chapter 23 Flashcards
What is a magnetic field?
A field surrounding a permanent magnet or current carrying conductor in which magnetic objects experience a force.
How do magnetic fields lines map to magnetic fields
- Arrows point from north to south pole.
- The closer the arrows are the stronger the magnetic field is.
- uniform magnetic fields have lines equally spaced.
How do magnetic fields form in permanent magnets and current-carrying conductors?
- In a permanent magnet, the electrons in the atoms making up the atom align and spin around the nuclei in an ordered manner to produce an overall magnetic field.
- In a current-carrying conductor, the field is created by the electrons moving through the wire.
Magnetic field line pattern in flat coil, current-carrying wire and solenoid.
- Follow direction of flat circle, straight line, and long coil with right hand rule.
- For solenoid the four fingers represent the current and the one finger represents the magnetic field.
Magnetic force formula.
F = BILSinX.
Where X is the angle between the current and magnetic field lines.
What is magnetic flux density? (With unit symbol and SI unit).
B is the symbol for magnetic flux density
Unit: Teslas, T
SI Unit: 1 N A-1M-1
Derive the formula for the force on an electron in a magnetic field.
F =BIL
Length of a region wire can be written as Vt
The current of a shaded region can be written as NQ/t
These substitutions combine to give F = BNQV
For a single electron this becomes F = BeV
A practical for determining magnetic flux density
Two magnets are put side to side on a scale and it is zeroed.
A current-carrying wire is fixed and perpendicular to the magnetic field.
A current is ran through the wire and is varied, the force on the wire = equal to the force of the magnet downwards and hence the reading on the scale.
Vary current and force, then plot force against current and the gradient = BL
Relation of the Magnetic force to centripetal force
The centripetal force is provided by the magnetic force.
So for the electron, BeV = mv2 / r
How does a velocity selector function?
Two magnets are put side to side to create a magnetic field, and two charged plates are put to create a perpendicular electric field
The electric field force is F = EQ
The magnetic field force is F = BQV
So when the forces balance, E/B = V and this is the particular velocity which will travel straight
Magnetic flux, with units
The product of the component of the magnetic flux density perpendicular to the area and the cross sectional area
Φ = BCosXA
X is the angle between the normal to the surface area and the magnetic flux density
The weber, Wb = 1 T m2
Magnetic flux linkage
The product of the number of turns and magnetic flux, Magnetic flux = NΦ
Faradays law
The emf induced is proportional to the change in magnetic flux over change in time
It is equal to the negative change in magnetic flux over change in time
Lenz’s law
The direction of the induced e.m.f. or currrent is always opposite to the change producing it
Formulas for transformers
Ns/Np = Ip/Is = Vp/Vs
