7.3/4 Magnetic fields + Electromagnetism Flashcards
What’s Magnetic flux?
‘Magnetic Flux’ is the amount of flux that passes through a given area
‘Flux’ is the flow or the amount passing
So Magnetic flux measures how much of the magnetic field passes through a surface
ϕ=BA
ϕ is the Magnetic flux (Weber (Wb))
B is the magnetic flux density (T)
A is area (m^2)
What’s Magnetic flux density?
The strength of the magnetic field
The density by the magnetic field lines, or magnetic flux density at that point
Magnetic Flux density, B = Force/ Current x Length of wire
B=F/IL
F=BIL
Where Magnetic Flux density, B is measured in Tesla, T
Where F=Magnetic Force
What is a magnetic field?
A magnetic field is a region where a magnetic force is experienced by a moving charge or a magnetic material (e.g., iron)
What are the 4 metals with magnetic properties?
Steel
Iron
Cobalt
Nickel
What happens if you put a metal with magnetic properties in between a magnet?
The metal gets magnetised and will have its own North and South pole
What happens to magnetic flux when the coil is rotating?
The magnetic flux varies, as the area will not pick up the magnetic flux at at some angles
It is a maximum when the magnetic field lines are perpendicular to the coil area
It is at a minimum when the magnetic field lines are parallel to the coil area
What happens to the magnetic flux, φ when the area isn’t perpendicular to the magnetic field lines?
The angle, θ is measured from if the magnetic field lines are perpendicular to area, A
φ = BAcosθ
So the bigger the angle the less magnetic flux passing through the area
What does induced mean?
A magnet and a coil of wire can be used to produce an electric current.
So an induced emf is a potential difference made from a conductor (e.g wire) experiencing a change in magnetic flux
An induced current comes from this induced emf
What’s a conventional current vs electron flow current?
Conventional current is the movement of positively charged particles from the positive to negative terminal
- this is not real, but its what used to think in history
- but, its important in A-Level to use this, especially in Flemings left hand rule, as its based for conventional current
Electron flow current is the movement of negatively charged electrons from the negative terminal to the positive
- this is the reality
Hence, they’re opposite
What is Fleming’s left hand rule?
Uses FBI
Force = thumb
B = Magnetic field
I = CONVENTIONAL current = movement of positively charged particles
How can we tell the 3d direction of the magnetic field?
A dot represents the magnetic field is coming towards us (out of page)
A cross represents the magnetic field is moving away from us (into page)
Think of an arrow
How do we calculate the magnetic force of a moving charged particle?
Magnetic Force = Bqv
F=Bqv
q=charge of particle
v=velocity of particle
What’s the Magnetic Force?
The force on charged particles (such as electrons or protons) when they are moving through a magnetic field
The force exerted on the particle due to the magnetic field
What happens to the Magnetic force if B and v aren’t perpendicular?
F=Bvqsinθ
Where θ is the angle from magnetic field direction to direction of the current
Its important to know that F and B are ALWAYS perpendicular for the movement of charged particles, so this won’t be an issue
What happens to the movement of electrons in a magnetic field?
As it’s velocity is perpendicular to the magnetic field, when the force pushes the electron upwards, it pushes the velocity of the electron upwards as well
To stay perpendicular, the electron moves in a circular path
Creation in circular motion means mv^2/r = Bqv
mv/r = Bq
As q and m are constants, r ∝ v
and r ∝ 1/B
This is the same for the movement of positive charged particles, but they move in same direction as conventional current, not opposite
What’s a solenoid?
A long coil of wire in a spiral shape
Creates a uniform magnetic field inside the coil
The amount of turns of wire in the solenoid = N
What’s magnetic flux linkage?
The product of the magnetic flux and the number of turns of coil
Only used for coils/ solenoids
Refers to how much magnetic flux passed through a coil
Nφ
What’s Faraday’s Law?
The induced emf is equal to the rate of change of magnetic flux linage
ε=- d(Nφ)/dt
What’s electromagnetic induction?
The phenomenon when an emf is induced into a closed circuit when there’s a change in magnetic flux (linkage)
When a conductor (wire) moves through a magnetic field
Where is electromagnetic induction used?
Electrical generators
- convert mechanical energy to electrical energy
Transformers
- used in electrical power transmission
How does electromagnetism work in a conductor (wire)?
If current flows through a conductor (wire), a magnetic field is produced
The Right Thumb Rule is used to tell the direction of the magnetic field
How does electromagnetism work within a solenoid?
The solenoid acts as a magnet
Has a North and South Pole
- Lenz’s law helps us decide what pole is where
Has a strong magnetic field going through it (South to North)
Has a weaker field around it (North to South)
What’s Lenz’s Law?
Used to predict the direction of an induced emf in a coil
The induced emf is in the direction that opposes the charge carrying it
If an external’s magnet’s North Pole is pushed through the solenoid, there will be a force opposing this, hence this will be a North Pole (as 2 Norths oppose each other)
The induced current direction is shown by the Right Hand Grip Rule
How do we use the direction of the induced current to figure what way the magnetic field flows in a coil?
Look at what way the current is travelling around the coils
Use the right hand grip rule to see what way the magnetic field will run inside of the coil
The side where magnetic field ends up at is North
The one where it enters is the South
How do we use the Right Hand Grip Rule to show what way the induced current moves in a coil?
Look at which poles are what
Realise what way the field needs to travel
Knuckles point towards field
Thumb points towards induced current
Make sure wire is at the front
make sure you point into the centre
How does Lenz’s Law the consequence of the principle of energy?
Work has to be done in order to overcome the repulsive effect
- As the electromagnetic effects will not create electrical energy out of nothing
What’s an alternating current (a.c)?
A current which periodically varies between a positive and negative value
Varies as a sine curve
The time period (for whole λ ) is 1/50 seconds
- So 50 Hz
What’s the peak voltage, V(0) in the Uk?
325V
The rms = 230V
What’s Root-Mean-Square (rms) Current & Voltage?
Finding the average of the voltages/ currents in an a.c current
Squaring values, then finding the mean and rooting it finds the rms
The shortcut to do it is:
V(rms)=V(0)/√2
The rms values are the equivalent of the Direct Current (d.c) that produces the same power
How is an induced emf created in transformers?
An a.c is ran through the primary coil
Creates an alternating magnetic field within the iron core, which strengthens it
Passes through to secondary coil
Leads to change I’m flux linkage, creating an induced emf
Induced emf is higher if there’s more coils on the secondary coil
less coils = less emf
What’s the equation for the induced emf through a wire?
emf = BLV
B=Magnetic flux density
L=length of wire
v=velocity of current
What is an alternating generator?
An external force causes a coil between a magnetic field to rotate
As the coil rotates, theres a constant change in flux, according to Faradays law causes an induced emf
And flemings left hand shows an induced current is created
As the coil spins, direction of induced current reverses periodically producing an AC voltage
Slip rings are used to maintain a continuous electrical connection between the coil and the output (e.g bulb)
