Fields Flashcards
What is a force field?
A region where an object will experience a non-contact force
What do force fields cause?
Interactions between particles
What force acts between masses?
Gravity
Is force a vector or a scalar?
Vector
What do field lines show?
Direction of the force
What is a radial field?
Where the force decreases the further away you are
What happens to field close to the Earth’s surface?
They become (almost) uniform
What is G?
The gravitational constant
What mathematical property does Newton’s Law of Gravitation have?
It is an Inverse Square Law
Force ∝ 1/(r^2)
What has a bigger impact in Newton’s Law of Gravitation; distance or mass?
Distance
What is gravitational field strength a measure of?
Force per unit mass
In a radial field, what is the relationship between g and r?
g is inversely proportional to r^2
What is gravitational potential a measure of?
Potential energy per unit mass
Why is gravitational potential negative?
You have to do work against the field to move the mass away
What is the gravitational potential energy an infinite distance from the mass?
Zero
Is gravitational potential positive or negative?
Negative
What is gravitational potential measured in?
J/kg
Why do you sometimes see positive gravitational potential energy?
It is the gain in potential energy
What does the gradient of a Gravitational Potential-Distance graph represent?
g
What is the energy required to move a mass through a gravitational potential difference (V)?
mV
What is an Equipotential?
A line/surface that joins all the points with the same potential (V)
What is the gravitational potential difference (V) when you travel along an Equipotential?
0
What is the work done when you travel along an Equipotential?
0
How are Equipotentials oriented in relation to field lines?
Perpindicular
What keeps an object moving in circular motion?
Centripetal force
What is the centripetal force for satellites?
Gravity
What is the force acting on an object in circular motion equal to?
F = (mv^2)/r
What is the speed of a satellite in a gravitational field?
v = sqrt((GM)/r)
What is the orbital period (T) of a satellite?
T = (2πr)/v
How do you find the relationship between T and r?
Substitute the satellite speed formula into the orbital period formula
In elliptical orbit, how does a satellite’s kinetic energy change with potential energy?
Kinetic energy (speed) increases as potential energy (height) decreases
What is the orbital period of a geostationary satellite?
24 hours
What kind of orbit does a geostationary satellite have?
Synchronous orbit
What is a synchronous orbit?
Orbital period of satellite = rotational period of primary
Where must a geostationary satellite be positioned above the primary?
Directly above the equator
Why are geostationary satellites useful for TV signals?
The satellite is stationary relatively, so the angle of the transmitter/receiver doesn’t need to change
How low does a Low Orbit Satellite orbit?
Below 2000 km
What are the benefits of Low Orbit Satellites?
Useful for communications
Cheaper to launch
What are the drawbacks of Low Orbit Satellites?
Multiple satellites working together needed
Are Imaging Satellites geostationary or Low Orbit?
Low Orbit
If the charged object is a sphere, where can you assume all of the charge acts?
At the centre
If force F acts on Q1, what is the force acting on Q2?
-F
What mathematical property does Coulomb’s Law of Gravitation have for point charges?
It is an Inverse Square Law
Force ∝ 1/(r^2)
What is electric field strength (E) a measure of?
Force per unit charge
In what direction does the vector electric field strength (E) point?
The direction that a positive charge would move
What kind of field does a point charge have?
A radial field
What is the relationship between E and r in a Radial Field?
E ∝ 1/(r^2)
What is the relationship between E and d in a Uniform Field?
E ∝ 1/d
What is the formula for E in a uniform field?
E = V/d
What is the path of a charged particle moving through and electric field?
Curved
In what direction does a positive charge curve through an electric field?
Direction of the field lines
In what direction does a negative charge curve through an electric field?
Opposite to the direction of the field lines
If a particle enters an electric field at right angles, what direction does it accelerate?
Perpendicular to initial motion
When is electric potential (V) negative?
When Q is negative and the force is attractive
What is the relationship between g and V?
g = -V/r
In what direction do magnetic field lines go?
North to South
What is induced around any wire carrying an electric current?
Magnetic Field
How do you determine the direction of a magnetic field induced by an electric current?
Right-Hand Rule
What shape does the magnetic field around a coiled wire take?
Doughnut-shaped
What is a solenoid?
A cylindrical coil of wire
What word describes a “coil with length”?
Solenoid
Describe the magnetic field around a solenoid
Like a bar magnet, with a north and south pole
What happens to a wire carrying a current in an external magnetic field?
It will experience a force
Why does a current-carrying wire in an external magnetic field experience a force?
The field around the wire and the external magnetic field “add together”. Because the resultant field is not uniform, a force is produced.
How do you determine the direction of the force on a current-carrying wire in an external magnetic field?
Fleming’s Left-Hand Rule
What is the force on current-carrying wire parallel to an external magnetic field?
0N
What is the relationship between the force on a current-carrying wire in an external magnetic field and the magnetic flux density?
F ∝ B
Define magnetic flux density
The force on one metre of wire carrying a current of one amp at right angles to the magnetic field.
What is the equation linking force, current, length of wire and flux density of a current-carrying wire in a perpendicular magnetic field
F = BIL
Is flux density a scalar or a vector?
Vector
What is magnetic flux density measured in?
Tesla
Define the Tesla
Wb/(m^2)
Why does a current-carrying wire experience a force in a magnetic field?
Force acts on all charged particles moving in a magnetic field
What is the mathematical condition for circular motion?
Force is perpendicular to direction of travel
For a charged particle moving through a magnetic field, how do you adapt Fleming’s left-hand rule?
Second finger is direction of motion of a POSITIVE charge (as opposed to current)
For a negatively-charged particle moving through a magnetic field, in which direction does your second finger point with Fleming’s left-hand rule?
Opposite to direction of travel
How do you find the radius of the circular path of a charged particle in a magnetic field?
Combine the equations for the force on a charged particle in a magnetic field and the force on a particle in circular motion
What is magnetic flux density a measure of?
Strength of a magnetic field
like a “density” of field lines per area
When does magnetic flux (Ф) = BA?
B is perpendicular to A
What is N in magnetic flux linkage?
Number of coils
What is induced by a flat coil/solenoid moving closer to/further away from a magnet?
e.m.f.
How do you induce an e.m.f. with a flat coil or solenoid?
Move it closer or further away from a magnet
What is the magnitude of induced e.m.f. equal to?
Rate of change of flux linkage
What is the rate of change of flux linkage equal to?
Magnitude of induced e.m.f.
What is Faraday’s Law?
The induced e.m.f. is directly proportional to the rate of change of flux linkage
Why does the electrons in a conducting rod accumulate at one end of the rod when there is relative motion with a magnetic field?
Force acts on all charged particles moving in a magnetic field
What is electromagnetic induction?
Electrons accumulate at one end of a conducting rod moving in a magnetic field, inducing an e.m.f. across the ends of the rod
What is magnetic flux linkage?
The product of the number of turns in a coil and the magnetic flux
What is the unit of both flux linkage and magnetic flux?
Weber (Wb)
What is Lenz’s Law?
The induced e.m.f. is always in such a direction as to oppose the movement that caused it
How do you use Fleming’s left-hand rule to find the direction of induced e.m.f. in a conductor?
Thumb = resistive force (Lenz’s Law)
What is the equation for the force on a charged particle in a magnetic field?
F = BQv
How does a generator convert kinetic energy into electric energy?
Rotating a coil in a magnetic field
What flows through a coil that is part of a complete circuit moving in a magnetic field?
An induced current
What transfer of energy occurs in a generator?
Kinetic to electrical
What is an Alternator?
A generator of alternating current
How does an alternator connect the coil to an external circuit?
Slip rings and brushes
What is the phase difference between flux linkage and induced voltage in an Alternator?
90 degrees
Where do you measure θ between for BANcosθ?
Normal to the coil and B
Coil is perpendicular when θ = 0
How do you know that the field lines are going into the paper?
Circle with a cross in it
What is the waveform for voltage with an alternating current?
Sinusoidal
What is the waveform for e.m.f. with an alternating current?
Sinusoidal
What is the waveform for flux linkage with an alternating current?
Sinusoidal
What is the time period of an alternating current?
The time between peak voltages
What kind of current is Mains Electricity?
Alternating current
What kind of current does a battery supply?
Direct current
What does a transformer do?
Changes the size of the voltage for an alternating current
What happens in a transformer’s primary coil?
An alternating current produces magnetic flux
What is another name for the primary coil?
Input coil
Describe a transformer
A primary coil (connected to an a.c. current) around one side of an iron core, with a secondary coil on the other side
How is an alternating voltage induced in a transformer’s secondary coil?
Changing magnetic field is passed through the iron core, inducing an alternating voltage in the secondary coil
How do transformers change the voltage?
Having a different number of turns on each coil
What causes small losses of power from the transformer?
Eddy currents in the iron core
How do eddy currents dissipate energy?
By generating heat
How can you reduce the effect of eddy currents?
Laminate the core with layers of insulation
How is heat generated in a transformer?
Resistance in coils, eddy currents
How do you reduce the resistance in coils in a transformer?
Use a thick copper wire, which has low resistance
What is the efficiency of a transformer?
Ratio of power out to power in
How must you express transformer efficiency?
As a percentage
Are transformers 100% efficient?
No
What current is electricity from power stations sent round the country in the national grid?
Lowest possible current
Why is electricity sent in the national grid at the lowest possible current?
Reduced the power lost due to resistance
How are transformers used in the national grid?
Step-up voltage to the nation grid
Step-down voltage for domestic use
