1
Q
What are the two things you either need to transmit a huge amount of power like the national grid does?
A
You either need a high potential difference or a high current
(P=VI)
2
Q
What would be the problem if the national grid used a high current?
A
lots of energy would be lost as the wires heat up and transferred to the thermal energy store of the surroundings
3
Q
Why does the national grid use a really high potential difference?
A
It’s much cheaper to boost the potential difference really high (400,000V) and keep the current as low as possible
For a given power, increasing the pd decreases the current, which decreases energy lost making the national grid more efficient at transferring energy
4
Q
What does the ‘demand’ of electricity mean?
A
Electricity usage (demand) changes throughout the day
This is the amount of energy power stations have to produce so there is enough energy for everyone when they need it
5
Q
When is demand for electricity predicted to increase?
A
- when people get up in the morning
-come home from school or work - when it gets dark or cold outside
-popular events like a sporting final being shown of TV
6
Q
Name 2 ways power stations cope with the changing demand?
A
-Power stations often run well below their maximum power output (spare capacity to cope with high demand) - even if another power station unexpectedly shuts down
- There are lots of smaller power stations on standby that can start up quickly
7
Q
What is electric current? -units?
A
The flow of electric charge
units: amperes (amps) A
8
Q
What does electric charge need to flow?
A
- a complete circuit
-a source of potential difference
9
Q
What is potential difference?- units?
A
- the driving force that pushes the charge around
-units: volts, V
10
Q
What is resistance? units?
A
Anything that slows down the flow of current
units: ohms
11
Q
What does the current flowing through a component depend on?
A
- the resistance of the component
- the potential difference across the component
12
Q
What does a greater resistance across a component cause?
A
A smaller current that flows *for a given potential difference across the component
13
Q
What is the national grid?
A
A giant system of cables and transformers that covers the UK and connects power stations to consumers (anyone who is using electricity)
14
Q
What does the national grid do?
A
The national gris transfers electrical power from power stations all over the grid (the supply) to anywhere it is needed(the demand) e.g. homes and industry
15
Q
What does the total energy transferred by an appliance depend on?
A
- the power of the appliance
- how long the appliance is on for
16
Q
What is the power of an appliance?
A
the energy that it transfers per second
more energy transferred in a given time means more power
17
Q
What happens when you add resistors in parallel?
A
Adding resistors in parallel reduces the total resistance
- if you have two resistors in parallel, their total resistance is smaller than the resistance of the smallest of the two resistors
18
Q
Explain how resistance decreases when you add resistors in parallel?
A
- all resistors have the same potential difference across them as the source.
- adding another loop gives the current another direction to go in
-increases the total amount amount of current that can flow around the circuit - using V=IR will show how the resistance is decreasing
19
Q
What happens to potential difference in a parallel circuit?
A
Potential difference is the same across all components
-all component get the full source pd
- identical bulbs connected in parallel will be at the same brightness
20
Q
What happens to current in a parallel circuit?
A
Current is shared between branches
-total current flowing around the circuit is equal to the sum of currents through the separate components
21
Q
Current in parallel:
What happens at junctions?
What will be the current of two identical components in parallel?
A
- at junctions the current either splits or rejoins- total current going into a junction is the same as the total current leaving
-they will have the same current flowing through them
22
Q
What is a thermistor?
A
A thermistor is a temperature dependant resistor
23
Q
How does temperature affect a thermistor?
A
-In hot conditions, the resistance drops
- In cool conditions, the resistance goes up
24
Q
What is a use of a thermistor?
A
They make useful temperature detectors (e.g. car engine temperature sensors and electronic thermostats)
25
What two things does the energy transferred by an appliance depend on?
- the power of the appliance
- how long the appliance is on for
26
What is the power of an appliance?
-the energy that it transfers per second
- so, the more energy it transfers in a given time, the higher its power
27
What are appliances often given?
- a power rating
- they are labelled with the maximum safe power they can operate at
- you can usually take this to be their maximum operation power
28
What does the power rating of an appliance tell you?
the maximum amount of energy transferred between stores per second when the appliance is in use
29
How do power ratings help customers?
- It can help them choose between models
- lower power ratings mean an appliance uses less electricity in a given time so is cheaper to run
30
Why are higher powers of appliances not always useful?
- a higher power doesn't always transfer more energy usefully
- a more powerful appliance may still be less efficient, it might only transfer the same amount of energy (or even less) to useful stores
31
work done is the same as...
energy transferred
32
energy transferred is the same as ...
work done
33
Why does a moving charge transfer energy?
the charge does work done against the resistance of the circuit
34
Explain how a kettle is designed to transfer energy to components in the circuit when a current flows?
Kettles transfer energy electrically from the mains ac supply to the thermal energy store of the heating element inside the kettle
35
Explain how a handheld fan is designed to transfer energy to components in the circuit when a current flows?
- energy is transferred electrically from the battery of a handheld fan to the kinetic energy store of the fan's motor
36
What factor causes the energy to be transferred by appliances less usefully?
- Current
- the higher the current, the more energy is transferred to the thermal energy stores of the components (and then the surroundings)
37
What does the term 'I-V characteristic' refer to?
a graph that shows how the current flowing through a component changes as the potential difference across it is increased
38
Give an example of a linear component?
a fixed resisistor
39
Give two examples of a non- linear component
filament lamp
diode
40
What is the name of components with a straight line I-V characteristic?
linear components
41
What is the name of components with a curved I-V characteristic?
non-linear components
42
Describe the resistance of ohmic conductors
the resistance of ohmic conductors (e.g. wire or a resistor) doesn't change with the current
43
Describe the current and pd of an ohmic conductor at a constant temperature?
At a constant temperature:
the current flowing through an ohmic conductor is directly proportional to the potential difference across it
(R is constant in V=IR)
44
How does the resistance of a filament lamp change when a electrical charge flows through it?
- some energy is transferred to the thermal energy store if the filament which is designed to heat up
- resistance increases with temperature, so as the current increases, the filament lamp heats up more and the resistance increases
45
What does the resistance depend on for diodes?
- the resistance depends on the direction of the current
-they will easily let current flow in one direction, but have a very high resistance if it is reversed
46
What is LDR short for?
Light dependent resistor
47
What is an LDR?
- a resistor that is dependent on intensity of light
- In bright light, the resistance falls
- In darkness, the resistance is highest
48
State 3 applications of LDRs
- automatic night lights
-outdoor lighting
- burglar detectors
49
What is a thermistor?
- a thermistor is a temperature dependent resistor
- in hot conditions, the resistance drops
- in cool conditions, the resistance goes up
50
State 2 applications of thermistors
- They make useful temperature detectors:
- e.g. car engine temperature sensors
- and electronic thermostats
51
INVESTIGATING FACTORS AFFECTING RESISTANCE:
Name 2 factors that can affect the resistance of a circuit
- whether the components are in series or parallel
- length of wire
52
INVESTIGATING FACTORS AFFECTING RESISTANCE:
What is the function of an ammeter?
How is it connected in the circuit?
- to measure the current (in amps) flowing through the test wire
- the ammeter must always be placed in series with whatever you're investigating
53
INVESTIGATING FACTORS AFFECTING RESISTANCE:
What is the function of the voltmeter?
How is it connected in the circuit?
- measures the potential difference across the test wire in volts
- must always be placed in parallel with whatever you're investigating, not around any other bit of the circuit
54
INVESTIGATING FACTORS AFFECTING RESISTANCE:
Describe the relationship between resistance and length of wire?
resistance is directly proportional to length
- the longer the wire, the greater the resistance
55
INVESTIGATING FACTORS AFFECTING RESISTANCE:
Why may the graph not go straight through the origin?
- it could be because the clip is no attached a exactly 0cm - this is a systematic error
56
PARALLEL CIRCUITS:
Explain how components are connected in in parallel?
- each component is connected separately to the +ve and -ve of the supply
(except ammeters which are always connected in series)
57
PARALLEL CIRCUITS:
What will happen if you remove or disconnect a component in parallel?
- it will hardly affect the others at all
58
PARALLEL CIRCUITS:
Explain why most things are connected using parallel circuits?
What do everyday circuits often include?
- you have to be able to switch everything on and off separately
- for example, in cars and household electronics
- everyday circuits often include a include a mixture of series and parallel circuits
59
SERIES CIRCUITS:
Explain how components are connected in series circuits
- components are connected in a line, end to end, between the +ve and -ve of the power supply
(except for voltmeters, which are always connected in parallel, but they don't count as part of the circuit)
60
SERIES CIRCUITS:
Why are very few things connected in series circuits in practice?
- If you remove or disconnect one component, the circuit will be broken and all components stop working
61
SERIES CIRCUITS:
Describe how potential difference is distributed through a series circuit?
- the total pd if the supply is shared between the various components
- the potential differences round as series circuit always add up to equal the source pd
62
SERIES CIRCUITS:
Describe how current is distributed through a series circuit?
What determines the size of the current?
- In series circuits, the same current flows through all components
-the size of the current is determined by the total pd of the cells and the total resistance of the circuit (I=V/R)
63
SERIES CIRCUITS:
What happens to resistance in series circuits?
Resistance adds up
- the total resistance of two components is just the sum of their resistances
64
SERIES CIRCUITS:
Explain why resistance adds up in series circuits?
- by adding a resistor in series, the two resistors have to share the total pd
- the pd across each resistor is lower so the current through each resistor is also lower
- current is the same everywhere, so the total current in the circuit is reduced when a resistor is added, this means the total resistance of the circuit increases
- the bigger a components resistance , the larger its share of the total pd
65
SERIES CIRCUITS:
What happens to cell potential differences?
- Cell potential differences add up
- there is a bigger pd when more cells are in series, if they're all connected the same way
For example, two cells with a pd of 1.5V connected in series supply 3V between them
66
What are the two types of electricity supplies?
- alternating current (ac)
- direct current (dc)
66
What is an ac supply?
- Where the current is constantly changing direction.
- alternating currents are produced by alternating voltages in which the positive and negative ends keep alternating
67
What is the UK mains supply and pd?
An ac supply at around 230V
68
What is the frequency of the ac mains supply?
50 Hertz, Hz
50 cycles per second
69
What type of current do cells and batteries supply?
direct current
70
What is direct current?
- a current that is always flowing in the same direction, it's created by a direct voltage
71
How are most electrical appliances connected to the mains supply?
- by three core cables
- the each have a core of copper and a coloured plastic coating
72
What does the colour of a cable tell you?
The colour of the cable insulation tells you its purpose
73
Why are the colours of cables the same for every appliance?
So it is easy to tell the different wires apart
74
LIVE WIRE:
Colour?
Purpose?
Potential difference?
- brown
- provides the alternating potential difference from the mains supply
- at about 230V
75
NEUTRAL WIRE:
Colour?
Purpose?
Potential difference?
- blue
- completes the circuit, when the appliance is operating normally, current flows through the live and neutral wires
- at around 0V
76
EARTH WIRE:
Colour?
Purpose?
Potential difference?
- green and yellow
- protecting the wiring and for safety - is stops the appliance casing from becoming live. It doesn't usually carry a current, only when there is a fault
- also at 0V
77
How can the live wire give you an electric shock?
Like the earth, your body is at 0V
If you made contact with the live wire, your body would provide a link between the supply and the earth
a large pd is produces across you body and a current flows through you
causing a large electric shock
78
What does a large electric shock cause?
it could injure or even kill you
79
How can a plug socket or light switch be dangerous even if it is turned off?
- there is still a danger of an electric shock
- there isn't a current flowing but there is still a pd in the live wire
- making contact with the live wire provides a link between the supply and earth, so a current would flow through you
80
How can a connection between live and earth become dangerous?
- If the link creates a low resistance path to earth, a huge current could flow, which could result in a fire
81
REMEMBER:
throughout the day, electricity usage (the demand) changes, power stations have to produce enough electricity for everyone to have it when they need it
82
What is the national grid?
a giant system of cables and transformers that cover the UK and connects power stations to consumers (anyone who si using electricity)
83
How does the national grid get the pd up to 400,000V for efficient transmission?
- transformers are used
(and big pylons with huge insulators)
84
How are transformers structured?
They have two coils:
- a primary coil
- and a secondary coil
- with an iron core
85
How are step up transformers structured?
They have more turns on the secondary coil than the primary coil
86
What happens at a step-up transformer?
pd is incresed by the transformer
the current is decreased
87
What happens at the local consumer end of the national grid?
pd is then reduced again at the step-down transformer
the current is therefore increased
88
How are step down transformers structured?
they have more turns on the primary coil than the secondary coil
89
How do you calculate the power of the primary and secondary coil?
primary coil = power = pd * current
- transformers are nearly 100% efficient so,
power in primary coil = power in secondary coil
90
What causes a build up of static?
- static is built up from friction
- negatively charged electrons tranfer from one insulating material to another
- this leaves the materials electrically charged, with a positive static charge on one and an equal negative tatic charge on the other
91
What factor controls which way electrons are transferred in the build up of static electricity?
the two materials involved
92
What is an example of two materials that can build up static?
polythene and acetate rods being rubbed against a cloth duster
93
Static electricity: what two charges can be produced by the movement of electrons?
+ ve and -ve electrostatic charges
94
Static electricity:
How can one material becoe positively charged?
What happens to the other material?
a positive static charge is caused by the loss of electrons
- the material that gains the electrons has an equal negative charge
95
Explain how too much static can cause sparks
- as electric charge builds on an oblect, the pd between the object and the earth (which is at 0V) increases
- is the potential difference gets large enough, electrons can jump across the gap between the charged object and the earth
- this is the spark
96
Explain how you can get static shocks getting out of a car?
- elecrons can jump to any earthed conductor that is nearby
- a charge build's up o the car's metal frame and when you touch the car, the charge travels through you to the earth
97
What is lightning an example of?
a really big spark
98
What happens when two electrically charged objects are brought close together?
they exert a force on one another
99
What will two like charges do to each other?
repel
100
What will two things with opposite electric charges do to each other?
they will attract each other
101
The force between two charged objects gets weaker...
the further apart they are
102
What will the force between two electrically charged objects do ?
What is this known as?
- it will cause the objects to move if they are able to do so
- this is known as electrostatic attraction / repulsion (and is a non-contact force)
103
If you suspens a rod with a known charge from a piece of string:
What will an object do with the same charge placed nearby?
repel the rod
the rod will move away from the object
104
If you suspens a rod with a known charge from a piece of string:
What will an object do with the opposite charge placed nearby?
- it will attract the rod and cause it to move towards the object
105
Where is an electric field created?
- around any charged object
106
Where is the electric field of an object strongest?
the closer to the object
107
Where is the electric field of an object weakest?
the further from the object
108
What do you draw to show an electric field around an object?
field lines
109
What does an 'isolated' sphere mean?
it's not reacting with anything
110
What happens when a charged object is placed in the electric field of another object?
what causes thus?
it feels a force
the electric fields of each charged object iteracting with each other
111
electric fields: What determines the force on n objects?
the strength of the electric field it is in
112
What happens as you increase the distance between two charged objects?
the strength of the field decreases and the force between them gets smaller
113
When are sparks caused?
when there is a high enough potentila difference between a charged object and the earth (or an earthed object)
114
Explain sparks in terms of electric fields?
- a high pd causes a strong electric field between the charged object and the earthed object
- the strong electric field causes electrons in the air partices to be removed (known as ionisation)
- air is normally an insualtor, but when it is ionised it is much more conductive, so a current can flow through it - this is the spark
GCSE Physics
flashcards
Decks in class (21)
# Cards
-
Topic 1 - Energy (1)34
-
Topic 1 - Energy (2)31
-
Topic 1 - Energy (3)80
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Topic 2-Electricity115
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Topic 4 - Atomic structure70
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Topic 5 - Forces (1)31
-
Topic 5 - Forces (2)17
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Topic 5 - Forces (3)47
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Topic 5 - Forces (4)35
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Topic 5 - Forces (5)45
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Topic 5 - Forces (6)38
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Topic 6 - Waves (1)28
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Topic 6 - Waves (2)35
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Topic 6 - Waves (3)42
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Topic 6 - Waves (4)36
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Topic 6 - Waves (5)41
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Topic 7 - Magnetism and Electromagnetism34
-
Topic 8 - Space Physics (1)29
-
Topic 8 - Space Physics (2)9
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PAPER 1 - past paper mistakes21
-
PAPER 2 - past paper errors15
