Fields Flashcards
Gravitational Potential
Work done (against the field) per unit mass to move an object from infinity to a point in the field.
Newtons law of gravitation
The force of attraction between two point masses is proportional to the product of their masses and inversely proportional to the square of the distance between them.
Properties of geostationary orbit
- a type of geosynchronous orbit
- orbits over equator
- fixed position relative to earth
- time period of one day
- about 36000km from earths surface
Faradays Law
The induced e.m.f is directly proportional to the rate of change of flux linkage.
Lenz’s Law
The induced e.m.f is always in such a direction as to oppose the change that caused it.
Coulomb’s law
force between two (point) charges is proportional to (product of) charges and inversely proportional to the square of their distance apart
The energy of an orbiting satellite is…
constant
Why is the energy of a satellite in circular orbit constant
Because the distance from the mass is constant
Why is the energy of a satellite in elliptical orbit constant
Because as height decreases, a satellite speeds up, so its kinetic energy increases and potential energy decreases (so energy remains constant)
Properties of low orbit satellites
- Below 2000km
- Used for communication
- Orbit over north and south pole
The energy of a satellite
Potential + kinetic energy:
E = - GMm/2r
Electric field strength
Force per unit charge on a positive test charge placed at that point, (N/C)
E is proportional to
Q/A
Electric potential
Work done per unit positive charge on a positive test charge when it is moved from infinity to that point
Gravitational Field strength
The force per unit mass on a small test mass placed in the field
Capacitance
Charge stored per unit pd
Time constant
Time take for capacitance to fall to 0.37 of its initial value
Dialectrics
Insulating materials that increase the ability of a capacitor to store charge (increasing capacitance of the capacitor) because each molecule of the dielectric becomes polarised. The positive side of the dielectric attracts more electrons from the battery onto the negative plate. The negative side of the dielectric pushes electrons back to the battery from the positive plate.
To increase the capacitance of a capacitor
Make the area as large as possible, make the plate spacing as small as possible and fill the space between the plates with a dielectric with a high relative permittivity.
Flux linkage and Emf are
90 degrees out of phase
What happens in power stations
magnet spins instead of a coil, (50 Hz), different phases have different emfs
Back emf
The emf that is induced to oppose the current that caused it
Opposing emf is low in a motor when
The motor is spinning slowly due to high load
Opposing emf is high in a motor when
The motor is spinning fast when unloaded
V supplied in a motor (IR)
V supplied - Back Emf
Left hand rule
Motor effect - current makes force
Right hand rule
Generator rule - force makes current
North pole produces
anti-clockwise current
South pole produces
Clockwise current
Describe how a downwards travelling north facing magnet is affected when travelling through a coil
North pole created at the top of the coil (repulsion due to Lenz’s law), terminal velocity reached, halfway through, the polarity of coil switches and north tries to prevent the magnet from leaving.
How are masses detected? (equation too)
Heavier atoms have a larger radius when subject to a magnetic field
r = mv/BQ
Velocity selector equation
V = E/B
Cyclotron equation
Need radius to increase as they cross the gap, ac current allows polarity to switch every 1/2 turn. f = BQ/2piM and T = 2piR/V
A free electron in a magnetic field will undergo
Circular motion, speed and ke are constant, F is perpendicular to the displacement
What is a cyclotron and how does it work
A cyclotron consists of two semicircular electrodes placed inside a uniform magnetic field. Charged particles are fired into the centre of the cyclotron where they are deflected by the magnetic field into a circular path. When they reach the edge of one semicircular electrode they accelerated by a potential difference between the electrodes until they reach the second semicircular electrode. The magnetic field again deflects the particle so it still follows a circular path in the second electrode, but the radius of the particle’s path is larger as its speed has increased. It returns to the first electrode and this process continues until the particle has been accelerated to a high speed and is then released from the cyclotron.
If a coil is parallel to the field and is turned by a small angle, will it produce a maximum or minimum magnitude of emf and why?
Maximum, because when it is parallel, the flux linkage in at a minimum (as no field lines are being crossed). As soon as it turned by a small angle, there is a big change in flux linkage resulting in a high emf being produced.
If a coil is perpendicular to the field and is turned by a small angle, will it produce a maximum or minimum magnitude of emf and why?
Minimum, because when it is perpendicular, the flux linkage in at a maximum (as the maximum field lines are being crossed). As soon as it turned by a small angle, there isn’t a massive change in flux linkage as most lines are still being crossed resulting in a minimum emf.
