4. Imagining The Invisible Flashcards
1
Q
I) (1 mark) How would an change in external magnetic field lead to faults in other electrical circuits inside a television?
A
Leads to heating effects, damage and additional magnetic fields.
2
Q
I) What physical properties of the earth can be monitored to discover what lies underneath?
A
Electrical resistivity
Velocity of sound waves transmitted through the ground
Reflection of radio waves, allowing distance measurements
Gravitational field strength
Magnetic field strength
Radioactivity
3
Q
I) What is an anomaly?
A
A reading that deviates from the expected reading.
4
Q
What measurements of the physical properties of the earth could you take to survey what is underneath? (6 methods)
A
- Electrical resistivity
- Velocity of sound waves through the ground
- Reflection of radio waves, allowing distance measurement
- Gravitational field strength
- Magnetic field strength
- Radioactivity
5
Q
Describe how a geophysical survey can be made over a piece of land.
A
-Use a grid system and measure the geophysical property at regular intervals
6
Q
What may an anomaly in a geophysical survey indicate?
A
A dense body or a less dense region (lighter minerals or a cave or a previous excavation).
7
Q
State Newton’s gravitational law (equation)
A
F=GMm/r^2
8
Q
What is the definition of the gravitational field strength
A
The gravitational force on a mass of 1kg at that point
9
Q
How can the overall gravitational force on a 1kg mass be found?
A
Adding up vectorially all the forces exerted on the 1kg mass (for particles of the same mass, those closest to the 1kg mass will exert a greater force than one further away)
10
Q
Why do variations in the earth’s gravitational field occur?
A
- Due to the earth not being perfectly spherical
- Due to the earth’s rotation
11
Q
What are the factors that affect the magnitude of the change of the value of g caused by an object eg mineral deposit below the ground?
A
- Difference in density of the body
- Volume
- Distance from the surface
12
Q
What is one gal equivalent to?
A
- 0.01Nkg^-1
- Equal to an acceleration of free fall of 1cms^-1
- Normal gravitational field strength would be 981 gal
13
Q
Describe how a free fall gravimeter works
A
- Measures the absolute value of g at that point
- Object falls in free fall in a vacuum
- Laser at the bottom shines light through a partially reflecting mirror which is reflected off the falling object
- Partially reflecting mirror reflects light to a reference mirror which is to the side of the partially reflecting mirror
- Light from the reference mirror and partially reflecting mirror are reflected to a light sensor by the partially reflecting mirror
- Light sensor detects the interference fringes produced by the superposition of the laser light reflected from the reference mirror and a mirror attached
- As the object falls the detector monitors the changing light intensity
- FOR A CHANGE OF ONE WAVELENGTH OF THE LASER LIGHT, THE OBJECT WOULD HAVE MOVED DOWN HALF A WAVELENGTH
14
Q
Modern gravimeters can measure changes in g to 1 part in 1000. What is the equivalent precision in gals?
A
0.001mgal
15
Q
How can a pendulum be used to measure g? What are the limitations?
A
- Length of string cannot be determined accurately enough to make measurement to the accuracy required for gravitational surveying
- g is proportional to 1/T^2 (from the pendulum equation) so as g increases, the period decreases
- Timing a large number of oscillations could give you the value for the change in gravitational field
16
Q
What happens to the percentage uncertainty when a quantity is squared in an equation?
A
Percentage uncertainty is doubled
17
Q
What is the direction of a magnetic field defined as?
A
The direction of the force on a magnetic North pole due to the field
18
Q
What is the angle of dip?
A
The angle between the direction of the earth’s magnetic field and the plane of the horizon.
19
Q
What kind of field lines would represent a (relatively) strong magnetic field?
A
Field lines very close together
20
Q
What causes a magnetic field? Give an example of how a magnetic field could be produced in the lab
A
The movement of charge
Running a current through a wire- the movement of the charged electrons creates a magnetic field
21
Q
What ‘rule’ can be used to find the direction of a magnetic field caused by an electrical current through a wire?
A
The corkscrew rule. The point of the corkscrew is pointing in the direction of CONVENTIONAL CURRENT and the way it turns reveals the orientation of the field produced
22
Q
What form does the magnetic field produced by a current flowing through a wire take?
A
It is a magnetic field in the form of closed circles, NOT a helix, around the wire.
23
Q
How is a magnetic field produced by a bar magnet?
A
- Movement of electrons inside atoms
- Atoms arranged so they line up
- Net magnetic field produced as they do not cancel out like they do in normal objects
24
Q
What is residual magnetism?
A
Rocks in the earth used to be molten and in this state, the atoms aligned with the magnetic field that existed at that time.
This residual magnetism can now be detected and give geologists information about the past.
Anomalies can be created by residual magnetism in old clay pottery as the atoms aligned when they were in the kiln- could indicate an ancient civilisation was at that site
25
What reasons could there be for varied magnetic fields?
- Electrical devices we use produce a magnetic field
- We communucate through electromagnetic waves
- Electromagnets used in motors to power machines
- Electricity conducted in overhead and underground cables for power supply
- Magnetic storms due to solar flares
26
```
For a flat coil what does
-Adding more turns do
-Increased current do
-Larger radius do
To the magnetic field strength produced
```
- Each turn produces its own field and these add up together to increase the overall field strength
- An increased current means a grater flow of charge so a higher magnetic field
- Larger radius increases the distance of the current from the centre so the mag field strength is decreased
27
How can a flat coil be modified to produce a stronger mag field?
Use an iron core
28
If current is increased to increase the magnetic field in a wire, how is energy dissipated?
Through IR^2 heating
29
What is magnetic flux?
Magnetic flux (ΦB) is the number of magnetic field lines (also called "magnetic flux density") passing through a closed surface
30
What is the SI unit of magnetic flux?
The webber
| 1 Wb is equal to 1 Tm^2
31
What is the SI unit for magnetic flux density?
Tesla
32
Which factors increase the magnitude of a force experienced by a length of wire perpendicular to a magnetic field?
- Current increasing
- Length of wire perpendicular to the field increasing
- Magnetic flux density increasing
33
What is one Tesla equal to?
1T= one newton per metre per amp
| One Tesla causes a force of 1 newton on a wire of length 1 metre carrying a current of 1 amp
34
Because changes detected in geophysics are often very small, the unit used is the 'gamma' what is one gamma equal to?
It corresponds to a flux density of 10^-9T
35
Describe an experiment to determine the value of B for a magnet (between two poles of two magnadur magnets)
- Set up the magnets on a yoke standing on a set of weighing scales with a wire passing through the gap between the magnets
- When current runs through the wire which is perpendicular to the magnets, the value shown on the electrical balance will increase or decrease (doesn't matter which as it will be of the same magnitude)
- Using the values for current supplied, length of wire and the force produced (calculated from the mass change), B can be calculated with the smallest detectable change being 9.81x10^-4 N of the precision of the scales is +/-0.1grams
36
How is B resolved?
-B is a vector quantity so it is resolved using only the component perpendicular to the direction of the magnetic field
37
Describe how a hall probe works
- Small slice of semiconductor carrying a steady current
- When a magnetic field exists perpendicular to the probe, charge carriers are deflected to one side
- The potential difference exists perpendicular to the current caused by the power supply
- Magnitude of the potential difference is proportional to the magnetic flux density
- If calibrated, it can measure the actual value of B
38
Describe how a proton magnetometer works
- Protons have a charge so they can behave like tiny magnets
- Large current passed through paraffin
- Strong magnetic field causes protons in he nuclei of the hydrogen atoms to line up in the direction of the magnetic field
- Power switched off and the protons precess around the direction of the earth's magnetic field
- The frequency of precession (Lamor frequency) is proportional to the earth's magnetic flux density at that point
- This precession induces an emf in the coil. Lamor frequency calculated by timing the number of oscillations in the induced emf in a known time
- Freq can be used to directly compare strengths of fields or converted into values of flux density
39
What is precession?
the slow movement of the axis of a spinning body around another axis due to a torque (such as gravitational influence) acting to change the direction of the first axis.
40
State Faraday's first law (two ways of being expressed)
- When a conductor cuts magnetic flux, an emf is induced in the conductor
- When the magnetic flux linking a coil changes, an emf is induced in the coil
41
State Faraday's second law (two ways of being expressed)
Magnitude of induced emf depends on:
- Rate at which flux is cut
- Rate of change of flux linkage
42
What happens when an electrical conductor cuts magnetic flux perpendicular to the conductor?
An emf is induced only when the conductor is moving
43
How can the magnitude of an induced emf be increased?
- Using a stronger magnet (increasing the FLUX DENSITY)
| - Moving the wire faster (greater rate of flux cutting)
44
How does an induced emf arise?
As the wire moves, electrons moving with the wire experience a force- electrons are forced to one end, making one end positive and the other end negative
45
Changing flux linkage
See textbook pg 72
46
What did Lenz discover about induced currents?
He found that induced currents are in a direction that produces a force to oppose the change that is producing the current
47
Explain why Lenz's law is a consequence of the conservation of energy
It is a consequence of the conservation of energy
- Energy has to be supplied to a magnet to move it into a coil of wire because of the repulsive force experienced as the magnet moves
- Thus work is done on the magnet and this becomes electrical energy dissipated in the circuit connected to the coil
48
Describe how a geophone is used to measure the movements of the earth
- Magnet has a large mass and is suspended between 2 soft springs (low spring constant)
- Magnet and spring mounted inside a circular coil that is attached to the case of the geophone
- If the magnet moves relative to the case, there is a change in flux linkage through the coil
- EMF induced, the greater the movement relative to the coil, the greater the magnitude of the induced emf
- Magnet oscillates producing an alternating emf
- The voltage pulse produced by the geophone can be used to start or stop a timing device
49
What is an eddy current?
a localized circular electric current induced in a conductor by a varying magnetic field.
50
Describe an instance in which there can be a flux change without movement.
- An electromagnet (coiled wire)(primary coil) and a secondary coil, both with an iron core running through both of them
- Current run though primary coil
- Produces a magnetic field which passes through the secondary coil
- Therefore there is a change in flux linking the first and second coils, inducing an emf in the secondary coil
- When flux in the primary coil reaches its max. the emf ceases (overall there is a voltage pulse across the secondary coil)
- DIRECTION OF CURRENT IN THE SECONDARY COIL IS IN THE OPPOSITE DIRECTION TO THE CURRENT IN THE PRIMARY COIL AS IT MUST PRODUCE A FLUX WHICH OPPOSES THE INCREASE IN FLUX CAUSING THE INDUCED CURRENT
- When the current is switched off, flux collapses and another emf is induced across the secondary coil in the opposite direction
51
How can you increase the emf induced in a secondary coil in a transformer?
Increasing the ratio of turns on the secondary coil to the primary coil
52
In a practical transformer, how are energy losses reduced?
- Using low resistance coils
- Winding the coils on top of each other to maximise the flux linkage
- Laminating the core to reduce eddy currents (which would cause the temperature of the core to rise)
53
How can an alternating magnetic field be investigated?
- Use a search coil
- Coil needs a small area so a field in a small region can be investigated
- Many coils to produce a measurable induced emf
- High frequencies from a signal generator used so that there is a larger rate of change of flux (so larger induced emf)
54
What is the effect of an alternating magnetic field on eddy currents?
-Charged particles made to go in circular paths first one way then the other
55
What is the change in flux linkage in an eddy current proportional to?
Change in flux linkage and therefore emf is proportional to the area (pi r squared)
56
What does the magnitude of an eddy current depend on? What is the current proportional to?
- The resistance of the material in the loop (which is proportional to the length of the loop)
- Proportional to the radius
57
What sort of heating do eddy currents cause?
-They cause IR^2 heating
58
How does a maglev train function using eddy currents?
- Direction of induced currents opposes the change in the magnetic field producing them
- These currents produce repulse force which provides the magnetic levitation that is used in maglev trains
59
Give one reason why the conductivity of the soil can vary
Electrical resistance of the ground can vary according to the number of ions present in it
60
Describe how a metal detector works
- A current pulse is sent through the transmitting coil which produces a magnetic field
- Magnetic field induces an eddy current in a metallic object
- This current takes time to decay and as it does so, it generates an induced emf across the receiver coil
- Eddy current in a good conductor takes a long time to decay as it is dissipated slowly
- Length of received pulse indicates whether there is metal and may give an idea of the type of material being detected
61
How does resistivity surveying work? (use a non uniform wire as an example)
- When a potential difference is applied across a uniform wire the pd is uniform across the wire
- When the wire is non uniform- thicker in parts and less thick in others, the pd varies as the resistance changes due to the diameter of the wire changing
62
During resistivity surveying using 2 probes, what is the resistance given by? (equation)
R= p/2πd
R=resistance
p=density
d=spacing of the electrodes
You do not need to memorise this equation
63
Describe the process of resistivity surveying (steps)
- Insert the probes a set distance apart eg 1m
- Measure the potential difference and the current
- Calculate the resistance using RV/I
- Calculate soil resistivity using the formula 2πdR
- Move the probes along the grid lines being investigated and measure the resistance of the next 1m length
64
Why is the process of resistivity surveying unreliable?
- Electrode contact resistances
- Polarisation due to the production of gases or other deposits on the electrodes
- Emfs may be generated at the electrodes as the metals are in contact with salts
65
How can contact resistance and polarisation in resistivity surveying be reduced?
- Using an interface made from a solution of ions (eg CuSO4 solution)
- Increasing the moisture content near the electrodes
66
How can a 'potential divider' technique be used to more accurately measure soil resistivity?
- Set up 2 electrodes 1m apart
- Potential difference between 2 electrodes is measured along the line joining them using small regular intervals
- This shows more accurately small localised deviations in resistivity and also bigger deviations indicating the presence of buried objects
67
How may soil resistivity be surveyed using 4 equally spaced electrodes?
- 4 equally spaced electrodes all moved together at 1m intervals along the grid lines
- pd between inner electrodes (connected to a voltmeter) divided by the current in the outer electrodes (connected to an ammeter) remains constant if the resistance of the soil remains constant
- Increase or decrease is an anomaly indicating a buried object
68
How can an earthquake be detected using a seismometer (horizontal movement produced by an earthquake as opposed to the vertical movement which can be detected using a geophone)
- Pendulum with a large mass suspended inside a case
- Case attached to building, when building move case moves with it however the pendulum doesn't move at first due to its large inertia
- Horizontal movement of the case produces a horizontal force on the suspended mass, causing it to oscillate
- Pen fixed to the pendulum marks the paper on a rotating drum as the pendulum moves
69
What is a P wave and what is an S wave?
- P waves are longitudinal
| - S waves are transverse
70
Which type of wave moves more quickly, a P wave or an S wave?
-P waves travel more quickly than the S waves
71
Which 2 factors of a material determine the speed at which P and S waves travel?
- Density
| - Elasticity
72
What happens to waves travelling through the soil when they encounter, for example, a lump of granite?` Which direction in relation to the normal does the wave bend?
- Some energy of the waves is reflected and some is refracted
- The wave bends away from the normal
73
What is the critical angle for a wave travelling from soil to granite given by?
sin c=speed of waves in soil/speed of waves in granite
74
Describe briefly how an object could be detected using waves through the earth.
- Small explosion or hammer hitting a geophone
- Two geophones placed some distance away
- Waves arriving at the two geophones produce electrical pulses which start and stop a timing device respectively
- Comparing to what the time taken should be can indicate what materials/objects are underneath eg a piece of granite- the waves would travel faster in the granite
75
Calculating the depth of granite
See textbook
76
(2 marks)
| Explain how an emf is generated in a geophone and how the magnitude of the emf is related to the speed of the case.
- Flux cutting induces emf across the coil
| - Larger velocity= larger rate of change of flux
77
(3 marks)
| What properties of the magnet, springs and coil are necessary to make the geophone sensitive?
- Low resistance coils
- Many turns on coil
- Springs with low spring constant
- Magnet with large mass (allow inertia)
- Strong magnetic field
78
(1 mark)
Seismic refraction is only applicable where seismic speeds of layers increase with depth. Explain what is meant by ''seismic speeds of layers increase with depth''
Wave speed within a layer is greater in deeper layers
79
(3 marks)
Equipment is set up to measure the strength of a magnetic field. A current balance is used: circuit passing through a pair of strong magnets and is the thrust produced on the wire by this magnetic field is balanced on the other side by a weight. State and explain how this apparatus could be modified to be more sensitive to changes in the magnetic flux density.
```
-Replace wire with coil
>longer wire inside field
-Use asymmetrical balance
>longer left arm would give a greater torque for the same force
-Larger current
>Larger magnetic force generated
```
80
(1/2 marks)
| Explain what is meant by precession.
The slow movement of the axis of a spinning body around another axis due to a torque (such as gravitational influence) acting to change the direction of the first axis.
81
```
(2 marks)
Compared to a proton magnetometer, suggest one advantage and one disadvantage of using these methods
-Metal detection
-Resistance surveying
-Ground penetrating radar
-Gravitational field variation methods
```
Metal detection:
- limited depth
+ simple well known technique/portable/works with all types of metal
Resistance surveying:
- contact resistance/variation of material/ cumbersome apparatus/rocky terrain difficult to insert probes/slow to operate
+ simple technique
```
Ground penetrating radar:
- interference from spurious
reflections/frequency dependent/object size
dependent/can diffract round objects
+ covers large area quickly/immediate
imaging/computer image
```
Gravitational field variation methods
- significant density variation/large mass
anomalies needed
+ simple technique
82
(6 marks)
Explain how ride designers use knowledge of physics to ensure riders experience thrilling changes in motion. Go on to explain any limits or constraints, to the ride design, made to ensure the riders are safe.
```
• rapid change in direction
• rapid change in speed
• changes in height above the ground
• weightlessness
• large accelerations/ big g forces
• acceleration equates to thrill
• larger accelerations mean larger forces
• forces can harm rider
• repetitive vibrations act like driving oscillator
• resonance of body can lead to injury
• strapped in to reduce effects of inertia when
direction changes
```
83
(1 mark)
A pendulum is set up with a magnet as the bob which moves in simple harmonic motion and induces an alternating emf in a nearby coil.
Explain how the pendulum may be modified to double the frequency of oscillation of the magnet.
-Shorten the pendulum to 1/4 the original length
84
(2 marks)
A pendulum is set up with a magnet as the bob which moves in simple harmonic motion and induces an alternating emf in a nearby coil.
The frequency of oscillation of the magnet is increased without changing the amplitude of oscillation. Explain why this increases the maximum emf induced.
- Speed of magnet increases
| - Rate of change of flux increases
85
(2 marks)
Oscillating magnet system suspended by a pendulum string inducing an alternating emf in a nearby coil. State ways of increasing the maximum induced emf
- More TURNS in the coil
- Coil with a greater area
- Soft iron core in the coil
- Larger amplitude oscillation of the magnet
- Increased frequency of oscillation of the magnet
86
(2 marks)
A pendulum is set up with a magnet as the bob which moves in simple harmonic motion and induces an alternating emf in a nearby coil.
Explain how the output from the coil would have to be processed in order to store the information in a computer memory.
- Output voltage sampled OR sample at a frequency much higher than that of the pendulum changed to digital/ binary form
- OR processed by an analogue to digital converter
87
(2 marks)
(Ultrasound pulse transmitted through a patient)
Give two possible reasons why the amplitude of the received pulse is lower than that which is transmitted.
- Pulse spreads out/ not unidirectional
- Energy absorbed by body/ tissue
- Some ultrasound reflected by skin as gel was not used
