Chapter 15 - electromagnetic machines Flashcards
What is a magnetic field?
A region where a force is exerted on a magnetic material
How can magnetic fields be represented?
field lines?
In which direction do field lines travel?
North to South
How is the strength of a magnetic field represented by field lines?
The stronger a field is, the closer together the field lines will be
What is a neutral point?
A point at which the magnetic fields cancel out
What will be generated around a wire carrying an electric current?
A magnetic field
What is the right hand grip rule used for?
Finding the direction of a magnetic field around a current carrying wire
What is the right hand grip rule?
Direction of thumb = current
Curl of fingers = magnetic field
What is a solenoid?
A form of electromagnet - coil/multiple coils of wire which act like a bar magnet when carrying a current
What happens when a current carrying wire is in an external magnetic field?
The wire experiences a force
(because the magnetic fields around the wire and the external magnetic field interact, so the field lines around the magnet contract)
What happens when a current carrying wire is in an external magnetic field, but the current is parallel to the magnetic field lines?
No force will act
When a wire is in an external magnetic field, in what direction will the force on the wire be?
perpendicular to both the magnetic field and the current direction
What is Fleming’s left hand rule used for?
Finding the direction of force on a current carrying wire in a magnetic field
What is Fleming’s left hand rule?
thumb = force
first finger = field
second finger = current
What is the equation for finding the size of the force on a current carrying wire in an external magnetic field?
F = BIL force = magnetic field strength x current x length of wire
What is the symbol for magnetic field strength?
B
What is the definition of magnetic field strength?
The force on one metre of wire, carrying a current of one amp at right angles to the magnetic field
What is another name for magnetic field strength?
Magnetic flux density
What is magnetic field strength/magnetic flux density measured in?
Teslas, T
What are the equivalent units to one Tesla?
Wbm^-2
Webers per metre squared
What is magnetic flux density?
The number of flux lines per unit area
the magnetic field strength
Is magnetic field strength a vector or scalar quantity?
vector
How does a motor work?
A current carrying loop is placed in an external magnetic field, which causes it to rotate
A split ring commutator reverses the current every half turn to keep the motor spinning in a constant direction
What does a split ring commutator do?
Reverses the current in a motor’s loop with every half turn, to keep it spinning in a constant direction
How does an induction motor work?
The magnetic field is altered around a coil of wire which is free to move, inducing a current and causing the wire to rotate
What is the magnetic flux density?
The magnetic field strength per unit area
What is the equation for finding the magnetic flux?
Φ = BA
magnetic flux = magnetic field strength x area
What is the symbol for magnetic flux?
Φ
What is the flux linkage?
The flux in a number of linked coils (eg. in a solenoid0
What is the equation for flux linkage?
flux x number of turns
ΦN
What happens when a conductor is moved through an external magnetic field?
An electromotive force (emf) is INDUCED across the conductor
If it is part of a complete circuit then a current will flow
Why is an emf induced in a current carrying wire moving through an external magnetic field?
The electrons experience a force, causing them to accumulate at one end of the wire
This induces a potential difference -emf- across the wire
What is electromagnetic induction?
When an electromotive force is induced (by a conductor moving through a magnetic field) and a current flows (if the circuit is complete)
What causes an electromotive force to be induced?
Changes in flux
Where there is relative motion between a conductor and a magnet
What happens when lines of flux are ‘cut’?
An emf is induced
and a current will flow if the circuit is complete
What is Faraday’s law?
The induced emf is directly proportional to the rate of change of flux linkage
What is the equation for emf induced?
Faraday’s law
emf = flux change/ time taken emf = Δ(ΦN)/Δt
How can the size of the emf be found graphically?
The gradient of a flux linkage vs time graph
ΦN/t
How can the flux change be found graphically?
The area under an emf vs time graph
emf/t
Why can induced emf be given by Blv?
The flux is given by Φ = BA A = xl, which is the length x distance moved Φ = Blx emf = Φ/t = Blx/t v = x/t emf = Blv
What is Lenz’s law?
The induced emf is always in such a direction as to oppose the change that caused it
How can Lenz’s law and Faraday’s law be combined to give an equation for induced emf?
emf = -d(ΦN)/dt
the minus sign is just to show that the emf is in the opposite direction to the changes which caused it
In which direction will current flow if an emf is induced in a complete circuit?
The same direction as the emf
When using Fleming’s left hand rule for induced emf, in what direction does the thumb point?
The opposite direction to the motion of the conductor, because the resistance force is in the opposite direction tot he direction of motion
What are transformers used for?
Changing the voltage for an alternating current
In transformers, how is an alternating current produced in the second coil?
The alternating current in the first coil produces a changing magnetic field. This is passed through the iron core. The change in flux experienced by the second coil causes an induced alternating emf, which causes an alternating current to flow
What is the equation for an ideal transformer?
Vp/Vs = Np/Ns (N = number of turns in a coil)
What do step-up transformers do?
How?
Increase the voltage
By having more turns on the secondary coil
What do step-down transformers do?
How?
Decrease the voltage
By having fewer turns on the secondary coil
What is the symbol for permeance?
Λ
What is the equation for permeance?
Λ = μA/L permeance = (permeability x cross sectional area)/length
What does a higher material permeance mean?
A higher flux induced in it for a given number of current turns
How would you increase the permeance of a transformer core?
- Larger cross sectional area
- Shorter length
- Made out of a material with a greater permeability (eg iron)
Will a magnetic circuit still work in a vacuum/ in air?
yes
but the permeability is much lower, so amount of flux will be dramatically decreased
What problems would making the transformer core fatter (to increase permeance) cause?
The conductance of the copper wire coils would be decreased, causing greater energy loss
The copper coils have the greatest conductance when there is the most turns with the shortest wire length, ie tight coils with a low radius
Are transformers 100% efficient?
No
What happens to the energy lost in transformers?
It is mostly lost as heat from eddy currents/current in the copper wires passing through a resistance
What are Eddy currents in transformers?
Currents are induced by changing flux in the iron core, in the opposite direction to the initial current
How can Eddy currents in transformers be minimised?
The core is laminated with layers of insulator. The magnetic circuit will not be broken as it can pass through insulators, but the electrical circuit for the eddy currents will be broken, meaning they can’t flow properly
How is heating in the coils of a transformer minimised?
Thick copper wire is used because it has a low resistance
Why is electricity from power stations transmitted at the highest possible voltage?
P = IV so this means the lowest possible current
P = I^2R so a lower current means less power lost to heat when the current passes through a resistance
It is more efficient
What voltage of electricity is used in homes?
230V
Roughly what voltage is used for transmission in the national grid?
400, 000V
What is an alternator?
A generator of alternating current
What do generators/dynamos do?
Convert kinetic energy to electrical energy
How do generators/dynamos work?
An electric current is induced by rotating a conducting coil in a magnetic field
What do alternators have instead of a split ring commutator?
slip rings and brushes
For an alternator, how often does the direction of the output voltage and current switch?
Every half turn
