Magnetic Fields Flashcards
Right hand rule
Used to determine the direction of a magnetic field around a current-carrying wire
Thumb represents direction of current
Curled fingers represent direction of field
Increasing current
Also used to determine the direction of a magnetic field in a solenoid - fingers represent current, thumb for field direction
Fleming’s left hand rule
Used to determine direction of force when a current-carrying wire has a flow of current parallel to the direction of the external magnetic field
Thumb - force direction
First finger - direction of uniform magnetic field
Second finger - direction of current
Magnetic flux density
AKA magnetic field strength - increases with current
The strength of the magnetic field that produces a force of 1 N on 1 metre of wire carrying a current of 1 Amp perpendicular to the magnetic field (Tesla definition)
Measures in teslas, T
1 Tesla = 1 Newton per Amp metre
T=N/Am
Moving a charged particle in a circular path
F=mv^2/r and F=BQv
r = mv/BQ
Radius of curvature increases (less deflection) with increases in mass or velocity
Radius of curvature decreases (more deflection) with increases in field strength or charge
Line of force - magnetic field line
A line along which a North Pole would move in the field
Motor effect
The force on a current-carrying conductor due to a magnetic field
Investigating flux density
Use a wire connected to a mass on a balance going through a magnetic field perpendicular to the field
Record initial mass and length
Record mass for each value of current
Covert mass to force = mg
Plot a graph of force against current - gradient = Bl
Magnetic force on a charged particle
Results in a circular orbit - force of the magnetic field is at right angles to the motion
Doesn’t change the speed no work is done
F = BQv F = mv^2/r
r = (mv)/BQ
Cyclotron
Made of two semicircular electrodes with a uniform magnetic field applied perpendicular to the plane of the electrodes
An alternating potential difference is applied between the electrodes
Used to produce radioactive tracers or high energy radiation beams - radiotherapy
How does a cyclotron work?
Charged particles are fired into than electrode the magnetic field makes them follow a semicircular path and leave the electrode
P.d accelerated the particles across the gap into the other electrode
This increases the speed so it’s path has a larger radius
P.d reversed and the process repeated increasing speed and spiralling outwards until it leaves the cyclotron
Magnetically hard and soft
Hard -> Permanent magnets
Soft -> temporary magnets
Magnetic materials
Iron
Nickel
Cobalt
Steel
Inducing magnetism
Magnetism can be induced of a material is placed in a magnetic field
Increasing field strength of a solenoid
Increase the current
Increase the number of turns in the coil
Increasing force on a wire in a magnetic field
Increase magnetic field strength - flux density
Increase current
Increase length (e.g. More coils)
Mass spectrometer
Used to analyse the type of atoms present
The atoms are ionised and directed at a narrow beam into a uniform field
Ions are all at the same velocity - a velocity selector ensures this
Each ion is deflected by a different amount onto the detector - allowing them to be identified
Velocity selector (for mass spectrometer)
Only allows ions through travelling at a certain velocity, v
Same velocity is important when identifying ions as it impacts radius
v must be such that the force from the electric field is equal to the force from the magnetic field
BQv = EQ
Alternating current
The charge carriers in a circuit repeatedly reverse their direction if an alternating p.d. is applied
Frequency of a.c. = number of cycles per second
Mains electricity has a frequency of 50Hz
Peak value of p.d. or current is maximum value in either direction
Peak to peak value = 2 X peak value
Peak value of mains = 325 V
Root mean square value of mains = 230V
Fleming’s left hand rule for a moving electron
Remember current direction is in the OPPOSITE direction to the direction of the electrons motion
Then just use the left hand rule to find the direction of the force
When is the magnetic force on a charge when in a magnetic field 0?
Charge is stationary
Charge is moving parallel to the field
