Magnetic Fields Flashcards
Topic 20
Magnetic field
an example of a field of force produced either by moving charges or permanent magnets
Conventions of field line representation
from North to South
the denser the field lines, the stronger the field at that point
Equation for a current-carrying conductor in the region of a magnetic field
F = BILsinθ
F –> force
B –> magnetic intensity
I –> current in the conductor
L –> length of the conductor
θ –> angle of magnetic field lines through the plane
Magnetic flux density
the force acting per unit current per unit length on a wire placed at right-angles to the magnetic field
Magnetic field (flux) intensity at a point
the amount of flux lines passing perpendicularly through the unit area at that point; it is a vector quantity
B = Φ/A
Units: Wb/m² or T for Tesla
Equation for a charged particle moving in the region of a magnetic field
F = BQvsinθ
F –> force
B –> magnetic field intensity
Q –> charge on the particle
v –> velocity of the particle
θ –> angle between B-axis and v-direction
Direction of a force on a charged particle can be worked out by:
–> the use of Fleming’s left-hand rule
Keep in mind: must use direction of flow of positive charge
–> the direction of the force will always be perpendicular to the plan of ‘B’ and ‘v’
Solenoid
a long current-carrying coil used to generate a uniform magnetic field within its core; using a ferrous core, such as iron, increases the strength of the magnetic field
Right-hand grip rule
a rule for finding the direction of the magnetic field inside a solenoid; if the right hand grips the solenoid with the fingers following the direction of the conventional current around the solenoid, then the thumb points in the direction of the magnetic field
Right-hand rule
a rule for finding the direction of the magnetic field around a straight, current carrying wire; if the right hand grips the wire, with the thumb pointing in the direction of the current, the fingers will curl around in the direction of the magnetic field
Fleming’s left-hand (motor) rule
a rule used to predict the force experienced by a current-carrying conductor placed in an external magnetic field
thuMb –> Motion
First finger –> magnetic Field
seCond finger –> conventional Current
Motor effect
the term used when a current-carrying wire in the presence of a magnetic field experiences a force
Hall effect
the production of a potential difference across an electrical conductor when an external magnetic field is applied in a direction perpendicular to the direction of the current
Hall voltage
the potential difference produced across the sides of a conductor when an external magnetic field is applied perpendicular to the direction of the current; the Hall voltage, Vᵸ, is directly proportional to the magnetic flux density, B
Derive and state the Hall voltage equation
Fᴮ = Bqv & Fᴱ = Eq
When no more charge moving to sides due to force from magnetic field (Vᵸ is constant): Fᴱ = Fᴮ
Substitute to give: E = Bv
E = Vᵸ/d
Substitute to give: Vᵸ/d = Bv
I = nAvq –> v = I/(nAq)
Substitute to give: Vᵸ/d = (BI)/(nAq)
A = td (cross-sectional area through which current is flowing)
Substitute to give: Vᵸ/d = (BIq) / (ntdq)
Vᵸ = (BI) / (tqn)
[remember: bye-bye, Grade 10!]
Describe the motion of a charged particle moving in a uniform magnetic field perpendicular to the direction of motion of the particle
Explain how electric and magnetic fields can be used in velocity selection
For the beam of charged particles not to deviate, the force acting on the particles due to magnetic field intensity must be equal and opposite to the force acting on the particles due to the electric field.
Therefore Fᴮ = Fᴱ.
Bqv = Eq
v = E/B <– the speed needed to fire
Can you sketch magnetic field patterns due to the currents in a long straight wire, a flat circular coil and a long solenoid?
The magnetic field in a solenoid can be increased by…
increase current
increase number of turns on the solenoid
add a ferrous core
Explain the origin and direction of the forces between current-carrying conductors in the examples of
i) like current
ii) unlike current
i) they attract; they try to nullify each other’s effect, so there is a void of sorts and the conductors will be squashed toward the empty space
ii) they repel (this is different to everything else we have studied that have unlike properties!)
Magnetic flux definition/equation
magnetic flux is the product of the magnetic flux density and the cross-sectional area perpendicular to the direction of the magnetic flux density
Φ = BA
Unit: Weber (Wb)
Fleming’s right-hand (generator) rule
a rule used to predict the direction of the current caused by induced e.m.f. in a conductor moved at right angles to a magnetic field
thuMb –> Motion
First finger –> magnetic Field
seCond finger –> induced conventional Current
Expression for the component of the magnetic flux density B perpendicular to the plane of the cross-sectional area, where θ is the angle between the normal to the area and the magnetic field
Bcosθ
Magnetic flux linkage
the product of magnetic flux for a circuit and the number of turns
Unit: Weber (Wb)
