Topic D Key Terms Flashcards
Gravitational Field
Region of space when a mass will experience a non-contact force
What is an assumption of using the equation F = -Gmm/r^2
Masses act as point masses at their centre. This is true as long as we are at the surface or outside the mass
Gravitational field strength (g)
The force per unit mass for a small point mass
or
The force upon a point unit mass
W = m g (in a uniform field)
g=W/m
Kepler’s first law
The orbit of the planets are elliptical and the sun is a focus of the ellipse
Perihelion
The earth is at its closest to the sun and is fastest at this point (this is the winter solstice for the Northern Hemisphere)
Aphelion
The earth is at its furthest from the sun and is slowest at this point. (This is the summer solstice for the Northern Hemisphere)
Kepler’s Second Law
The area swept out by the radius in unit time is a constant
Kepler’s third law
T^2 is proportional to R^3
Gravitational potential Energy
Gravitational potential Energy (J) is the work done in moving a given mass from infinity (GPE = 0) to that point in the gravitational field
Gravitational Potential (J.kg^-1)
The work down in moving a unit point test mass from infinity to that point in the gravitational field.
Gravitational potential energy of a system
The work done in assembling the objects in the system from infinity (GPE = 0) to that point in the system
What is field strength
Field strength is the potential gradient
Why are equipotential lines circular and concentric
Closer to the earth, the equipotential lines become closer together. because field strength is the potential gradient and the gravitational field strength is stronger closer to the earth.
Newtons universal law of gravity:
Every mass in the universe attracts every other mass with a force that is directly proportional to each mass and inversely square to their displacement.
F = - Gm1m2/r^2
g = -GM/r^2
(radial field)
Gravitational potential energy and gravitational potential
In a unifrom gravitational field the change of GPE is the work done against the weight. GPE = mxgxh
The (absolute) GPE is the work done in bringing mass m from infinity to displacement r away from mass M. GPE = -GMm/r
The gravitational potential is the work done in bringing a point unit mass from infinite to displacement r from mass M V = -GM/r
Total energy of the orbit
KE = 1/2 GMm/r
GPE = -GMm/r
Total energy = KE + GPE
Total energy = 1/2 GMm/r + (-GMm/r)
Total Energy = - 1/2 GMm/r
What happens when you move to a lower stable orbit and energy
- Satellite initially slows down
- Total energy decreases
- Moves to a lower orbit
- Potenetial energy decreases
- Loss of potential energy changes into kinetic energy
- Satellite now has a lower orbital radius but is rotating with a greater speed
Escape Velocity
The minimum instantaneous velocity given to an object at a planet’s surface that will allow it to reach zero gravitational potential
What is a field
A field is a region (volume/space) within which a non-contact force occurs
Fields are vectors
What are the fundamental forces and strengths of them in order
Strong Nuclear Force
Electromagnetic Force
Weak Nuclear Force
Gravity
What is an electric field
A region within which a non-contact force is exerted upon a unit point positive charge (test charge)
Coulomb’s Law
Force between any two charges is directly proportional to each charge and inversely proportional to the square of the displacement between them.
What is an assumption of Coulomb’s law
Charges act as point charges at their centre
What is the difference between Coulomb forces and gravity
Gravity is only attractive. Coulomb forces can be attractive or repulsive, so there is no need for a + or - in the formula, because the charges decide that
Coulomb forces can vary with the material between the two charges, but the force of gravity is independent of the medium
What is electric field strength
The force upon a unit point positive test charge
How can charges be transferred
- Friction/rubbing
- Contact
- Induced charge
What is electrical potential
The work done in bringing a unit, point, positive test charge from infinity to that displacement in the electric field
Where is there zero electric potential
Zero potential is at infinite distance where there is no coulomb force
Electrical potential is a scalar as it is energy per coulomb
What is the root mean square (r.m.s)
r.m.s current/pd/emf is the equivalent value of the constant DC current/pd/emf that would dissipate equal power in the same circuit
why would you take the root of the mean of the square?
mathematically, the average of sinusoidal functions are zero. hence we take the root of the mean of the square to get an average of the supply
What are some key observation of a transformer
Transformers are STEP-UP when they INCREASE the voltage.
Transformers are STEP-DOWN when they DECREASE the voltage.
The purpose of the iron core is to STRENGTHEN the magnetic field strength.
The wires must be INSULATED to stop the iron core conducting a current and short circuiting the coils.
AC is essential because with an AC input the current changes, creates a changing magnetic field which induces through the secondary coils and produces a voltage.
A transformer does not work with steady DC because a DC current would produce a constant magnetic field which would not change and would not induce through the secondary coils, so no voltage would be produced.
Explain Self Inductance
- The switch is closed and a current starts to run through the coil. The magnetic field in the iron core which was initially zero, but now starts to increase.
- This strengthening magnetic field creates a change of flux linkage through the coil and this INDUCES an emf.
- According to Lenz’s Law the induced EMF must oppose the change. The induced EMF is a “BACK EMF” and opposes the PD from the supply. This causes the current to build slowly.
- The magnetic field reaches its full strength and is now constant; there is no longer a change of flux linkage and now no back EMF.
- When the switch is disconnected, the collapsing field creates a change of flux linkage for the coil and induces a FORWARD EMF that keeps the circuit powered for a few seconds.
How does a Transformer work?
The alternating PD creates an alternating current in the primary coil.
This creates an alternating magnetic field around the iron core.
This changing magnetic field travels forwards and backwards through the secondary coil.
According to Faraday’s Law of Electromagnetic Induction, the alternative EMF induced in the secondary coil is equal to the rate of change of flux linkage.
The greater the number of turns in the secondary coil, the greater the flux linkage and the greater the induced EMF. Hence, a transformer can change the PD of any alternating PD.
