3.7 Fields and their consequences Flashcards
(161 cards)
1
Q
What is a force field?
A
An area in which an object experiences a non-contact force
2
Q
How can a force field be represented?
A
- A vector, describing the direction of the force that would be exerted on the object, which allows the direction of the field to be deduced
- Field lines, where the distance between the field lines represents the strength of the force exerted by the field in that region
3
Q
How are force fields formed?
A
During the interaction of masses, static charge or moving charges
4
Q
How are gravitational fields formed?
A
During the interaction of masses
5
Q
How are electric fields formed?
A
During the interaction of charges
6
Q
What are the similarities between gravitational and electrostatic forces?
A
- Both follow an inverse square law
- Both use field lines to be represented
- Both have equipotential surfaces
7
Q
What are the differences between gravitational and electrostatic forces?
A
- In gravitational fields, the force exerted is always attractive, while in electric fields the force can be either repulsive or attractive
- Electrostatic forces act on charges, while gravitational forces acts on masses
8
Q
What is gravity?
A
The universal attractive force acting between all matter
9
Q
What is Newton’s law of gravitation?
A
The magnitude of force between two masses is directly proportional to the product of the masses, and inversely proportional to the square of the distance between them
10
Q
Describe a uniform gravitational field.
A
- Equally spaced parallel lines, with arrows pointing towards the surface
- The same gravitational force is exterted on a mass everywhere in the field
11
Q
Describe a radial gravitational field.
A
- Equally spaced lines with arrows pointing towards the centre of the object
- Force exerted depends on position of object in field - as an object moves further away from the centre, the magnitude of the force decreases, as the distance between field lines increases
12
Q
What is gravitational field strength?
A
The force per unit mass exerted by a gravitational field on an object
13
Q
What is the difference between the gravitational field strength of a uniform field and a radial field?
A
It is constant in a uniform field, but varies in a radial field
14
Q
What is the gravitational potential at a point?
A
The work done per unit mass required to move an object from infinity to a given point
15
Q
What is the gravitational potential at infinity?
A
Zero
16
Q
Why is gravitational potential always negative?
A
The gravitational potential at infinity is zero, and as an object moves from infinity to a point, energy is released as the gravitational potential energy is reduced, and the gravitational force is always attractive, so the system is always doing the work
17
Q
What is gravitational potential difference?
A
The energy per unit mass needed to move an object between two points
18
Q
What are equipotential surfaces?
A
Points of equal potential joined together, so the potential on the surface is constant everywhere
19
Q
What is the gravitational potential difference when moving along an equipotential surface?
A
Zero, so no work is done
20
Q
What is the relationship between gravitational potential and the distance between two objects?
A
The gravitational potential is inversely proportional to the distance between the centres of the two objects
21
Q
What is the gradient of a V-r graph?
A
-g
22
Q
What is Kepler’s 3rd law?
A
- The square of the orbital period is directly proportional to the cube of the radius
- T²/r³=k
23
Q
How do you derive Kepler’s 3rd law?
A
- Centripetal force = gravitational force to find v²
- v² = (2πr/T)² to find T² = (4π²/GM) r³
24
Q
What is the total energy of an orbiting satellite made up of?
A
Kinetic and potential energy
25
Why is the total energy of an orbiting satellite always constant?
If the height is decreased, its gravitational potential energy decreases, but it will travel at a higher speed, so kinetic energy increases, so total energy is always kept constant
26
What is escape velocity?
- The minimum velocity an object must travel at in order to escape the gravitational field at the surface of a mass
- The velocity at which the object’s kinetic energy is equal to the magnitude of its gravitational energy
27
What are synchronous orbits?
Where the orbital period of the satellite is equal to the rotational period of the object it is orbiting (24 hours if orbiting Earth)
28
What are geostationary satellites?
Satellites that follow a specific geosynchronous orbit, their orbital period is 24 hours so they always stay above the same point on Earth, as they orbit directly above the equator
29
What are geostationary satellites used for?
Sending TV and telephone signals
30
What are low-orbit satellites?
Satellites that have lower orbits than geostationary satellites, so they travel much faster making their orbital periods much smaller
31
What are low-orbit satellites used for and why?
- Monitoring weather
- Making scientific observations about unreachable locations
- Military applications, spying
- They require less powerful transmitters and are able to orbit across the entire Earth’s surface
32
What is Coulomb’s law?
The magnitude of the force between two point charges in a vacuum is directly proportional to the product of their charges, and inversely proportional to the square of the distance between them
33
What is air treated as when using the force between two charges equation?
A vacuum
34
For a charged sphere, where is charge assumed to act?
At the centre of the sphere
35
What is the nature of the force if charges have the same sign?
Repulsive
36
What is the nature of the force if charges have different signs?
Attractive
37
Compare the magnitude of gravitational and electrostatic forces between subatomic particles.
The magnitude of electrostatic forces between subatomic particles is magnitudes greater than the magnitude of gravitational forces, because the masses of subatomic particles are incredibly small, whereas their charges are much larger
38
What is electric field strength?
The force per unit charge experienced by a positive charge in an electric field
39
What is the general equation to find electric field strength?
E=F/Q
40
Derive the equation ΔW=QΔV.
- W=Fd
- F=EQ
- E=ΔV/d, d=ΔV/E
- W=(EQ)(ΔV/E), W=QΔV
41
What does the equation ΔW=QΔV represent?
The work done by moving a charged particle between the parallel plates of a uniform electric field
42
Describe the trajectory of a moving charged particle entering a uniform electric field initially at right angles.
The charged particle will experience a constant electric force either in or opposite to the direction of the field (depending on its charge), causing it to accelerate, so it follows a parabolic shape
43
What is absolute electric potential?
The potential energy per unit charge of a positive charge at a point in the electric field
44
Where is the absolute magnitude of electric potential greatest?
The surface of a charge
45
What is the electric potential at infinity?
Zero
46
What does the value of electric potential being positive or negative depend on?
The sign of the charge
47
What is the nature of the electric potential and charge if the charge is positive?
Electric potential is positive, and the charge is repulsive
48
What is the nature of the electric potential and charge if the charge is negative?
Electric potential is negative, and the charge is attractive
49
On a V-r graph, if V is initially positive, what is the nature of the force?
Repulsive
50
On a V-r graph, if V is initially negative, what is the nature of the force?
Attractive
51
What does the gradient of a V-r graph represent?
The value of electric field strength at that point
52
What is electric potential difference?
The energy needed to move a unit charge between two points
53
What does the area under an E-r graph represent?
Electric potential difference
54
What is capacitance?
The charge stored by a capacitor per unit potential difference
55
What is a capacitor?
An electric component which stores charge
56
What does a capacitor consist of?
Two conducting parallel plates with a gap between which may be separated by a dielectric (insulating material)
57
What happens when a capacitor is connected to a source of power?
Opposite charges build up on the two parallel plates causing a uniform electric field to be formed
58
What is permittivity?
A measure of the ability to store an electric field in a material, a property of dielectrics
59
How do you find relative permittivity?
By finding the ratio of the permittivity of the dielectric to the permittivity of free space, εr= ε/ε0
60
What is relative permittivity?
The dielectric constant of a dielectric, used to calculate the capacitance of a capacitor
61
What is a dielectric formed of?
Polar molecules
62
What are polar molecules?
Molecules with one end which is positive and one which is negative
63
How are polar molecules arranged when there is no electric field?
In random directions
64
What happens to polar molecules when an electric field is present in a capacitor?
- They move and align themselves with the field, with the negative ends rotated towards the positive plate of the capacitor and the positive ends to the negative plate
- Each molecule now has its own electric field, which now oppose the field formed by the capacitor, reducing this field
65
What does the electric field strength of a polar molecule depend on?
The dielectric’s permittivity
66
What happens as a result of polar molecules reducing the electric field formed by the capacitor?
The electric field strength decreases, decreasing the potential difference required to charge the capacitor, causing capacitance to increase, as C=Q/V
67
What does the area under a graph of charge against potential difference (Q-V graph) represent?
The electrical energy stored by a capacitor
68
What does a Q-V graph of a capacitor look like?
A straight line through the origin, as potential difference is directly proportional to charge
69
Describe a circuit in which you can charge a capacitor and take relevant measurements.
A series circuit with a power supply, ammeter, resistor and a capacitor, with a voltmeter in parallel with the capacitor
70
What does the area under an I-t graph represent?
Charge
71
What does the gradient of a Q-t graph represent?
Current
72
Describe what happens in a capacitor once it has been connected to a power supply.
- Current starts to flow and negative charge builds up on the plate connected to the negative terminal
- On the opposite plate, electrons are repelled by the negative charge building up on the initial plate, so these electrons move to the positive terminal
- An equal but opposite charge is formed on each plate, creating a potential difference
- As the charge across the plates increases, the pd increases but the electron flow decreases due to the force of electrostatic repulsion also increasing, so current decreases and eventually reaches zero
73
How do you discharge a capacitor?
It must be connected to a closed circuit with just a resistor
74
Describe the I-t graph of a charging capacitor.
Negative exponential curve, with current tending to zero
75
Describe the V-t graph of a charging capacitor.
Positive exponential curve, with potential difference tending to V0
76
Describe the Q-t graph of a charging capacitor.
Positive exponential curve, with charge tending to Q0
77
Describe the I-t graph of a discharging capacitor.
Negative exponential curve, with current tending to zero
78
Describe the V-t graph of a discharging capacitor.
Negative exponential curve, with potential difference tending to zero
79
Describe the Q-t graph of a discharging capacitor.
Negative exponential curve, with charge tending to zero
80
What happens when a capacitor is discharged?
Current flows in the opposite direction, and the current, charge and pd across the capacitor will fall exponentially, so it will take the same time for the values to halve
81
What is the equation involving current for a charging capacitor?
I = I0e^-t/RC
82
What is the equation involving current for a discharging capacitor?
I = I0e^-t/RC
83
What is the equation involving potential difference for a charging capacitor?
V = V0(1 - e^-t/RC)
84
What is the equation involving potential difference for a discharging capacitor?
V = V0e^-t/RC
85
What is the equation involving charge for a charging capacitor?
Q = Q0(1 - e^-t/RC)
86
What is the equation involving charge for a discharging capacitor?
Q = Q0e^-t/RC
87
What is I0?
The initial current
88
What is V0?
The initial potential difference
89
What is Q0?
The initial charge
90
What is the time constant?
- The product of resistance and capacitance, RC
- The value of time taken to discharge a capacitor to 1/e of its initial value of Q, I or V
- The value of time taken to charge a capacitor to (1 - 1/e) of its initial value of Q or V
91
What is the gradient of a graph of ln(Q) against t?
-1/RC
92
What is the time to halve, T1/2?
- The time taken for the current, charge or potential difference of a capacitor to discharge to half of the initial value
- T1/2 = -ln0.5RC = 0.69RC
93
What is induced when a current passes through a wire?
A magnetic field
94
What do the magnetic field lines induced by current passing through a wire form?
Concentric rings
95
How can the direction of the magnetic field around a current carrying wire be determined?
The right hand grip rule
96
What is magnetic flux density (B)?
A measure of the strength of a magnetic field, T
97
What is one Tesla?
A force of 1N on 1m of wire carrying 1A of current perpendicular to a magnetic field
98
What happens when a current-carrying wire is placed in a magnetic field?
A force is exerted on the wire
99
When is no force exerted on a current-carrying wire in a magnetic field?
If the current is travelling parallel to the magnetic field, because no component of the field is perpendicular to the current.
100
What is the formula for the force on a current-carrying wire in a magnetic field when the field in perpendicular to the current?
F = BIl
101
How can the direction of the force exerted on a current-carrying wire in a magnetic field be determined?
Fleming’s left hand rule
102
What is Fleming’s left hand rule?
- Thumb - represents the direction of the motion
- First finger - represents the direction of the field
- Second finger - represents the direction of the conventional current
103
What is the direction of a magnetic field on a magnet?
Arrows on field lines point from the North Pole to the South Pole.
104
What is the formula for the force acting on a moving charged particle in a magnetic field when the field is perpendicular to velocity?
F = BQv
105
How can the direction of the force on a moving charged particle in a magnetic field be determined?
Fleming’s left hand rule, where the second finger represents the direction of travel (v), and if the particle is negatively charged, the direction of the second finger must be reversed.
106
How does the force exerted on a moving charged particle in a magnetic field cause the particle to move?
The charged particle follows a circular path
107
Why do moving charged particles in a magnetic field follow a circular path?
The force exerted is always perpendicular to the motion of travel, acting as a centripetal force.
108
How can the radius of a moving charged particle’s circular path in a magnetic field be found?
r = mv/BQ
109
What is an application of the circular deflection of charged particles in a magnetic field?
A type of particle accelerator called a cyclotron
110
What are the uses of a cyclotron?
- Producing ion beams for radiotherapy
- Radioactive tracers
111
What makes up a cyclotron?
- Two semi-circular electrodes called “Dees”
- Uniform magnetic field acting perpendicular to the plane of the electrodes
- High frequency alternating voltage applied between the electrodes
112
Describe what happens in a cyclotron.
- Charged particles move from the centre of one of the electrodes
- They are deflected in a circular path by the magnetic field
- Once they reach the edge of the electrode they begin to move across the gap between the electrodes
- Here they are accelerated by the electric field, so the radius if their circular path will increase as they move through the second electrode
- When they reach the gap again, the alternating electric field changes direction allowing the particles to be accelerated again
- The process is repeated several times until they reach the required speed and exit the cyclotron
113
Why do charged particles in a magnetic field move in a circular path at a constant speed?
- The force exerted by the magnetic field is always perpendicular to the direction of travel
- No work is done on the particles by the force, so kinetic energy is constant
114
Why is there an alternating electric field between the electrodes in a cyclotron?
- As the force exerted by the magnetic field is always perpendicular to the direction of travel, the particle’s speed will not increase due to the magnetic field
- So it is needed to accelerate the particles
115
What is magnetic flux (Φ)?
A value which describes the magnetic field or magnetic field lines passing through a given area.
116
What is magnetic flux linkage?
The product of magnetic flux and the number of turns, N, of a coil
117
What does θ represent in magnetic flux linkage?
The angle between the magnetic field and the normal to the plane of the coil.
118
Why does the magnetic flux density of a coil decrease as it rotates in a uniform magnetic field from 0° to 90°?
The number of field lines passing through the coil decreases as it is rotated up to 90°.
119
What is generated when a coil is rotated through a uniform magnetic field?
EMF
120
How is an EMF induced in a conducting rod when it is moved relative to a magnetic field?
The electrons in the rod will experience a force, as they are charged particles, and build up on one side of the rod, causing an emf to be induced.
121
What is electromagnetic induction?
When the relative movement of a conducting rod in a magnetic field causes an EMF to be induced in the rod.
122
What is induced if a bar magnet moves relative to a coil of wire?
EMF
123
What condition is needed for current to be induced in a coil of wire as a bar magnet moves relative to it?
If the coil forms a complete circuit
124
What two laws govern the effects of electromagnetic induction?
Faraday’s law and Lenz’s law
125
What is Faraday’s law?
The magnitude of induced EMF is equal to the rate of change of flux linkage
126
What is Lenz’s law?
The direction of induced EMF is such as to oppose the motion causing it
127
How can Lenz’s law be demonstrated?
- By measuring the speed of a magnet falling through a coil of wire, and its speed when falling from the same height without falling through the coil
- The magnet will take longer to reach the ground when it moves through the coil
128
Describe what happens when a bar magnet falls through a coil of wire.
1. As the magnet approaches the coil, there is a change of flux through the coil so an EMF is induced
2. Due to Lenz’s law, the direction of induced EMF is such as to oppose the motion of the magnet, so the same pole as the pole of the magnet approaching the coil is induced to repel the magnet, slowing it due to electromagnetic forces of repulsion
3. As the magnet passes through the centre of the coil, there is no change in flux so no EMF is induced
4. As the magnet begins to leave the coil, there is a change in flux, so an EMF is induced that opposes its motion, so an opposite pole is induced by the magnet, causing it to slow down due to electromagnetic forces of attraction
129
Why is the formula ε = -NΔΦ/Δt negative?
Due to Lenz’s law, the direction of the induced EMF will act to oppose the change in flux that created it.
130
Derive a formula for the magnitude of EMF induced by a straight conductor of length l, moving in a magnetic field of flux density B.
s = vΔt => A = lvΔt
ΔΦ = BA = BlvΔt
ε = ΔΦ/Δt = BlvΔt/Δt = Blv
131
What is the formula for finding the EMF induced in a coil rotating at a constant frequency in a magnetic field?
ε=BANωsin(ωt)
(The derivative of the formula for magnetic flux linkage with respect to time)
132
What type of EMF is induced when a coil rotates in a magnetic field?
Alternating
133
What does alternating mean?
Changes direction with time
134
What type of current can be shown on an oscilloscope?
Any type (direct or alternating)
135
What does an oscilloscope show?
The variation of voltage with time
136
What happens when the time base is turned off on an oscilloscope?
The trace will show all the possible voltages at any time in one area
137
Why might turning the time base off on an oscilloscope be useful?
To take measurements
138
What does the trace of a direct current look like on an oscilloscope with the time base turned on?
A straight line parallel to the x-axis, at the height of the output voltage.
139
What does the trace of a direct current look like on an oscilloscope with the time base turned off?
A dot at the height of the output voltage.
140
What does the trace of an alternating current look like on an oscilloscope with the time base turned on?
A repeating sinusoidal waveform
141
What does the trace of an alternating current look like on an oscilloscope with the time base turned off?
A straight vertical line
142
How can measurements be made easier to take on an oscilloscope?
By changing the scale on the Y-axis or X-axis
143
What is the peak voltage (V0) on an oscilloscope?
The distance from the equilibrium to the highest (or lowest) point.
144
What is the peak-to-peak voltage on an oscilloscope?
The distance from the minimum point to the maximum point
145
What is root mean square voltage?
- The average of all the squares of the possible voltages
- Gives you the average value of voltage output by the supply in either direction
146
What is the potential difference of the electricity supplied in homes in the UK?
230 V
147
Is the electricity supplied in UK homes from an alternating supply?
Yes
148
What makes up a transformer?
- A primary coil, attached to the input voltage
- A secondary coil, connected to the output voltage
- An iron core
149
How does the primary coil in a transfer induce a voltage in the secondary coil?
- The alternating current in the primary coil provides a changing magnetic field
- This passes through the iron core and interacts with the secondary coil to induce a voltage in it
150
What does a step-up transformer do?
Increases the input voltage by having more turns on the secondary coil than the primary.
151
What does a step-down transformer do?
Decreases the input voltage by having less turns on the secondary coil than the primary.
152
What are the causes of energy loss (efficiency loss) in a transformer?
- Eddy currents
- Resistance in the coils
- A core that is not easily magnetised
153
How are eddy currents induced?
By the alternating magnetic field in the primary coil
154
What do eddy currents form?
A loop in the iron core
155
How do eddy currents cause energy loss in a transformer?
- Due to Lenz’s law, they oppose the field that produced them, reducing the flux density of the field, and therefore the voltage induced in the secondary coil
- They generate heat, causing energy loss
156
How can the effects of eddy currents be reduced?
- By using a laminated iron core
- Using a core made out of a high resistivity metal
157
How does a laminated iron core reduce the effects of eddy currents?
The eddy currents cannot pass through the insulator, so their amplitudes are reduced, reducing the decrease in the flux density of the field, and therefore the voltage in the secondary coil.
158
How can energy loss in a transformer due to resistance in the coils be reduced?
By using a thick wire, which has low resistance, reducing the heat loss.
159
How can energy loss in a transformer due to a core that is not easily magnetised be reduced?
By using a magnetically soft iron core, to allow easy magnetisation and demagnetisation.
160
How can unnecessary energy losses when transferring electrical power be reduced?
By reducing the current to a minimum value, as power will be lost due to current.
161
Why are step-up transformers used when transmitting electricity over long distances?
As P=IV, as the voltage is stepped up, the current will decrease, reducing energy loss.
