Gravitational and Electric Fields

What is a force field?

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

What does the length of each vector represent in the field diagram?

The magnitude of the gravitational field strength

What is gravitational potential?

The work done to move a 1kg mass from infinity to a point in a field

The gravitational potential, V at a point is the gravitational potential energy that a unit mass at that point would have.

When the distance from the source mass increases what is the effect on the gravitational potential?

V increases, becoming less negative.

What type of field is around a planet?

Radial so the g depends on distance

Below the surface of a planet what is the relationship between the gravitational field strength and distance

Linear then as the distance from the earths centre increases the gravitational field strength decreases exponentially but never reaches 0

What 2 things does the magnitude of the gravitational force between 2 objects depend on?

Mass (proportional)

Distance (inversely proportional- inverse square law)

What is the escape velocity?

The minimum speed an unpowered object needs in order to leave the gravitational field and not fall back towards the gravitational field due to gravitational attraction.

The increase in gravitational potential energy comes from initial kinetic energy to escape the gravitational field of the planet.

What are equipotentials?

2D lines and 3D surfaces that join together all of the points with the same V. This means that as you travel along the equipotential, your potential doesn’t change so no loss or gain of energy. The gravitational potential difference is 0.

For a uniform spherical mass the equipotential are spherical surfaces. The equipotential and field lines are always perpendicular.

What is a satellite?

Any smaller mass which orbits a larger mass

What is a low orbiting satellite?

Any satellite which orbits between 180 and 200km above Earth

What is a geostationary satellite?

Examples of a synchronous orbit which is when an orbiting object has an orbital period equal to the rotational period of the object it is orbiting. - Always above the same point on the Earth, travels at the same angular speed as the Earth turns below them and in the same direction(West to East)

Exam definition of escape velocity

(i) (Minimum) Speed (given at the Earth’s surface) that will allow an object to leave / escape the (Earth’s) gravitational field (with no further energy input)

State 2 reasons why rockets launched from the Earth’s surface do not need to achieve escape velocity to reach their orbit

(Not given all their KE at Earth’s surface) energy continually added in flight / continuous thrust provided / can use fuel (continuously) .

Less energy needed to achieve orbit than to escape from Earth’s gravitational field / it is not leaving the gravitational field

Explain why E_p values are negative

Zero potential at infinity.

Energy input needed to move to infinity from the point.

Work has been done by the field moving object from infinity so potential energy falls as object moves from infinity.

Explain why V values are negative

Gravitational potential is the work done per unit mass in bringing an object from infinity to a point. The potential at infinity is 0.

Work has been done by the field so potential at all points closer than infinity is negative.

State Newton’s law of gravitation

Force of attraction between 2 point masses is proportional to product of masses, inversely proportional to square distance between them.

Describe properties of geo-synchronous orbit and the advantages it offers when a satellite is used for communications

Orbits westwards over equator

Maintains a fixed position relative to the surface of the Earth

Period is 24h

Offers uninterrupted communication between transmitter and receiver

Describe and explain what happens to the speed of a satellite when it moves to an orbit that is closer to the Earth.

Speed increases, loses PE but gaines kinetic energy

What does electric field strength mean?

The force per unit positive charge

How are electric field lines drawn?

They are drawn to show the direction of the force that would act on a positive charge. For a positive point charge the field lines point away from the point charge, and for a negative point charge they point towards it. For parallel plates the field lines point from the plate with the more positive voltage to the less voltage one.

What is electric potential?

work done per unit charge to move a small positively charged particle from infinity to that position

Recall Similarities and differences between electrical and gravitational fields

• g is force per unit mass where as E is force per unit positive charge

• Newton’s law of gravitation is an inverse square law and so is coulomb’s law

• Gravitational potential is potential energy per unit mass and is 0 at infinity. Absolute Electric potential; is potential energy per unit positive charge and 0 at infinity.

• The equipotential for a uniform spherical mass forms a spherical surface and the equipotential for a point charge forms a spherical surface.

• Gravitational forces are always attractive, whereas electrostatic forces can be attractive or repulsive.

Explain how equipotential surfaces in the diagram (increasing distance from centre of moon and V becoming less negative) show that the gravitational field is not uniform

The potential difference between the lines is constant, but the distances are not

The equipotential surfaces are curved

State axes of kepler’s third law

log(T) against log(R)

Is gravitational potential a vector or scalar quantity?

Scalar quantity.

Gravitational potential is a measure of the potential energy per unit mass at a point in a gravitational field. It describes how much potential energy a mass would have at a specific location in the field, without specifying any direction.

State 2 reasons why rockets launched from the Earth’s surface do not need to achieve escape velocity to reach their orbit

Not given all their KE at Earth’s surface, energy continually added in flight

Less energy needed to achieve orbit than to escape from Earth’s gravitational field, it is not leaving the gravitational field.

Explain why astronauts in an orbiting space vehicle experience the sensation of weightlessness

Both astronaut and vehicle are travelling at the same orbital speed(are in freefall)

No normal reaction between astronaut and vehicle.

Describe essential features of the orbit needed for the satellite to appear geostationary

Equatorial orbit

Period 24 h

Moving West to East

Describe and explain the path than an alpha particle would take in a uniform electric field between 2 plates

An alpha particle is positively charged so in the uniform electric field it would experience a constant force towards the negatively charged plate. F=EQ

This is the only force acting on the alpha particle so it accelerated towards the negative plate while the horizontal component of velocity remains constant so follows a parabolic path.

State one use of geostationary satellites and explain why a synchronous nature of its orbit around Earth is useful for this purpose

They can be used to send TV signals. A satellite in a synchronous orbit remains above the same point on earth so the receiver on earth doesn’t have to change its angle to remain pointing at the satellite

Explain why satellites uses very little fuel once it has reached its intended orbit

The total energy of the satellite is constant once it is in orbit so it would only need to use fuel for additional movement