Section 6 - Fields Flashcards
Gravitational fields, Electric fields, Capacitors, Magnetic fields, Electromagnetic induction.
What is a gravitational field?
The force field round a mass.
What is a field line (gravitational)?
The path a smaller mass would follow if pulled towards a larger mass.
Define the strength of a gravitational field (g)
The force per unit mass on a small test mass placed in the field.
Describe a radial field
The field lines are like the spokes of a wheel, towards the centre.
What happens to g in a radial field?
Decreases with increasing distance from the massive body.
Describe a uniform field
Field lines are parallel and equally spaced.
What happens to g in a uniform field?
Magnitude and direction of g is constant.
Define gravitational potential energy
The energy of an object due to its position in a gravitational field.
Define gravitational potential
Gravitational potential at a point is the work done per unit mass to move a small object from infinity to that point.
What are equipotentials?
Surfaces of constant potential.
Define potential gradient (gravitational)
Potential gradient at a point in a gravitational field is the change in potential per metre at that point.
What is Kepler’s Third Law?
For all planets r^3/T^2 is the same.
What are the assumptions in Newton’s law of gravitation?
Always attractive
Proportional to the mass of each object
Proportional to 1/r^2 (r is distance between objects)
What is the name for G?
Universal constant of gravitation
Define escape velocity
The minimum velocity an object must be given to escape from the planet when projected vertically from the surface.
What is the formula to remember for escape veloctiy?
v(esc)=sqrt(2gR)
g is gravitational field strength
R is the radius of the planet
What is the height of a geostationary satellite above the Earth?
35600km
What do electrical conductors contain a lot of?
Free electrons
What do electrically insulating materials not contain?
Free electrons
What is a field line (electric)?
The path a free positive test charge follows.
Describe a field between two oppositely charged plates
Field lines run parallel
At right angles to the plates
At the edges of the field the lines are slightly curved
What kind of a field is it between two oppositely charged plates?
Uniform field
Define electric field strength at a point in the field
The force per unit charge on a positive test charge placed at that point.
Electric field strength between plates is proportional to what?
Q/A
Q - charge
A - surface area
Define electric potential at a point in an electric field
Work done per unit positive charge on a positive test charge when it is moved from infinity to that point.
Define potential gradient (electric) at a point in an electric field
The change of potential per unit change of distance in a given direction.
How is potential gradient effected if the field is non-uniform?
Varies according to position and direction.
Closer equipotentials means greater potential gradient at right angles to them.
How is potential gradient effected if the field is uniform?
Equipotentials between the plates are equally spaced parallel to the plates.
Potential gradient is constant.
Define electric field strength
The negative of the potential gradient.
In Coulomb’s law, force is proportional to what?
Proportional to the product of the charges
Inversely proportional to the square of the distance between them
What is a point charge?
An expression for a charged object in a situation where distances under consideration are much greater than the size of the object.
If two forces are in the same direction, what is the resultant electric field strength formula?
E(1) + E(2)
If two forces are in opposing direction, what is the resultant electric field strength formula?
E(1) - E(2)
If two forces are at right angles to each other, what is the resultant electric field strength formula?
sqrt(E(1)^2 + E(2) ^2)
What is the constant of proportionality in electrostatic fields?
1/4πε(0)
What is a capacitor?
A device designed to store charge.
How does a capacitor work?
Two parallel metal plates placed near each other. When connected to a battery, electrons move through the battery and are forced onto one of the plates. On the other plate, an equal number of electrons leave that plate.
What is the symbol for a capacitor in a circuit?
Two equal length parallel lines.
How can a capacitor’s pd be measured when it is charging?
Use a stopwatch and measure the voltmeter at set times.
Use a data logger.
What would a charge against pd graph for a capacitor look like?
A straight proportional line passing through the origin.
Define capacitance
The charge stored per unit pd.
How is the energy stored in a capacitor obtained from a graph?
Charge against pd graph
Area under is a triangle of QV/2
In what proportionality does current, charge and pd of a capacitor decrease?
Exponentially
What is the time constant of a circuit?
RC
When t=RC in a capacitor’s discharge, what percentage charge remains?
37%
When t=2RC in a capacitor’s discharge, what percentage charge remains?
14%
At what point, is a capacitor considered discharged?
When t=5RC
