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A level Physics > Unit 4.2 - Electrostatic and gravitational fields > Flashcards

Unit 4.2 - Electrostatic and gravitational fields Flashcards

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1
Q

What’s similar about electrostatic and gravitational fields?

A

Both have very similar forms

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2
Q

What does the gravitational field of the earth provide?

A

A constant acceleration of 9.81ms^-2

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3
Q

When is using the constant acceleration of 9.81ms^-2 from the gravitational field of the earth a useful method?

A

When we consider small changes in height above the surface of the earth

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4
Q

What is a field in physics?

A

A model of a physical quantity, typically a number, that has a value for each point in space and time

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5
Q

Scalar fields

A

Will have a value only at different points

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6
Q

Example of a scalar field

A

The temperature at different positions around the country

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7
Q

Vector fields

A

Have a value and a direction related to their position

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8
Q

Describe and give an example of a vector field

A

On a weather map, the surface wind velocity is described by assigning a vector to each point on a map. Each vector represents the speed and direction of the movement of air at that point.

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9
Q

What is a gravitational field?

A

A model used to explain the influence that a massive body extends into the space around itself, producing a force on another massive body (a body that has mass)
= force per unit mass

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10
Q

What is used to explain gravitational phenomena?

A

A gravitational field

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11
Q

What is a gravitational field measured in?

A

Newtons’s per kilogram

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12
Q

How has the way gravity has been explained changed over time?

A

Original concept —> gravity was a force between point masses
Then —> gravity was some kind of radiation field or fluid
Now —> gravity is taught in terms of a field model rather than a point attraction

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13
Q

Describe what a field model is and what happens in it

A

Rather than two particles attracting each other, the particles distort space-time via their mass, and this distortion is what is perceived and measured as a “force”

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14
Q

What is measured as a force in a field model?

A

Particles distorting space-time via their mass

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15
Q

What does mass do to space-time?

A

Curves it

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16
Q

Describe space- time when there’s no mass

A

Flat

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17
Q

Describe space-time where there is a mass

A

Curvature towards the mass

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18
Q

What will happen to two masses in a field model and why?

A

Will attract each other due to the curvature caused by the mass

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19
Q

Where will masses fall in the field model with a large and small mass?

A

The smaller mass will fall towards the large mass

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20
Q

How does matter move in the field model?

A

In certain ways in response to the curvature of space-time

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21
Q

Describe the gravitational force in the field model

A

There is either no gravitational force or gravity is a fictuous force

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22
Q

Way of imagining the field model

A

Consider a heavy mass on a flexible surface

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23
Q

What is the only thing that’s significant to the object causing the curvature of space-time in field model?

A

The mass of the object

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24
Q

What is not significant to the curvature of space-time caused by an object in the field model?

A

The size of the object

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25
Q

What can we consider the mass of a body to be in the field model?

A

The point at the centre of the body - the centre of the mass

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26
Q

Where do we consider the weight of objects to act?

A

At the centre of gravity

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27
Q

Why is the field theory used to explain the origin of the solar system?

A

All planets orbit in the same direction (in the example of the heavy mass on a flexible surface, it’s the friction that causes the objects to eventually fall)

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28
Q

Field lines

A

“Imaginary” force lines which describe the effect of a field at that point

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29
Q

What can we consider field lines to be?

A

The gradient of the field

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30
Q

Example of field lines in real life

A

On maps, contour lines join areas of equal height above sea level

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31
Q

What does the gradient of field lines relate to?

A

The proximity (nearness) of the contour lines

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32
Q

What do closer field lines indicate?

A

A higher area (steepest slope)

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33
Q

What do further away field lines indicate?

A

Lower areas (less steep slope)

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34
Q

As field lines reperesent height, what do they also represent?

A

Potential

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35
Q

What does a higher field line mean?

A

A higher potential

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36
Q

When we consider a point mass, describe the space-time surrounding it

A

Is curved in a radial manner surrounding it

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37
Q

What is a point mass in the field model the opposite of and why?

A

The opposite of a “hill” on a map since other masses “fall” towards it

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38
Q

Where is a point mass on a field model and why?

A

At the bottom of the “well” since it’s causing the curvature

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39
Q

What are the lines going into the well on a field model?

A

Field lines

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40
Q

Would do field lines represent?

A

The direction that an object would move

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41
Q

What do the circular lines on a field model represent?

A

Equipotentials

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42
Q

What do equipotentials on a field model show?

A

That an object has the same potential anywhere along the line

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43
Q

In which direction would a mass placed anywhere in a field experience a force?

A

Towards the potential well

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44
Q

What is the height from a point mass on an equipotential diagram associated with?

A

A potential energy

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45
Q

What happens to the height as you go further out from the centre on an equipotential diagram and why?

A

Is increasing because the point mass is at the lowest point in the field

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46
Q

If there’s a small gradient in the field model, describe the force

A

Small

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47
Q

If there’s a big gradient in the field model, describe the force

A

Big

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48
Q

Which lines represent the direction a mass would experience a force?

A

Field lines

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49
Q

What do field lines represent?

A

The direction a mass placed here would experience a force

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50
Q

What can the value of the force on an object in a field be calculated by?

A

The potential gradient

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51
Q

What is field strength measured in?

A

Force per kg

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52
Q

What is field strength the same as?

A

The potential gradient

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53
Q

Where is the highest gradient in an equipotential diagram?

A

In the middle of

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54
Q

Describe the change in force when going out from the centre of an equipotential diagram

A

There isn’t a linear change in force - it follows the inverse square law

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55
Q

Explain with an example the inverse square law in an equipotential diagram

A

Double the distance, the field is 4x weaker

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56
Q

What type of forces can we have in an electrical field?

A

Attractive and repulsive forces

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57
Q

What are the only type of forces we can have in a gravitational field?

A

Attractive,

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58
Q

In what type of field can we only have gravitational forces and in what type can we have repulsive forces too?

A

Gravitational
Electrical

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59
Q

What type of region is a gravitational field?

A

A region where a mass exerts a force on another mass

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60
Q

Why are the equipotential lines always formed into a well in gravitational fields?

A

Since gravity is always attractive

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61
Q

Why is electric force defined as having an effect on?

A

A positive charge

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62
Q

What does it mean since electric force is defined as having an effect on a positive charge?

A

There are two possible illustrations for the field surrounding point charges, dependent on their polarity

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63
Q

What are the two fields in electrical fields

A

The field due to a positive charge and the field due to a negative charge

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64
Q

What will a positive charge do at the field due to a positive charge

A

Will fall down the gradient into the potential well

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65
Q

What does a negative charge do in the field due to a negative charge

A

Goes up the gradient out of the potential well (anti-gravity)

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66
Q

What is the size of a gravitational force calculated according to?

A

Newton’s law of universal gravitation

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67
Q

Newton’s law of universal gravitation

A

Every massive particle in the universe attracts every other massive particle with a force that is directly proportional to the products of their masses and inversely proportional to the square of the distance between them

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68
Q

How do we measure the distance between objects when calculating the gravitational force?

A

We take the distance between them from the centre of mass

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69
Q

What should the particles be when using the universal law of gravitation and when does this not matter?

A

Spherical
If there’s a great enough distance between them, it doesn’t matter much

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70
Q

Equation to describe Newton’s law of universal gravitation

A

F = GM1M2/r^2

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71
Q

Equation to calculate the force between two masses

A

F = GM1M2/r^2

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72
Q

G

A

Universal gravitational constant

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73
Q

r^2 in the equation for the force between two masses

A

The distance between the two masses

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74
Q

How do we know that the equation for the force between two masses is an inverse square law?

A

It includes r^2

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75
Q

Why is the force between two masses almost nothing and when does this change?

A

Since the vale of G s tiny
The force is bigger when masses are present

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76
Q

Gravitational field strength

A

The force per unit mass on a small test mass placed at a point

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77
Q

Gravitational field strength equation

A

g = GM/r^2

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78
Q

Why is the gravitational field strength equation how it is?

A

We consider M2 to be 1kg

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79
Q

What is the distance between two objects that are touching if we need to calculate the force between them and why?

A

Equal to the sum of their radii
It’s the distance between the centres of gravity of the objects

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80
Q

What does the electrical field only apply to?

A

Charged object

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81
Q

Do the equations have a similar form for both electrical and gravitational fields? What is different?

A

Yes, it’s just hat electrical fields only apply to charged objects

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82
Q

Coulomb’s law

A

The force between two point charges is directly proportional to the product of the magnitude of the charges and inversely proportional to the square of the distance between them

83
Q

How is Coulomb’s law different to Newton’s law of universal gravitation?

A

They’ve both got the same wording, it’s just that “masses” and “charges” are used

84
Q

Equation for the force between two charges

A

F = 1/4piε0 Q1Q2/r^2

85
Q

What will happen if both of the charges are the same when calculating the force between two charges?

A

They will repel

86
Q

Electrical field strength equation

A

E = 1/4piε0 Q/r^2

87
Q

Why is the electrical field strength equation how it is?

A

Q2 is assigned the value of +1C

88
Q

What is the electrical field strength equation true for?

A

A field in a vacuum or air

89
Q

What would happen if an electrical field strength were in another substance that isn’t air to the electrical field strength equation?

A

εr would change the value, but we won’t encounter this

90
Q

What can 1/4piε0 be taken as as an approximation?

A

9x19^9 (which is in the data book)

91
Q

Potential

A

The energy per unit charge/mass at that point in the field

92
Q

Where in a field is there no effect?

A

At the level of zero potential

93
Q

Describe zero potential

A

Flat space

94
Q

Describe the work when the field does the work

A

Negative work

95
Q

Describe the work when you have to put work into the field

A

Positive work

96
Q

What work will be required to take an object from infinity to another point?

A

Negative

97
Q

What does it mean if the work is negative?

A

The field does the work

98
Q

How is the potential defined at infinity?

A

Zero

99
Q

When would the work don in a field be even higher?

A

If moving to a point nearer the centre of the mass

100
Q

If a mass has moved even closer to the centre of the mass, describe the work

A

Larger negative number

101
Q

What type of value is the potential anywhere due to any mass going to be?

A

Negative

102
Q

Why is the potential anywhere due to any mass going to be negative?

A

Since the potential decreases as we approach the mass and it starts at zero at infinity

103
Q

Does potential have a direction? Why?

A

No, since potential is a scalar field (like the temperature map)

104
Q

What is field strength equal to?

A

Potential gradient

105
Q

Describe the force an object experiences at a higher potential gradient

A

Bigger

106
Q

At what potential gradient does an object experience a bigger force?

A

Higher potential gradient

107
Q

How do we define potential in electrical fields compared to gravitational fields?

A

Gravitational fields = unit mass
Electrical fields = unit positive charge

108
Q

What does the work needed to bring a unit positive charge to a point in an electrical field depend on?

A

Whether it is a positive or negative charge causing the field

109
Q

What will a positive charge require to move the positive charge closer to it?

A

Positive external work

110
Q

What will a negative charge require to move a positive charge towards it and what does this mean?

A

Negative external work
The field does the work

111
Q

What do we need to consider to calculate the value of the potential in both electrical and gravitational fields?

A

The field strength

112
Q

Field strength

A

Force per unit charge/mass

113
Q

Describe the forces in a gravitational field

A

Small (but big masses)

114
Q

describe the forces in an electrical field

A

Big (but small charges)

115
Q

Comparing the different lines in a field to the map analogy, what can the different lines be compared to?

A

Equipotentials (lines of equal potential) are equivalent to the contour lines
Force lines are equivalent to the gradient on the map

116
Q

What is force in a field?

A

The potential gradient

117
Q

Potential gradient equation

A

Potential gradient = Change in potential/distance = force

118
Q

Expression for the potential at a point in a gravitational field

A

Vg = -GM/r

119
Q

Difference between forces and potentials equations

A

Potentials —> r
Forces —> r^2

120
Q

Why is the negative sign in the expression for potential at a point in a gravitational field?

A

Since all potentials are negative

121
Q

Equation for the potential in an electrical field

A

VE = 1/4piε0 Q/r

122
Q

What will the polarity of the potential in an electrical field come from?

A

The polarity of the charge present

123
Q

Explain how the polarity of the potential in an electric field will come from the polarity of the charge present

A

If it is a negative charge, the potential will be negative
If it is a positive charge, the potential will be positive

124
Q

What is done to calculate the potential energy of an object of a given mass/charge?

A

The potential is multiplied by the value of the field potential at the point where the object lies

125
Q

Equation for the potential energy of an object of a given mass/charge

A

Vg = -GM1M2/r

126
Q

Equation for the potential energy between two charges

A

VE = 1/4piε0 Q1Q2/r

127
Q

Where will there be no work done in n electrical field?

A

At the very top of the dip before entering it

128
Q

Describe the work done when moving towards the mass

A

Work is always done by the field

129
Q

Vg

A

Gravitational potential

130
Q

VE

A

Electrical potential

131
Q

Which sign is always negative - Vg or VE?

A

Vg

132
Q

What does the sign of VE depend on?

A

The size of the charge causing the field

133
Q

How, basically, do we work out potential energy?

A

Potential x mass/charge

134
Q

What can we use potential energy to work out?

A

The maximum kinetic energy

135
Q

At what angle do fields come from point charges every time?

A

90 degrees

136
Q

What come from point charges at 90 degrees every time?

A

Point charges

137
Q

Where does an electric field go from and to?

A

From the positive charge to the negative charge

138
Q

Where is in the highest force in an electric field pattern?

A

In the negative well

139
Q

4 electric field patterns to learn

A

Point positive charge
Point positive and point negative charge
Row of negative charges and point positive charge
Row of positive and negative charges (capacitor)

140
Q

Why is calculating the net potential due to several masses or several charges very straightforward?

A

They are simply overlapping scalar fields

141
Q

What is the analogy to explain calculating the net potential due to several masses or several charges/

A

Imagine placing a temperature map of a winer’s day over a similar map in the smear, as if both days happen simultaneously. The net effect will give the total temperature of both days.

142
Q

What is the net effect of the overlap of two potential maps?

A

The addition of the potential due to each field

143
Q

Explain an example where the potential would decrease in a specific point when working out the net potential due to more than one mass/charge

A

If one field is negative and the other positive (only possible for charges), then the potential is reduced at that point.

144
Q

What is the only situation in which it’s possible for one field to be negative and one to be positive when calculating the net potential?

A

Only possible for charges

145
Q

When does the net potential become zero?

A

When both of the fields are equal and opposite

146
Q

How do we calculate the potential at a point?

A

Just calculate the potential due to each mass or charge present and add them up

147
Q

What are all of the potential values with gravitational potential?

A

Negative

148
Q

What do the + and - charges depend on when working out net potential with charges?

A

Depend on the charges

149
Q

What type of field is the force field and what does this lead to?

A

Vector field
Direction has to be considered when adding the net effect of fields due to more than one mass/charge

150
Q

When does direction have to be considered when working out its net amount and why?

A

For net field strengths or net forces
They’re vector fields

151
Q

How is wind pattern observed in reality?

A

It’s a resultant of all the individual pressure gradients due to the difference in the density of the air over the country

152
Q

What would we get if both weather systems “overlap” and what is this used as an analogy of?

A

A “resultant wind”
Net field strength or net force due to more than one mass/charge

153
Q

How is wind pattern observed?

A

Is a resultant of all the individual pressure gradients due to the difference in the density of the air over the country

154
Q

Why is the direction of the field due to each mass always obvious with the gravitational force?

A

It’s always attractive

155
Q

How is the size of field strength calculated?

A

Using g = GM/r^2
M = mass of the body
r = distance from the point to the centre of the body

156
Q

Calculation for working out the net field force, for example between the earth and the moon

A

g(earth)-g(moon) and then if the value is negative then g(moon)>g(earth) and the field direction would be towards the moon

157
Q

When is the vector addition with net field strength or net force straightforward?

A

When the masses are in a line

158
Q

What do we do in order to work out the net field strength or net force when the point lies off the line joining the two masses?

A

Then the horizontal and vertical components of the field are added as vector - just like with forces in unit 1

159
Q

What types of forces are there for charges?

A

Both attractive and repulsive

160
Q

What will most exam questions be on in terms of net field strength or net force calculations?

A

Charges

161
Q

What is the field direction with charges?

A

The direction in which a positive charge would move

162
Q

When is there an attractive force with charges?

A

When negative

163
Q

When is there a repulsive force with charges?

A

When positive

164
Q

How is the magnitude of the force between charges calculated?

A

Using E = 1/4piepsilon0 Q/r^2

165
Q

In some situations, how is it easier to get the direction of the field when working out the net field strength or net force due to more than one mass/charge?

A

It may be easier not to include the polarity of the charge and simply use arrows to get the direction of the field

166
Q

How can we plot equipotentials due to different combinations and shapes of charges?

A

A point charge can be produced using the tip of a wire
A line of charge can be produced using a strip of conductor

167
Q

How can we produce a point charge?

A

Using the tip of a wire

168
Q

How can we produce a line of charge?

A

Using a strip of conductor

169
Q

How are field lines plotted compared to the equipotential lines?

A

At 90 degrees

170
Q

What are field lines at 90 degrees to?

A

The equipotential lines

171
Q

What is field strength equal to?

A

Force and potential gradient

172
Q

If we plot a graph of potential at various points in a field, what will the gradient show?

A

The field strength

173
Q

Unit of field strength

A

Arbitrary units

174
Q

What does a steeper slope when plotting a graph of potential at various points in a field mean?

A

A stronger field

175
Q

Where are the peaks on 3D graphs of field strength?

A

At point positive charges

176
Q

Where are the troughs on 3D graphs of field strength?

A

At point negative charges

177
Q

What will a graph of field strengths with a row of positive charges and a row of negative charges show and what does this represent?

A

The graph will be a steady slope from high to low voltage
Represents a capacitor

178
Q

How is the direction of the field visualised when creating graphs of field strengths?

A

Since the direction of movement of a positive charge placed here is clear to see

179
Q

What would we do in the case of the following question?
“Calculate the work done when a +1.0nC charge is brought from a large distance way and placed at P”

A

Since it’s coming from a “large distance away”, the change in potential will be from zero to that point

So since W = qdeltaV, we just need to work out V at P
Vp = 1/4piepsilon0 x Q/r
Then multiply V by the charge (1nC)

180
Q

When do we know that we can calculate a change in potential as from zero to that point?

A

If the object is coming from a large distance away

181
Q

What are the two ways in which we could calculate the work needed to raise a mass of 1kg through a distance of 2m near to the earth?

A
  1. Using work done = energy transferred and Ep =mgh
  2. Using work done =energy transferred and Ep = Vgm

Both will give the same answer

182
Q

Is Ep = mgh valid?

A

For most everyday calculations, yes

183
Q

Worded and equation definition of an electrical field

A

The electric Coulomb force per unit charge
E = f/q

184
Q

Worded and equation definition for a gravitational field

A

The gravitational force per unit mass
g = f/m

185
Q

What do we do if there are ever any questions in this unit involving protons?

A

Remember that they have the same charge as an electron, just positive

186
Q

Charge of a proton

A

Same as an electron just positive

187
Q

What does the sum of kinetic and potential energy always make up and what does this mean?

A

Always remain constant at each point of motion, and so any change in PE will be equal to the KE which we can use to work out other factors

188
Q

Electrical field unit

A

NC-1

189
Q

Gravitational field unit

A

Nkg-1

190
Q

Electric potential unit

A

JC-1

191
Q

Gravitational potential unit

A

Jkg-1

192
Q

What will the resultant electric field strength be a long distance away from two charges and why?

A

Zero
The fields from the charges will cancel

193
Q

What happens to the resultant force from charges with increasing distance and why?

A

Decreases
Inverse square law

194
Q

Why does the resultant force from charges decrease with distance

A

Due to the inverse square law

195
Q

Gravitational potential at a point

A

Work done per unit mass from infinity

196
Q

Electric potential

A

The work done per unit charge in bringing a charge from infinity to that point

197
Q

Direction of magnetic fields

A

Positive to negative

198
Q

Equation for working out the force in an electric field + explain

A

F = EQ

It comes from the F = 1/4pi x Q1Q2/r^2 equation, where Q2 becomes the other Q
E is E = V/d

199
Q

What’s similar about gravitational and electric fields?

A

Vectors
Obey inverse square law
Both are attractive fields (electric can be repulsive too)

200
Q

Electric field strength E

A

The force per unit charge on a point positive charge (vector)

201
Q

Electric potential V

A

The work done per unit charge in bringing the charge from infinity to that point

202
Q

What do we always need to say when describing potentials?

A

Work done

203
Q

Similarities and differences between electric and gravitational fields

A

Similarities:
-same shape field
-obey the inverse square law
-vectors
-both lead to PE
-PE proportional to 1/distance
-fundamental force laws
-vectors

Differences:
-E.fields = force per unit charge
G.fields = force per unit mass
-E. fields = attractive and repulsive
G. fields = only attractive
-Gravity is far weaker on a small scale, but electrical negligible on a large scale
-E potential = work done per unit charge
G potential = work done per unit mass
-E fields depend on permittivity
-G fields depend on dark matter

204
Q

Explain why all gravitational potentials are negative

A

Since the potential at infinity is zero and work is done in an object to get to infinity

A level Physics (29 decks)

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