Magnetic fields Flashcards
Conventional current vs electron flow
Conventional current is the flow of electrons from the positive terminal (the long thin terminal) to the negative terminal (the short fat terminal), even though in reality we know the electrons travel from negative to positive due to their attraction.
In exams, when given a diagram remember these rules. If the charge carrier is an electron it travels towards the negative side, and if the charge carrier is a proton, it travels towards the positive.
Magnetic field signs
O (dot) is out of the page.
X is into the page.
Magnetic flux density
The magnetic flux density B of a field is defined as:
The force per unit length, per unit current on a current carrying wire at right angles to the magnetic field lines.
Magnetic flux density is measured in teslas (T)
One tesla, 1 T, is defined as:
The flux density that causes a force of 1 N on a 1 m wire carrying a current of 1 A at right angles to the flux
The higher the flux density, the stronger the magnetic field i.e. regions where flux lines are closer together
Magnetic field lines
Arrows point out of a north pole and into a south pole. (South seeking)
Flux lines are drawn closer together to represent where the field is stronger ( the field is strongest at the poles of a bar magnet where the field lines are most dense)
The motor effect
A current-carrying conductor produces its own magnetic field:
When interacting with an external magnetic field, it will experience a force
The force F on a conductor carrying current I at an angle θ to a magnetic field with flux density B is defined by the equation
F = B I L sin theta
This equation shows that the force is the strongest when the force and the field are at right angles ( sin 90) and as you decrease the angles, the force decreases)
Flemmings left hand rule
Ensure your fingers are at right angles to each other.
Where your thumb point will show the direction of the force.
Point your index North to south.
Point your middle finger in the direction of the CONVENTIONAL current.
The couple on a coil in a magnetic field
When we place a coil of wire in a magnetic field, the magnetic field exerts a force on the wire. Each side of the coil experiences a force, which is equal in magnitude, but opposite in direction. Because the forces act at different points, they don’t cancel each other out, but instead cause a turning effect.
Why does the reading on a balance change when a current-carrying wire is placed between the poles of a magnet?
The wire experiences a force F=BIL. According to Newton’s Third Law, the magnet exerts an equal and opposite reaction force on the balance.
This reaction force causes the reading on the balance to change.
Balance Reading Increases or Decreases:
If the Lorentz force pushes the wire up, the magnet is pushed down, increasing the balance reading.
If the Lorentz force pushes the wire down, the magnet is pushed up, decreasing the balance reading.
What happens to charged particles as they enter a magnetic field ?
When charged particles pass through a uniform magnetic field, at right angles to the field lines, each particle experiences a force.
Show the derivation of the force on a charged particle
The movement of charged particles is a current, the equation for current is I = Q/t.
Each particle has a velocity, v and from the equation speed = distance / time, we can see that the charged particles travel a length vt, in the time given. l=vt
Now we can sub this into the equation F=BIL
F = B (Q/t) (vt)
F = Bqvt/t
F=Bqv
What are the conditions for a charged particle moving perpendicular to a magnetic field to feel a force?
- The particle must be charges, it cannot be a neutron.
- The particle must be moving. It cannot be stationary.
- The particles must be moving perpendicular to the magnetic field lines.
Why do charged particles move in a circular path when entering perpendicular to a magnetic field?
When the charged particle enters the magnetic field, it experiences force = Bqv
This force is always perpendicular to the particles velocity, and he velocity is always changing due to the direction constantly changing.
When the particle enters the field, the force Bqv deflect it, perpendicular to its motion, and as the particle deflects and curves, its velocity changes. The force continues to act perpendicular to this velocity and this causes further deflection. This process repeats and his results into a stable circular trajectory.
The force Bqv provides the exact centripetal force and is directed towards the centre and it causes acceleration of the particles towards the centre.
Why is no work done when charged particles move perpendicular to magnetic field lines?
The equation for Work done = Force x distance (the parallel distance in the direction of the force)
However, the force is always perpendicular to the particles displacement and therefore no work is done.
We define work done as the change in kinetic energy. If work done is 0 this means there is no change in kinetic energy, and this means the speed of the particle speed not change.
What is the equation for the radius of the circular orbit?
Centripetal force = mv^2/r
Magnetic force = Bqv
Equate these
Bqv = mv^2/r
Rearrange for r
r = mv/Bq
Describe what happens inside a cyclotron, take your particle as a proton
A proton is released from the source in the centre of the cyclotron. This proton passes the uniform electric field that is present between the dees, and this electric field causes the proto to accelerate towards the negative dee due to attraction.
When the proton reaches the dee, it follows a circular trajectory with a constant speed and radius of mv/Bq. Now when the proton reaches the gap again, the alternating voltage supply switches the polarity of the dees, and the proton passes the electric field and accelerates again. Now it has a greater speed and as it enters the dee, it has a greater radius according to mv/Bq.
This process repeats, the proton accelerates, increases in speed, therefore increases in its radius, and eventually the proton leaves the cyclotron.
