23 - Magnetic Fields Flashcards
What causes a magnetic field?
- Permanent magnets
- Moving charges
What do we use to map magnetic fields?
Magnetic field lines
Describe the magnetic field lines for a straight wire.
Concentric circles around the wire
Describe the magnetic field lines for a bar magnet
Loops from north pole to south pole
Describe the magnetic field lines for a ring (circle)
Concentric circles around ring obeying RHR
Field lines going through centre of ring
Describe the magnetic field lines for a solenoid.
Like bar magnet but with field lines going through core of solenoid.
What is the magnetic polarity of the Earth’s south pole (Antarctica)>
North
they are reversed
What do the fingers stand for in Flemming’s left hand rule?
First finger - Force
seCond finger - Current
thuMb - Motion
When a current carrying wire is placed in a magnetic field, what experiences a force?
The wire and the magnet creating the field both experience an equal and opposite force.
What does the equation F = BILsin0 give us?
Why does it contain sin0?
The force experienced by a current carrying wire in a magnetic field.
Because current and magnetic field must be perpendicular for a force. Sin0 gives us the perpendicular component of the current if it is not already 90, where sin90 = 1.
When do we use the equation F=BIL?
When the current and magnetic field are at right angles.
sin90 - 1
How can you experimentally determine the force exerted on a wire in a magnetic field and therefore find magnetic flux density?
Place opposite poles on a digital balance + set to zero.
Run wire through gap between poles.
Force on balance equal to force experienced by wire.
Mass on balance * g = F
B = F/IL (Measure current and length with ammeter + ruler)
What does B stand for?
Magnetic flux density
What is the unit for magnetic flux density?
T (Tesla)
What is the formula for the force on a single charged particle?
F = BQv
F=BIL I=NQ/t F=BNGL/t distance / time (L/t) = v (velocity) F=BNQv Force per particle = /N F=BQv
Describe and explain the path of a charged particle moving in a uniform magnetic field.
Circular motion.
Force will always be at right angle to both current (direction of travel) and magnetic field.
Therefore, force will act as a centripetal force, keeping the particle in circular motion.
How do we show something travelling into a plane (into paper)?
X
cross
How do we show something travelling out of a plane (out of paper)?
.
dot
How do we calculate the radius of circular motion of a particle in a magnetic field.
BQv = mv^2/r
r = mv/BQ
Describe how a velocity selector works.
A magnetic field is placed across an electric field.
Charged particles are fired into the selector through a small gap with different velocities.
They experience a force in opposite directions due to the two different fields.
Because F = BQv, the force caused by the magnetic field is proportional to the velocity.
Particles travelling only at a certain velocity will experience an equal force from both fields and will travel in a straight line, emerging from the selector.
What equations must we relate for a velocity selector?
BQv = EQ
v = E/B
Describe briefly how mass spectrometers work.
Rough understanding for context
Velocity selector - all with same v
Enter magnetic field and experience circular motion.
Since r = mv/BQ, r is proportional to m.
All of the particles of different masses are deflected by a different amount onto a detector.
Particles can then be categorised by their masses.
How can an emf be induced in a coil?
When there is a change in the magnetic flux linking the circuit.
Define magnetic flux.
EQUATION
The product of the component of the magnetic flux density perpendicular to the area, and the cross sectional area.
Φ = BAcosθ
When do we use Φ = BA?
When the magnetic field is normal to the area.
What is flux linkage?
The magnetic flux multiplied by the number of turns in the coil.
NΦ
Unit Wb turns
What is the unit for magnetic flux?
Webber (Wb)
What can change in order to induce an emf?
Φ = BAcosθ
B, A or cosθ can be changed to change the magnetic flux and induce an emf.
What is Faradays law?
Emf is directly proportional to the rate of change of magnetic flux linkage.
ℰ = -ΔΦ/Δt
What is Lenz’s law?
The direction of the induced emf and current is always such as to oppose the change producing it.
(This is why the constant of proportionality in Faraday’s law is -1)
Why must Lenz’s law be true?
Because of the conservation of energy.
In order to produce electrical energy, work must be done against the direction of motion.
How does a simple AC generator work?
As a coil of wire rotates in the magnetic field, cosθ changes. This means that Φ changes and so an alternating emf is induced.
How can we calculate the emf of an AC generator from a graph of magnetic flux linkage against time?
The gradient
Differentiate a cosine graph = sine graph
What will happen to the voltage if there are double the number of output coils on a transformer?
The potential difference will double.
How does a simple transformer work?
AC current supplied to primary coil.
Causes alternating magnetic flux linking the two coils.
Alternating magnetic flux linkage in the secondary coil induces an alternating emf and therefore current.
Why do transformers have an iron core?
So that none of the magnetic flux linking the two coils is lost.
What is a laminated iron core?
A core made up of layers of iron separated by an insulator.
What does a laminated iron core achieve?
Prevents currents in the core itself where energy would be lost through heating.
Improves the efficiency of the transformer.
Why are large voltages and low currents used for transmitting energy over the national grid.
To minimise heat loss.
Define the Tesla.
The magnetic flux density when a wire of 1m, perpendicular to the magnetic field ,experiences a force of 1N when a current of 1A flows through it.
Define magnetic flux density.
The force experienced by a wire perpendicular to a magnetic field, per unit length of the wire and per unit current through it.
