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Mag&electric fields
| Question | Answer |
|---|---|
| Name 5 magnetisable materials | -Iron -Cobalt -Nickel -Steel -Neodymium |
| Define flux line | The path taken by a small test magnet when placed in that field |
| Define Test magnet | A north pole test magnet is modelled to experience a magnetic force, it is small enough to have no effect on that magnet |
| If flux line density increases, magnetic field strength | Increases |
| Where is magnetic field strength highest | At the pole |
| What are the three ways to demagnetise a magnet | -A physical shock will randomise the dipoles -Raising the temperature above the curie temperature if we induce a field -We can semi-randomise the magnet by alternating the direction of the core for ever-decreasing amounts of time (de-gaussing) |
| What is the difference between a soft iron core and a hard iron core | Soft iron-temporary magnet, increases flux line density within a solenoid Hard iron-permanent magnet |
| What is the generator effect | -Flux lines cut by coil -Due to relative motion between coil and magnet -A p.d is produced -If circuit is complete current will flow |
| How do we increase the p.d of a generator | -More turns on coil-> More flux cut -Higher field strength magnet -Turn the magnet faster-> Faster relative motion so increased rate of flux cut |
| Explain how the bell circuit works | -Circuit on and complete therefore current flows -Solenoid magnetised -Armature attracted to solenoid -Hammer hits ball -Circuit incomplete -Demagnetises solenoid -Hammer moves back and completes the circuit |
| What is the equation for force on a current carrying wire | F=BIlsinθ |
| Define magnetic flux density | The force per unit length per unit current on a current carrying wire at right angles to a magnetic field |
| Define tesla | The strength of a magnetic field that produces a force of 1N in a wire of length 1m with 1A flowing perpendicular to the field |
| Define ampere | One unit current produced in two infinitely long straight wires 1m apart in a vacuum which produces a force of 2x10^-7N/m in each wire |
| In Fleming's left and right hand rule what do the different fingers mean | ThuMb-Motion First finger-Field SeCond finger-Current |
| What is the hall effect | A charged particle is travelling in a magnetic field would experience a force proportional to -Magnetic flux density(B) -The charge on the particle(Q) -The velocity of the particle(v) |
| What is the equation for the hall effect | F=BvQ |
| What is the equation relating emf to magnetic fields | Ɛ=Bvl |
| Describe the hall slice | -Consider a piece of conducting material in a mag field of flux density B -Field is north-south -Current flows right-left -Electron flow is left-right -Force on e- acts towards you (out of paper) -+ve charge builds up on front face -P.d is induced |
| What is the equation for the hall voltage | Vhall=BI/net |
| What do the thumb and fingers mean in the right hand grip rule | Thumb-> Current Fingers-> Field |
| Define relative magnetic permeability | The field strength with the material divided by the field strength without the material in a toroidal (endless) solenoid |
| What is the value of magnetic permeability of free space | 4πx10^-7 |
| What is the equation for magnetic permeability of a wire | B=μ0I/2πr |
| What is the equation for magnetic permeability at the centre of a long solenoid | B=μ0nI |
| What is the equation for magnetic permeability in a short coil | B=μ0NI/2r |
| What are the equations for magnetic permeability at the end of a long coil | B=μNI/2l B=μnI/2 |
| What are the equations for Faradays law | Ɛ=BA/Δt Ɛ=ϕ/Δt |
| Define Faradays law | Induced emf in a circuit is equal to the rate of change of flux linkage |
| Define magnetic flux | The product of flux density and area |
| What is the equation for magnetic flux | ϕ=BA |
| Define Lenz law | The induced emf is always in a direction so as to oppose the change that caused it |
| What are the applications of Lenz law | Electromagnetic breaking Protection of motor coils |
| Explain how back emf occurs | -When flux is cut, emf is induced -Emf acts in the opposite direction to the applied p.d (Lenz) -The size of this emf is proportional to the rate of flux cut (Faraday) |
| What is the equation for power of a motor | P=IƐ+I^2r |
| How can we reduce power loss in a motor | -Low resistance windings -Laminated iron core to reduce eddy currents |
| What are the equations for flux linkage | Φ=ϕN Φ=BAN |
| What is Newman's equation | Ɛ=-Nϕ/Δt=-Φ/Δt |
| Explain what happens when you drop a magnet through a coil | -Magnet passes through coil -Flux cut -Induced emf ∝ rate of flux cut -Magnet approaches coil p.d generated in + direction as north cuts flux -Magnet leaves coil opposite happens - -p.d > +p.d as it has >v -Area under peaks equal as same flux cut |
| On the graph of magnet passing through long coil why is there a flat section in the middle of the graph | Both ends of the magnet are passing through the coil so the north and south cancel out |
| Why do we use step up transformers | Increase the voltage and therefore decrease the current so that less energy is dissipated through the heating effect |
| Why do we use step down transformers | Decrease the voltage and therefore increase the current to prevent sparking |
| Why do we use an AC current in transformers | -An AC current is used in the primary coil -This produces a continually changing magnetic field -That cuts across the secondary coil -Induces an alternating voltage in the secondary coil -Which is different to the primary coil |
| What is the transformer equation | Vp/Vs=Np/Ns |
| How do you calculate RMS current | Irms=I0/√2 |
| How do you calculate RMS voltage | Vrms=V0/√2 |
| Explain charging by friction | -Rag is rubbed on rod -Rag gives/takes electrons -Rod and rag becomes charged |
| Explain charging by induction | -Introduce a negatively charged rod -Electrons in object are repelled and move away -Therefore object has an overall positive charge -Therefore object is attracted to rod |
| Why can more charge be concentrated on a pointed object | They have a higher surface area |
| Explain the action of a point | -Pin is placed on van de graft generator -Van de graft generator is negatively charged -Electrons in the air are repelled -Air is ionised -Electrons in the pin are attracted to the ionised air causing electron streaming |
| Explain how a lightning conductor works | -Thundercloud is -vely charged --ve charges in building repelled -Building left with +ve charge -Electrons attracted to building -Lightning hits lightning conductor as it's the highest point -Lightning conductor also takes the -ve charge out of cloud |
| Explain how the gold leaf electroscope works | -Negatively charged rod introduced -Negative charges in the plate move onto conducting rod and gold leaf -Conducting rod and gold rod repel -The amount by which the gold leaf moves is proportional to charge |
| Define electric field line | -Path taken by a positive test charge when place in that field |
| Define coulomb | -The charge passing a point each second when a current of 1A is passed |
| What is the charge of an electron | 1.6x10^-19C |
| How many electrons are there in one coulomb | 6.25x10^18 |
| What do we use to measure small charges | Electrometers |
| How can we detect tiny charges | Gold leaf electroscopes (they can be detected but not measured) |
| Define electric field strength | The force per unit charge on a positive test charge in that field |
| What is the equation for electric field strength | E=F/Q |
| How can we tell if a field is uniform | -E is the same at all points in a field -Field lines are parallel -F on a test charge is constant |
| Define electric potential | The potential energy per unit charge of a positive test charge placed at that point |
| What is the equation for acceleration in an electric field | a=EQ/m |
| What is the equation for electric potential | V=E/Q |
| What is the equation for electric field in uniform field | E=V/x |
| What is the electric field strength within a charged sphere | 0 as the overall effect of the charges would cancel |
| What is the equation for electric field strength of radial fields | E=Q/(4πƐ(0)r^2) |
| What is the equation for electric potential in a radial field | V=Q/4πƐ(0)r |
| Define potential gradient | The potential gradient at any point in a field is the change of potential per unit distance at that point |
| What are the equations for potential gradient | Potential gradient=V/x=F/Q |
| What is the equation for work done in a radial field | W=Er |
| Describe Millikan's oil drop experiment | -Oil drops are squirted by vaporiser -Oil drops are charged by friction -Oil drops go through hole in between plates -If it is switched off they fall at T.V -If it is on they are held stationary by the electric field |
| What is the equation for Millikan's oil drop experiment when it is switched off | mg=6πrηv |
| What is the equation for Millikan's oil drop experiment when it is switched on | 6πrηv=VQ/x |
| What is the equation for charge in Millikan's oil drop experiment | Q=6πrηv(term)x/V |
| What is the equation for terminal velocity in Millikan's oil drop experiment | v(term)=(2(r^2)g(ρ(oil)-ρ(air))/9η |
| What is the equation for charge in Millikan's oil drop experiment (NO r) | Q=(6πrηv(term)x/V)√((9ηv(term))/(2gρ(oil))) |
| What is the equation for coulombs law | F=(1/4πƐ(0))(Q1Q2/r^2) |
| Compare gravitational and electric fields | G all mass: E all charge F=mg:F=QE G=F/m:E=F/C G equi line equal G energy: E equi line equal E energy F=Gm1m2/r:F=kQ1Q2/r G Monopolar:E dipolar Point mass radial G:Point charge radial E Near sphere; uniform:Near sphere charge/parallel platesuniform |
Created by:
Liam P
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