7.3/4 Magnetic fields + Electromagnetism

Question 1

What’s Magnetic flux?

Answer 1

‘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)

Question 2

What’s Magnetic flux density?

Answer 2

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

Question 3

What is a magnetic field?

Answer 3

A magnetic field is a region where a magnetic force is experienced by a moving charge or a magnetic material (e.g., iron)

Question 4

What happens to magnetic flux when the coil is rotating?

Answer 4

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

Question 5

What happens to the magnetic flux, φ when the area isn’t perpendicular to the magnetic field lines?

Answer 5

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

Question 6

What does induced mean?

Answer 6

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

Question 7

What’s a conventional current vs electron flow current?

Answer 7

Conventional current is the movement of positively charged particles from the positive to negative terminal

Electron flow current is the movement of negatively charged electrons from the negative terminal to the positive

Hence, they’re opposite

Question 8

What is Fleming’s left hand rule?

Answer 8

Uses FBI
Force = thumb
B = Magnetic field
I = CONVENTIONAL current = movement of positively charged particles

Question 9

How can we tell the 3d direction of the magnetic field?

Answer 9

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)

Question 10

How do we calculate the magnetic force of a moving charged particle?

Answer 10

Magnetic Force = Bqv
F=Bqv

q=charge of particle
v=velocity of particle

Question 11

What’s the Magnetic Force?

Answer 11

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

Question 12

What happens to the Magnetic force if B and v aren’t perpendicular?

Answer 12

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

Question 13

What happens to the movement of electrons in a magnetic field?

Answer 13

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

Question 14

What’s a solenoid?

Answer 14

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

Question 15

What’s magnetic flux linkage?

Answer 15

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

Question 16

What’s Faraday’s Law?

Answer 16

The induced emf is equal to the rate of change of magnetic flux linage

ε=- d(Nφ)/dt

Question 17

What’s electromagnetic induction?

Answer 17

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

Question 18

Where is electromagnetic induction used?

Answer 18

Electrical generators
- convert mechanical energy to electrical energy
Transformers
- used in electrical power transmission

Question 19

How does electromagnetism work in a conductor (wire)?

Answer 19

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

Question 20

How does electromagnetism work within a solenoid?

Answer 20

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)

Question 21

What’s Lenz’s Law?

Answer 21

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

Question 22

How do we use the Right Hand Grip Rule to show what way the induced current moves in a coil?

Answer 22

Figure out 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

Question 23

How does Lenz’s Law the consequence of the principle of energy?

Answer 23

Work has to be done in order to overcome the repulsive effect
- As the electromagnetic effects will not create electrical energy out of nothing

Question 24

What’s an alternating current (a.c)?

Answer 24

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

Question 25

What’s the peak voltage, V(0) in the Uk?

Answer 25

325V
The rms = 230V

Question 26

What’s Root-Mean-Square (rms) Current & Voltage?

Answer 26

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

Question 27

How is an induced emf created in transformers?

Answer 27

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

Question 28

What’s the equation for the induced emf through a wire?

Answer 28

emf = BLV

B=Magnetic flux density
L=length of wire
v=velocity of current