Magnetic Fields Flashcards
What does the magnitude of the force on a wire depend on
- Current
- Strength of the magnetic field
- Length of the wire
- Angle between the lines of the field and the direction of the current
How can a wire induce a magnetic field
when a current flows through it
When will a current carrying wire experience a force
If it is placed at a non-zero angle to the filed lines of another magnetic field. The field lines from the wire and the external magnet interact, this causes the field lines from the external magnet field to contract, so the field lines are closer together, causing a force on the wire.
Define Magnetic Flux Density
Magnetic Flux Density, B, is the force per unit length, per unit current, on a current carrying conductor at right angles to the magnetic field lines. It is a vector quantity and measured in teslas, T. Effectively, it is the strength of the magnetic field.
Define the Tesla
The tesla is the flux density that causes a force of 1N on a 1M wire carry a current of 1A at right angles to the flux
Why do charged particles follow a circular path in a magnetic field
Charged particles in a magnetic field follow a circular path with the force always acting towards the centre of curvature; this is because the force on the charge from the magnetic filed is always at right angles to the direction of the motion of the particle.
This also means that no work is done on the particle by the field; the speed and kinetic energy of the particle are unchanged
What is a cyclotron
A cyclotron is a particle accelerator that accelerates charged particles through a spiral path using a fixed magnetic field and an alternating potential difference
They are used with heavier particles like alpha and protons
Explain how a cyclotron works
- The particles experience a potential difference when they travel across the gap, and gain energy equal to QV
- Since the particles have more kinetic energy, they move faster and accelerate to the next dee
- The ac voltage is timed to change direction every time the particle reach the gap with the dees – it must alternate to accelerate the particles each time the reach the gap
- Particle spend the same time inside each dee, but the radius of their path increases after each gap and they travel further in the same time
How does special relativity effect a particle’s speed in a cyclotron
- The effect of special relativity limits a particle’s speed in a cyclotron.
- Particles get more massive as they travel close to the speed of light.
- As particles move faster and their mass increases, the time spent in each dee increases and the more massive particles get out of step with the alternating potential difference
- Magnets are used
- Magnetic field acts perpendicular to the path of the particles - meaning the magnetic force acts perpendicular to their path
- The force required depends on the speed of the particles - larger speed - stronger force - stronger magnets
- F=BQv
Define Magnetic Flux
Magnetic flux, Φ, is defined as the magnetic flux density multiplied by the surface areas of the surface, where this area is perpendicular to the lines of flux. Magnetic flux is measured in Weber’s (Wb)
What factors will increase the number of flux lines a wire cuts through each second
- length L is longer
- The wire moves faster
- The magnetic flux is stronger
Define Magnetic Flux linkage
Magnetic flux linkage is defined as NΦ, where Φ is the number of flux lines that pass through with each turns of the coil of N turns.
Measured in Weber Turns
What factors effect the size of Flux Linkage
o The flux density
o The orientation of the coil and flux lines
o The coils cross-sectional area
o The number of turns on the coil
Explain how electromagnetic induction occurs
- When a length of wire moves between two magnets,
electromagnetic induction occurs - An e.m.f is induced in the wire because of an electric
charge moving perpendicular to the magnet
experiences a force, BQv - The electrons in a wire move towards one end of the
wire when the wire moves perpendicular to the
magnetic field - This leaves one end of the wire negatively charged
overall and the other end positively charged, creating a
potential difference across the wire - A current can then flow if the wire is part of a complete
circuit
What is Faradays law
Faradays law states that the e.m.f is equal to the rate of change of magnetic flux linkage
Explain Faraday’s law - when and when won’t an e.m.f be induced in a coil
o Relative movement between a magnet and a coil changes the flux linkage in the coil. This generates an e.m.f
o Rotating a coil in the plane perpendicular to the field changes the cross-sectional area – through which the flux passes – this changes the flux linkage and generates an e.m.f
o Increasing the relative motion, or the speed at which the coil rotates, increases the rate of change of the flux linkage, which increases the induced e.m.f
o If there is no relative movement or rotation, the flux linkage does not change, so no e.m.f is generate
What is Lenz’s law
Lenz’s law indicates the direction of the induced e.m.f causes effects that oppose the change producing it
Explain Lenz’s Law
Pushing a magnets south pole into the coil produces a south pole – this repels the magnet
Pulling a south pole out of the coil induces a north pole – this attracts the magnet
In a pratical situation:
- Where the south pole of a magnet is pushed into the
coil, a current is induced in the wire, which becomes an
electromagnet - If the south pole of the electromagnet faces the moving
magnet, the poles repel, and work must be done to keep
pushing the magnet into the coil of wire
What are Eddy currents
Eddy currents are circulating electric currents flowing in a plane of metal
They are caused by the change of flux linkage when the metal moves perpendicular to the field, and the currents flow in a direction to oppose the motion creating them
How can you calculate the area swept out through a conductor moving in a magnetic field and use this to find the e.m.f induced
When a conductor moves at a velocity, v, perpendicular to the flux lines, Faraday’s law applies and an e.m.f is generated
For a conductor of length, travelling in a flux density B, the area swept out per second is length x velocity
The induced e.m.f equals rate of change of flux linkage so:
ε=Blv
What equation is used to calculate the induced a.c. voltage in an coil rotates in a magnetic field
ε=BANωsinωt
e = induced e.m.f (volts) B = magnetic flux density (Teslas) A = cross sectional area of the coil N = number of turns on the coil ω= angular speed of the rotating coil t = time (seconds)
- the equation is as long as the axis of rotation is at right angles to the field.
How do you convert degrees into radians
angle x π/180
