7. Fields and their Consequences Flashcards
1
Q
When can charge transfer between two objects?
A
When they slide past each other
2
Q
When two objects slide past each other, how is charge transferred?
A
Electrons leave one surface and join the other
3
Q
What is earthing?
A
When electrons move to or from the earth to balance charges on charged objects
4
Q
What happens, when charge is transferred between objects, if one of the objects is an insulator?
A
The charge can build up
5
Q
Why does charge build up on the dome of a Van de Graaf generator?
A
The dome is metal, but charge builds up as it is isolated
6
Q
What happens when a charged polythene rod is brought near to uncharged objects?
A
It attracts the object
7
Q
How do you get charge onto an electroscope?
A
∙ charged object e.g. polythene rod is brought near the metal plate
∙ gold leaf moves towards rod
∙ electroscope is earthed so charge is stored
8
Q
How can electric fields be represented?
A
Field lines
9
Q
What do field lines show?
A
The direction of the force that would be felt by a small positive charge
10
Q
What do equally spaced field lines show?
A
A uniform field
11
Q
What practical can be done to plot field lines?
A
Electrolytic tanks and conducting paper
∙ damp filter paper, potassium permanganate and 250V electrodes
∙ plot equipotential lines using a point probe attached to a voltmeter (field lines plotted perpendicular to equipotential line)
12
Q
What are the rules about field lines?
A
∙ never start or stop in empty space
∙ never cross
∙ density of field lines shows strength of field
13
Q
Where do field lines stop and start?
A
Either on a charge or at infinity
14
Q
Why do field lines never cross?
A
If they did, a small positive charge place there would feel forces in different directions, which could be resolved into the one true direction of the field there
15
Q
What is a neutral point?
A
A point exactly between two like charges where no field exists
16
Q
What is the equation for electric field strength?
A
E = F / Q
17
Q
What is field strength?
A
The force per unit (positive) charge exerted by the field
18
Q
What are the units for electric field strength?
A
N C-1
19
Q
Summary of Coulomb’s law?
A
The force between two point charges is
∙ directly proportional to each of the charges Q1 and Q2
∙ inversely proportional to the square of their separation
20
Q
What is the equation for Coulomb’s law?
A
F = kQ1Q2 / r²
21
Q
What does k depend on in F=kQ1Q2 / r²?
A
The permittivity of the substance separating the charges
22
Q
What permittivity does every insulating material have?
A
Greater than the permittivity of free space
23
Q
What does it mean, in terms of charges, that the permittivity of water is about 80 times the permittivity of free space?
A
Makes the forces between charges 1/80th of value - when salt is put in water the forces are reduced and crystal structure collapses
24
Q
What equation do you get when the electric field and coulombs law equation are combined?
A
E = kQ / r²
25
What does k equal in E = kQ / r²?
1 / 4πℇ
26
How can the strength of the uniform field between parallel plates be made stronger?
∙ increase p.d. across plates
∙ moving plates closer together
27
Equation for strength of a uniform field?
E = V / d
28
What can electric field strength be measured in?
Vm-1 or NC-1
29
Why can electric field strength be measured in Vm-1 or NC-1?
Work is done moving an object, and work is done when a charge moves through a p.d.
30
How are electrons used in TVs, oscilloscopes and X-ray machines?
∙ electron gun produces electrons by thermionic emission
∙ then they are accelerated by an electric field
∙ as electrons accelerate across field, they lose potential energy but gain kinetic
(gain in Ek = loss in Ep)
31
What happens in an electron deflection tube?
Moving electrons pass through an electric field between two plates
32
What is the equation for the force on an electron in an electron beam, and why is this?
∙ F = e V / d
(as E=V/d and force on an electron charge is F=Ee
33
What is the shape of the path when electron beams are deflected?
Curved, as the force (F=eV/d) is constant
34
What is the difference between electric and gravitational fields, in terms of what 'feels' the force?
∙ electric - charge q
∙ gravitational - mass m
35
What is the difference between electric and gravitational fields, in terms of their definition?
∙ electric - force per unit charge
∙ gravitational - force per unit mass
36
What is the difference between electric and gravitational fields, in terms of their constant of proportionality?
Both inversely proportional to r²
37
What is the difference between electric and gravitational fields, in terms of their force equation?
∙ electric - F=Eq
∙ gravitational - F=mg
38
What is the difference between electric and gravitational fields, in terms of their direction of force?
∙ electric - like charges repel, unlike charges attract
∙ gravitational - all masses attract
39
What is the difference between electric and gravitational fields, in terms of their relative strength?
∙ electric - strong at close range (responsible for chemical bonding)
∙ gravitational - weak except for massive bodies (responsible for motion of planets)
40
Do all points in an electric field have an absolute electric potential?
Yes
41
What is absolute electric potential?
The electric potential energy that a unit positive charge would have at that point
42
What is absolute electric potential calculated using?
V = 1/4πℇ x Q/r
43
What does r equal in V = 1/4πℇ x Q/r?
Distance from the charge Q
44
What does the sign of electric potential depend on?
The sign of Q (V is +ve when Q is +ve and the force is repulsive)
45
When will V (electric potential) be zero?
When r is
46
For a V-r graph, what does the gradient of a tangent to the graph give?
The field strength at that point
47
When will two points in an electric field have an electric potential difference between them?
If they have a different absolute electric potential
48
What is electric potential difference?
The energy required to remove a unit charge between two points in an electric field that have a different electric potential
49
How can electric potential difference be found?
Using the area under the graph of E against r
50
Where are field lines in relation to equipotential lines?
Perpendicular
51
What happens in terms of energy when you travel along an equipotential line?
No work is done so no energy is transferred
52
For a charged sphere, where may charge considered to be?
At the centre
53
When is electric potential zero?
At infinity
54
What equation links V to E?
E = ΔV/Δr
55
What is G?
The universal gravitational constant
56
What is the unit of G?
N m2 kg-2
57
What does the formula F = Gm1m2/r2 apply to?
Point masses, but spherical masses can be treated as point masses with all their mass concentrated in the centre
58
Is the force in F = Gm1m2/r2 attractive or repulsive?
Always attractive
59
What is a field?
A region of space around an object where other bodies feel a force due to it
60
Is your pull on the earth the same as the earth's pull on you?
Yes, but your gravitational field is far weaker
61
What is field strength?
A point in a body's field as the gravitational force exerted on an object placed at that point, per kg of the object's mass
62
If two objects of different masses are placed at the same point in a field will they experience the same field strength and gravitational force?
Will experience same field strength, but different gravitational forces
63
When calculating gravitational field strength, what is the mass that matters?
The mass below you only
64
Where does g = GM/r2 come from?
When objects are a distance from earth e.g. other planets
* F = Gm1m2/r2
* small mass m, and planet mass M - F = GMm/r2
* but F=mg so mg = GMm/r2
* therefore g = GM/r2
65
What is Kepler's third law?
The time of one orbit T, and the distance from the planet to the sun r, are related by T² ∝ r³
66
How is T² ∝ r³ proven?
* planet remains in orbit due to centripetal force (F=mv2/r)
* F=Gm1m2/r2
* GMm/r2 = mv2/r
* but v = 2πr/T
* so GM/r2 = 4π2r/T2
* T2 = (4π2/GM) r3
67
What are geostationary satellites used for?
Communications
68
How many geostationary satellites would be able to cover the entire earth?
3 placed into orbit 120 degrees apart above the equator
69
Why are the energy and costs required for launching a satellite into a geostationary orbit high?
Because they have to be launched so high
70
Equation for GPE in a uniform field?
Change in GPE = mgh
71
What is the value of GPE at infinite distance?
0
72
What is GPE proportional to, in terms of r?
1/r
73
What is the equation for GPE in a radial field?
GPE = –GMm/r
| note this is not change in GPE
74
When GPE = –GMm/r, what happens to GPE as you move closer to M?
GPE becomes more negative as at infinite distance GPE=0
75
Why did someone come up with gravitational potential?
To get an expression involving energy which is independent of the mass placed in a field
76
What are the two ways gravitational potential can be thought about?
1. GP at a point in a field is the energy per unit mass (V=Ep/m so V=gh)
2. GP is work done per unit mass in moving a small object from infinity to that point (ΔV=ΔE/m)
77
Is gravitational potential a scalar or vector?
Scalar
78
Is gravitational field strength vector or scalar?
Vector
79
Is the force between two masses vector or scalar?
Vector
80
Is gravitational potential energy vector or scalar?
Scalar
81
What is the unit for gravitational potential?
J kg-1
82
What do equipotentials do?
Join points of equal potential
83
When V = -GM/r is used to calculate V at earth's surface, V = -63 MJ kg-1. What does this mean?
63mJ of work needs to be done in order to move 1kg from the earth's surface to infinity
84
What type of graph is the graph of V against r?
A 1/r curve, not an inverse-square
85
What is the derivation for the escape velocity equation?
* Ep = -GMm/r and Ek=1/2mv²
* energy is conserved so Ep+Ek=0
* -GMm/r + 1/2mv² = 0
* so v = √2GM/r
* or √2gr as gr=GM/r
86
What is a null point?
A point between two masses where the resultant g force is zero
87
What are low earth orbits?
* satellite systems used in telecommunications
| * orbit between 400 and 1,000 miles above the earth's surface
88
What is a synchronous orbit?
An orbit where the satellite has a period equal to that of the body being orbited
89
What is the radius of a geostationary orbit?
approx 42000km
90
What do capacitors do?
Store electric charge
91
Where are capacitors used?
In almost all electric circuits
92
What does a capacitor consist of?
Two parallel metal plates separated by an insulator called a dielectric
93
What is a dielectric?
The insulator in a capacitor that separates the two metal plates
94
When will a capacitor have a greater capacitance?
When it can store more charge
95
What is the capacitance of a capacitor?
The charge stored per unit of potential difference across it
96
What is the equation for capacitance?
C = Q/V
97
What is the unit of capacitance?
the farad, F
| =1CV⁻¹
98
Are farads large or small units?
Very large
99
How are farads usually marked?
In pico or microfarads
| 1 pF = 10⁻¹²
100
What may a capacitor have marked on it?
A working voltage which must not be exceeded
101
Roughly, how much electric charge do capacitors store?
Small amounts, providing power for a short amount of time
102
Why can charged capacitors be dangerous?
They can discharge all of their charge in a fraction of a second
103
Where can capacitors be used?
* camera flash
* back up power supply
* to smooth dc power supplies
104
What happens in terms of electrons when a capacitor is charged?
* electrons are pushed onto one plate and off the other
* the power supply does work on the electrons so the Ep increases
* this Ep is stored in the electric field between the plates
105
In a V-Q graph, what is the area underneath the graph?
Energy stored
106
Explain how a capacitor 'gets' charge.
* during charging electrons flow from -ve terminal of power supply to one plate of capacitor and from other plate to +ve terminal
* switch closed, charging starts, rate of flow of charge is large (i.e. big current) and this decreases with time and plates become more charged so resist further charging
107
What happens, when charging a capacitor, to the rate of flow of charge?
* at first rate of flow of charge/current is large
| * this decreases with time as the plates become more charged so resisting any further charging
108
What effect would adding a resistor in the circuit when charging a capacitor have?
Only affects time taken for capacitor to become fully charged and not the eventual p.d. across it
109
What does the gradient of the tangent to the curve at a point on a Q-t graph give?
The current at that time
110
How to find current at a particular time using a Q-t graph?
Gradient of tangent at a certain point
111
On a current-time graph, what does the area underneath the curve give?
The charge stored
112
What is T½ on a Q-t graph?
The time for the charging current to half
113
What actually happens in a discharge circuit?
* as soon as switch is closed, 'large' current flows and p.d. across capacitor drops
* as charge flows from one plate to the other through the resistor the charge is neutralised and so the current falls
* rate of decrease of p.d. also falls
114
When is a capacitor fully discharged?
When the charge on the plates is zero and the current and p.d. are also zero
115
How does a resistor affect the discharge of a capacitor?
The value of the resistor doesn't affect the final p.d., only the time it takes to reach this value
116
How does a larger resistor affect the time taken for a capacitor to discharge?
The bigger the resistor, the longer time taken to discharge
117
What can a discharge curve apply to?
V, Q or I against time
118
What is the area under the discharge curve of a I-t graph equal to?
The charge that has flowed
119
What is the gradient of the tangent on a Q-t graph equal to?
The current at that point in time
120
What is half life equal to?
0.69RC
121
What is the time constant?
The time taken for the p.d. across the capacitor and the charge stored on the capacitor to drop to 1/e of their original values
122
What is the gradient of the graphs of loge(current) etc. graphs?
All have same gradient; -1/RC
123
What equation shows the factors affecting the capactiance of a parallel plate capacitor?
C = Aε0εr/d
124
What does C equal in C = Aε0εr/d?
Capacitance (F)
125
What does A equal in C = Aε0εr/d?
Cross sectional area of overlap of the plates (m²)
126
What does ε0 equal in C = Aε0εr/d?
Permittivity of free space (Fm⁻¹)
127
What does εr equal in C = Aε0εr/d?
Relative permittivity of the dielectric
128
What does d equal in C = Aε0εr/d?
Distance between the plates (m)
129
What is relative permittivity also known as in terms of capacitors?
The dielectric constant of the material
130
What is εr calculated from?
εm / ε0
where εm is the permittivity of the material used as the dielectric
131
What is the unit of εr?
No unit
132
What is a dielectric material (dielectric for short)?
An electrical insulator that can be polarised by an applied electric field
133
What happens when a dielectric is placed in an electric field?
Electric charges do not flow through the material as they do in a conductor, but only slightly shift from their average equilibrium positions causing dielectric polarisation
134
When does dielectric polarisation occur?
When a dielectric is placed in an electric field and the electric charges shift from the average equilibrium positions
135
What is the effect of dielectric polarisation?
* +ve charges are displaced toward field and -ve shift in opposite direction
* creates internal electric field that reduces overall field within dielectric so reducing p.d. across capacitor
136
How is an internal electric field created in a capacitor?
The dielectric is placed in an electric field, dielectric polarisation occurs and creates an internal electric field
137
What is the effect of an internal electric field in a dielectric?
It reduces the overall field within the dielectric itself, reducing the p.d. across the capacitor
138
How can the p.d. return to its original value after dielectric polarisation?
Requires the addition of more charge onto the plates
139
What effect does dielectric polarisation have on the capacitor overall?
The capacitor can store more charge for the same p.d. and increase its capacitance
140
When will a wire in a magnetic field experience a force?
When it carries a current
141
How can the direction of the force on a current carrying wire be worked out?
Using Fleming's left hand rule
142
What does each finger mean in Fleming's left hand rule?
First finger = field
Second finger = Current
Thumb = Thrust/Force
143
What equation is used to work out the magnitude of force on a current-carrying wire?
F = BIL
144
What does F mean in F=BIL?
Force (N)
145
What does B mean in F=BIL?
Flux Density (T)
146
What does L mean in F=BIL?
Length of conductor in field (m)
147
What does I mean in F=BIL?
Current (A)
148
How is the tesla defined using words?
One newton per amp per metre
149
How is the tesla defined using the equation?
1T = 1 N/Am
150
How can the strength of a magnetic field be measured?
By the force per unit current per unit length acting on a current carrying conductor placed perpendicular to the lines of a uniform field
151
How can magnetic flux density B be measured?
By the force per unit current per unit length acting on a current carrying conductor placed perpendicular to the lines of a uniform field
152
What does the right hand thumb rule determine?
The direction of the magnetic field in a direction of current
153
What directions are field and current relative to each other in F=BIL?
Perpendicular
154
How is a wire with the current coming towards you drawn?
A circle with a dot in
155
How is a wire with the current travelling away from you drawn?
A circle with a cross in
156
What happens to a charged particle when it moves through a magnetic field?
It experiences a force
157
When a charged particle moves through a magnetic field and experiences a force, what is this force proportional to?
* B - magnetic flux density
* Q - charge on the particle
* v - velocity of the particle
158
When a charge is moving in a magnetic field, how can the force be calculated?
F = BQv
159
What condition must be true when F=BQv is used to calculate the force on a charge?
The charge must be moving at 90° to the field
160
In the equation F=BQv, what is the direction of the force given by?
Fleming's left hand rule
161
When a charged particle moves at right angles a magnetic field, what is the constant force perpendicular to?
Both the velocity and the field
162
Why does the constant force on a particle in a magnetic field change the particle's direction of motion and has no effect on speed?
The constant force is perpendicular to both the velocity and field
163
Does the force from a magnetic field on a charged particle affect the particle's speed?
No
164
Does the force from a magnetic field on a charged particle affect the particle's direction of motion?
Yes
165
What is the result when a constant force acts on a charged particle in a magnetic field?
The particle travels in a circular path
166
What does it mean for a charged particle in a magnetic field, that the constant force is perpendicular to the velocity and field?
The particle travels in a circular path
167
How is the equation for the radius of the circular path which a charged particle follows derived?
* BQv = mv²/r
| * r = mv/BQ
168
What is the equation for the radius of the circular path which a charged particle follows?
r = mv/BQ
169
How is the equation for the time period for an electron in a magnetic field to make one rotation derived?
* time = distance/speed
* time = length of circular path/speed of electron or T=2πr/v
* since r = mv/BQ, T = 2πmv/vBQ
* T = 2πm/BQ
170
Equation for time period of an electron in a magnetic field to make one rotation?
T = 2πm/BQ
171
In a magnetic field of constant flux density, does the time period of an electron depend on its speed?
No
| a faster moving electron moves in a circle of larger radius, but takes the same time to make one revolution
172
Where might the circular path of charged particles in a magnetic field be applied?
Cyclotron
173
What is magnetic flux given by?
Φ = BA
174
What does Φ mean in Φ=BA?
Magnetic flux (Wb)
175
What does A mean in Φ=BA?
Cross sectional area (m2)
176
What does B mean in Φ=BA?
Flux density (T)
177
What is magnetic flux measured in?
weber (Wb)
178
What is 1 weber defined as?
1 Wb = 1T m2
179
What happens to the equation Φ = BA if the plane of the area is not perpendicular to the field?
Φ = BAcosΘ
180
What is flux linkage?
The magnetic flux linking the coil
181
What is flux linkage given by?
Flux linkage = NΦ
182
What does Faraday's law state?
The magnitide of the induced emf is equal to the rate of change of flux linkage
183
What does N mean in flux linkage = NΦ?
The number of turns in a coil i.e. the number of turns cutting the flux
184
What is the equation relating to Faraday's law (words)?
Induced emf = change in flux linkage/time taken
185
What is the equation relating to Faraday's law (symbols)?
E = ΔNΦ / Δt
186
What does Faraday's law essentially tell us?
The size of the induced emf
187
How can we find the direction of the induced emf described in Faraday's law?
Using Lenz's law
188
What is Lenz's law
The direction of the induced emf is such that it will try to oppose the change in flux that is producing it
189
Equation that relates to Lenz's law?
E = - ΔNΦ / Δt
190
What does the minus sign in Lenz's law equation show?
That the emf is always induced in a direction so as to oppose the change in flux
191
What does a generator do?
Converts kinetic energy to electical energy
192
What do the slip rings in a generator do?
Rotate with the coil and press against stationary carbon brushes
193
In a generator, which side of the coil makes contact with which brush?
Each side of the coil always makes contact with the same brush
194
In a generator, what happens to flux linkage as the coil rotates at a steady rate?
The flux linkage constantly changes
195
In a generator, how much a.c. does one revolution of the coil give?
One revolution of the coil gives one cycle of a.c.
196
Overall, what increases the peak emf of a generator?
By increasing the rate of change of flux linkage of the coil as it spins
197
How can the rate of change of flux linkage of the coil in a generator be increased, in order to increase peak emf?
* using a coil with more turns
* using a coil with a larger cross-sectional area
* increasing the strength of the magnetic field
* increasing the frequency of rotation of the coil
198
What will the frequency of rotation of the coil affect in a generator?
The rate of change of flux linkage of the coil, as well as the frequency of the a.c. signal
199
In a generator, what is the relationship between the frequency of rotation of the coil and the frequency of the ac signal?
They are directly proportional
200
What are the equations for flux linkage when looking at generators?
* flux linkage = BANcosΘ
* flux linkage = BANcosωt
* flux linkage = BANωsinωt
201
In the equation flux linkage = BANcosΘ, what does Θ depend on? What equation does this result in?
The angular speed ω of the coil
giving flux linkage = BANcosωt
202
What does ω stand for?
Angular speed in rad s-1
203
How does induced emf in a generator vary? Why is this?
Sinusoidally, as max. change happens when Θ=90°
204
What type of current do cells and batteries supply?
Direct
205
What is a direct current?
Current flowing in one direction only
206
What type of current does mains supply?
Alternating
207
What is the peak value of ac current or pd?
The maximum in either direction
208
How can peak value of ac be measured?
From the wave as the amplitude
209
How is peak current denoted?
I₀
210
How is peak voltage denoted?
V₀
211
What is the peak to peak value of current or pd?
The range of values - the distance from the peak above the zero line to the peak below the zero line
212
What is the time period of an ac current?
The time taken for one complete cycle/wave
213
What is the root mean squared?
A value of current that produces the same heating effect in a resistor as the equivalent dc
214
Why, for an ac current, is it impossible to assign a value to pd and current for a fixed value of time? What is done instead?
Current and pd is constantly changing so average would be zero
- root mean squared produces same heating effect in a resistor as the equivalent dc
215
What is an oscilloscope used for?
To show the sizes of voltages and currents in both dc and ac circuits
216
What does a dc trace on an oscilloscope look like? Why?
A straight line, as the current is constant, so the voltage is constant
217
What does an ac trace on an oscilloscope look like? Why?
A wave, as the current is constantly changing from maximum flow in one direction to maximum flow in the other direction, so voltage does the same
218
What controls do we use on an oscilloscope?
* volts/div dial
| * time base dial
219
What does the volts/div dial on an oscilloscope allow you to do?
Change how much each vertical square is worth
220
What does the time base dial on an oscilloscope allow you to do?
Change how much each horizontal square is worth
221
How can you measure the voltage of a dc supply using an oscilloscope?
Counting number of vertical squares from origin to line and multiply by volts/div
222
How can the time for one period be measured using an oscilloscope?
Counting how many horizontal squares one wavelength is and multiply by time base
223
What does a transformer do?
Changes the value of ac voltage
224
What do transformers consist of?
Two coils wound around a soft iron core
225
How does a transformer work?
* ac current flows in the primary coil
* producing a changing magnetic flux in the soft iron core
* meaning the flux linkage of the secondary coil is constantly changing
* and so an ac voltage is induced across it
226
What does a step-up transformer do? Why is this?
Increases ac voltage, as secondary coil has more turns than primary coil
227
What does a step-down transformer do? Why is this?
Decreases ac voltage, as secondary coil has fewer turns than primary coil
228
In a transformer, what is the ratio of voltages equal to?
The ratio of turns
229
What equation shows the ratio of voltages in transformers?
Vs / Vp = Ns/ Np
230
What happens in terms of energy losses in an ideal transformer?
No energy is lost
231
What is the equation for power in an ideal transformer, and under what condition?
VpIp = VsIs
| provided no energy is lost
232
What are eddy currents?
Looping currents induced by the changing magnetic flux in the core
233
What do eddy currents do?
Create a magnetic field that acts against the field that induced them
234
How do eddy currents dissipate energy?
By generating heat
235
How can the energy loss from eddy currents be reduced?
By laminating the core
236
How can the efficiency of a transformer be calculated?
Ratio of power out to power in:
E = IsVs / IpVp
237
What is voltage stepped up to through the National Grid?
400 000 V
