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Physics > 3.7 Fields and their consequences > Flashcards

3.7 Fields and their consequences Flashcards

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1
Q

What is the definition of a force field?

A

A region in which a body experiences a non-contact force.

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2
Q

In which situations to force fields arise?

A

Between objects of mass
Between static charges
Between moving charges

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3
Q

What are the similarities between gravitational and electrostatic forces?

A

Both involve inverse square law
Use of field lines
Use of potential
Equipotential surfaces

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4
Q

What are the differences between gravitational and electrostatic forces?

A

Masses always attract but charges can also repel

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5
Q

What is the definition of gravitational force?

A

The force of attraction between 2 masses is proportional to the product of the 2 masses and inversely proportional to the distance between their centres squared.

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6
Q

What is gravity?

A

A universal attractive force acting between all matter.

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7
Q

What is Kepler’s 1st law?

A

The path of any object in an orbit follows the shape of an ellipse, with the orbited body at one of the foci.

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8
Q

What is Keplers 2nd Law?

A

An imaginary line from the sun to the planet sweeps out equal areas in equal times.

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9
Q

What is Kepler’s 3rd law?

A

The square of a planet’s orbital period is directly proportional to the cube of the average radius from the sun.

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10
Q

What is gravitational potential?

A

Work done per unit mass when a mass is moved from infinity to a point which is a distance, r, from the centre of a body. It is scalar.

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11
Q

What is an equipotential surface?

A

A surface of constant potential. (No work needs to be done to move across it.)

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12
Q

What are the properties of equipotential lines?

A

The gravitational potential is the same anywhere on the line.
They are perpendicular to field lines.
Concentric circles
Distance between them increases

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13
Q

What is potential gradient?

A

At a point in a gravitational field, it is the change of potential per metre at that point. (It is equivalent to g, it decreases as you get further form the earths surface.)

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14
Q

Why is gravitational potential negative?

A

At infinity, potential is 0. So, energy must be put in to move from infinity.

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15
Q

What is the definition of a gravitational field?

A

A region of space where an object of mass experiences a force.

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16
Q

What is the definition of gravitational energy?

A

The energy of an object due to its position in a gravitational field.

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17
Q

What is the work done on a body in a gravitational field?

A

It is equal to the gravitational potential energy gained.

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18
Q

What is escape velocity?

A

The minimum velocity an object must be given to escape from the planet when projected vertically from the surface.

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19
Q

What is the total energy for an object in orbit?

A

Total energy is half the gravitational potential energy. (You add kinetic energy and gravitational potential.)

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20
Q

What are the 3 different types of orbits?

A

Geostationary
Low/ polar
Geosynchronous

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21
Q

What are the properties of a geostationary orbit?

A

The orbital period of the satellite is the same as the rotational period of the object.
The axis of rotation of the satellite is the same as the axis of the planet.
The satellite must orbit in the plane of the equator.

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22
Q

What are the uses of geostationary orbits?

A

Communication
TV
Weather

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23
Q

What are the features of a geosynchronous orbit?

A

The period of the orbit is the same as the time period of one rotation of the planet on its axis.
The satellite return to the same point in the sky at the same time each day.

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24
Q

What are the features of a low orbit?

A

Travel relatively close to the earth and take around 90min to complete one orbit.
An example of a low orbit is a polar orbit which orbits the poles of the earth.

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25
Q

What are the uses of low orbits?

A

Spying
Imaging
Weather
Geological prospecting
GPS

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26
Q

What factors affect orbital period?

A

Speed
Circumference

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27
Q

What is the effect of increasing distance on orbital speed and period?

A

↑ distance → ↓ orbital speed

↑ distance → ↑ orbital period

28
Q

What is the definition of a coulomb? Is charge scalar or vector?

A

The charge transported by 1 amp of current in 1 second. Charge is scalar.

29
Q

What is Coulomb’s law?

A

The magnitude of force, F, between charges Q1 and Q2 is proportional to the product of Q1 and Q2 and inversely proportional to the square of the distance, r, between their centres.

30
Q

What is an electric field?

A

Where a charged particle feels a force.

31
Q

What is the definition of electric field strength?

A

The electric force per unit charge on a positive charge. It is a vector quantity.

32
Q

What is the null point?

A

In the case of two like charges, there’s a point between the 2 charges where the electric field strength is 0. It is farther from the greater charge.

33
Q

What is a uniform electric field?

A

A charge will feel the same force wherever it’s placed in the field and the force acts parallel to the field lines. The electric field between 2 parallel charged plated is uniform.

34
Q

Which factors affect uniform electric field strength?

A

Potential difference between the plates
The distance between the plates

35
Q

What happens when a charged particle enters a uniform field?

A

The horizontal component of its velocity remains unchanged. It accelerates in the vertical direction. Gravity is negligible.

36
Q

Describe Millikan’s oil drop experiment.

A

The oil drops were placed in a uniform electric field.
The drops were given a negative charge.
This produces an upwards force in the field, which balances the weight.
This enables the charge on an electron to be calculated.
All charges were found to be a multiple of e.

37
Q

What happens inside an electron gun?

A

The positive anode attracts the e-. It is circular with a hole in the center.
The attractive force on the e- is horizontal.
The hot negative cathode releases e- by thermionic emission.

38
Q

What is the definition of electric potential energy?

A

The work done, on a positive test charge, when it is moved, from infinity, to that point in the field.
It is a scalar quantity.

39
Q

What is the definition of electric potential?

A

At a point in an electric field, it is the work done per unit positive charge on a positive test charge when it’s moved from infinity to that point in the field.
It is scalar.

40
Q

In which situations will potential difference be negative or positive?

A

V is positive when a force is required to move a positively charged object to a point.
V is negative when the charge will move anyway due to the electrostatic attraction of the other charge.

41
Q

What is potential gradient?

A

The gradient of a V-r graph which is also equal to electric field strength. As you go up a potential gradient, you travel in the opposite direction to the field.

42
Q

What does the area under an E-r curve represent?

A

The electric potential at that point.

43
Q

What is the definition of capacitance?

A

The ability of a body to hold an electric charge.

44
Q

What is a farad?

A

The capacitance when a p.d of 1V appears across a capacitors plates when a charge of 1C is stored.

45
Q

What is the basic structure of a capacitor?

A

It contains 2 parallel plates separated by an insulating material, a dielectric.

46
Q

What are the uses of capacitors?

A

The flash in a camera
Smoothing surges in electric currents
Allows data to be stored in a computer if the power is cut
Used to start nuclear reactions, by powering intense laser beams
(Some hybrid cars)

47
Q

How is a capacitor charged?

A

Electrons gather on the plate attached to the negative terminal of the battery.
So, electrons are drawn from the positive plate of the capacitor.
So, a net negative charge forms on one plate and a positive on the other.
A current flows until the p.d across the capacitor is the same as the EMF.

48
Q

What does the gradient and area under the curve of a a charge-voltage graph represent?

A

Gradient= capacitance
Area under the curve= energy stored

49
Q

What happens when the voltage across the capacitor doubles?

A

The charge also doubles causing energy stored to quadruple.

50
Q

What factors affect capacitance?

A

Area
Distance
Permittivity

51
Q

What effect does increasing the area of the plates have on capacitance?

A

By increasing the area of the plates, the charge that can be stored increases for a particular voltage. So, capacitance increases.

52
Q

What effect does increasing the distance between the plates have on capacitance?

A

The effect that the plates have on each other decreases. So, capacitance decreases.

53
Q

What effect does increasing the permittivity of the dielectric have on capacitance?

A

This increases the strength of the electric field between the plates and increases the capacitance.

54
Q

What are the common dielectrics?

A

Mica
Wax paper

55
Q

What is the purpose of the dielectric?

A

To insulate the 2 plates when storing a charge.
To allow an electric field to form.

56
Q

What is the permittivity of air?

A

It is equivalent to the permittivity of free space.

57
Q

How do dielectrics work?

A

The material is made up of polar molecules.
When charge is applied, an electric field forms between the plates, so, molecules rotate and align with the electric field.
The molecules have their own electric field, opposing that of the capacitor.
The larger the permittivity, the larger the opposing field as electric field decreases which reduces the p.d needed to charge the capacitor, increasing the capacitance.

58
Q

What happens when the dielectric is removed from a charged capacitor?

A

εr falls to 1
The capacitance falls by the same factor
The p.d increases by the same factor
Q remains constant
E increases by the same factor
Energy is conserved as work is done to separate the negatively charged surface of the dielectric from the positively charged plate.

59
Q

How is a capacitor discharged?

A

When it is connected to a complete circuit, it discharges the stored charge, creating a current.

60
Q

Which factors affect discharge time?

A

Capacitance
Resistance

61
Q

What effect does capacitance have on discharge time?

A

↑Capacitance → ↑ Discharge time

62
Q

What effect does resistance have on discharge time?

A

↑Resistance → ↑ Discharge time

63
Q

What is the time constant?

A

The time taken for the charge, current or voltage to decrease by a factor of e^-1

64
Q

How do you find the time constant from a graph?

A

Find V_0
Find e^-1V_0
The value for time at e^-1V_0 is the time constant

65
Q

What is the time taken for Q,I and V to halve when a capacitor is discharged?

A

0.69RC

Physics (7 decks)

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