r α m)
do particles with a greater charge move in smaller or bigger circles
particles with a greater charge move in smaller circles ( r α 1/Q)
do stronger magnetic fields make the particles move in smaller or bigger circles
stronger magnetic fields make the particles move in smaller circles (r α 1/B)
what equations are useful when solving problems including charged particles in circular paths
F=BQv
F=ma
F=mv^2/r
F=2πr/T
why is a velocity selector required in a mass spectrometer
so that the atoms emerge with the same speed v before entering a uniform magnetic field
describe how a velocity selector works
-two parallel plates produce uniform electric fields
- uniform magnetic field is applied perpendicular to the electric field
- charged particles enter through a slit
-if they travel at the right speeds the deflections cancel and they emerge from the second narrow slit
what is electromagnetic induction
electromagnetic induction occurs whenever there is a change in the magnetic flux linking the circuit
describe what happens when a magnet is pushed towards a coil of wire
an emf is induced across the ends of the coil
describe what happens when a magnet is pulled away from a coil of wire
a reverse emf is induced
what happens when a magnet is repeatedly pushed and pulled in a coil of wire
an alternating current will be induced in the coil- the faster the magnet is moved the larger the emf is
how is energy conserved when a magnet is pushed and pulled in a coil of wire
some of the work done to move the magnet is transferred into electric energy
define magnetic flux
the product of the component of the magnetic flux density perpendicular to the cross sectional area
explain why a changing magnetic field induces a current in a wire
the motion of the coil (and the electrons in it) relative to the magnetic field makes the electrons move because they experience a magnetic force given by BEv. the moving electrons constitute an electrical current within the coil
how is magnetic flux calculated
Φ=B x A
Φ=B x A cosθ
whats the SI unit of magnetic flux
Wber (Wb) -> Tm^2
what is the value of the magnetic flux when the magnetic field is normal to the area being considered
B x A
what is the value of the magnetic flux when the magnetic field is parallel to the area being considered
zero
define magnetic flux linkage
the product of the number of turns in the coil N and the magnetic flux Φ
what can you change to induce an emf
B A θ
whats the difference between magnetic flux density and magnetic flux
magnetic flux density is the strength of the field whereas magnetic flux density is the product of the magnetic flux density perpendicular to the cross sectional area
state Faraday’s law
the magnitude of the induced emf is proportional to the rate of change of magnetic field linkage
state Lenz’s law
the direction of the induced emf is always such as to oppose the change producing it
Whats Faradays law in the form of an equation
ε α ΔNΦ/Δt
whats the equation combining faradays law and lenzs law
ε = -ΔNΦ/Δt
whats the purpose of a transformer
change alternating voltages to higher or lower values
what does a transformer consist of
a laminated iron core, a primary (input) coil and a secondary (output) coil
how does a transformer work in terms of changing magnetic flux
-an alternating current is supplied to the primary coil which produces a varying magnetic flux in the soft iron core -the secondary coil is linked by the changing flux. -the iron core ensures that all the magnetic flux in the primary coil links the secondary coil and none is lost.
what does faradays law tell us about transformers
a varying emf is is produced across the ends of the secondary coil
state the turn-ratio equation
ns/np=Vs/Vp for an ideal transformer
what is a step up transformer
a step up transformer has more turns on the secondary than the primary coil and Vs>Vp - increases the voltage
what is a step-down transformer
a step-down transformer has fewer turns on the secondary than on the primary coil and Vs<Vp - decreases the voltage
what is the relationship between current and voltage in a 100% efficient transformer
VsIs=VpIp or Ip/Is=Vs/Vp
what does it mean if a transformer is 100% efficient
the output power from the secondary coil is equal to the input power into its primary coil
why are transformers not 100% efficient
power loss due to the heating effect of the current
which design features of a transformer increase efficiency and why
-low resistance windings
-making a laminated core with layers of iron separated by an insulator helps to minimise currents induced in the core itself
-core is made out of soft iron whidh iw very easy to magnetise which helps overall efficiency of the transformer
