Module 6 Magnetic Fields Flashcards
What is the symbol used for magnetic fields?
B fields
What are the two ways of making a magnetic field?
With a permanent magnet
Moving chargers such as electric currents through wires
What is the direction of magnetic field lines?
north to south
How do you draw a magnetic field or current at 90º into a piece of paper?
a circle with a cross in it
How do you draw a magnetic field or current at 90º out of a piece of paper?
circle with a dot
How to remember which way is which for circles with dots and crosses when showing directions in and out of paper?
cross is into paper as that’s what you would see if you fired an arrow into a paper
What is Maxwell’s right hand grip rule?
make a thumbs up. Direction of thumb is direction of current in a wire and fingers are direction of magnetic fields around the wire
How can RH grip rule be used for solenoids?
thumb is direction of magnetic field, fingers are direction of current in the wire
What is a solenoid?
a coil of wire with length much greater than the diameter
How can the B field inside a solenoid be described?
strong and uniform
What is a flat coil?
literally a single coil of wire
What happens when a current carrying wire which is at 90º to a magnetic field?
wire will experience a force at 90º to both
What is the formula for the force exerted on a wire by a magnetic field?
F=BILsinθ
What are the units for magnetic fields?
Tesla (T)
Define the tesla?
a flux density of one Tesla produce a force of 1 Newton on a wire of length 1 metre carrying a current of 1 Ampere at 90º to the field
How to remember which finger represents what In Fleming’s left hand rule?
F B I thumb first second just like in the equation
What happens if the magnetic field and current are in opposite directions?
no force is generated
(Current parallel to B field)
What is another name for magnetic field strength?
magnetic flux density
symbol B (in Tesla)
Describe an experiment to measure magnetic flux density
Place two magnetic poles at 90º to a current carrying wire over an electronic balance
Zero the balance once current is zero
Power supply is turned on to increase current which is measured on the ammeter
For each current measured the mass reading is taken on the electronic balance
Length of the wire inside the field is measured
Plot a graph of force against current, gradient = BL
Describe how an electric motor works
a square wire with a current flowing through it is placed into a magnetic field
A force is exerted onto the wires which are at 90º to the magnetic field
This is considered a couple
Makes wire rotate (couple causes torque)
Show that the torque of a wire in an electric motor is = BI(area)
Torque = Fd with F = BIL
Torque = BILd
When using Flemings left hand rule for a moving negative particle, which direction is the current taken as?
opposite to velocity of negatively charged particle (ie conventional current)
Explain why the motion of a moving charge in a magnetic field can be considered as circular motion?
F remains at 90º to direction of motion
(no displacement in direction of the force, no work done, constant KE and speed)
Explain how the formula for centripetal force can be used with F = BQv
F = mv^2/r
This is the same F as in F = BQv
As F remains at 90º to direction of motion and thus can be considered a centripetal force
What is another notation for F = BQv for an electron?
F = Bev
What is a velocity selector?
When a moving charged particle
Passes through a B field and E field which are perpendicular to each other
And experiences no deflection if v=E/B
Derive is the formula for a velocity selector?
BQv=EQ
Q’s cancel out
Bv=E
v = E/B
Draw the arrangement of magnetic field and metal plates for electric fields to produce a velocity selector (for a negative particle)
Force from magnetic field B is vertically downwards
Force from electric field E is vertically upwards
What are the three stages to mass spectrometer?
Acceleration of charged particle
Velocity selector
Deflection
What happens during acceleration in mass spectrometer?
Acceleration of charged particles
QV = 1/2mv^2 so ions gain a speed towards a positive anode
Some ions pass through holes in an anode to form a narrow beam
What happens during the velocity selector section of the mass spectrometer?
Crossed E and B fields
No deflection for ions whose speed is v = E/B
What happens during deflection section of the mass spectrometer?
Only a magnetic field is present
Ions that pass through have a circular path
BQv = mv^2/r
What does the value of r radius and d diameter depend on in the final stage of mass spectrometer?
mass of ion, magnetic flux density
and speed (although all ions have the same speed after the velocity selector)
When drawing concentric rings around a wire what must you do?
Use the right hand grip rule to find direction of B field
Rings get further apart as B field gets weaker
