Magnetic Fields Flashcards
What happens when a current-carrying wire goes in a magnetic field ?
the current-carrying wire will experience a force
State the condition for F = BIL
- the direction of the force acting in a current-carrying wire is PERPENDICULAR to a magnetic field
- the current flow in wire must be perpendicular to a magnetic field
Fleming’s Left hand rule ?
First finger =Field
Second finger = current
Thumb = force (motion)
What happens when charged particle moves through a magnetic field ?
the charged particle will experience a force
- current I = Q/t
- distance L = vt
F = BQv
What happens when charged particle moves perpendicular into a magnetic field ?
it will flow a circular motion
- the force is always towards the centre of the circle
- the direction of the particle is the Second finger (normally current)
- if negative charge then point finger in the opposite direction to its motion
How is the radius and frequency of the circular motion of the charged particle affected ?
RADIUS r = mv/BQ
-mass or velocity increase = radius decrease (less deflect)
- magnetic field/charge increase = radius increase
FREQUENCY f=BQ/2πm
- frequency of rotations is independent of its velocity.
What is Magnetic Flux density (B) ?
force acting on one metre of wire carrying a current on one amp
- is a vector
- measured in teslas (T)
- known as magnetic field strength
What is one Tesla equal to ?
equal to one newton per amp per metre (1T = 1N/Am)
what is the right hand rule used for ?
to work out the direction of a magnetic field around a current -carrying wire
- thumb = current
= fingers = magnetic field
Representation of magnetic field on magnet and paper ?
MAGNET
- the magnetic field lines go from NORTH to SOUTH
- NORTH = anticlockwise current (N)
- SOUTH = clockwise current (S)
PAPER
- the magnetic field goes INTO paper when a +
- the magnetic field goes OUT paper when a •
How particles accelerators such as CYCLOTRONS make use of the force that acts on charged particles moving perpendicular to a magnetic field ?
HOW
- two electrodes
- charged particle start in centre - circular motion out
- particle accelerates between electrodes so potential difference create and particle increase speed
- spend equal time in each electrode as frequency is independent of velocity
Uses of CYCLOTRONS ?
USES
- Medicine
- Provide radioactive traces
- High energy beams for use in radiotherapy
What is Magnetic Flux (Φ) ?
The magnetic flux passing through an area is given by the magnetic flux density multiplied by the area
Φ = BA
- Measured in Wb (Weber’s)
- the number of magnetic field lines passing through the area
When is an E.M.F induced across a conductor ?
when the conductor moving through the magnetic field or a conductor inside a changing magnetic field
- if the conductor in a circuit then a current will flow
What is Magnetic Flux linkage (NΦ) ?
NΦ = BAN
N - number of turns on the coil cutting the wire
- Measured in Wb (Weber’s)
What does the Rate of change in Flux linkage tell you ?
It tell you how strong the electromotive force (EMF) will be in volts
- A CHANGE IN FLUX LINKAGE OF ONE WEBER PER SECOND WILL INDUCE AN EMF OF 1 VOLT IN A LOOP OF WIRE
How to calculate either the Magnetic Flux or Flux linkage of a rectangle coil when the coil is not normal to B ?
1) Φ = BACOSѲ
2) NΦ = BANCOSѲ
What is the Radius and frequency equation in magnetic field circular motion ?
r = mv/BQ f = BQ/2πm
What is Faraday’s Law ?
The induced EMF = the rate of change of Flux linkage in a conductor
E = NΔΦ/Δt
- Measured in Volts (V)
- Need to know GRAPHS for Φ/t and E/t
What is Lenz’s Law ?
The DIRECTION of the induced EMF is such that it will try to oppose the change of flux that is producing it
E = -(NΔΦ/Δt)
- agrees with the conservation of energy - they energy used to pull a conductor though a magnetic field, against the resistance caused by magnetic attraction, is what produces the induced current
What happens when you drop a magnetic through a solenoid ? - Lenz’s Law
1) north and north end meet so repulsion - A
What happen if no current is induced in circuit ?
- no current = no magnetic field
- but still EMF induced
- fall at acceleration = g
How is an EMF induced in a rotating coil ?
- rotating caused alternating EMF is induced
E = BANwSINwt - graph is E/t - sin graph - EMF is greatest when coil is parallel to Magnetic field (NΦ - Fluxage linkage= 0)
What is Alternating Current ?
Changing direction with time
How to operate an Oscilloscope ?
It displays voltage
1) Alternative Display
- y-gain = 2v per division
- time base = 1ms per division
- time basee off - straight line vertical
2) Direct Display
- time base on - straight line horizontal
- time base off - dot
Who to calculate Time period, Peak Voltage and Peak-Peak Voltage from graph ?
1) Time period - one complete oscillation
2) Peak Voltage = Vo - amplitude
3) Peak-Peak Voltage = Vpp = whole vertical
How to calculate ‘root mean square current and potential differences ?
Vrsm = Vo/√2 (main electricity - 230v) Irsm = Io/√2 Prms = Po/2 or Po = Vo x Io
What is a Transformer and the shape of it ?
a device that makes use of electromagnetic induction to change the size of the Voltage for an alternating current
- Its a laminated iron core with a magnetic field around it and Primary and Secondary coil around
How to calculate the number of turns on coils or voltage across coils ?
Ns/Np = Vs/Vp
Who does a step-up and step-down transformer work ?
STEP-UP TRANSFORMER - increase voltage ( reduce power loss P=VI)
- having more coils turns (N) on Secondary coil
STEP-DOWN TRANSFORMER - decrease voltage
- having more coils turns (N) on Primary coil
What causes Energy loss in transformers and how to reduce them ?
1) EDDY CURRENT (heat lost) - Laminating core so current can’t flow
2) Electrical resistance - Thick copper (low r)
3) magnetism/de-magnetism core - SOFT IRON
4) Air gaps/impurities - high quality iron
How to calculate the efficiency of a transformer ?
Power = V x I - POWER IN = POWER OUT - IpVp = IsVs EFFICIENCY = IsVs/IpVp - energy lost - E = Pt
How is electrical power is transmitted in the National Grid ?
Power station -> Step-up -> Pylogens -> Step-down -> House (230V)
- Transformers step-up = high voltage = low current = LOW ENERGY LOSSES
