Magnetic Fields Flashcards
What is a Magnetic field?
a region where a force is exerted on a magnetic material
What is the direction of the magnetic field?
North to South
What do the field lines look like on a current carrying wire?
the field lines are concentric circles centred around the wire
How can we find the direction of the magnetic field for a current carrying wire?
right hand rule
What is a solenoid?
consists of a wire coiled up into a spiral shape
What happens when you place a current carrying wire in a magnetic field?
the field around the wire and the field from the magnets interact. the field lines from the magnet contract to form a “stretched catapult” effect where the flux lines are closer together. i.e causes a force on the wire
what happens if the current is parallel to the magnetic field lines?
- what condition must be kept?
no force acts
- the current has to be perpendicular
How do you find the direction of the force?
- what does each finger represent?
- Flemings Left Hand rule
- thumb: force
first finger: magnetic field
middle finger: current
What is the equation for the Force on a current-carrying wire in a magnetic field?
F= BIL
What is the magnetic flux density?
Units?
- the force per unit length per unit current on a current carrying conductor at right angles to the magnetic field lines.
- Tesla, T
what is magnetic flux density a measure of?
the number of field lines passing through the region of space per unit cross-sectional area
What is the equation for the Force on charged particles moving in a magnetic field?
F= Bqv
How are charged particles deflected in a magnetic field?
charged particles are deflected in a circular path
what is an application for the circular deflection of charges particles in a magnetic field?
a cyclotron, used for radiotherapy
What is the function of a cyclotron?
uses a magnetic field to keep charged particles moving in a circle and electric field to accelerate
-produces X-rays
what is the equation for the radius of curvature of a charged particle?
r= mv/BQ
How does a cyclotron work?
There are two Dees, which are electric plates that cause charged particles to move in a circle and increase in speed between the plates (due to the magnetic force) This increases their radial displacement. the polarity of the plates are reversed to produce a spiral motion.
what is the equation for the time period of a particle in an orbit in a cyclotron?
t= 2πm/BQ
what is a velocity selector?
a device that uses perpendicular electric and magnetic fields to select charged particles with a specific velocity
What feature does the ion need to have to go through a velocity selector?
equal top charge
what is magnetic flux?
the magnetic field lines passing through a given area
what is magnetic flux linkage?
the product of the magnetic flux and the number of turns on a coil through which the field passes
what is the equation for magnetic flux?
φ= BAcosθ
what is the equation for magnetic flux linkage?
Nφ= BANcosθ
what is the equation for Faraday’s law?
emf= N Δφ/Δt
What is Faraday’s law?
the magnitude of induced emf is equal to the rate of change of flux linkage
What is Lenz’s law?
the direction of induced current is such as to oppose the motion causing it
what is the equation for Lenz’s law?
emf= - (N Δφ/Δt)
explain Lenz’s law with the magnet approaching a coil analogy
- As the magnet approaches the coil, there is a change of flux through the coil so an emf and a current is induced.
- The direction of induced current is opposite to the motion of the magnet. (with repulsion)
- As the magnet passes through the coil there is no change in flux
- As the magnet leave the coil, there is a change in flux, so current is induced which opposes the motion of the magnet (with attraction)
what is the equation for maximum emf?
ε= ωBAN
what is the equation for minimum emf?
ε= -ωBAN
What is the equation for emf?
ε= ωBANsin(ωt)
what is an alternating current?
current that changes direction
what is an oscilloscope?
device which shows the variation of voltage with time
-what would an oscilloscope show for a direct current?
-what about when the time base is switched off?
-trace will show a straight line parallel to the axis
-only a dot will be seen on the screen
-what would an oscilloscope show for an alternating current?
-what about when the time base is switched off?
-a sinusoidal waveform
-a straight vertical line
what is peak voltage on an oscilloscope?
distance from the equilibrium to the highest point
what is peak to peak voltage on an oscilloscope?
distance from the minimum point to the maximum point
what is the root mean square (rms) voltage on an oscilloscope?
the average of all the squares of the possible voltages
What is the equation for rms current and rms voltage?
Iᵣₘₛ= I₀/√2
Vᵣₘₛ=V₀/√2
what is the electricity supplies in homes in the UK in volts?
230Vᵣₘₛ
what is the equation for power with peak current?
P=1/2 x I₀²R
how does a transformer work?
- an alternating current flows in the primary coil
- this causes an alternating magnetic field in the soft iron core which means the flux linkage in the secondary coil is constantly changing therefore and alternating emf is induced in the secondary coil
-what is an ideal transformer?
-what is the equation for an ideal transformer?
-all the flux linkage produced by the primary coil passes through the secondary coil
I₁V₁=I₂V₂
what is the transformer equation?
V₂/V₁=N₂/N₁
what is the efficiency equation for a transformer?
e(I₁V₁)=I₂V₂
what is a step up transformer?
increases the a.c/p.d as the secondary has more turns
what is a step down transformer?
decreases a.c/p.d as the secondary has less turns
what are eddy currents?
when there is a continuous change in flux, an emf is induced, causing currents known as eddy currents to occur
what the main causes of energy loss in a transformer and their solutions?
-eddy currents (laminated core)
-resistance in the coils (solved by using thick wire)
-if the core is not easily magnetised (magnetically soft iron core is then used)
how can the effect of eddy currents be reduced?
-using a laminated iron core, meaning that the core is made using layers of iron between layers of an insulator so amplitude is reduced
-core made of a high resistivity metal
