PAPER 3 Flashcards
1
Q
Describe the Dalton model
A
- 1870s
- an atom is a very strong indestructible sphere
- all the atoms in an element are the same
- the atoms in one element are different from the atoms of all other elements
2
Q
Describe the Thomson model
A
- 1897
- investigating rays given out by hot elements (cathode rays) - was NOT investigating atoms
- discovered that cathode rays are made out of particles with less that 1/1000 the mass of a hydrogen atom
- found a particle called an electron & decided that it must have come from within the atom
- found out that an electron has a negative charge
- concluded that atoms have no electrical charge (overall neutral)
- 1904
- plum-pudding model
- positively charged matter in an atom is evenly spread out (bc. atom is neutral)
- electrons are spread out randomly inside the atom
3
Q
What is a femotmetre?
A
1fm = 1 x 10^-15 m
4
Q
Describe the Rutherford model
A
- 1899
- Alpha Scattering Experiment
- results couldn’t be explained using the plum pudding model but his results did not account for the electrons
- Alpha particles = helium nuclei (2 protons and 2 neutrons so an overall positive charge)
- Alpha particles are emitted by some substances, which are radioactive
- apparatus was in a vacuum to prevent air molecules from absorbing the alpha particles
- Most ∝-particles passed straight through foil (so atom is mostly empty space)
- Few ∝-particles deflected through small angles (so most of mass of an atom is in the centre / nucleus)
- Very few ∝-particles deflected through more than 90 degrees (nucleus is positively charged)
5
Q
Describe the Bohr model
A
- Rutherford’s experiment did not account for the electrons of an atom
- In 1913, Niels Bohr suggested the electrons moved in fixed orbits (electron shells) around the nucleus of an atom
6
Q
What is the typical size (order of magnitude) of an atom?
A
1 x 10^-10 m
7
Q
Define density
A
- a measure of how much matter is contained within a given volume
8
Q
What is the formula for density?
A
density (kg/m^3) = mass (kg) / volume (m^3)
9
Q
What is temperature?
A
- A measure of how hot or cold something is
- 0 oC = 273 K
- average kinetic energy of the particles
10
Q
What is the energy in a thermal store?
A
- measured in joules
- depends on the arrangement of particles and how fast they are moving / vibrating
11
Q
What is specific heat capacity?
A
The energy (J) needed to raise the temperature of 1kg of a substance by 1K or 1oC (1J/kgK OR 1J/kgoC
It depends on:
- the type of material
- the mass of the material
- the temperature rise
change in thermal energy (J) = mass (kg) x specific heat capacity (J/kgoC or J/kgK) x change in temperature (K or oC)
- Tells you how resistant a material is to a change in temperature
12
Q
What is kinetic energy?
A
Energy due to motion of the particles
13
Q
What is potential energy?
A
Energy due to the position of particles (due to the strength of bonds between the particles)
14
Q
What energy is changed during a state change?
A
- potential energy
- energy change calculated using specific latent heat
15
Q
What energy is changed during a temperature change?
A
- kinetic energy
- energy change calculated using specific heat capacity
16
Q
What is Specific Latent Heat of Fusion?
A
- heat energy transferred when 1kg of a substance changes from the solid state to the liquid state (or vice versa)
- energy transferred (J) = mass of substance (kg) x specific latent heat (J/kg)
17
Q
What is Specific Latent Heat of Vaporisation?
A
- heat energy transferred when 1kg of a substance changes from the liquid state to the gas state
- energy transferred (J) = mass of substance (kg) x specific latent heat (J/kg)
18
Q
What is pressure?
A
Pressure (Pa) = force (N) / area (m2)
1 Pa = 1 N/m2
1 kPa = 1000 Pa
Pressure (Pa) = force normal to the surface (N) / area of that surface (m2)
19
Q
Can you reach ‘absolute zero’?
A
No, absolute zero = 0K
20
Q
Describe the relationship between the volume of a gas and the pressure
A
As the volume doubled, the pressure halves.
As the volume halves, the pressure doubles.
So they are inversely proportional.
21
Q
What is the equation relating the pressure wand volume of a given mass of a gas at a constant temperature?
A
Pressure (Pa) x volume (m3) = constant
22
Q
What does pressure produce?
A
a net force at right angles to any surface
23
Q
How can you increase the internal energy of a gas?
A
- heating it
- doing work on it
24
Q
explain how doing work on a gas can increase its temperature
A
If you apply a force to the pump and move it in, you do work on the gas and it gets hotter.
The average speed of the particles will increase because their kinetic energy increases when they collide with the moving piston, so the temperature is higher
25
describe a simple model of the Earth's atmosphere
an assumption of uniform density
The atmosphere = a single layer of gases that covers the Earth of about 700 km
26
What is atmospheric pressure?
- The pressure exerted by gases of the atmosphere.
| - On the surface of the Earth, atmospheric pressure = 100 kPa = 100,000 N/m2
27
explain why atmospheric pressure varies with height above the surface of the planet
- On the surface of the Earth, atmospheric pressure = 100 kPa = 100,000 N/m2
- Near the top of Mount Everest, atmospheric pressure = 33 kPa
- this is because there is less air above pushing down
28
What is the equation for liquid pressure?
Pressure (Pa) = height of column (m) x density of liquid (kg/m3) x gravitational field strength (N/kg)
gravitational field = 10N/kg near the Earth's surface
29
describe the factors which influence floating and sinking
- weight
| - upthrust
30
explain why pressure in a liquid varies with depth and density and how this leads to an upwards force on a partially submerged object
If the pressure difference together with the area is big enough, then the net force will be enough to balance the weight
31
An object floats if ...?
(Pressure at bottom x area at bottom) - (pressure at top x area at top) = weight
32
How can you use ultrasound to measure distance?
- ultrasound = high-frequency sound waves beyond the range of human hearing (20,000 Hz)
- device measures time taken for a pulse to travel there and back
- if uses the time taken and the speed of the pulse to work out the distance
33
What can you used to measure very short periods of time?
Light gates
34
What is uniform motion?
Motion at a constant speed in a straight line
35
What is the equation for average speed?
Average speed (m/s) = total distance (m) / total time (s)
36
How many metres are in 1 mile?
1609m
37
How many seconds are in an hour?
3600 seconds = 1 hour
38
What is a scalar quantity?
Magnitude (size) only
- distance
- speed
39
What is a vector quantity?
Magnitude (size) and direction
- displacement
- velocity
40
What is acceleration?
The change in velocity per second
Acceleration (m/s2) = change in velocity (m/s) / time (s)
41
Interpret a distance-time graph
Speed = gradient (change in distance / time)
42
Interpret a displacement-time graph
Gradient = Velocity (change in displacement / time)
43
Interpret a velocity-time graph
Gradient = acceleration (magnitude and direction)
Area underneath = displacement
44
Interpret a speed-time graph
Gradient = acceleration (magnitude only)
Area underneath = distance travelled
45
What is the equation of motion that relates final velocity, initial velocity, acceleration and displacement
(Final velocity (m/s))^2 - (initial velocity (m/s))^2 = 2 x acceleration (m/s^2) x displacement or distance (m)
46
What is the equation for kinetic energy?
Kinetic energy (J) = 0.5 x mass (kg) x (speed (m/s))^2
47
recall examples of ways in which objects interact
- electrostatics
- gravity
- magnetism
- by contact (including normal contact force and friction)
48
Why does electrostatic force occur?
The electric force between stationary charged bodies is the electrostatic force
It is an attractive and repulsive force between particles due to their electric charges
49
Why does gravity occur?
Things with mass or energy are attracted to each other
50
What do forces act as?
- forces act as vectors so they are represented with an arrow.
- the length of the arrow = magnitude of force
- direction of arrow = direction of the force
51
What is Newton's First Law?
- An object will continue to stay at rest or move with uniform velocity unless a force acts on it.
- It takes a resultant force to change the motion (speed or direction) of an object
- so if resultant force = 0, then the speed or direction of the object will not change
- The Law is about the profile of inertia
52
What is inertia?
- the inertia of an object is a measure of how difficult it is to change its velocity
53
What is the inertial mass of an object?
the ratio of force over acceleration
54
What is equilibrium?
When the resultant force is zero
55
What is terminal velocity?
The velocity that a moving object achieves when the resultant force is zero
56
What happens if the resistance force of an object is not zero?
- the speed of the object will change
- the direction of motion of an object will change
- the speed and direction of motion of an object will both change
57
What is Newton's Second Law?
The acceleration that the resultant force produces on an object depends on:
- the size of the resultant force
- the mass (inertia) of the object
Force (N) = mass (kg) x acceleration (m/s2)
58
What is the equation that relates force, mass and acceleration?
Force (N) = mass (kg) x acceleration (m/s2)
59
Why do objects move in a circle?
An object moving in a circle at a constant speed is still accelerating because it is constantly changing direction even if the speed is constant.
A force directed towards the centre of the circle acts on the object
60
What is the Law of Conservation of Momentum?
In any collusion, momentum is conserved so the momentum before is the momentum afterwards
61
What happens when objects collide?
- in an elastic collision, no energy is transferred to other stores (energy in kinetic store stays the same)
- in reality, some some energy is transferred (maybe to thermal store) so the collision is not perfectly elastic
62
What happens when objects collide and join together?
- in an inelastic collision, some energy is transferred to other stores
- eg. When snooker balls collide, and energy is transferred by sound to a thermal store
- eg. A collision after which the velocity of the combined objects is less than that of the original objects
63
What is momentum?
- a vector quantity
| - momentum (kg m/s) = mass (kg) x velocity (m/s)
64
What are the formulae relating force, mass, velocity and acceleration?
```
Force = mass x acceleration
Force = mass x (velocity/time)
```
65
What is work done?
A transfer of energy between stores, as a result of a force acting on an object.
If the work is done on an object, it gains energy
If the work is done by an object, it loses energy.
The total amount of energy remains the same (it is conserved).
Work done (J) = force (N) x distance (m)
66
convert between newton-metres and joules
1 J = 1 Nm
67
What is power?
The rate at which energy is transferred
Power (W) = work done (J) / time (s)
= energy transferred (J) / time (s)
Power (W) = potential difference (V) x current (A)
= (energy transferred/charge) x (charge/time)
= (current x resistance) x current
= current^2 x resistance
68
recall and apply Newton's third law
For every action, there is an equal and opposite action
69
How do you stretch, bend or compress an object?
Forces can stretch, bend or compress objects but more than one force has to be applied for this to happen
70
What is elastic deformation?
- when a material that returns to its original shape when the forces are removed
71
What is plastic deformation?
- when a material stays deformed or distorted even after the forces are removed
72
What is the limit of proportionality?
- also called the elastic limit
- below the point, when the forces are removed, the spring returns to its original length
- above the point, when the forces are removed, the spring does not return to its original length and is permanently deformed
73
What is Hooke's law?
The extension of a spring is proportional to the force until you reach the limit of proportionality
Force exerted by a spring (N) = spring constant (N/m) x extension (m)
74
Interpret a 'force applied and extension' graph
The gradient is the spring constant which tells you how stiff the spring is, or how difficult it is to stretch
75
calculate the work done in stretching
Energy transferred is work done
Energy transferred in stretching (J) = 0.5 x spring constant (N/m) x (extension (m))2
76
What matter has a gravitational field?
- all matter has a gravitational field that causes attraction
- the field strength is much greater for massive objects
77
What is the formula for gravity?
gravity force (N) = mass (kg) x gravitational field strength (N/kg)
78
What is the formula for gravitational potential energy?
gravitational potential energy (J) = mass (kg) x height (m) x gravitational field strength (N/kg)
79
What is the formula for the resultant force?
resultant force (N) = mass (kg) x acceleration due to gravity (m/s2)
80
What is weight?
- the force of the Earth on an object when it is on the Earth' s surface
- measured using a scale or a newtonmeter
- weight (N) = mass (kg) x gravitational field strength (N/kg)
- gravitational field strength (g) is 10N/kg on the Earth's surface
81
What is gravitational field strength?
- also known as gravity constant
- represented as 'g'
- a measure of the force on 1kg of mass when it is in a gravitational field due to another mass
82
What is a gravitational field?
- a region where a mass experiences an attractive force
- the same size force acts on each object
- the force is bigger if:
- the mass of one or both of the masses is bigger
- the distance between the masses is smaller
83
What does gravitational field strength depend on?
- mass of the Earth
| - radius of the Earth
84
recall the acceleration in free fall
- free fall = any motion of a body where gravity is the only force acting upon it
- weight (N) = mass (kg) x gravitational field strength (N/kg)
- force = mass x acceleration
- acceleration (m/s2) = force (weight) (N) / mass (kg)
- when there is no air resistance, all objects fall at the same rate regardless of their mass
- near the Earth the rate is the acceleration of free fall, 10 m/s2
- Due to the Earth's gravity, the speed of an object dropped from a height will increase at a rate of 10 m/s every second as it falls (10m/s2)
85
What is a moment?
- the turning effect of a force
- moment of a force (N m) = force (N) x distance (m)
- distance is normal to the direction of the force
86
What is the principle of moments?
- an object is balanced if the anticlockwise moments are equal to the clockwise moments around a pivot
- a pivot is also called a fulcrum
87
What is the effort?
- the force you exert onto the lever
88
What is the load?
- the force you exert onto the lever
89
Describe interaction pairs for levers
- if the pivot is close to the load, you only need a small effort to lift it
- pushing down on the lever produces a turning force on the load, but your hand moves further than the load
90
What is the formula for mechanical advantage?
mechanical advantage = load / effort
work done by you = work done on the load (if you ignore friction between the lever and the pivot)
91
What is the formula for effort?
effort (N) = (load x distance from pivot to load) / (distance from pivot to effort)
92
What is a lever?
- a force multiplier
| - transmits forces by rotating about a pivot
93
How do gears transmit forces?
- a gear is like a lever that rotates
- if you make a smaller cog rotate with a certain force, it will make a larger cog rotate
- the larger cog will exert a greater force but will not move so far (force x distance of both cogs is equal)
- ratio of diameters of cogs tells you the ratio of the effort and the load
94
What can you use cogs to do?
- change the direction in which the rotating force acts
| - change the speed at which the rotating force rotates
95
Describe a hydraulic machine
- a machine that uses a liquid to transmit a force
- made of two pistons connected by a pipe
- when the push on the piston, the pressure is transmitted through the liquid and the other piston moves
- a hydraulic machine is a force multiplier (hydraulic lifts or car jacks enable you to use a small force to lift heavy objects)
96
What is charge?
- charge is a property of matter
| - there are positive and negative charges
97
What does the pressure in fluids (gases and liquids) cause?
- causes a net force at right angles to any surface
98
Describe the production of static electricity
- static charge only builds up on insulators
- when you rub two insulators together, electrons are transferred from one insulator to the other (positive charges don't move)
- one object ends up with extra electrons
- the other force ends up with not enough electrons to cancel out he positive charge
99
How do you discharge an object?
- need to remove the charge
- connect it to something that allows the charges to flow (eg. A piece of metal)
- sparks also discharge charged objects
100
What is a spark?
A flow of charge ( a current ) through the air
101
explain the concept of an electric field and how it helps to explain the phenomena of static electricity
- there is an electric field around a charged object or particle
- if another charged object is put in this field, it will be attracted or repelled (even if the objects are not touching) and the field lines will 'stretch'
- the force on an object will be in the direction that causes the field lines to shorten and straighten
- close together field lines = stronger electric field
- direction of field lines = direction of the force on a positive charge
102
What is current?
- rate of flow of charged particles (or charge)
- in metal wires of a circuit, electrons move
- current in a single closed loop is the same everywhere
103
What are the conditions required for a current?
- source of potential difference
| - closed circuit
104
What is conventional current?
- opposite to electron flow
- when you draw the direction of the electric current on a circuit diagram, you draw it going from the positive terminal of the battery to the negative terminal
105
What is the equation for current?
Charge flow (coulombs) = current (amps) x time (seconds)
106
What are small currents measured in?
- milliamperes (mA)
| - 1 mA = 1 x 10^-3 A
107
What is the formula linking energy transferred, potential difference and charge?
Energy transferred (J) = potential difference (V) x charge (C)
- energy transferred = work done
- measured when a voltmeter is connected across a component in a circuit
108
What is a series circuit?
- has only one loop
- current is the same everywhere (measured with ammeter)
- potential difference (measured by voltmeter, connected to both sides of a component)
109
What is a parallel circuit?
- has more than one loop
- each loop can be worked independently
- the current is different at different points in a parallel circuits (the current in the loops add up to the current near the battery)
110
What does current depend on?
- V = IR- resistance (R) (in some resistors, this value remains constant but it can change as the current changes)
- potential difference (V)
- the units in which these are measured
111
What is potential difference?
- a difference in electrical potential produced by the separation of charge
- a potential difference produces an electric field which produces a force on charged particles in it
- the positive terminal of a cell is at a higher electrical potential than the negative terminal
- measured in volts (V)
- when you apply a p.d between the ends of a piece of wire, an electric field is set up (very quickly) in the wire, so the electrons start to move straight away
- measured with a volt meter
112
What is resistance?
- a measure of how difficult it is for an electric current to pass through a component
- measured in ohms
- resistance = potential difference / current
113
Why does the resistance of some components change?
- a metal is made up of +ve ions arranged in a regular layers
- ions are formed when electrons leave the outer shell of metal atoms and become delocalised (are free to move through the structure of the metal)
- resistance is produced when electrons collide with the ions in the lattice, which explains why the resistance of some components changes with current
114
What is a variable resistor?
- a circuit component that changes the amount of wire or other resisting material
- every time you use a dimmer switch, you use a variable resistor
115
Describe a non-linear characteristic graph
- a characteristic graph = current (y-axis) against potential difference (x-axis)
- a wire is a linear circuit element (it's resistance does not change as the potential difference changes)
- a resistance wire or a resistor can be used if you need the resistance to be constant
116
What is Ohm's Law?
- the current is proportional to the potential difference if the temperature does not change
- So if the wire gets hot, the resistance varies and the graph is not a straight line
- for a non-linear circuit element, the resistance is not constant
117
Describe the characteristic graph for a lamp
The current increases as the potential difference increases, but at a slower rate
118
What is a diode?
- an electrical component that only allows a current to flow one way
- some diodes emit light (LEDs = light emitting diodes)
- long leg of an LED should be connected to the positive terminal of a battery
119
What are sensing circuits?
- thermistors can be used in sensing circuits to produce a potential difference that changes with temperature
- output potential difference depends on the potential difference of the battery, the magnitude of each resistor and temperature
120
How can you calculate the potential difference in a sensing circuit?
1. Calculate net resistance
2. Calculate current
3. Calculate potential difference across a particular resistance
121
What is alternating current?
- current constantly changes direction
- mains electricity is an AC supply
- UK mains supply is about 230 V (has a frequency of 50 Hz, which means that it changes direction and back again 50 times a second)
122
What is direct current?
- current flows in only one direction
- batteries and solar cells supply DC electricity
- a typical battery may supply 1.5 V
123
Describe a diode characteristic graph
- as a potential difference is applied in the forward direction, very little current flows but then suddenly increases
- if the potential difference is reversed, there is no current (because a diode ensures that a current only flows in one direction)
124
What is a zero error?
- if an analogue meter doesn't read zero before you connect it, there will be an error in all measurements taken (a zero error)
- if there is a zero error, the graph will not go through the origin
125
What is a thermistor?
- made of a semiconducting material (eg. Silicon)
- electrons in the atoms of a semiconductor do not need much energy to escape from the atoms to form a current
- a thermistor is at the end of many digital thermometers
- resistance of a thermistor depends on the temperature
- thermistors come in different shapes and sizes and detect different ranges of temperature
- as heat is applied, many electrons gain energy energy to escape the atoms in the semiconductor
- low temperature = small current = thermistor's resistance is high (if potential difference is maintained)
- high temperature = high current = thermistor's resistance is low (if potential difference is maintained)
126
What are thermistors used for?
- monitor temperature in ovens and refrigerators
| - help prevent devices from overheating
127
What is the symbol for an LDR?
Circle with rectangle and 2 arrows pointing inwards
128
What is a permanent magnet?
- made up of many small magnetic regions (domains) that all line up
129
What is an induced magnet?
- made up of many small magnetic regions (domains) that only line up when they are in a magnetic field
- in hard magnetic materials, the domains continue to be lined up when you remove the magnetic field
- in soft magnetic materials, the domains return to their original direction
130
How do magnetic fields explain the behaviour of magnets?
- magnetic field lines represent magnetic flux
131
What is magnetic flux density?
- also called magnetic field strength
| - the number of lines passing through a particular area
132
What is an LDR ?
- light dependant resistor
- resistance depends on light intensity
- made of a semiconducting material
- light causes electrons to be relayed into the circuit to increase the current
- as light intensity increases, more electrons are released into the semiconductor and resistance decreases
- used to control lights in a building
133
What is a compass?
- a north-seeking pole
134
Describe the relationship between Earth and magnetism
- earth behaves as if it has a larger bar magnet at its centre
- field could be produced by convention currents in the molten iron core of the Earth (but scientists aren't 100% sure)
- many compasses are weighted
- the 'dip' = angle between the field lines and a line horizontal to the surface of the Earth
- the 'dip' = 90o at the north and south magnetic poles
- the 'dip' = 0o at the magnetic equator
135
describe the characteristics of the magnetic field of a magnet
- the direction of the magnetic field is tangent to the field line at any point in space
- field strength is proportional to the line density
- field lines cannot cross
- magnetic field lines are continuous, forming closed loops without beginning or end (go from the north pole to the south pole)
136
describe how to show that a current can create a magnetic effect
- right hand rule
| - if the current is coming towards you, the field lines are anti-clockwise
137
Magnetic field strength
- depends on the magnitude of the current (bigger current = a stronger field)
- depends on the distance from the conductor/wire (nearer the wire = a stronger field)
- measured in teslas
138
explain how solenoid arrangements can enhance the magnetic effect
- a solenoid = a coil of wire
- magnetic field in the centre is a straight line
- adding together many fields produces a much stronger field than that of a single wire
- putting a magnetic material inside the core makes the magnetic field even stronger and causes it to produce an induced magnet (an electromagnet which is stronger than any permanent magnet)
139
What happens when you combine fields?
- you can combine the field due to a wire with the field due to a permanent magnet
- this produces a force on the wire
- if the two fields are in the same direction, they add up but if they are in opposite directions, they cancel out
140
Fleming's left hand rule
- left hand
- current, magnetic field and force are all at right angles to each other
- thumb = force (movement)
- index finger = field (north to south)
- middle finger = current (positive to negative)
141
What does the strength of a field around a wire depend on?
- the current through it
142
What does the force on a wire depend on?
- the current
- the field that it is in
- the length of wire in the field (assuming the wire and the field are at 90o)
143
What is the equation for calculating the force acting on a wire?
force on a conductor (at right angles to a magnetic field) carrying a current (N) = magnetic flux density (T) x current (A) x length (m)
144
How is rotation caused in electric motors?
- a combination of magnetic fields can cause a force on a wire that has a current flowing through it
- make a piece of wire into a loop and place the loop into a magnetic field
- when the wire is connected to a battery, a current flows
- one side of the wire goes upwards and the other side goes downwards
- the coil will start to rotate the other way as soon as it passes the vertical position (this means that the motor would not spin well)
145
What will a change in the magnetic field around a conductor do?
- it can give rise to an induced potential difference across its ends, which could drive a current, generating a magnetic field that would oppose the original change
146
How does an electric motor work?
- current needs to move in from the right and out from the left at all times, but still allowing the coil to spin
- split-ring commutator enables the current to flow the same way from the battery, but change to different halves of the coil as it spins
- this ensures that the force on the left-hand side of the coil is always upwards, and the force of the right-hand side of the coil is always downwards
147
How can you change the speed of an electric motor?
By changing:
- the magnitude of the current flowing in the coil
- the strength of the magnetic field
- the number of coils of wire
- the length of the coil
148
What is electromagnetic induction?
The process of producing an induced potential difference in a conductor due to the conductor cutting magnetic field lines
- requires a changing magnetic field
149
How do you increase the induced potential difference?
Depends on:
- the length of wire in the field
- the rate at which field lines are cut
SO potential difference can be increased by:
- moving the wire faster (cutting more field lines per second)
- using a stronger magnetic field (cutting more field lines per second)
- using more wire (more loops/coils) (inducing a potential difference in each loop, so total potential difference increases)
150
What direction does an induced potential difference follow in?
- magnetic field produced is in the opposite direction to the field that produces the potential difference
151
How does an alternator work?
- changing magnetic field required to induce a potential difference
- output of the coil is a potential difference that changes direction
- an alternator = an alternating current generator
- an alternator = the coil of wire that spins between the poles of a magnet (equivalent to moving the magnet in and out of the magnetic field)
- brushes are not attached to the slip rings (brush against the slip rings so the voltmeter is always connected to the ends of the coil, but the coil does not become tangled)
152
How does a Dynamo work?
- a direct current generator
- potential difference produced does not change direction (the coil is connected to a split-ring commutator) but does change in magnitude
153
How can the output be increased in alternators and dynamos?
- using a stronger magnetic field
- using more turns on the coil
- spinning the coil faster
154
How does a transformer work?
- used to increase or decrease a potential difference
- can change the potential difference if an alternating current
- step-up transformer increases the potential difference
- step-down transformer reduces the potential difference
- made of 2 coils of wire (a primary coil from the AC input and a secondary coil leading to the AC output)
- wires are wound around an iron core (not electrically connected) - a loop of iron with two coils
- iron core is easily magnetised and can carry magnetic fields from the primary coil to the secondary coil
155
What happens when a transformer is working?
- magnetic field is trapped inside the iron core
1. A primary potential difference across the primary coil drives an alternating current through the primary coil
2. The primary coil current produces a magnetic field, which changes as the current changes
3. The iron core increases the strength of the magnetic field
4. The changing magnetic field induced a changing potential difference in the secondary coil
5. The induced potential difference produces an alternating current in the external circuit
156
How does the output of a transformer depend on the number of turns on the coils?
Potential difference across primary coil (V) / potential difference across secondary coil (V) = number of turns in primary coil / number of turns in secondary coil
157
Explain the action of a microphone
- a device that converts sound waves into electrical signals
- use the generator effect to induce a changing current from the pressure variations of sound waves.
1. pressure variations in sound waves cause the flexible diaphragm to vibrate
2. the vibrations of the diaphragm cause vibrations in the coil
3. the coil moves relative to a permanent magnet, so a potential difference is induced in the coil
4. the coil is part of a complete circuit, so the induced potential difference causes a current to flow around the circuit
5. the changing size and direction of the induced current matches the vibrations of the coil
6. the electrical signals generated match the pressure variations in the sound waves
158
explain the action of headphones and loud speakers
- use the motor effect
- variations in an electric current cause variations in the magnetic field produced by an electromagnet which causes a cone to move, which creates pressure variations in the air and forms sound waves
Alternating current supplied to the loudspeaker creates sound waves in the following way:
1. a current in the coil creates a magnetic field
2. the magnetic field interacts with the permanent magnet generating a force, which pushes the cone outwards
3. the current is made to flow in the opposite direction
4. the direction of the magnetic field reverses
5. the force on the cone now pulls it back in
6. repeatedly alternating the current direction makes the cone vibrate in and out
7. the cone vibrations cause pressure variations in the air - which are sound waves
To make a loudspeaker cone vibrate correctly, the elastic current must vary in the same way as the desired sound.
159
Practical Activity Group 2: Investigate the effects of forces on the extension of springs and magnets.
1) Secure a clamp stand to the bench using a G-clamp or a large mass on the base.
2) Use bosses to attach two clamps to the clamp stand.
3) Attach the spring to the top clamp, and a ruler to the bottom clamp.
4) Adjust the ruler so that it is vertical, and with its zero level with the top of the spring.
5) Measure and record the unloaded length of the spring.
6) Hang a 100 g slotted mass carrier (weight 0.98 N) from the spring. Measure and record the new length of the spring.
7) Add a 100 g slotted mass to the carrier. Measure and record the new length of the spring.
8) Repeat step 7 until you have added a total of 1,000 g.
EQUATION
extension = length - unloaded length
PRECAUTIONS
1) Use a G-clamp to secure the stand
BECAUSE
Heavy objects falling on feet could bruise or fracture
2) Wear eye protection, support and gently lower masses whilst loading the spring
BECAUSE
Sharp end of spring may recoil if the spring breaks which may lead to damage to eyes, cuts to skin
3) Gently lower load onto spring and step back
BECAUSE
Masses may fall to floor if the spring fails and could bruise or fracture or fracture feet
160
Practical Activity Group 3: Measure speeds and accelerations.
AIM OF EXPERIMENT
- To investigate the acceleration of an object on an angled ramp.
METHOD
1) Set up a ramp balanced on a wooden block at one end (avoid making the ramp too steep as this will cause the trolley to roll too quickly which could make measuring difficult)
2) Mark out 30 cm at the end of the ramp.
3) Practise recording the time it takes for the trolley to travel the length of the ramp. To do this, release the trolley from the top of the ramp, start the stop clock and record the time taken for the trolley to move the whole distance of the ramp.
4) Repeat this twice more and record all results in a table similar to the one below.
5) Then calculate the mean time and record this also. Remember that these are practise results.
6) Next, record the time it takes for the trolley to travel the final section of the ramp. To do this, release the trolley from the top of the ramp again but this time start the stop clock when the trolley reaches the last 30 cm of the ramp.
7) Record the time it takes for the trolley to travel the last 30 cm of the ramp in a table like the one shown below. These are your final results.
8) Repeat this twice more, and record a mean time for the trolley to travel the last 30 cm of the ramp.
9) Calculate the speed of the trolley when it was descending the last 30 cm of the ramp using the equation: speed (m/s) = distance (m) ÷ time (s) (Remember to first convert 30 cm into metres)
ANALYSIS OF RESULTS
1) Use the result from above as the final speed and take the initial speed of the trolley as 0 m/s.
2) Calculate the acceleration of the trolley when descending the entire length of the ramp using acceleration (m/s2) = change in speed (m/s) ÷ time taken (s).
Remember that the change in speed is from 0 m/s to the calculated result.
PRECAUTIONS
Keep feet well away from the end of the ramp
BECAUSE
the trolley falling may cause injury to feet
161
Practical Activity Group 6: Investigate the characteristics of circuit elements.
To investigate the relationship between current and potential difference for a resistor, bulb and diode.
1) Connect the circuit as shown in the first diagram.
2) Ensure that the power supply is set to zero at the start.
3) Record the reading on the voltmeter and ammeter.
4) Use the variable resistor to alter the potential difference.
5) Record the new readings on the voltmeter and ammeter.
6) Repeat steps three to four, each time increasing the potential difference slightly.
7) Reverse the power supply connections and repeat steps two to six.
8) Plot a graph of current against potential difference for each component.
9) Repeat the experiment but replace the fixed resistor with a bulb.
162
Practical Activity Group 6: Investigate the characteristics of circuit elements
Filament Bulb - Analysis and Evaluation
ANALYSIS
- Non-linear relationship (current is not proportional to potential difference)
EVALUATION
- In a filament bulb, the current does not increase as fast as the potential difference.
- Doubling the amount of energy does not cause a current twice as fast.
- The more energy that is put into the bulb, the harder it is for the current to flow - the resistance of the bulb increases.
- As the potential difference increases, so does the temperature of the thin wire inside the bulb, the filament.
- The increased vibrations of the ions in the filament because of the increased temperature make it harder for the electrons to get past.
163
Practical Activity Group 6: Investigate the characteristics of circuit elements
Fixed Resistor - Analysis and Evaluation
ANALYSIS
- When a graph of current against potential difference is plotted, it shows a linear relationship passing through the origin.
- These graphs are indicators of direct proportionality.
- Such a relationship means that both variables change in the same way, ie if the potential difference is doubled, the current doubles as a result.
Evaluation
- For a fixed resistor, the potential difference is directly proportional to the current.
- Doubling the amount of energy into the resistor results in a current twice as fast through the resistor.
- This relationship is called Ohm's Law and is true because the resistance of the resistor is fixed and does not change.
- A resistor is an ohmic conductor.
164
Practical Activity Group 6: Investigate the characteristics of circuit elements
Semiconductor diode - Method, results analysis and evaluation & precautions
METHOD
1) Connect the circuit as shown in the diagram having chosen a suitable protective resistor (between 100 Ω and 500 Ω).
2) Set the variable resistor to give the lowest potential difference and record the readings on the voltmeter and milliammeter.
3) Alter the variable resistor to increase the potential difference by 0.2 V.
4) Record the new readings on the voltmeter and milliammeter.
5) Repeat steps - three to four, each time increasing the current slightly.
6) Reverse the power supply connections and repeat steps two to six.
7) Plot a graph of current against potential difference for the diode.
ANALYSIS OF RESULTS
When the graph is plotted this time, it shows that the diode does not allow any current to flow until the potential difference reaches a certain value (usually around 0.7V).
EVALUATION
- A semiconductor diode only allows current to flow in one direction.
- If the potential difference is arranged to try and push the current the wrong way (also called reverse-bias) no current will flow as the diode's resistance remains very large.
- Current will only flow if the diode is forward-biased.
- When forward-biased, the diode's resistance is very large at low potential differences but at higher potential differences, the resistance quickly drops and current begins to flow.
PRECAUTIONS
Do not touch the resistance wire whilst the circuit is connected and allow time for the wire to cool
BECAUSE
the resistance wire is heated which may cause burns to the skin
165
Practical Activity Group 6: Investigate the resistance of metallic wires.
Independent Variable = length of wire.
Dependent Variable = resistance of the wire.
Control Variables = the material, the cross section area and the temperature of the wire. These are kept the same by not changing the wire during the experiment, by keeping the current small and opening the switching between readings
Equipment
- 1m length of constantan wire
- a metre rule
- a low voltage power pack
- a voltmeter
- an ammeter
- connecting leads
- a switch
- 2 crocodile clips
- Sellotape.
1) Set up the circuit, as shown above. Attach the flying lead at the 20 cm mark so that the length of wire the current flows through is 20 cm. Record this length in a suitable table.
2) Adjust the power pack until the current on the Ammeter is 0.4 A. Record the current in the table.
3) Read the corresponding value of voltage across the wire on the voltmeter and record in the table.
4) Switch the switch off to prevent the temperature of the wire rising.
5) Switch on again and repeat the reading of voltage. Record in the table. Switch off and calculate the average voltage.
6) Calculate the resistance of this length of wire and record in the table.
7) Switch on again. Ensure that the current is still 0.4 A and repeat current and voltage reading for lengths of 40 cm, 50 cm 60 cm 80 cm and 100 cm.
8) Calculate the resistance for each length, remembering to switch off between each reading.
PRECAUTIONS
1) To ensure the temperature of the wire does not increase, switch off between readings and keep the current as low as possible.
THIS IS BECAUSE
The temperature of the wire must be kept constant.
AND
- Whenever a current flows through a conductor there is a heating effect.
- Electrical energy is converted to heat energy.
2) Do not set up the experiment near taps, sinks etc.
THIS IS BECAUSE
There is a risk of electric shock
3) Do not handle the wire. Switch off between readings.
THIS IS BECAUSE
There is a risk of minor burns because the wire gets hot
CONCLUSION
- As the length of wire increases, the resistance will increase.
- The greater the length of wire the greater the number of collisions between the free electrons and metal ions.
- This will result in greater resistance
