6.3 Electromagnetism Flashcards
what is a magnetic field?
a magnetic field is the region around a permanent magnet or a moving charge in which another body with magnetic properties will feel a force
what do magnetic field lines show?
- the shape of the field
- the direction of the field
magnetic field lines always go from…
NORTH to SOUTH
what is the symbol for current going into the page?
a circle with a cross, think arrowhead moving away from you
what is the symbol for current coming out the page?
a circle with a dot, think arrowhead coming towards you
what do you always get around a wire carrying an electric current?
a magnetic field
what is a solenoid?
a coil of wire
what rule can be used to work out the magnetic field around a current carrying wire? and what are the two differences in the rule for a simple wire and a solenoid?
you use the right hand rule where…
thumb = current
fingers = field direction
FOR A STRAIGHT CONDUCTOR^
thumb = field direction
fingers = current (curl your fingers)
FOR A SOLENOID^
when you have a current perpendicular to a magnetic field what happens?
a force is induced
(this force comes from the interaction between the magnetic field of the wire and the magnetic field of the external magnets)
what rule can you use to work out induced force from a current carrying wire being perpendicular to a magnetic field?
Fleming’s left hand rule, where…
thumb = force
index = field
middle = conventional current (positive to negative)
what is the equation for the induced force on a current carrying wire in a magnetic field?
F = BILsinϴ where F = induced force B = magnetic flux density I = current in the wire L = length of current carrying wire in the field
when is the induced force from a current carrying wire in a magnetic field at its maximum?
when the current carrying wire and magnetic field are perpendicular, i.e F = BILsinϴ when ϴ = 90
outline an experiment to investigate flux density (calculate B)
- set up a digital balance with a coil of wire in a magnetic field at 90 degrees on top of the balance, connect the wire to a circuit to produce a current, have an ammeter connected in series
- the power supply should be connected to a variable resistor to be able to alter the current, zero the balance when there is no current so the mass reading is only due to the electromagnetic force not the weight force, then turn on the power supply
- note the mass and current, then change the current by altering the variable resistor and record the new mass and current, do this for a range of currents and repeat to get averages
- convert the mass into force using w = mg, plot a graph of F against I and the gradient will be equal to B x L (because of F = BIL)
- measure gradient and divide by length, L, of wire in the field to get B, magnetic flux density
what is the equation for magnetic flux when the area cuts through the field at 90 degrees?
Φ = B x A magnetic flux (Φ) = magnetic flux density (B) x area at right angles to the flux (A)
what is magnetic flux density measured in?
tesla, T
what is magnetic flux measured in?
weber, Wb
what is the equation for magnetic flux when the area that has been cut is not at 90 degrees to each other?
Φ = BAcosϴ
what is magnetic flux density defined as? (in words)
magnetic flux density is a measure of the strength of the magnetic field and is defined by the equation for the force on a current-carrying conductor in a magnetic field, F = BILsinϴ
what is 1 tesla equal to?
1 tesla is when a wire perpendicular to the field with a current of 1 ampere experiences a force of 1 newton per metre of length
is tesla a large or small unit?
a very LARGE unit, the Earth’s magnetic field is roughly equal to about 60 micro tesla
what is the equation for the induced force on a single charged particle in a magnetic field?
F = BQV
how do you derive F = BQV?
F = BIL I = Q / t V = L / t so L = Vt F = B x Q x Vt / t F = BQV
how do charged particles move in a magnetic field? and why?
they are deflected in a circular path (circular motion) because the force is acting at right angles to the direction of motion, no net force so particles have constant speed