Chapter 23 - Magnetic Fields Flashcards
Define a Magnetic Field
A magnetic field is a region of space in which moving charged particles are subject to a magnetic force. They are created by moving charges.
Permanent Magnet
Made from a magnetised material that creates its own persistent magnetic field.
Visualising Magnetic Fields
Compass or Iron filings
Electromagnet
Uses the field generated by a current carrying wire to create a magnetic field that can altered in strength by changing the current.
Electromagnets are usually a solenoid with a soft magnetic core.
Solenoid
A solenoid is a coil of wire.
Magnetically Soft and Hard Materials
Magnetically soft materials can be magnetised but do not stay magnetised after the original field is removed.
Magnetically hard materials can be magnetised but become permanently magnetised.
Magnetic Field of Earth
The north pole is actually the magnetically south pole and vice versa.
Earth magnetic field is incredibly important because it protects us against solar wind.
Drawing Magnetic Fields
Magnetic Fields can be represented by lines of magnetic flux which point from the north pole to the south pole. The density of these lines represents the strength.
North and South interactions with Magnets
Opposites poles attract.
Like poles repel.
Magnetic Field around a current carrying wire
We can use the right hand grip rule.
Right hand thumbs up.
The thumb represents the direction of current.
Fingers represent the direction of circular magnetic field.
Magnetic Field around a Solenoid
We can use the right hand grip rule.
Right hand thumbs up.
The direction of the fingers represents the circular flow current.
The thumb shows the direction of the magnetic field.
Fleming’s Left hand Rule
Current carrying wired in a perpendicular magnetic field, experiences a force due the interaction of the two magnetic fields.
With your left hand; have your thumb up, your first finger straight out and your second finger perpendicular to the first finger.
Thumb - Direction of Force
First Finger - Magnetic Field
Second Finger - Direction of Current
Force on a Current Carrying Wire in a Magnetic Field
F = BILsinθ
B is the magnetic flux density
I is the current
L is the length of wire
θ angle between current flow and magnetic field
Magnetic Flux
Magnetic Flux ϕ is measured in Webers (Wb), is the measure of the total magnetic field.
ϕ = BAcosθ
B is Magnetic Flux Density
A is Cross Sectional Area
θ is the Angle between Flow of Current and Magnetic Field
Magnetic Flux Density
Magnetic Flux Density is a measure of magnetic flux density per area. Measured in Teslas (T).
Investigating Magnetic Flux Density
· Horseshoe magnet on a digital set of scales, and 0 the scales.
· Set up a circuit with the wire passing through the magnetic field, an ammeter, dc power supply and a variable resistor.
· Using Fleming’s Left Hand have the current flowing so the force produced acts down towards the balance.
· The scales will produce a mass.
· Plot a graph of Current against mass, we know mg = BIL so gradient of graph is BL/g . So times gradient by gravity and divide by the length of wire exposed to the magnetic field to get B.
Motion of Charged Particles in a Uniform Magnetic Field
F = BIL and I = Q/t
F = BQL/t
L (distance) / t = v
F= BQv
Mean Drift Velocity to Fₑ = -Bev
I = -Anev and F=BIL
F = -BAnevL
n = N/V
F = -BA(N/V)evL
1/L = V/A
F = -B(1/L)NevL
F = -BNev
F/N = -Bev
F/N = Force per delocalised electron
Motion of a Free Particle in a Magnetic Field
· Force is perpendicular to its velocity, therefore travelling in circular motion.
F = mv/r² and F = BQv
mv²/r = BQv
r = mv/BQ
Velocity Selectors
· Velocity selectors use a combination of magnetic and electric fields to isolate particles with a specific velocity.
· Fields act perpendicular to each other.
F = EQ and F = BQv
F has to be equal for particle to go through slit.
EQ = BQv
v = E/B
Magnetic Flux Linkage
Magnetic Flux Linkage Φ is a measure of the flux in the whole solenoid and so is the sum of the magnetic flux through each turn of the coil.
Nϕ = Φ = BANcosθ
B is Magnetic Flux Density
A is Cross Sectional Area
N is the number of Coils
θ is the Angle between Flow of Current and Magnetic Field
Electromagnetic Induction
A current is induced due to a change in the magnetic flux linkage (cutting magnetic field lines), either by having a varying magnetic field or by moving the wire through a non uniform magnetic field.
This is because the magnetic force acts on the delocalised electrons in the metal. Fleming’s Left hand rule can be used as force as the direction of motion.
Faraday’s Law
The induced e.m.f is proportional to the rate of change of magnetic flux linkage.
ε ⍺ Δ(Nϕ)/Δt
Lenz’s Law
The induced e.m.f is generated in a direction so that it opposes the change that produced it. This is due to the conservation of energy.
Current will be the opposite the Fleming’s Left Hand rule.
Current is created from the work done to move the wire into the field.
Combining Faraday’s and Lenz’s Law
ε = - Δ(Nϕ)/Δt
Inducing a current from a Bar Magnet and Solenoid
The current will be produced in such a way that the magnetic field of the solenoid opposes the magnetic field of the bar magnet.
So if north is put to on side of the solenoid that side will become the north pole.
The right hand grip rule can then be used to work out the direction of current.
Investigating Magnetic Flux
AC Generators
· Converts Kinetic Energy into Electrical Energy.
· Coil of wire is rotated in a uniform magnetic field, the area perpendicular to the magnetic field is constantly changing. Causing a constantly changing magnetic flux linkage.
· When the coil is perpendicular to the magnetic field there is maximum flux linkage and 0 e.m.f, because this is where there is minimal change in linkage.
· When the coil is parallel to the magnetic field there is maximum voltage and 0 magnetic linkage, this is because there is maximum change in linkage.
Electric Motors
· Converts Electrical Energy into Kinetic Energy.
·Current is place through a coil, which is in a perpendicular magnetic field. Causing a force to be produced, Fleming’s Left hand rule.
· Current flows in a different direction for each side of the coil, causing the coil to spin. As these forces are equal and opposite about a pivot.
· If AC is used, the coil will rotate at the same frequency as the AC.
· If DC is used, a split ring communicator is used to reverse the direction of current after the coil has travelled through 180°.
Transformers
Step up and Step down transformers raise or lower the voltage of alternating current through electromagnetic induction.
They are made from a soft iron core, in the shape of a square ring, with primary input coils on one side and secondary output coils on the opposite side.
An alternating current is passed through the primary coils, inducing an alternating magnetic field. This magnetic field induces a current on the secondary coils (Faraday’s Law).
The voltage outputted depends on the number of coils:
Nₛ/Nₚ = Vₛ/Vₚ = Iₚ/Iₛ
Uses of Transformers
Transformers are used to reduce power loss in the national grid.
Pₗₒₛₛ = I²R
Step up transformers, increase the voltage from the power station and decrease the current, so the power loss is reduced.
Step down transformers are used to then decrease the voltage for consumers to use.
Efficiency of Transformers
Efficiency = Pout/Pin x 100
= (IₚVₚ/IₛVₛ) x 100
Increasing the Efficiency of Transformers
· Using lower resistance wires, less power loss.
· Laminated iron core, reduces eddy currents.
· Soft iron core, easier to magnetise and demagnetise.
