Magnetic Fields Flashcards
What is a magnetic field?
A field of force that is created either by:
- Moving electric charges
- Permanent magents
Why is a magnetic field created around a current carrying wire?
Due to the movement of electrons in it
What charges do not produce magentic fields?
Stationary ones
What are magnetic fields represented by?
Magnetic field lines
What are the key features of magnetic field lines?
- The lines come out of the north pole and into the south pole
- The direction of the field lines shows the direction of force that a free magnetic north pole would experience at that point
- The field lines are stronger the closer the lines are together
- The field lines are weaker the further apart the lines are
- The lines never cross since the magnetic field is uniquely is unique at any point
- They are continuous
What is a uniform magnetic field?
Where the magentic field strength is the same at all points
What rule do you use to find field lines for a current carrying wire?
Maxwells right hand grip rule
What is the direction of current taken to be in a current carrying wire?
Conventional current
What do field lines around a solenoid look like?
- They emerge from the north pole and return to the south pole
- Which is north and south pole depends on the direction of the current
What is a flat circular coil?
One of the coils of a solenoid
How can you increase the strength of a magnetic field in a solenoid?
- Adding a core made from from ferrous (iron-rich) material
- Adding more turns in the coil
What is Flemings left hand rule?
What does dots on the page mean in reference to the direction of the magnetic field?
The field is coming OUT of the page
What do X’s on the page mean in reference to the direction of the magnetic field?
The field is going into the page
Where is the maximum magnetic force on a moving charge directed?
Perpendicular to the velocity
When the magnetic force is perpendicular to a charges velocity, what happens?
The force causes the particle to move in a circle - the magnetic force is acting as a centripetal force
Where is the conventional current/ velocity of a positive charge always directed?
In the opposite direction to the movement of electrons
What happens to a current carrying conductor when it comes into contact with an external magnetic field?
A magnetic force will be exerted onto the conductor
When will a current carrying wire experience the maximum magnetic force from an external magnetic field?
When the current through it is perpendicular to the direction of magnetic flux lines
What is the magnetic force, magnetic flux density, current and length equation?
F = BILsinθ
F = magnetic force on the current carrying conductor (N)
B = magnetic flux density of external magnetic field (T)
I = current in conductor (A)
L = length of the conductor in the field (m)
θ = angle between the current carrying conductor and the external B-field
When does the minimum force occur for a current carrying conductor being effected by an external B-field?
This is when sinθ = 0 so the conductor is parallel to the B-field and a force of 0 is created. The conductor will experience 0 force if it is parallel with the external B-field
How can you determine magnetic flux density?
What is magnetic flux density?
The force acting per unit current per unit length on a current carrying conductor placed perpendicular to the magnetic field
What is magnetic flux density measured in?
Tesla
What is a Tesla?
The magnetic flux density when a wire carrying a current of 1A placed perpendicular to the magnetic field experiences a force of 1N per metre of its length
What is the equation for the magnetic force on an isolated moving charge given by?
F = BQv (when the charge is moving parallel to the magentic field)
F = BQv sinθ
F = magnetic force on the particle N
B = magnetic flux density T
v = speed of particle ms^-1
θ = angle of moving charged to the magnetic field lines
What is the size of the magnetic force if the charge is moving parallel to the magnetic field?
0
When is the size of the magnetic force maximum for a moving charge?
When it is perpendicular to the magnetic field
What kind of path does a charged particle in a uniform magnetic field, with the magnetic force perpendicular to the motion of the particle, move in?
A circular one - the magnetic force will always be perpendicular to velocity so the magnetic force in the centripetal force
How do you relate magnetic force and centripetal force?
They equal one another so
mv^2 / r = BQv
so r = mv / BQ
What is a velocity selector?
A device consisting of perpendicular electric and magnetic fields lines, where charged particles with specific velocities can be filtered
In an area with both electric and magnetic fields, what do the forces depend on?
- Electric force does not depend on the velocity
- Magnetic force does depend on the velocity - it is greater for particles moving at greater speeds
What is the speed in which a particle will remain undeflected given by?
The ration of the electric and magnetic field strength
v = E/ B
What force can be ignored in a region of both magnetic and electric fields?
Gravitational force - it is negligible compared to the other 2
What happens if a particle has a speed greater than or less than v (the speed in which a particle remains undeflected?
The magnetic force would defect it and it would collide with one of the charged plates. This would remove the particles in the beam that are not at the exact speed v
What does the quantity magnetic flux signify?
How much of a magnetic field passes perpendicularly through an area
What is magnetic flux?
The product of the magnetic flux density and the cross-sectional area perpendicular to the direction of the magnetic flux density
What is the equation for magnetic flux?
Φ = BA
or Φ = BA cosθ when magnetic flux is not completely perpendicular to the area
Φ = magnetic flux , Wb
B = magnetic flux density, T
A = cross-sectional area, m^2
θ = angle between magnetic field lines and the line perpendicular to the plane of the area
When does the magnetic flux maximum occur?
When the magnetic field lines are perpendicular to the plane of the area
When does magnetic flux minimum occur?
When the field lines are parallel to the plane of the area
What is magnetic flux linkage?
The product of the magnetic flux and the number of turns of the coil
What is the equation for flux linkage?
Flux linkage = ΦN = BAN
where N = number of turns of the coil
What is the unit for magnetic flux linkage?
Webber turns, Wb turns
What is Faraday’s law?
The magnitude of the induced emf is directly proportional to the rate of change of magnetic flux linkage
What is Lenzs law?
The induced emf is set up in a direction to produce effects that oppose the change causing it
What does Lenzs law detect?
The direction of the emf and hence the current, must be set up to oppose an incoming magnet. If a north pole approaches a coil face, the emf must be set up to create an induced north pole. This is because the 2 north poles will repel one another.
What is the equation from combining Lenzs’ and Faradays’ law?
ε = - Δ(NΦ) / Δt
ε = induced emf
NΦ = magnetic flux linkage
t = time
What does the negative sign in the Faraday/ Lenz law equal?
Lenzs law - the induced emf is set up in an opposite direction to oppose the changing flux linkage
What would the gradient of a graph of magnetic flux linkage against time equal?
The induced emf
What happens when a coil rotates in a uniform magnetic field?
The flux through the coil will vary as it rotates. Since emf is the rate of change of flux linkage, this means the emf will also change as it rotates. The maximum emf is when the coil cuts through the most field lines
What is the equation with emf, flux linkage and angular velocity?
How do you investigate magnetic flux?
What is the generator effect?
If a coil of wire is rotated inside a magnetic field by an external force, an emf will be generated in the wire which causes current to flow within the coil
What is a simple alternator?
A type of generator that converts mechanical energy to electrical energy in the form of alternating current
How does an AC generator work?
- A rectangular coil that is forced to spin in a uniform magnetic field
- The coil is connected to a centre reading meter by metal brushes that press on 2 metal commutator rings. The commutator rings and brushes provide a continuous connection between the coil and the meter
- When the coil turns in one direction the pointer defects one way first, and then the opposite way and back again. This is because the coil cuts through the magnetic field lines and a potential difference, and therefore current, is induced in the coil
- The pointer deflects in both directions because the current in the circuit repeatedly changes direction as the coil spins
- The induced potential difference and the current alternate because they repeatedly change direction
What is a transformer?
A device that changes high alternating voltage at low current to low alternating voltage at high current and vice versa
What are transformers designed to do?
To reduce heat energy lost why electricity is transmitted down electrical power lines from power stations to the national grid
What is a transformer made up of?
- A primary coil
- A secondary coil
- A soft iron core
How does a transformer work?
- In the primary coil, an alternating current producing an alternating voltage is applied
- This creates an alternating magnetic field inside the iron core and therefore a changing magnetic flux linkage
- A changing magnetic flux linkage passes to the secondary coil through the iron core
- This results in a changing magnetic flux linkage in the secondary coil and hence an emf is induced
- An emf produces an alternating output voltage from the secondary coil
- The output alternating voltage is at the same frequency as the input voltage
- The magnitude of the emf is determined by the number of coils
What is a step-up transformer?
More coils in the secondary than the primary and the secondary voltage is smaller than the primary voltage
What is a step-down transformer?
Has more coil in the primary then the secondary and the secondary voltage is smaller than the primary voltage
