Paper 2 Topics Flashcards
(284 cards)
1
Q
What is 1 mole?
A
A collection of 6.02×1023 molecules
(Avogadro’s constant)
2
Q
What is the molar mass of a substance?
A
The mass of each mole (every 6.02×1023 molecules)
Eg for He each mole has a mass of 4g
3
Q
How do you calculate the molar mass of a compound eg NO2
A
Add up the nucleon numbers
(14+16+16=46gmol-1)
4
Q
How do you calculate the number of molecules in a substance?
A
N = n × NA
(Number of molecules = moles × Avogadro’s constant)
5
Q
What is the molecular mass and how is it calculated?
A
The mass of each molecule of the substance
m = M/N
(molecular mass = total mass / number of molecules)
6
Q
How is the total mass of a substance calculated?
A
M = n × mr
(Total mass = moles × molar mass)
7
Q
How do you convert a temperature from °C to K?
A
T(K) = T(°C) + 273
8
Q
Define absolute zero
A
The point at which an ideal gas exerts no pressure
(0K, -273°C, molecules have no kinetic energy)
9
Q
What is Boyle’s Law?
A
The pressure in a gas is inversely proportional to the volume it occupies
at a fixed temperature
and a fixed mass of gas
(P ∝ 1/V)
10
Q
What does the P-V graph look like for an ideal gas?
A
11
Q
How do you prove Boyle’s law graphically?
A
Plot a graph of P against 1/V
Should be a straight line passing through the origin
12
Q
What is Charles’ Law?
A
Volume a gas occupies is directly proportional to the temperature of the gas
at a fixed pressure
and a fixed mass of gas
(V ∝ T)
13
Q
How do you prove Charles’ law by graph?
A
Plot a graph of V against T
Should be a straight line passing through the origin
14
Q
For an ideal gas, what does a graph of V against T(°C) look like?
A
Note: x-intercept represents absolute zero
15
Q
What is the Pressure law?
A
The pressure of a gas is directly proportional to the temperature of the gas
at a fixed volume
and a fixed mass of gas
16
Q
How do you prove the pressure law graphically?
A
Plot a graph of P against T
Should be a straight line passing through the origin
17
Q
For an ideal gas, what does a graph of P against T(°C) look like?
A
Note: x-intercept is absolute zero
18
Q
What is the ideal gas relationship?
A
19
Q
When can you use the ideal gas relationship?
A
If the mass of the gas is constant
20
Q
How do you calculate the work done compressing or expanding a gas?
A
Calculate the area under the curve
21
Q
What is the general equation for pressure?
A
P = F / A
(Pressure = Force / Area)
22
Q
How does a gas exert a pressure on a container?
A
- The gas molecules collide with the container walls changing their momentum.
- This creates a force on the molecule and the wall
- Exerting a pressure
23
Q
What are the 5 conditions for an ideal gas?
A
- Volume of the molecules must be much smaller than the volume of the gas itself
- The intermolecular forces are negligible
- The collision time of molecules with each other and the walls is much less than the time between them
- The collisions are elastic (no loss in KE)
- The molecules’ motion is random
24
Q
How does Brownian motion explain the random motion of smoke?
A
- Air molecules are moving randomly
- They collide with the smoke changing momentum and exerting a force on the smoke particles
- If at one moment there are more collisions on one side than the other
- The smoke particle has a resultant force so accelerates in that direction
25
Explain Boyle's Law using the molecular Kinetic Theory
* When volume of container is decreased
* More collisions **per second**
* So total momentum change bigger (▲p)
* So force exerted bigger
* So pressure bigger (From P = F/A)
26
Explain Charles' Law using the molecular kinetic theory
* When temperature is increased
* Volume increases to increase the distance travelled between collisions
* Molecules have greater kinetic energy but travel further so frequency stays same
* Change in momentum (▲p) stays constant
* So pressure is constant (P = F/A)
27
Explain the Pressure law using the molecular kinetic theory
* As temperature increases
* The average kinetic energy of the molecules increases
* Increasing the number of collisions **per second** with container walls
* So greater change in momentum
* Greater force and pressure exerted (P = F/A)
28
How would you use this equation to work out the density of a gas?
29
How do you calculate crms from a list of speeds?
1. Square the speeds and add up
2. Take a mean of the squares
3. Square root the value
30
How is cms calculated?
cms = (crms)2
31
What are the units of cms?
[m2s-2]
32
What does the **maxwell-boltzmann distribution** tell us about gases?
Molecules have a **range of kinetic energies**.
So temperature of the gas is a measure of the **average kinetic energy.**
33
For these equations how do you **calculate the internal energy** of the gas?
Multiply each by the **number of molecules of the gas**.
34
How do two objects brought into contact reach thermal equilibrium?
* There is a **net flow** of thermal energy from the hotter object to the colder object
* **Until** both objects are at the **same temperature**
* And there is now **no net flow of thermal energy**
35
Define specific heat capacity
The energy required to increase 1kg of a substance by 1K [Jkg-1K-1]
36
When would you use this equation?
To calculate the **mass flowing per kg** of a fluid
37
Why does the temperature of a substance changing state not increase?
The thermal energy is used to **break some of the intermolecular bonds** (solid → liquid) or the **rest of the intermolecular bonds** (liquid → gas)
38
Define specific latent heat of fusion
The energy required to change the state of 1kg of a solid to a liquid **at its melting point.**
39
Define specific latent heat of vaporization
The energy required to change the state of 1kg of a liquid to a gas **at its boiling point.**
40
What is wrong with this?
Haven't considered the change of states. Need to break it into 3 equations:
41
How are these two gravitational fields similar? How are they different?
Both are **uniform** (constant field strength)
**Closer field lines represent stronger field**
42
How are radial and uniform fields different?
**Radial fields** have a **decreasing field strength**
(Field lines increasing in separation)
**Uniform fields** have a **constant field strength**
(Field lines constant\ separation)
43
In gravitational fields when can you use the equation EP = mgh?
Over **small distances**
When radial fields are approximately uniform
And **g is approximately constant**
44
Why can't SUVATs be used for radial gravitational fields?
SUVATs need a **constant acceleration**
Radial fields have a variable field strength and so a variable acceleration
45
What are equipotentials and how are they related to field lines?
An equipotential has **the same potential along that line**
(So no work is done moving along the equipotential)
They are **always perpendicular to field lines**
46
What is Newton's Universal Law of Gravitation?
**Force** acting between two bodies is:
1. **Directly proportional to the product of their masses (F∝m1m2)**
2. **Inversely proportional to the square of their separation (F∝1/r2)**
47
Define gravitational field strength and state its units
The force acting per unit mass on an object in a gravitational field
[NKg-1] or [ms-2]
48
In the gravitational field strength equation what does M represent?
The **mass** of the object **creating the field**
49
If the Earth is exerting a force on the rocket of 5000N,
What force is the rocket exerting on the Earth?
5000N also. An equal and opposite force from Newton's 3rd Law
(Which has little effect on the Earth because it has so much more mass)
50
How do you calculate the resultant gravitational field strength at a point between two bodies?
1. Calculate the field strength for each body in turn (ignoring the other one)
2. Calculate the difference between the field strengths (g is a vector)
51
How do you you calculate the **field strength (or force) neutral point** between two bodies in a gravitational field?
52
What is the definition of and the equation for **absolute potential energy in a gravitational field?**
The **work done** moving an object **from infinity** to that point in the field
53
Why is **gravitational** **potential energy always negative?**
* Gravitational **potential energy is 0 at infinite distance**
* **And decreases inwards** as you move towards object creating field
* (So must go negative)
54
What is **gravitational potential?**
The **work done per unit mass** moving an object **from infinity** to that point in a field
55
In this equation for **gravitational potential** what object is represented by mass M?
The **mass** of the object **creating the field**
56
Which astronaut has a **greater loss in gravitational potential energy?**
Neither. **Potential energy (and potential) are scalar** quantities so are unaffected by the path
**Both decrease by 1440MJ**
57
What is the mistake here?
In the second stage the **mass of the satellite must be used**
(Not the Earth's mass again)
58
If a **gravitational fields question uses the word ‘height’** what must you do?
Height is the distance above the surface
So you **must add on the radius** of the planet/star/object
59
**Why can't two objects have a neutral point for gravitational potential?** (or GPE)
**Gravitational potential from both is negative**
So they **combine**
**To increase the magnitude** of the potential
60
When can you use these proportionality equations in Gravitational fields?
When the **mass or masses are constant**
61
In gravitational fields What does a **force-separation graph** look like?
And what else does the graph tell you?
The **area under the curve is the change in potential energy** moving between the two separations
62
In gravitational fields What does a **field strength-separation** graph look like?
And what else does the graph tell you?
The **area under the curve is the change in potential** moving between the two separations
63
In gravitational fields What does a **potential energy-separation graph** look like?
And what else does the graph tell you?
The **gradient of a tangent is the magnitude of the force** at that point
64
In gravitational fields What does a **potential-separation** graph look like?
And what else does the graph tell you?
The **gradient of a tangent is the field strength** at that point
65
What is the equation for **gravitational field strength within a planet?**
**(r ≤ R)**
This part of the graph is linear as g ∝ r
66
How do you **derive the equation for gravitational field strength inside a planet?**
* Use general equation for density (M/V)
* With V as the volume of a sphere (4/3πr3)
* Sub into general equation for field strength
67
How do you **derive Kepler's 3rd Law?** (r3 **∝** T2)
1. Equate centripetal force to force due to gravity
2. Substitute in angular speed formula (from circular motion)
3. Rearrange
68
How do you derive the formula for the **velocity of a satellite orbiting** a planet or star?
1. Equate centripetal force to force due to gravity
2. Rearrange
69
Which planet has the greatest orbital velocity and why?
Mercury
It is closest to Sun so **smallest r**
70
How do you **derive the formula for the escape velocity** of a planet or star?
71
In these 3 equations what does the mass refer to?
The **mass of the object creating the field**
72
Why does a satellite not need to be above the escape velocity to reach low Earth orbit?
1. Escape velocity only applies to objects without engines (that can't increase their KE)
2. Satellite isn't escaping the field (so doesn't need as much KE)
73
How do you calculate the Kinetic Energy of a satellite orbiting a planet?
Substitute orbital velocity into equation for kinetic energy
74
How do you calculate the total energy of an orbiting satellite?
Add the kinetic and potential energy together…
75
What is the difference between a **geosynchronous and geostationary orbit?**
Both have orbital periods of 24 hours (the same as the Earth)
76
How are **geostationary and polar satellites** different?
**Geostationary satellites orbit above the same point of the equator** and have an orbital period of 24 hours
**Polar satellites orbit over the North and South pole** with an orbital period of much less (around 2 hours)
77
What are polar satellites used for?
* Communication for high latitude regions (close to the poles)
* Espionage (spying)
* Meteorology (weather)
78
What are Geostationary satellites used for?
* Satellite television
* Mobile Phone Communications
* GPS
79
What 4 things do magnetic fields affect?
1. Charges moving in the field
2. Conductors with a current passing through
3. Other magnets
4. Magnetic materials
80
What do the field lines for a bar magnet look like?
Field lines **always** act **North → South**
81
What do the field lines look like between two opposite poles?
**Field is uniform** between the poles
82
What do the field lines look like between two like poles?
83
How are field lines represented ‘going into the page’?
84
How are field lines represented ‘coming out of the page’?
85
This conductor in a magnetic field has a current passing through
But doesn't experience a force
Why?
Because it is **parallel to the field lines**
86
When do you use Fleming's Left hand Rule?
1. Looking at DC motors
2. Looking at charges moving in a magnetic field
87
How do you calculate the force on a **conductor placed at an angle** in a magnetic field?
First use trigonometry to calculate the **perpendicular component** of its **length**
88
Why does the reading on the balance increase when a current runs through the conductor?
The magnetic field pushes up on the conductor
So the conductor pushes the magnets down (**Newton's 3rd Law**)
89
How can you increase the mechanical energy produced by the DC motor?
Increase the **torque** by:
90
What do the **commutator rings** do in the DC motor?
**Switch connections** of the bars every 180°
So **direct current is produced**
91
What happens if the commutators are removed from the DC motor?
Force on each bar won't change
So coil will reach **equilibrium in vertical** position
And **won't continue spinning**
92
Why does an electron move in a circular path in a magnetic field?
**Force** from magnetic field **perpendicular** to **velocity** of electron
93
How do you apply **Flemming's left hand rule to a negative charge** moving in a field?
**Current** acts **opposite** to the **velocity**
94
How do you apply **Flemming's left hand rule to a positive charge** moving in a field?
**Current** acts in the **same direction** as the **velocity**
95
When should you use each equation?
**F=BIL on a conductor** in a magnetic field (with current)
**F=BQv on a charge** in a magnetic field (moving)
96
How do you calculate the **radius of the orbit** of a charge moving in a magnetic field?
**Equate** the magnetic and centripetal **forces**
97
How do you explain the different curvatures of radiation () passing through a magnetic field?
**Greater the specific charge** → **Smaller r** (Bigger deflection)
98
How do you calculate the speed of a charged particle accelerated through an electric field?
99
How much work does a magnetic field do on a moving charge?
0J because the **force and velocity vectors are perpendicular**
So the charge does not increase its kinetic energy
100
In a **mass spectrometer** how does the **velocity selector** work?
Unless an ion's velocity = E/B, it will travel in a parabola and miss the gap
101
In a **mass spectrometer** how does the **mass separation** work?
The ions have the **same velocity** (from the velocity selector)
So **deflect by specific charge**
102
In a **particle accelerator** why are both **magnetic and electric fields needed?**
103
In the **cyclotron** what is the **purpose of the alternating current and magnetic field?**
Alternating current → Electric Field between 'Dees' → Increases kinetic energy
Magneti Field → Moves particle in circular path in ‘Dees’ → Containing particle
104
In the **cyclotron** why is the **frequency of the alternating current constant?**
As the charge speeds up → Travels further in each Dee → So takes **same time**
105
How do you calculate the **AC frequency of the cyclotron?**
**Note:** f is independent of v
So the **frequency is constant**
106
How do charges interact in these situations?
1. **Like** charges **repel**
2. **Opposite** charges **attract**
107
Which direction will these charges move?
**Electric field lines** shows **direction** of Force on **+ve charges**
**(-ve charges** are **opposite)**
108
How are **radial and uniform electric fields** different?
1. **Radial** fields have a **varying field strength** (weaker when further apart)
2. **Uniform** fields have a **constant field strength**
109
For electric fields, how are are **equipotentials related to the field lines?**
**Equipotentials always perpendicular** to field lines
110
How can you change this situation to **increase the force on the charge?**
1. Increase field strength
2. Increase magnitude of charge
111
What field lines are produced by…
a) +ve charge
b) -ve charge
Field lines always act…
* **Away from +ve**
* **Towards -ve**
112
How do the field lines look for these two interacting oppositely charged particles?
113
How do the field lines look for these two interacting like charged particles?
**NOTE: Field lines never cross**
114
What is the **electric field strength** at the following points?
**Field strength is constant between parallel plates** (capacitor)
## Footnote
**E1 = E2 = E3**
115
What is the **electric potential** at the following points?
**Electric potential linearly increases between parallel plates** (capacitor)
116
Define **Coulomb's Law**
117
Define **electric field strength**
**Force per unit charge** acting on a small **positive charge**
118
What force would act on a 5C charge placed at 6NC-1?
F=Eq → 5x6 = **30N**
119
What is wrong here?
**Electric field strength ≠ acceleration**
120
How do you calculate **electric field strength outside a conducting sphere?**
**Treat it as a point charge**
121
How do you calculate **electric field strength inside a conducting sphere?**
**E=0 everywhere!!!**
122
What is the graph of **electric field strength for a conducting sphere?**
123
How is electric field strength calculated here?
For **parallel plates calculate E first if possible**
124
How do you work out the **resultant field strength** between charges?
1. Work out **field strength from each**
2. Label vectors
3. **Add or subtract** field strengths
125
Can you use SUVATs here?
Yes!
Field strength constant → Acceleration constant
126
Why is **potential energy here +ve?**
**Ep = 0 at** **∞**
**Increases as charge moves closer**
127
Why is **potential energy here -ve?**
**Ep = 0 at** **∞**
**Decreases as charge moves closer**
128
Why is **potential energy here +ve?**
**Ep = 0 at** **∞**
**Increases as charge moves closer**
129
Why is **potential energy here -ve?**
**Ep = 0 at** **∞**
**Decreases as charge moves closer**
130
Define **electric potential**
**Work done per coulomb** moving **positive charge** from **infinity to that point**
131
Why does the moving charge's potential energy increase?
Equipotentials show the change in potential of a +ve charge
If +ve charge → decreases
If -ve charge → increases
132
How are these charges different?
**Q is the charge creating** the field
**q is the charge moving** in the field
133
What is wrong here?
Electric potential is scalar
But they are **opposite → must be subtracted**
134
How do you calculate the **neutral electric field strength (or force) point** between charges?
135
How do you calculate the **neutral electric potential point** between charges?
136
What is the graph of **electric force for a conducting sphere?**
**Same as field strength** graph
137
What is the graph of **electric potential for a positively charged conducting sphere?**
138
What is the graph of **electric potential for a negatively charged conducting sphere?**
139
What two situations **produce a uniform electric field?**
1. Radial field over a short distance
2. Field between 2 parallel plates
140
Define **Capacitance**
**Charge stored per unit Volt [F]**
141
What do the gradient and area under this graph represent?
**Gradient → Capacitance**
**Area → Work done** (Energy Stored)
142
What is wrong with this?
**C = capacitance →** not the charge!!!
143
When building a capacitor how do you **maximize the capacitance?**
1. **Increase the area** of the plates
2. **Decrease** the plate **separation**
3. Place **dielectric** between plates
144
What does it mean if the **relative permittivity of a dielectric (****εr****)** is 5.0?
The capacitor stores **5x more charge with the dielectric** between the plates!
145
How does **adding a dielectric increase the capacitance** of a capacitor?
1. Dielectric contains **polarised molecules**
2. They **align with the field** between the plates
3. Bigger negative charge **attracts more electrons** onto negative plate
4. Repels more electrons away from positive plate
5. **V same** but **Q has increased**
146
What happens if the **dielectric is removed**?
(Capacitor **still connected to battery**)
1. Polarised molecules removed
2. **Some electrons leave negative** plate
3. **Attracts more electrons to positive** plate
4. **Q has decreased** but **V same**
5. **C decreases (C=Q/V)**
147
What happens if the **dielectric is removed**?
(When the Capacitor is **disconnected from battery**)
1. Polarised molecules removed
2. But charge is trapped on plates
3. **Same Q** but with **lower C**
4. **V increases (V=Q/C)**
148
How does this capacitor charge?
(When switch 1 is closed)
1. **Electrons flow** from the negative terminal of the battery
2. **To the connected parallel plate** (right plate)
3. **Electrons are repelled** from the opposite plate (left)
4. And attracted **to the positive terminal** of the battery
5. Charge across Parallel plates
149
How does this capacitor discharge?
(When switch 2 is closed)
1. **Electrons flow** from the negative plate (right)
2. Through the resistor
3. **To the other plate** (left)
4. Decreasing charge difference across plates
150
Define **time constant**
Time constant is how long it takes for a capacitor to…
1. **Charge to 63%** of max charge (0.63Q0)
2. **Discharge 63%** of Q0 (down to 0.37Q0)
151
What **factors affect the time constant** of a circuit?
1. The **resistance of the components** in the circuit (Capacitor R=0)
2. **Capacitance** **of the capacitor**
152
Complete this **discharging curve for a capacitor**
153
Complete this **discharging curve for a capacitor**
154
Complete this **discharging curve for a capacitor**
155
Complete this **charging curve for a capacitor**
156
Complete this **charging curve for a capacitor**
157
Complete this **charging curve for a capacitor**
158
How do you read off the **time constant from this graph**?
Read off time when **charge (or current or voltage)** has **decreased to 37% initial**
159
How do you read off the **time constant from this graph**?
Read off time when **charge (or current or voltage)** has **increased to 63% final**
160
Explain why the **I-t graph is exponential** when a **capacitor discharges**
1. Potential difference across capacitor drives large current through resistor
2. Charge across plates decreases
3. Potential difference across the plates decreases
4. Current gets smaller and smaller
161
Explain why the **I-t graph is exponential** when a **capacitor charges**
1. Battery drives current round circuit
2. Charge build up on capacitor plates
3. Potential difference builds up across plates
4. Difference in PD between battery and capacitor gets less
5. So smaller push on electrons
6. Smaller current
162
What is wrong here?
80% is the decrease in charge (∆Q)
So it **discharges to 20% of initial** Q=0.2Q0
163
How do you make a **capacitor charge/discharge at a constant rate**?
Use a **variable resistor**
Decreasing resistance
To keep **charging/discharging current constant**
164
How do the graphs change if a **capacitor is charging at a constant rate**?
Current → Constant
Voltage and Charge → Linear
165
How do the graphs change if a **capacitor is discharging at a constant rate**?
Current → Constant
Voltage and Charge → Linear
166
How do you **show Q=0.37Q0 after 1 time constant?**
**Set t=RC**
167
What is wrong here?
Capacitor is **discharging at a constant rate**
So **current is constant**
168
How does the potential difference of the **resistor change as the capacitor charges**?
**NOTE: VR+VC=V0**
169
How does the potential difference of the **resistor change as the capacitor discharges**?
**NOTE: VR+VC=V0**
170
For a **discharging capacitor,** what does the **gradient of the Q-t** graph give?
**Current** at that instant
171
For a **charging capacitor,** what does the **gradient of the Q-t** graph give?
**Current** at that instant
172
For a **discharging capacitor,** what does the **area of the I-t** graph give?
**Charge lost** in that region
173
For a **charging capacitor,** what does the **area of the I-t** graph give?
**Charge gained** in that region
174
How is **magnetic flux** calculated?
Magnetic flux density x Perpendicular area
175
How do you calculate the effective flux through this coil?
Must take **component of field perpendicular** to surface
(Or component parallel to normal of surface)
176
How does the **number of turns** in a coil affect its flux?
**Flux doesn't change**
But **flux linkage increases**
177
How do you find the flux linkage through this coil?
Must take **component of field perpendicular** to coil
(Or component parallel to normal of coil)
178
How do you **induce an emf** in this coil?
**Moving the magnet** in and out of the coil
Causes a **change in flux linkage**
(Faraday's Law)
179
If the magnet is **stationary** why is there no emf induced?
Emf **only induced if the flux linkage is changing**
| (Faraday's Law)
180
Which way does the current act in this coil and why?
Current acts to create a magnetic field **opposing the increase in flux linkage**
(Tries to keep magnet from entering)
Due to **Lenz's law**
181
Which way does the current act in this coil and why?
Current acts to create a magnetic field **opposing the decrease in flux linkage**
(Tries to keep magnet from leaving)
Due to **Lenz' law**
182
Why does moving the magnet f**aster induce a larger emf**?
The **rate of change of flux linkage is greater**
| (Faraday's law)
183
What is Faraday's Law of **electromagnetic induction**?
The magnitude of the **emf is proportional to the rate of change of flux linkage**
184
What is **Lenz' law**?
The direction of an induced **emf tends to produce a current which opposes the**
**change** causing it
185
Why can't the current act in this direction?
The magnetic field produced attracts the magnet
Increasing its kinetic energy
Energy has been created out of nothing!!!
(Against Lenz' law)
186
What is the **right hand grip rule**?
A current in a coil produces a magnetic field as shown
187
What is wrong here?
**Flux linkage becomes negative** when coil flips!!!
188
How is the **induced emf related to the graph of flux linkage**?
(From Faraday's law)
189
What is the corresponding graph of induced emf?
**emf = -ve gradient**
190
If the coil moves across the field at a constant speed what do the flux linkage and emf graphs looks like?
**emf = -ve gradient**
191
How do you calculate the emf induced between the ends of this conducting bar moving across a magnetic field?
192
When do you use **Fleming's Right Hand Rule**
For **generators**!!!
193
For the **AC generator** when is the **max emf** induced?
When **change in flux linkage is max**
| (When flux linkage = 0)
194
For the **AC generator when is 0 emf induced**?
When **change in flux linkage is 0**
| (When flux linkage = max)
195
What is the corresponding **emf graph for the AC generator**?
196
How is the **max induced emf calculated for the AC generator**?
197
How do you **increase the max induced emf** of the AC generator?
**Increase any of the terms** in the equation:
198
How is the **AC generator different to the DC motor**?
**Generator: Kinetic energy -\> Electrical** energy
(Motor: Electrical energy -\> Kinetic energy)
**Generator: Slip rings** keep each side of coil connected to same side of circuit
(Motor: Commutator ring switch polarity every half cycle)
199
What are **eddy currents**?
**Currents produced in a conductor by magnetic fields**
(Lenz' law in conductors)
200
Why does the magnet take longest to fall through the full copper pipe?
1. Magnet in freefall
2. Plastic not a conductor so no eddy currents (still in freefall)
3. Copper pipe incomplete so can't create eddy currents (still in freefall)
4. Eddy current reduce acceleration
201
How are eddy currents created in this copper pipe?
1. Flux linkage decreasing above -\> current creates attracting field upwards
2. Flux linkage increasing below -\> current creates repelling field upwards
202
How does eddy current braking work?
1. Part of disk leaving field -\> current creates attraction to electromagnet
2. Part of disk entering field -\> current creates repulsion to electromagnet
203
When can **eddy current braking not be used**?
To **hold a car stationary** on a slope
| (no change in flux linkage)
204
How does the **oscilloscope trace look for an AC current**?
| (When the **time base is switched on**)
205
How does the **oscilloscope trace look for an AC current**?
| (When the **time base is switched off**)
206
How does the **oscilloscope trace look for an DC current**?
| (When the **time base is switched on**)
207
How does the **oscilloscope trace look for an DC current**?
| (When the **time base is switched off**)
208
For AC supply what is **Vrms and how is it calculated**?
The **equivalent DC voltage that would supply the same average power**
209
For AC supply what is **Irms and how is it calculated**?
The **equivalent DC current that would supply the same average power**
210
Label **V0 and Vp→p on this AC oscilloscope trace**
211
What is wrong with this calculation?
When using electricity formulas **must use rms values for voltage** (and current)
212
How does a **step up transformer** work?
1. AC current flows through primary coil
2. Magnetic field flows through secondary coil
3. Changing flux linkage in secondary coil larger
4. Greater emf induced (so bigger voltage)
213
How does a **step down transformer** work?
1. AC current flows through primary coil
2. Magnetic field flows through secondary coil
3. Changing flux linkage in secondary coil smaller
4. Smaller emf induced (so smaller voltage)
214
Why does a **transformer only work with AC supply**?
**If supply is DC**
**Flux linkage** in secondary coil **doesn't change**
So emf isn't induced (Faraday's law)
215
How do you calculate the voltage (rms) in the secondary coil?
This **equation always works (no matter what efficiency)**
216
How do you **calculate the efficiency of a transformer**?
(The **voltages and currents must be rms** values)
217
What are the main causes of **thermal loss in a transformer**?
1. Large Eddy currents in magnet (P=I2R)
2. Large currents in primary and secondary coils
3. Hysteresis losses (magnet's resistance to change in flux linkage)
4. Flux losses (not all flux passing through secondary coil)
218
How are the main causes of **thermal losses in a transformer reduced**?
1. Large Eddy currents in magnet
2. Large currents in primary and secondary coils
3. Hysteresis losses
4. Flux losses
1. **Laminate** the core
2. Use **wire with** **low resistance**
3. Use **soft iron core** for magnet
4. **Wind primary coil over secondary** **coil**
219
Why are **step up transformers used to transport electricity** over long distances?
**Smaller currents = smaller energy losses** to thermal
| (P=I2R)
220
What happened in the **Rutherford scattering experiment**?
1. **Most** alpha particles passed through gold leaf **undeflected**
2. **Some** were **slightly deflected**
3. A tiny proportion (**1 in 8000) reflected**
221
What did **Rutherford's experiment** reveal about the atom?
1. Atom is **mostly empty space**
2. Centre of atom is **small, dense and positively charged (nucleus)**
222
Compare the **ionising power** of the three main types **of radiation**
* *Alpha most ionising**
* *Beta medium ionising**
* *Gamma least ionising**
223
Compare the **penetrative power** of the three main types **of radiation**
* *Gamma most penetrating**
* *Beta medium penetrating**
* *Alpha least penetrating**
224
Compare the **range** of the three main types **of radiation**
```
Alpha = 3-7cm
Beta = 0.2-3m
Gamma = Very long distance
```
225
What are the main **sources of background radiation**?
226
How does a **Geiger-Muller tube** detect radiation?
1. **Radiation ionises gas** in tube
2. Negative **ions attracted** to metal rod
3. Positive ions attracted to casing
4. Small **current generated** in circuit
227
How is **radiation** used to **control the thickness of metal**?
Why can't alpha radiation be used?
If detector count too low -\> metal too thick -\> Rollers move closer together
If detector count too high -\> metal too thin -\> Rollers move apart
Alpha won't be detected
228
What materials can **shield** the three main types of **radiation**?
* *Alpha = Paper**
* *Beta = Aluminum**
* *Gamma = Lead or several meters of concrete**
229
How is **radiation** used in a **smoke detector**?
**Alpha radiation ionizes** air between detector
**Ionized air causes current** to flow
**Smoke blocks ionization** of air
Current stops
Alarm sounds
230
If the detector is moved 3x further away what will happen to the count rate?
9x smaller
**Gamma radiation follows inverse square law**
231
How should you **safely use radiation**?
1. **Minimise** exposure **time**
2. **Maximize distance** from source
3. Store in shielded containers
4. Don't consume food or drink near source
232
How is **radiation** used for **medical imaging**?
**Medical tracer with short half life** injected
Tumors absorb radionuclides and emit gamma
Gamma detected outside body
233
How is **radiation** used to **destroy tumors**?
**Gamma radiation focused on tumor**
High energy breaks apart tumor
Low levels through other tissue
234
Sketch the graph of **nuclear stability**
235
On this graph of nuclear stability highlight regions of the decays…
1. **α**
2. **β-**
3. **β+**
4. **Proton emission**
5. **Neutron emission**
236
What makes a **nucleus unstable?** (and radioactively decay)
1. An **incorrect balance of protons and neutrons** (off line of stability)
2. **Too many nucleons**
3. **Nucleus in excited state**
237
What is **electron capture** and what is it's equation?
**Proton** captures inner shell electron and **becomes neutron**
238
What two forms of **radiation are released after electron capture**?
**X-ray** → electron de-excites to fill inner shell
**γ** → Nucleus reorders and de-excites
239
How does **distance of closest approach** work?
KE at distance → PE closest
Use to get rough size of nucleus
240
What two graphs could you plot to prove this relationship?
241
What does **r0** represent?
**Average radius of each nucleon**
242
How do you calculate the **average density of a nucleus**?
243
Why is the **average nucleus density so large**? (∼2.3x1017kgm-3)
**Atom is mostly empty space**
244
How was **electron scattering** used to **determine nuclear diameter**?
Graph plotted
First minima used to calculate diameter
(Don't need to know equation)
245
How is **electron scattering better than alpha recoil** to determine nuclear radius?
**Alpha Recoil**
* Closest approach so only an estimate
* Recoil of nucleus not considered
* Effect of strong force not known
**Electron Scattering:**
* Not affected by strong force (leptons)
* Electron **λdb tunable**
246
Define the **decay constant** **λ**
The **probability** that an unstable isotope **decays in one second**
247
Define the **activity, A**, of a radioactive sample
The **total number** of unstable isotopes that **decay after one second**
248
What does the activity, A, of a radioactive sample depend on?
1. The **decay constant** **λ**
2. The **number if unstable isotopes N**
249
Define the **half life, T½,** of a radioactive sample
Time taken for either…
1. Activity of sample to halve
2. **Number of unstable isotopes remaining to halve**
250
How do you **derive the half life T½** equation?
Set **N as 0.5N0**
251
How do you **prove this graph is exponential**?
Find **multiple T½ and compare**
252
What's wrong with this calculation?
**Activity and time must be the same units**
253
1. Derive the equation of this graph
2. What are the **gradient** and **y-intercept**?
254
What is the **gradient** of this graph?
r0 → **average radius of nucleon**
255
What do the **gradient** and **y-intercept** of this graph represent?
where r0 → **average radius of nucleon**
256
When should you treat the **neutron and proton** as having slightly **different masses**?
Dealing with **fusion or fission**
(Calculating mass defects, binding energies or mass difference)
257
Define **mass defect (∆m)**
Mass lost when nucleons (protons and neutrons) **come together to form nucleus**
258
Define **binding energy**
**Energy released** **when nucleons** (protons and neutrons) **come together** to form nucleus
259
How is **binding energy** related to **mass defect (∆m)**?
260
Define **1 atomic mass unit**
Mass of **1/12 of a carbon-12** atom
(1.661x10-27kg)
261
What's the next step here?
**Multiply by 931.5MeV**
262
What's the next step here?
**Use E=mc2**
263
Sketch the **binding energy per nucleon graph**
264
Label the fusion and fission regions
Why are they there
Energy only released if **binding energy per nucleon increases**
265
Define **metastable state** in radioactive decay
**Long-lived excited state** of nucleus
Eventually it de-excites emitting **γ**
266
How many possible decays are there?
**2****β- and 3γ**
267
What is **nuclear fission**?
Heavy **nucleus splits into two** lighter nuclei **releasing energy and neutrons**
268
What are the 2 main isotopes used as fuel in nuclear reactors?
**U-235, U-238**
269
How do you calculate the **energy released in this fission reaction**?
Calculate the **mass difference (****∆****mdiff)**
270
How do you calculate if a reaction is possible?
**If mass difference is positive** reaction is possible
271
What are the main components of the **nuclear reactor**?
1. **Fuel rods** - U-235
2. **Control rods** - Boron
3. **Moderator** - water or graphite
4. **Coolant** - CO2 or water
272
What does the **moderator** do in a nuclear reactor?
**Reduce neutrons' speeds** to thermal speeds
(More likely to be absorbed by U-235)
273
What do the **control rods** do in a nuclear reactor?
**Absorb some neutrons**
Stop chain reaction occurring
274
What are the main **safety features of the nuclear reactor**?
1. Reaction happens inside **thick walled concrete** vessel
2. **Control rods fully inserted** if meltdown starts
3. **Reactor flooded with water** to remove thermal energy if meltdown starts
275
In nuclear reactors how are **spent fuel rods disposed**?
1. Removed remotely from reactor
2. Stored in **cooling ponds** for up to 1 year
3. **Vitrified** by mixing with molten glass
4. Sealed in **barrels**
5. Stored in mountains or **deep underground**
276
What is **nuclear fusion**?
**Two lighter nuclei are combined** to form one heavier nuclei and **release energy**
277
Why is nuclear fusion so difficult to achieve?
Requires **incredibly high temperatures and pressures**
1. To **ionise** the isotopes
2. To bring isotopes close enough to **overcome electromagnetic repulsion**
278
**In fusion how close** do the ionised isotopes need to get?
Close enough for the **strong force to be larger than electromagnetic**
## Footnote
**(\<1fm)**
279
What are the **two main fusion isotopes** in stars (in their main sequence)
Deuterium → H-2
Tritium → H-3
280
What does the **coolant** do in a nuclear reactor?
**Transfer heat** from fuel rods to the water that spins the turbines
281
How is **1 Tesla** defined?
The magnetic field that applies a **force of 1N to a 1 metre conductor** with a **1A current** flowing **perpendicular to the field** (B=F/IL)
282
If this unstable isotope of caesium **decays by** **α** **emission** where does it end up on the graph?
N → neutron number
Z → Proton Number
283
If this unstable isotope of caesium **decays by** **β-** **emission** where does it end up on the graph?
N → neutron number
Z → Proton Number
284
If this unstable isotope of caesium **decays by** **β+** **emission** where does it end up on the graph?
N → neutron number
Z → Proton Number
