Physics Year 10 Flashcards
1
Q
What’s a transverse wave?
A
A wave where the vibrations are at a right angle (90 degrees) to the wave’s direction of travel.
2
Q
What are longitudinal waves?
A
Waves where the vibrations travel along the same direction as the wave’s direction of travel.
3
Q
Are light waves and Mexican waves transverse or longitudinal waves?
A
Transverse waves
4
Q
What type of wave are sound waves a p-waves an example of ?
A
Longitudinal waves
5
Q
How do you measure the amplitude of a wave ?
A
The distance between 0 (the line) and the top point of the wave
6
Q
How do u measure the wavelength of a wave ?
A
The distance between one point on a wave to the same point at the end of a wave.
7
Q
What’s the equation for wave speed ?
A
Wave speed = frequency x wavelength
8
Q
What is frequency of a wave ?
A
The number of waves passing a point per second
9
Q
What does the reflection of waves mean?
A
Waves moving towards a solid barrier, reflect (bounce) off at an angle equal to the angle at which they hit the barrier.
10
Q
What is the angle of incidence equal to?
A
The angle of reflection
11
Q
When waves reflect does the frequency, wavelength, and speed of the wave change?
A
Yes.
12
Q
What is meant by refracted waves?
A
Change of speed in a wave that Causes a change in direction
13
Q
What causes the wave to change speed?
A
If the substance or medium that the wave is travelling through changes.
14
Q
Do water waves travel fast or slow in deep water?
A
Fast
15
Q
Do water waves travel fast or slow in shallow water?
A
Slow
16
Q
What happens to a wave when it travels from deep the shallow water?
A
The waves slowdown as they enter the shallow water and refract. The wavelength decreases but the frequency stays constant.
17
Q
What happens when waves travel from shallow to deep water?
A
The waves speed up as they enter the deeper water and refract. The wavelength increases but the frequency stays constant .
18
Q
When does the wave refract towards the normal
A
When travelling from deep to shallow
19
Q
When do the waves refract away from the normal?
A
When travelling from shallow to deep.
20
Q
What is the order of the electro magnetic waves
A
- radio waves
- microwaves
- infra-red
- visible light
- Ultraviolet
- X-ray
- Gamma rays
21
Q
What’s speed do the em waves travel in a vacuum?
A
At the speed of light
22
Q
What type of waves are the em waves
A
Transverse waves
23
Q
Which em waves are ionising?
A
Ultraviolet
X-ray
Gamma
24
Q
What does ionising radiation do?
A
Interacts with atoms and can damage cells
25
What is radiation?
An electro magnetic wave (e.g.infra red radiation) or the energy given out by radioactive materials.
26
What are uses of the em waves
Radio - radio broadcast and television
Microwave - mobile phone and satellite signals
Infra red - fibre optic signals and thermal imaging
Visible light - illumination and computer screens
Ultraviolet - killing bacteria and detecting forged bank notes
X-rays - medical x-rays and X-ray luggage and bags
Gamma rays - radio therapy and sterilisation
27
What are satellites in geostationary orbits used for?
A geostationary orbit is when something, like a satellite orbits the earth centre above the equator. From earth the satellite appears to be still, however as the earth rotates over 24 hours, the satellite orbits the earth at the same point in the earth surface.
28
What is the density equation?
Density = mass/volume
29
What is the definition of density?
Mass per unit volume.
30
What is the density equation
Density = mass/volume
31
How do you work out the density of a regular shaped solid?
* measure the length, width and height of the solid (with a ruler)
* calculate the volume of the solid
* measure the mass of the solid (with top pan balance)
* calculate the density of the solid
32
How do you find the density of a liquid?
•measure the volume of the liquid (using a measuring cylinder)
• find the mass of the liquid by weighing the measuring cylinder without the liquid then with the liquid. Then subtract the two values
.calculate the density
33
How do you find the density of an irregular shaped solid?
* measure the volume of the solid by placing water in a measuring cylinder and then placing the solid in there and work out the difference in volume.
* measure the mass of the solid (with a top on balance)
* calculate the density
34
When is heat energy transferred between objects?
When there is a temperature difference
35
Is heat energy transferred from hot to cold?
Yes
36
What happens to the density of an object when it heats up?
The object expands so the density decreases
37
Where does conduction occur?
In solids
38
What is conduction of heat ?
Conduction of heat is the process where vibrating particles pass on their extra vibration energy to neighbouring particles.
39
Is conduction slower or faster in non-metals?
Slower as they are insulators
40
What do free delocalised electrons do?
They transfer kinetic energy through the metal. These increases heat flow per second along the solid
41
Where does convection occur?
Liquids and gases
42
How does convection work?
Above the flame particles are heated and rise. Cooler particles travel down toward the flame. The smoke travels down through the left tube and comes out of the right tube. The air particles become less dense by the flame so rise. The particles become more cooler and fall as they become more dense. This is called a convection current which travels in a loop.
43
True or false - objects that are hotter then their surroundings will emit infrared radiation and objects that are colder than their surroundings will absorb infra red radiation?
True
44
True or false - radiation is the only thing that can travel through a vacuum?
True
45
How do you reduce heat lost from a house?
* loft insulation
* cavity wall insulation
* Double glazed windows
* carpet or underlay
* Draught excluders
46
What does double glazed windows work?
The windows cainita in a layer of air/vacuum between two panels of glass. This insulating layer reduces conduction to almost zero. Air is a poor conductor and use of a vacuum prevents heat loss by convection as well.
47
How does cavity wall prevent heat loss?
Cavity wall is when an insulating material is placed in the gap between the outer bricks and in the inside wall of a house. The insulating material is made up of lots of little fibres, meaning it contains pockets of trapped air which reduces conduction and convection. A thin layer of silver foil can be added to the cavity wall to reduce any heat loss via radiation as the heat is reflected back into the house.
48
How does loft insulation work?
Reduces conduction and convection similar to how cavity walls work
49
What is the payback equation?
Payback time = installation cost/ saving per year
50
What is the current like in a series circuit?
The same throughout the circuit.
51
What’s the voltage like in a series circuit?
The voltage across components add up In a series circuit.
52
What is the current like in a parallel circuit?
The current splits and re joins at a parallel branch
53
What is the voltage like In a parallel circuit?
The voltage is the same across components
54
In a parallel circuit what do the the current in each parallel branch add up to?
The total supply current
55
Where is an ammeter placed in a circuit?
In series
56
Where sí a voltmeter places in a circuit?
In parallel
57
What it the equation for current?
I = v/r
58
What does adding more components do to the resistance in a series circuit?
Increases resistance
59
What does adding more components do to the resistance in a parallel circuit?
Decreases the resistance
60
What is the equation for resistance in a series circuit?
R= R1 + R2
61
What is the resistance equation for a parallel circuit?
1/R = 1/R1 + 1/R2
62
What is power?
energy transferred per second
63
What is power measured in
Jules per second (J/s)
64
What is the equation for energy?
Energy = power x time
65
What are the other two equations for power?
Power = voltage x current ( P = v x I )
Power = current squared x resistance
( P = I 2 x r )
66
For a thermistor what happens to the resistance when the temperature increases?
The resistance decreases
67
For an LDR what happens to the resistance when the light intensity increases
The resistance decreases
68
What are the main renewable energy resources?
```
Solar
Wind
Hydroelectric
Geothermal
Water/waves
Biomass
Tidal
```
69
What are the advantages for using wind as a source for generating electricity?
No fuel cost (wind is free)
| No harmful gases are produced
70
What are the disadvantages for using wind as a source for generating electricity?
Eyesore (not nice to look at)
Can be noisy
The amount of electricity generated depends on the wind intensity and direction I.e. the electricity source is inconsistent and not entirely dependable. If there is very low wind no electricity would be generated
71
What are the advantages for using solar power as a source for generating electricity?
No fuel costs (sunlight is free)
| No harmful gases are produced
72
What are the disadvantages for using solar power as a source for generating electricity?
Exspensive and inefficient so the cost of electric is higher
| Doesn’t work at night
73
What are the advantages for using tidal, wave and hydroelectric power as a source for generating electricity?
No fuel costs ( the movement of tides waves and the water cycle are natural processes
No harmful gases are produced
Reliable sources of energy (tides are particularly predictable)
74
What are the disadvantages for using tidal, wave and hydroelectric power as a source for generating electricity?
* It has been difficult to scale up the designs for wave machines to produce large amounts of electricity.
* Tidal barrages destroy the habitat of river and estuary species, including wading birds.
* Hydroelectricity dams flood farmland and push people from their homes.
* The rotting vegetation underwater releases methane, which is a greenhouse gas.
75
What are the advantages for using biomass as a source for generating electricity?
Carbon neutral. Carbon released into the atmosphere was originally removed from the atmosphere during the growth of the tree.
Can be produced/grown locally reducing inefficiencies/costs/emissions associated with transport
76
What are the disadvantages for using Biomass as a source for generating electricity?
Doesn’t burn as efficiently as fossil fuels.
Still releases some harmful gases into the atmosphere.
Requires a lot of wood framing space to be done on a large scale.
77
What are the main non-renewable energy sources?
Coal
Oil
Gas
Nuclear
78
What are the advantages of using coal oil and gas (fossil fuels) as a source for generating electricity?
Release a lot of energy of when burned
| A lot of the worlds Infrastructure is built to rely on them.
79
What are the disadvantages of using coal, oil and gas (fossil fuels) as a source for generating electricity?
Not sustainable.
Release toxic gases into the atmosphere which are harmful to the environment.
Releases greenhouse gases like carbon dioxide into the atmosphere, promoting global warming.
80
What are the advantages of using nuclear power as a source for generating electricity?
Nuclear energy sources are very efficient.
They produce lots of energy from just a small amount of material.
No harmful gases ar released.
81
What are the disadvantages of using nuclear power as a source for generating electricity?
Nuclear waste is produced which remains radioactive and harmful for a long time. This material could be catastrophic if released into the environment through a nuclear reactor accident.
82
How is solar power used to generate electricity?
* Light and heat energy are released as a result of nuclear fission in the sun can be harnessed to generate electricity.
* Solar cells transfer light energy into electrical energy.
* solar panels use heat energy to heat water which can then be used in a domestic setting.
* they are good for small scale energy generation but need direct sunlight to work well.
83
How is wind used to generate electricity?
Wind can be used to turn wind turbines that directly turn an electrical generator.
84
How is tidal and waves used to generate electricity?
Tidal barrages are built in river estuaries. These fires water through turbines as the tide rises and falls, generating electricity. Kinetic energy from moving waves can also be harnessed in a similar way.
85
How is hydroelectric power used to generate electricity?
Hydroelectric power is produced when when water with gravitational potential energy from an elevated store through a dam. The potential energy is transferred to kinetic energy driving turbines and generating electricity.
86
How is biomass used to generate electricity?
Biomass energy is produced from burning organic material. Like fossil fuels it can be used to produce electricity in a power station or as a direct heat source. Biomass power stations operate in a similar way to fossil fuel power stations.
87
How are fossil fuels used to generate electricity?
When burned fossil fuels transfer stored chemical energy into heat energy that produces steam to drive turbines for generating electricity.
88
How is nuclear power used to generate electricity?
Nuclear power stations use radioactive elements such as plutonium and uranium undergoing nuclear decay (fission) to heat water. This water produces steam which can turn electricity generators. Heat energy is produced which facilitates the transport of hot gas (kinetic energy) that drives turbines generating electricity.
89
What are the main energy changes in a fuel based power station?
Coal(store of CHEMICAL energy) — energy transferred as heat — water and steam (store of HEAT energy) — energy transferred as MOVEMENT(KINETIC) — Turbine — energy transferred as movement — generator energy transferred as ELECTRICITY
90
What is the efficiency equation?
efficiency = (output energy/ total input energy ) x 100
91
What parts make up the national grid?
Powerstation - step-up transformer - pylons - step-down transformer - homes/industry
92
What are the voltage levels at certain points in the national grid?
Powerstations - 25,000v
Pylons - 400,000v
Home/industry - 230v
93
What are the advantages for using high voltage /low current electricity in the long distance cables of the national grid?
The low currents reduce the amount of heat loss through the power lines.
94
How do you know if a transformer is a step down or step up?
If yeh secondary voltage is higher than the primary voltage it’s a step up transformer?
95
What do transformers do?
A step transformer increases (steps up) the voltage to reduce the current to reduce over heating of the cables.
Step down transformers the reduce the voltage back to a safe level for domestic use (to be used in a home).
96
What does 1kW mean?
1 kilowatt (1000 Watts)
97
What does kWh mean and what is it a unit of?
Kilowatt hour and is a unit of energy
98
What is the equation for kWh?
kWh = power (kW) x time (h)
99
What is the equation for cost?
Cost = units used x cost per unit
100
What is energy banding?
Energy banding is when you put electrical appliances into categories depending on how efficient they are.
The higher efficiency the less energy it wastes. This can lead to lower environmental impact and will also be the cheapest to run.
101
What is the difference between alternating current (a.c) and direct current (d.c)?
D.C flows in a single direction around a circuit. It is the type of current typically supplied by batteries and solar cells.
A.C the direction of this current is constantly changing around a circuit. UK mains supply is an alternating current at a voltage of 230 and frequency of 50hz. This means it changes direction around the circuit 50 times every second.
102
What is the function of a fuse?
Fuses are used to detect too much current. If the fuse rating is exceeded the fuse wire melts and breaks the circuit. This isolates the device.
103
What the advantage of using a fuse?
Cheap to buy and replace.
104
What is the function of a miniature circuit breaker (mcb) ?
Mcb’s depend on an electromagnet which separates a pair of contacts when the current flowing exceeds the mcb rating.
105
What’s the advantages of using a mcb as a protection device?
* if an mcb trips it can be reset.
| * more precise tripping value than a fuse.
106
What’s the function of a residual current circuit breaker (rccb) ?
rccb’s are a type of circuit breaker that works by comparing the current going into the device to the current going out. If there is a difference in the two currents it trips.
107
What’s the advantage of using rccb’s as a protection device?
Rccb’s are more sensitive and reacts in even less time than mcb’s and fuses.
108
What equation do you use to work out appropriate fuse values?
Current = power / voltage
109
What’s the function of a ring main?
Yeh wiring in a house connects all appliances together in parallel so that each has 230v across it and can be used independently. Power sockets in a house are connected by means of a ring circuit. Where live, neutral and earth wired form a loop of cable going from the consumer unit to all of the sockets in turn and then back to the consumer unit.
110
What are the advantages of using a ring circuit?
* the cables can be made thinner because there are two paths for the current.
* Each part of the cable carries less current.
* sockets can be place anywhere on the ring.
111
What is the function a of the live wire?
The live wire carries current to a device.
112
What the function of the neutral wire?
The neutral wire carries current away from the device.
113
What is the function of the earth wire?
The earth wire is used on appliances with metal casings. If the live wire touches the metal casing the earth wire allows the current to flow harmlessly to earth. This prevents the user from getting an electric shock.
114
What’s the equation for payback time?
Payback time = cost of insulation/ money saved per year
115
What are the conditions for total internal reflection of light?
* The angle of incidence must be greater than the critical angle.
* the light is moving in the more dense medium striking the boundary with the less dense medium.
116
What is in optical fibre?
An optical fibre is a long, flexible rod, made of glass or transparent plastic, which is very narrow. Light sent in one end of an optical fibre travels through to the other end by undergoing total internal reflection along its length.
117
What are the advantages of optical fibres against satellites?
* cheaper and easier to repair
* provide direct route for sending signals.
* satellites are bar to repair
* higher security
118
What are the advantages of satellites against optical fibres?
* not restricted to point to point communication
* coverage can easily be extended go include additional users
* optical fibres can be damaged from fishing trawlers, anchoring and shark bites .
119
What em wave is used in optical fibres?
Infra red waves
120
What em wave is used in satellites?
Microwaves
121
What are optical fibres also used for?
Remote imaging including endoscopic medical examinations.
122
What are the argument for using CT scans against Endoscope for medical information?
```
Advantages
•3D
•overall imaging of the body
Disadvantages
• higher risk (radiation)
```
123
What are the argument for using endoscope against Ct scan for medical information?
Less harmful
| Specific areas of the body
124
What are p waves?
Longitudinal waves. They are the fastest seismic waves and are able to travel through solids and liquids
125
What happens to p waves as they travel further into the mantle?
They refract more
126
Why does a shadow zone of p waves occur?
The p waves shadow zones occur due to refraction effects
127
How do p waves suggest the inner core is a solid?
P waves speed up in the inner core
128
What are secondary (s) waves?
Transverse waves that are slower than p waves and only travel through solids
129
What does travelling deeper into the mantle cause a waves to do?
Refract more
130
How does s waves not being able to travel through liquids effect the S wave shadow zone?
It creates a huge shadow zone because S waves cannot travel through the liquid outer core.
131
How does the S wave shadow zone effect the knowledge of the earths structure?
It shows that the earths outer core is liquid
132
Do primary and secondary waves travel into or out of the earth?
Into the earth
133
What are the properties of surface waves?
They are the most destructive of the seismic waves. They can be a combination of transverse and longitudinal waves. They have very large amplitudes and Hanse large amount ms of energy associated with them. They are the slowest of the waves and travel along the crust of the earth
134
What paths do s and p waves travel through the earth?
Curved paths due to refraction
135
What does refractions of waves suggest about the earth structure?
It has many layers
136
What is the equation for pressure?
Pressure = force / area
137
What is pressure a measure of?
Pressure is a measure of how much a force acting on a surface is spread out.
138
What does pressure depend on?
Force - a large force causes high area
| Area - a large area of contact causes a low pressure
139
What are 2 examples of pressure? (Knife and camel)
- knives need to be very sharp to cut/ damage a surface because the contact area is small giving it a high pressure.
- camels feet are wide to stop them sinking into the sand. The contact area is large, giving a low pressure
140
What is pressure measured in?
N/m2 or Pa
141
What is force measured in?
Newtons (N)
142
How does pressure arise in the atmosphere?
As you go further down towards the earth,the pressures increases as there are more colliding particles.
- there are more particles towards the surface of the earth as there is a higher gravitational pull.
- every time there is a collision there is a change in momentum creating a force.
143
What effect does pressure have on the volume of gas?
-When the pressure on the gas increases, the volume decreases providing the temperature stays the same.
-As the pressure doubled the volume half’s. This means the volume of a fixed mass of gas is inversely proportional to pressure
- this means
P x V = constant or p1 x v1 = p2 x v2
144
When does pressure increase?
Pressure increase when a force applied to an area increases or when the size of the area it’s applied to decreases
145
When does pressure decrease?
Pressure decrease when the drive applied to an area decreases or when the size of the area it’s applied to increases
146
What happens when a substance changes state?
As a substance changes state, it’s mass remains fixed. However the volume it occupies can change, and therefore so does it’s density. This can be explained using kinetic theory
147
what is kinetic theory for a solid to liquid transition?
as a solid is heated, the connecting molecules gain energy and therefore vibrate more. eventually the vibration is so vigorous that the bonds holding molecules in a solid lattice are broken, allowing them to move more freely. in this liquid state, molecules form temporary bonds with one another. the solid liquid transition is generally associated with an increase in volume. therefore as a liquid, the same mass of a substance will generally occupy a greater volume and so the liquid form will generally have a lower density.
148
what is kinetic theory for a liquid to gas transition?
a somewhat similar process f solid to liquid takes place during the liquid - gas transition, whereby more heat energy is inputted into the system. as a result molecules have more kinetic energy and become more sparsely populated. consequently, gases occupy the greatest volume for a given mass of substance.
149
what happens when a substance is cooled?
when a substance is cooled, heat energy is removed and the internal energy decreases. when enough energy has been removed, bonds may begin to from between the particles causing liquids to freeze (liquid - solid) or gases to condense (gases - liquid), and perhaps freeze.
150
what is Charles' law related to behaviour of gases?
the law relates to the temperature and volume of a gas at constant pressure. if the temperature of a fixed mass of gas is increased at a constant pressure, the volume it occupies will also increase. therefore the ratio of volume to temperature will be constant.
151
what is the equation for Charles' law?
V1/T1 = V2/T2 (T is temperature in Kelvin (K).)
152
what is the equation for Charles' law?
V1/T1 = V2/T2 (T is temperature in Kelvin (K).)
153
what happens when you change the temperature of a fixed volume of gas?
- when the temperature is increased, the gas particles move faster and the collisions become harder and more frequent. this means that the pressure also increases.
- when the temperature is decreased, the gas particles move more slowly and the collisions are less hard and less frequent. this means that the pressure also decreases.
154
what happens at absolute zero (-273 degrees Celsius)?
the molecules stop moving and the gas does not exert ay pressure at all.
155
what is the average kinetic energy of the particles in a gas directly proportional to?
the kelvin temperature of the gas
156
what are the conversions between degrees Celsius and kelvin?
degrees c - kelvin = temperature + 273
| kelvin - degrees c = temperature - 273
157
why do many scientists use kelvin instead of degrees c ?
it makes calculations simpler as there are no minus numbers since it starts at absolute zero (-273)
158
what happens when a substance is heated?
- its internal energy increases
- the movement of its particles increases
- bonds between particles break when a substance melts or evaporates, or sublimes to form a gas to a solid.
159
what happens when a substance is cooled?
- its internal energy decreases
- the movement of its particles decreases
- bonds between particles form when a substance condenses or freezes, or sublimes from a solid to a gas.
160
what happens to the mass when substances change in temperature and state?
- the chemical nature of the substance itself stays the same.
- the mass of the substance stays the same.
161
what is thermal capacity of an object?
the thermal capacity of an object is the amount of heat required to change the temperature of the object by a certain amount. heating it and dividing the heat energy used by its temperature change can determine the thermal capacity of a block of copper.
162
what is thermal capacity of an object measured in?
joules per kelvin (J/K)
163
what is the difference between temperature and heat?
- temperature is the measure of how hot something is
| - heat is a measure of the thermal energy contained in an object.
164
what does temperature increase depend on?
-mass of the object
-substance the object is made from
-amount of energy transferred
to the object
(for a particular object, the more heat energy transferred to it, the greater its temperature increase)
165
what is specific heat capacity?
the specific heat capacity of a substance is the amount of energy needed to change the temperature of 1kg of the substance by 1 degrees Celsius. different substances have different specific heat capacities.
166
what is the equation for specific heat equation?
specific heat capacity = heat energy/ (mass x temperature difference)
C = Q/ (M X delta theta)
167
what are the units for specific heat capacity?
J/Kg degrees Celsius
168
what is the definition of changing state?
when you supply heat energy to a substance or you remove heat from it, you can cause the substance to change in state
when a substance is changing state, the temperature remains consant.
169
what is an example of the temperature staying constant?
340000J of energy are required to melt 1kg of ice at 0 degrees c into 1Kg of water at 0 degrees c. this is called Latent heat!!!!
170
what are the 4 different changes in state?
- fusion: the substance changes from a solid to a liquid
- freezing: the substance changes from a liquid to a solid
- vaporisation: the substance changes from a liquid to a gas
- condensation: the substance changes from a gas to a liquid
171
what is the equation for the specific latent heat?
specific latent heat (J/kg) = the heat supplied (J)/the mass of substance (kg)
L = Q/m
172
what is a heating curve?
a heating curve is a graph showing the temperature of a substance plotted against the amount of energy it has absorbed. you may also see a cooling curve, which is obtained when a substance cools down and changes state.
173
what will a heating graph show?
- the temperature stays the same when a solid is melting or a liquid is boiling during a change of state, even though heat energy is being absorbed.
( (for a cooling curve) the temperature also stays the same while a liquid freezes, even though heat energy is still being released into the surroundings.)
174
where is the strength of a magnetic field the strongest?
at the poles
175
what are the 2 rules of magnetic field lines?
magnetic field lines never cross
| they always point from north to south
176
how does swapping the direction of the current affect the magnetic field?
the direction of the magnetic field will change
177
what is the motor effect?
when a current carrying wire is placed into a magnetic field it experiences a force. the electric current in the wire produces a magnetic field around the wire.
178
how can the size of the force be increased?
- by increasing the strength of the magnetic field
| - by increasing the size of the current.
179
what happens if the wire carrying the current is parallel to the magnetic field?
it will not experience a force
180
what does flemmings left hand motor rule tell us?
it highlights the link between the direction of the current, magnetic field and force on a wire.
181
what does each finger represent in flemmings let hand motor rule?
- first finger = magnetic field direction (N-S)
- second finger = current direction (+ - -)
- thumb = movement (force direction)
182
what is the motor force equation?
Force (newtons) = magnetic field strength (tesla) x current (amps) x length of wire in the field (metres)
F = B x I x L
183
what is a d.c motor?
a dc motor uses the motor effect it turns electrical energy to kinetic energy, it has a split ring communicator. this allows the current in the coil of the wire to change every half turn . this ensures the forces on the coil are in the same direction. the coil spins
184
how do you predict the direction of the spin for dc motor?
by using flemmings left hand motor rule
185
in a dc motor when is the maximum force experienced on the coil?
when its horizontal
186
how do you make the dc motor coil spin in the opposite direction?
- swap the north and south poles
| - reverse the direction of the current
187
how do you make the coil in a dc motor spin faster?
- increase the current
- increase the strength of the magnetic field
- increase the number of coils
- wind the coil on an iron core
188
what is electromagnetic induction?
188
what is electromagnetic induction?
electromagnetic induction is when a changing magnetic field induces a voltage and a current flows in the wire
189
explain how electromagnetic induction works?
- when the magnet is pushed into the coil the current goes one way (positive current.) when the magnet is removed the current goes in the opposite direction (negative current).
- when the magnet and coil are stationary the wire cuts no magnetic field lines. no voltage induced. current is zero.
- the faster the magnetic field lines are cut, the larger the induced voltage and then a larger current is produced.
- moving the magnet causes alternating current.
190
what is an ac generator?
- in a generator, one side of a coil moves up during one half turn and then moves down during the next half turn. this means that as a coil is rotated in a magnetic field, the induced current reverses direction every half turn. this is called alternating current.
- when a coil of wire is spun within a magnetic field an alternating current and voltage is produced.
- the wire or coil cuts through the magnetic field lines
191
how can the size of the current or voltage produced be increased?
- by increasing the speed of rotation
- by increasing the strength of the magnetic field
- by increasing the number of coils
- by increasing the area of the coil
- by winding the coil on an iron core
192
what happens to the voltage output of an ac generator when the coil is horizontal and when the coil is vertical?
```
horizontal = maximum voltage induced (negative or positive)
vertical = zero voltage
```
193
how can the direction induced on the ac generator be determined?
using flemmings right hand generator rule
- first finger = magnetic field direction (N-S)
- second finger = current direction (+--)
- thumb = movement (force direction)
194
what are transformers used for?
transformers are used in the national grid to step up the voltage or stepdown the voltage.
high voltage = low current (heating effect in wires reduced)
low voltage = high current (safer for use in home)
195
how do transformers work?
- transformers are made of an iron core (magnetic material) with two separate coils of wire
- an alternating current/voltage in the primary coil induces an alternating magnetic field in the soft iron core.
- the changing or alternating magnetic field induces an alternating voltage/current in the secondary coil.
- transformers only work with A.C.
- secondary coil more turns than primary coil = step up transformer
- secondary coil less turns than primary coil = step down transformer
196
what is the transformers equations?
```
v1/v2 = n1/n2
p(w)=v(v)I(a)
-v1 = voltage on the primary coil (v)
-v2 = voltage on the secondary coil (v)
-n1 = number of turns on primary coil
n2 = number of turns on secondary coil
```
