Chapter 23 - Magnetic Fields Flashcards
Magnetic field lines
Much the same as electric field lines, point from north to south
Direct between, curve around the edges
Can be plotted using a plotting compass
Magnetic metals
Iron, cobalt, nickel
Magnetic field lines around conductors
Act around a wire carrying a conventional current
Circles that are closer nearer the wire as they are stronger
Use right hand grip rule for direction, thumb is current, curved fingers is field direction
Stronger with a greater current / coiled wire
Current into plane and out of plane symbols
In: circle with an x in it
Out: circle with a point in it
Coil field lines
Small circle around the sides, gradually flattening curves as you move inwards, straight line through the centre
Use RHG rule for direction
Solenoid field lines
Uniform lines through the middle, curving wider each line from north to south outside.
Fleming’s left hand rule
Thumb: Force
Index: Field direction
Middle: Current
Wire in a field force equation
F = BILsin(θ)
where B is the magnetic flux density in T and θ is the angle between the magnetic field and right angles to the current
Experiment for B
- Clamp a wire over a top-pan balance between two magnets
- Set the balance to 0 when the current in the wire is 0
- As the current flows, the wire experiences an upward force so the magnets experience a downward force (Newton’s third law)
- Vary the current and measure the force (mg)
- Plot graph of F against I and multiply the gradient by 1/l for the force (l is measured with a ruler)
Electron deflection tube
Thermionic emission causes electrons to be emitted and move into the magnetic field
A force acting perpendicular to the motion acts on the electron (constant speed)
There is a screen where you can observe the motion
Magnetic force on a charged particle
F = BQv
Circular motion on a charged particle
Force is perpendicular to velocity so follows a circular path
Equate force equations for r = mv/BQ
Velocity selector
Two plates with slits and a vacuum between with a potential difference between the plates and a magnetic field
V and B can be adjusted slowly so only one velocity passes through the slit in the second plate
Velocity selector equation
v = E/B
How to generate an electric current using a magnet
Move a magnet relative to a coil of wire so the field lines intersect the coil at different points
Why does moving a magnet generate current
Work is done to move the magnet, some of this is transferred to electrical energy
Using F=Bev, the relative motion of the coil and the magnet cause the electrons to move
Magnetic flux definition
The component of the magnetic flux density perpendicular to the area
Magnetic flux and magnetic flux linkage units
Webers (Wb)
Magnetic flux formula
Φ = BA cos(θ) where θ is the angle between the field lines and the normal to the surface
Magnetic flux linkage formula
NΦ
The number of coils x magnetic flux
Transformer process
- Pass an AC to the primary coil
- This produces a changing magnetic flux in the laminated iron core
- The coils are linked by the changing flux of the magnetic field
- Faraday’s Law states this produces a changing emf in the secondary coil
Transformer equation
n(s)/n(p) = V(s)/V(p)
Transformer if 100% efficient
V(s)/V(p) = I(p)/I(s)
Faraday’s law definition
The magnitude of the induced emf is directly proportional to the rate of change of the magnetic flux linkage
Faraday’s law formula
ε = -Δ(Nφ)/Δt
Lenz’s Law
The direction of the induced emf or current is always such to oppose the change producing it
e.g. move a magnet towards, near side has the same polarity
AC Generator
- Coil rotates at a steady speed
- Flux linkage changes with time
- Gradient of Nφ is emf
- ε ∝ Δ(cosθ)/t
Where does a magnetic force act in relation to the field?
Perpendicular
