Fields and Consequences

What is a field?

An area in which an object experiences a non-contact force

Are fields vectors or scalars?

Vectors

How can fields be represented on diagrams? 

Field lines 

How do field lines show the strength of a field? 

Closer lines represent a stronger field 

When is a gravitational field formed?

During the interaction of masses

When is an electric field formed?

During the interaction of charges

Is gravity an attractive or repulsive force? 

Always attractive 

State Newton’s law of gravitation.

The magnitude of the gravitational force between 2 masses is directly proportional to the product of the masses and inversely proportional to the square of the separation

State the equation for gravitational force.

F = GMm/r²

State the 2 types of gravitational field.

Uniform and Radial

What is a uniform field? 

The same gravitational force is exerted on a mass anywhere in the field 

What is a radial field?

The force exerted depends on the position of the mass in the field. As an object moves further away from the bigger mass M, the force on it decreases 

What type of gravitational field is around the earth?

Radial, but can be assumed to be uniform close to the surface

What is gravitational field strength?

The force per unit mass exerted by a gravitational field on an object

How does gravitational field strength vary in different types of fields?

In a uniform field, the field strength is constant

In a radial field, the field strength decreases with distance from the object 

State the formula for gravitational field strength in a uniform field.

g = F/m

State the formula for gravitational field strength in a radial field.

g = GM/r²

What is gravitational potential? 

The work done per unit mass against gravitational force to move an object from infinity to a given point

What is the value of V at infinity?

0

Why is gravitational potential always negative?

As an object is moved from infinity to a point, its GPE is reduced from 0

What is gravitational potential difference?

The energy required to move a unit mass between 2 points

State the equation for work done in a gravitational field. 

(delta)W = m x (delta)V

What is an equipotential surface?

A line in a field where the the potential is constant along it

What is the work done when moving along an equipotential surface? 

Zero 

Where are equipotential lines compared to field lines?

Perpendicular to them

Draw a graph to represent the relationship between V and r

How can g be calculated from a V against r graph?

Draw a tangent to the curve and calculate the gradient, then multiply by -1

Draw a graph to represent the relationship between g and r.

How can potential difference be calculated from a g r graph?

The area under the curve

State Coulomb’s law.

The magnitude of the force between 2 point charges in a vacuum is directly proportional to the product of their charges and inversely proportional to the square of the separation

State the equation for force between 2 point charges.

F = 1/4piE₀ x Q1Q2/r²

State the value of E₀.

6.67 × 10^-11

Where is charge assumed to act from in a charged sphere?

The centre

How can the charges on the points be used to determine the nature of the force between them? 

If they have the same sign, the force is repulsive, and if the have different signs, it is attractive 

Are electrostatic forces between subatomic particles greater or less than gravitational forces and why?

Greater, the mass of subatomic particles is very small compared to their charges

What is the difference between the electrostatic and gravitational forces between 2 protons? 

Electrostatic force is 1.24×10³⁶ x greater 

What is electric field strength?

The force per unit charge experienced by an object in an electric field

State the formulas for electric field strength in a uniform field.

E = F/Q or E = V/d

State the formula for electric field strength in a radial field.

E = 1/4piE₀ x Q/r²

Electric field lines show the direction of the force acting on what type of charge?

Positive charge 

What is the strength of a radial field dependent on?

The distance between the 2 charges

State the formula for work done to move a charged particles between 2 charged plates? 

(delta)W = Q x (delta)V 

How can charged plates be used to determine the charge on a particle?

Fire the particle in between the plates at right angles to the field and the particle will accelerate towards one of the plates in a parabolic shape depending on whether it is negatively or positively charged

What is electric potential? 

The potential energy per unit charge of a positive point charge a that point in the field

State the formula for electric potential in a radial field?

V = 1/4piE₀ x Q/r

What type of force corresponds to a positive potential?

A repulsive force

How can the electric field strength be calculated from a V r graph?

Draw a tangent to the graph ( E = (delta)V / (delta)r )

What is electric potential difference?

The energy needed to move a unit charge between 2 points

State the equation for work done in an electric field.

(delta)W = Q x (delta)V