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IB Physics HL Core > Topic 10: Fields (HL) > Flashcards

Topic 10: Fields (HL) Flashcards

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1
Q

What is the symbol used to denote a gravitational field?

A

g

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2
Q

What is the symbol used to denote an electric field?

A

E

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3
Q

What three types of field can field lines represent?

A
  • Electric
  • Magnetic
  • Gravitational
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4
Q

Define: gravitational field and its units

A

Force per unit test point mass at a particular point in a gravitational field.

N kg-1

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5
Q

Define: electric field and its units

A

Force per unit test point positive charge at a particular point in a field.

N C-1

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6
Q

How can field lines represent the magnitude of a force?

A

The density of the field lines (the closer they are, the stronger the force)

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7
Q

How can field lines represent the direction of a force?

A

The direction in which a field line point represents the direction in which a unit test point experiences the force.

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8
Q

If an object is moved along the field lines, what work is being done?

A

Work is done by the field.

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9
Q

If an object is moved against the field lines, what work is being done?

A

The work is being done against the field.

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10
Q

If an object is moved at right angles to the field lines, what work is being done?

A

No work is done by nor against the field.

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11
Q

What is potential?

A

The energy (per unit point object) that an object has as a result of the field.

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12
Q

Define: gravitational potential and its units

A

Gravitational potential, Vg, is the energy per unit mass (energy/mass)

J kg-1

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13
Q

Define: electric potential and its units

A

Electic potential, VE, is the energy per unit charge (energy/charge)

J C-1

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14
Q

Define: gravitational potential difference and its units

A

Gravitational potential difference is the difference in potential between two points in a gravitational field.

ΔVg = (work done moving a test mass/test mass)

J kg-1

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15
Q

Define: electric potential and its units

A

Electric potential difference is the difference in potential between two points in an electric field.

ΔVg = (work done moving a test charge/test charge)

J C-1

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16
Q

What are equipotentials?

A

Equipotentials are lines at which all the points on it have the same potential.

Equipotential lines are places at distances where the change in potential is the same (i.e. at 0, 20, 40 etc.)

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17
Q

How do equipotentials look like in a uniform field?

A

Straight, equidistant lines

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18
Q

How do equipotentials look like in a radial field?

A

Circles which (starting from the centre) get progressively more and more spaced apart.

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19
Q

What is the relationship between equipotentials and field lines?

A

They are perpendicular to each other.

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20
Q

What is the (GCSE) gravitational potential energy equation?

A

ΔVg = mgΔh

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21
Q

What does ΔVg=mgΔh assume? (2)

A
  • g is constant, however, Newton’s theory of universal gravitation states that the field must change with distance (so must only be used for small distances)
  • assumes potential energy = 0 at the Earth’s surface, this is not the case
22
Q

Where does gravitational potential energy = 0?

What does this mean gravitational potential energy must always be?

A

At a point that is infinitely away.

ΔVg must always be negative.

23
Q

Is potential a scalar or a vector quantity?

A

Scalar

24
Q

Is potential energy a scalar or a vector quantity?

A

Scalar

25
Q

What are three important things to remember about gravitational potential?

A
  • Potential energy is zero at a point infinitely away.
  • As m moves towards M, the force on m increases.
  • Potential energy decreases as the gravitational force does work.
26
Q

What is gravitational potential gradient?

What are its units?

A

The change in gravitational potential divided by the change in distance (from source).

ΔV/Δr

J kg-1 m-1 = N kg-1 = m s-2

27
Q

How is the potential gradient related to the gravitational field strength?

A

The gravitational field strength is equal to minus the potential gradient (as it is an attractive force).

28
Q

What is the graph for the kinetic, gravitational and total energy when in orbit?

A
29
Q

Derive the equation for total energy in orbit

A

Total energy = kinetic energy + potential energy

KE = 1/2mv2 and v = √GM/r (circular motion)

so KE = 1/2(GMm/r)

GPE = -GMm/r

Total energy = -1/2(GMm/r)

30
Q

What are three things to note about energies when in orbit?

A
  • In order to orbit, the magnitude of KE must be 1/2 magnitude of GPE
  • The overall energy of the object in orbit is negative (must be less than zero otherwise it would escape orbit)
  • In order to move from a small radius to a larger radius orbit, the total energy must increase (must be a negative number closer to 0)
31
Q

What is apparent weight?

A

What we experience as weight is the reaction force, if the reaction force decreases or increases, we feel our weight “change” accordingly.

32
Q

When can you feel weightless?

A

Apparent weightlessness can occur when the object you are in, e.g. lift, is travelling at the same acceleration downwards as you are, as the reaction force is zero.

33
Q

Why do astronauts in orbit feel weightless?

A

As gravity provides the centripetal force for the orbit, both the astronaut and satellite are in free fall, therefore, they feel weightless.

34
Q

What are three important things to remember about electric potential energy?

A
  1. Potential is zero at a point infinitely away
  2. As q gets closer to Q, the force on q increases
  3. As q gets closer to Q, potential increases as the charge is moved against repulsion (against force) - assuming positive charges
35
Q

How does electric potential behave outside of the sphere?

A

Behaves “normally”, following coulomb’s law and field lines.

36
Q

How does electric potential behave inside the sphere?

A

As there is no net contribution from charges outside of the sphere, the electric field is zero.

Therefore, the potential gradient is zero and every point inside the sphere has the same potential equal to that of the sphere.

37
Q

What is the equation for potential difference in terms of potentials?

What does the sign indicate?

A

ΔV = Vfinal - Vinital

If positive, work done against the field. If negative, work done by the field.

38
Q

How does electric field strength relate to the potential gradient?

A

Electric field strength = - potential gradient

39
Q

Diagram for electric field strength, from in to out of a sphere

A
40
Q

Diagram for electric potential, from in to out of a sphere

A
41
Q

How can you calculate the potential difference from an electric field - distance graph?

A

The area under the graph

42
Q

How can you calculate the electric field strength from a potential - distance graph?

A

The gradient of the curve

43
Q

What does the potential graph look like going from inside a mass to outside of it?

A

Potential is 0 at centre.

Linear progression to the surface.

Then follows the gravitational potential equation.

44
Q

What is a polar orbitting satelite?

A

A satellite that orbits close to the earth, usually over the poles at very fasts speeds.

45
Q

What is a geosynchronous orbiting satellite?

A

A satellite that orbits far from the earth above the same region of the sky, in a figure of 8 orbit

46
Q

What is a geostationary orbiting satellite?

A

A satellite that orbits far from the Earth which remains at a constant point in the night sky

47
Q

In the left-hand rule, what does each finger signify and in which direction (e.g. north)?

A

Thumb: points to the direction of force

Index: points in direction of the magnetic field (pointing from north to south)

Middle: point in direction of the conventional current (not electron movement)

48
Q

What is the equation for the magnetic force acting on a charged particle?

A

F = Bqv

Where:

B is the magnetic field strength

q is charge

v is velocity

49
Q

How can you equate the electric field force and the magnetic field force?

A

qE = Bqv

E = Bv

where:

E is electric field strength

q is charge

B is magnetic field strength

v is velocity

50
Q

What quantities are properties of gravitational fields?

A
  • Gravitational field strength
  • Gravitational protential
51
Q

What quantities are properties of mass in a gravitational field?

A
  • Newton’s law of gravitation
  • Gravitational potential energy

IB Physics HL Core (13 decks)

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