Magnetic Fields Flashcards
What 4 things do magnetic fields affect?
- Charges moving in the field
- Conductors with a current passing through
- Other magnets
- Magnetic materials
What do the field lines for a bar magnet look like?
Field lines always act North → South
What do the field lines look like between two opposite poles?
Field is uniform between the poles
What do the field lines look like between two like poles?
How are field lines represented ‘going into the page’?
How are field lines represented ‘coming out of the page’?
This conductor in a magnetic field has a current passing through
But doesn’t experience a force
Why?
Because it is parallel to the field lines
When do you use Fleming’s Left hand Rule and what does it actually show?
- Looking at DC motors
- Looking at charges moving in a magnetic field
B shows the direction of the field from the north to south pole.
I shows the direction of current from the +ve to -ve terminal.
However, this doesnt apply for single charges moving in circular motion.
How do you calculate the force on a conductor placed at an angle in a magnetic field?
First use trigonometry to calculate the perpendicular component of its length
It is not important to use sin θ for these situations, but rather to find either the perpendicular component of length or field strength
Why does the reading on the balance increase when a current runs through the conductor?
The magnetic field pushes up on the conductor
So the conductor pushes the magnets down (Newton’s 3rd Law)
How can you increase the mechanical energy produced by the DC motor?
Increase the torque by:
What do the commutator rings do in the DC motor?
Switch connections of the bars every 180°
So direct current is produced and the coil rotates in a circle instead of backwards and forwards
What happens if the commutators are removed from the DC motor?
Force on each bar won’t change
So coil will reach equilibrium in vertical position
And won’t continue spinning
Why does an electron move in a circular path in a magnetic field?
Force from magnetic field perpendicular to velocity of electron
How do you apply Flemming’s left hand rule to a negative charge moving in a field?
Current acts opposite to the velocity
How do you apply Flemming’s left hand rule to a positive charge moving in a field?
Current acts in the same direction as the velocity
When should you use each equation?
F=BIL on a conductor in a magnetic field (with current)
F=BQv on a charge in a magnetic field (moving)
How do you calculate the radius of the orbit of a charge moving in a magnetic field? (this is a derivation)
Equate the magnetic and centripetal forces
(this equation is only true for moving charges in a magnetic field)
How do you explain the different curvatures of radiation passing through a magnetic field?
Greater the specific charge → Smaller r (Bigger deflection)
How do you calculate the speed of a charged particle accelerated through an electric field?
How much work does a magnetic field do on a moving charge?
0J because the force and velocity vectors are perpendicular
So the charge does not increase its kinetic energy
In a mass spectrometer how does the velocity selector work?
Unless an ion’s velocity = E/B, it will travel in a parabola and miss the gap
In a mass spectrometer how does the mass separation work?
The ions have the same velocity (from the velocity selector)
So deflect by specific charge
In a particle accelerator why are both magnetic and electric fields needed?
In the cyclotron what is the purpose of the alternating current and magnetic field?
Alternating current → Causes the Electric Field between ‘Dees’ to switch which Dee is positively charged to cause the force on the electron to change direction each time current flips
Magnetic Field → Keeps particles moving in circular motion by applying a force perpendicular to velocity
In the cyclotron why is the frequency of the alternating current constant?
As the charge speeds up → Travels further in each Dee → So takes same time
How do you calculate the AC frequency of the cyclotron?
Note: f is independent of v
So the frequency is constant
How is 1 Tesla defined?
The magnetic field that applies a force of 1N to a 1 metre conductor with a 1A current flowing perpendicular to the field (B=F/IL)
How is a compass used to navigate?
The compass is made of a magnetic metal that aligns with the Earth’s magnetic field lines.
Therefore, the compass points to the geographical North Pole which is the magnetic south pole
What is magnetic flux density and its units?
The force per unit length on a conductor carrying a unit current in a direction perpendicular to the field.
Units of Tesla (T)
What are the stages of mass spectrometry?
- The sample is ionised by an electron gun in order for the electric field to be able to affect the sample
- The particles in the sample are accelerated due to their charge being affected by the electric field
- The velocity selection then happens as the electric and magnetic fields combining BQ=mv so only particles of a set velocity will pass through the end slit
- Mass separation occurs due to the magnetic field and particles with a greater specific charge travel with a larger radius and are detected later.
When must you consider negative values in magnetic fields?
Only when a question asks to ‘state the direction’ or if you are combining equations such as ‘potential’ equations
Which rule do you use for AC generators?
Fleming’s Right Hand Rule
Which rule do you use for DC generators?
Fleming’s left hand rule
What does each finger show in Fleming’s right hand rule?
F shows the force acting on the coil due to the field.
B shows the direction of the field (point in the same direction as the north pole points)
I shows the direction of current from the +ve to -ve terminal.
What do we call Field Strength and Field Lines in magnetic fields?
Field Strength - Flux Density (B) measured in Tesla (T)
Field Lines - Lines of Flux
What is the purpose of the brushes in a DC motor?
Maintain contact between the coil and the power supply while alowing the coil to rotate
What is the equation for Torque produced by a DC motor?
Torque (Nm) = BILNd
Where N is number of turns in coil and d is the distance between the coplanar forces
In a cyclotron when is the charge accelerating?
The charge only accelerates between the dees as the dees are hollow conducting disks and E=0 inside hollow conducting disks
