Electricity cgp Flashcards
1
Q
What’s current
A
The rate of flow of charge
2
Q
1 C is defined as
A
The amount of charge that passes in one second if the current is one ampere
3
Q
You always need to attach an Ammeter in
A
Series
So that the current through the ammeter is the same as the current through the component
4
Q
Potential difference is the
A
Energy/work done per unit charge
5
Q
Potential difference or voltage is defined as the
A
Work done per unit charge moved
6
Q
The potential difference across components in parallel is there
A
Same
7
Q
Define the volt
A
The potential difference across the component is one volt when you convert one joule of energy moving one coulomb of charge through the component
1V=1JC^-1
8
Q
Everythinghas
A
Resistance
9
Q
If you put a potential difference across an electrical component what will happen
A
A current will flow
10
Q
How much current you get for a particular potential difference depends on the… Of the component
A
Resistance
11
Q
You can think of a components resistance as a measure of how
A
Difficult it is to get a current to flow through it
12
Q
Resistance is measured in
A
Ohms
13
Q
A component has a resistance of one ohm if
A
A potential difference of 1 V make a current of one amp flow through it
14
Q
For an ohmic conductor resistance is a
A
Constant
15
Q
Conductors that obey Ohms law are called
A
Ohmic conductors (mostly metals)
16
Q
What’s ohms law
A
Provided the physical conditions such as temperature remains constant the current through an ohmic conductor is directly proportional to the potential difference across it
[therefore current is proportional to the voltage)
17
Q
The graph of current against potential difference is a straight line graph this means that
A
Resistance is constant
in other words V divided by I is always a fixed value
18
Q
Often factors such as… Will have a significant effect on resistance (the resistivity changes)
So you need to remember that ohms law is only true for ohmic conductors under……
A
Light level or temperature
Constant physical conditions e.g. temperature
19
Q
I /V graphs show how
A
How resistance varies
They show how the current flowing through a component changes as the potential difference across it is increased
20
Q
The IV graph for an ohmic conductor is a
A
Straight line through the origin
21
Q
At a constant temperature the current through and ohmic conductor e.g. metals is… to the voltage.This means that the i/v characteristics graph for an ohmic conductor at a constant temperature is a straight line through …
I.e. there resistance is
A
Directly proportional
The origin
Constant
22
Q
Check
If something is directly proportional then the graph would be
A
A straight line through the origin
23
Q
For a I/V graph a steep line means…resistance
For a V/I graph a shallow line means… Resistance
A
Low
Low
24
Q
The I/V characteristic for a filament lamp is a
It starts … but gets… As the voltage rises
A
Curve
Steep
Shallower
25
What is the filament in a lamp
Just a coiled up length of metal wire
26
Because the filament in a filament lamp is metal you might think it should have the same characteristic graph as a metallic conductor that's why doesn't it
Because it gets hot as current flowing through the lamp increases its temperaturetherefore a filament lamp
27
The resistance of a metal increases as the
Temperature increases
28
Describe the V/I graph for a filament lamp
The opposite to an I/V graph
| Start shallow and get steeper as the current and voltage increase
29
Semiconductors are used in
Sensors
30
How good are semiconductors at conducting electricity
| And Why
They are nowhere as near as good as metals
| Because there are far fewer charge carriers available
31
If energy is supplied to a semiconductor what happens and what does this mean
More charge carriers can be released and this means they make excellent sensors for detecting changes in their environment
32
Give two examples of semiconductor components
Thermistors and diodes
33
The resistance of a thermistor depends on
Temperature
34
What a thermistor
A resistor with a resistance that depends on temperature
35
What does NTC thermistors stand for
Negative temperature coefficient
36
What do negative temperature coefficient thermistors mean
The resistance decreases as the temperature goes up
37
why does the resistance decreases as the temperature goes up for negative temperature coefficient thermistors
Warming the thermistor give more electrons enough energy to escape from their atoms.
this means that there are more charge carriers available so the resistance is lower
38
What's the graph for resistance against temperature for a NTC thermistor
And what's the thermistor circuit symbol
Pg85
39
The gradient of a V – I graph does the opposite to the gradient of a… Graph
I/v
40
Draw the IV graph and VI graph for a NTC thermistor
Page 85
41
Diodes only let current flow in…
One Direction
42
Diodes I designed to let
Current flow in One Direction only
43
Give an example of a diode
Light emitting diode LED's
44
Give the symbol of a diode and an LED
Page 85
45
Diodes cont.) What is forward bias
Forward bias is the direction in which the current is allowed to flow
46
Most diodes require… Voltage of about… In the forward direction before they will…
A threshold voltage
About 0.6V
Conduct
47
In reverse bias The resistance of the diode is… And the current that flows is…
Very high
| Very tiny
48
What three things determine resistance
Length -The longer the wire the more difficult it is to make current flow
area -the wider the wire The easier it will be For the electrons to pass along it
resistivity-which depends on the material. The structure may make it easy or difficult for charge to flow
49
In general resistivity depends on
The material and Environmental factors such as light intensity and temperature
50
The resistivity of a material is defined as
The resistance of a 1 m length with a 1 m² cross sectional area
51
Resistivity is measured in
Ohm metres
52
What's the formula for resistivity
Pg.86
53
Typical values for the resistivity of conductors are
Really small
54
For copper at 25 degrees Celsius the resisititivity is
1.72x10^-6 ohm meters
55
To find the resistivity of a wire you need to find its
Resistance
56
The method of an experiment to find the resistivity of a wire
1.Assume that the wire is cylindrical and Find the cross-sectional area of the test wire using formula for area of a circle
Using micrometer measure the diameter of the test wire in at least three different points along the wire
Take average of the diameter and divided by 2 to get the radius make sure this is in metres
2.clamp test wire to a ruler with the circuit attached to the wire where the ruler reads 0
3. Attach flying lead to test wire
4.Record of the length of the test wire connected in the circuit ,the volt meter reading and The ammeter reading
5.use the readings to calculate the resistance which is v divided by I
6. Repeat this measurement and calculate average resistance for the length
7. Repeat for several different lengths
8. Plot your results on a graph of resistance against length and draw line of best fit
The gradient of the line of best fit is equal to R/L equals resistivity/Area. So multiply gradient of line of best fit by cross-sectional area of the wire to find the resitivitt of the wire material
57
The resistivity of a material depends on its temperature so you can only find a resistivity of a material at a
Certain temperature
58
Current flowing in the test wire can cause its temperature
Increase which can lead to random errors and invalid results in the experiment
Therefore try to keep temperature of test wire constant
59
In an experiment how can you keep the temperature of the test wire constant
By only having small current flows through the wire as current flowing in the test wire can cause his temperature to increase
60
Superconductors have
Zero resistivity
61
Normally all materials have some
resistivity
62
Even really good conductors like silver and… Have some…
That resistance means that when ever electricity flows through them they… And some of the electrical energy is…
Copper
Resistivity
Heat up
Wasted as thermal energy bracket heat bracket
63
You can lower the resistivity of many materials like metals bye
Cooling them down
64
If you call some materials e.g. mercury down to below a transition temperature there resistivity… and they become
Disappears entirely and they become a super conductor
65
Without any resistance none of the electrical energy is turned into… So this means that..
That means you can start a current flowing in a circuit using a magnetic field, take away the magnet and the current would…
Heat
None of It’s wasted
Carry on the flowing forever
66
Most normal conductors e.g. metals have transition temperatures below
Getting things that’s cold is
10kelvin (263 degrees Celsius)
Hard and very expensive
67
Uses of superconductors
Using superconducting wires you could make :
power cables that transmit electricity without any loss of power
Really strong electromagnets that don’t need a constant power source ( for use in medical applications and Maglev trains)
Electronic circuits that works really fast, because there is no resistance to slow them down
68
Power is the
Rate of transfer of energy
69
Power is measured in… Where one what is equivalent to to…
Watts
| One watt is equivalent to 1 J/s
70
Power is equal to
Energy over time
| Voltage times current
71
Potential difference is defined as the… Current is defined as the… Therefore potential difference times current is…
Energy transferred per coulomb
Number of coulombs transferred per second
Energy transferred per second i.e. power
72
Energy is equal to power times bye
Time
73
Batteries have
Resistance
74
Resistance comes from
Electrons colliding with atoms and losing energy to other forms
75
In a battery chemical energy is used to
As they move they
This means that batteries must have
This is called
Make electrons move
Collide with atoms inside the battery
Resistance
Internal resistance
76
The Electromotive force or EMF is the
Amount of electrical energy the battery produces for each coulomb of charge
77
The electromotive force isn’t actually a
It’s measured in
Force
Volts
78
Load resistance is the
Total resistance of all the components in the external circuit
79
You might see load resistance being called
External resistance
80
Chemical reactions in the battery produce
Electrical energy
81
The potential difference across the load resistance is the
This potential difference is called the
Energy transferred when 1C of charge flows through the load resistance
Terminal p.d. (V)
82
If there was no internal resistance the terminal PD would be
However in real power supplies there’s always some
The same as the EMF
Energy lost overcoming the internal resistance
83
The energy wasted per coulomb overcoming the internal resistance is called the
Lost colts
84
Conservation of energy tells us: Energy per coulomb transferred and load resistance+ Energy per coulomb wasted in internal resistance=
Energy per coulomb supplied by the source
85
For cells in series in a circuit you can calculate the total EMF of the cells bye
Because each charge goes through each of the cells and so gains
… From each one
Adding their individual EMf’S
Emf (electrical energy)
86
For identical cells in parallel in a circuit, the total EMF of the combination of cells is the same size as the
This is because
EMF of each of the individual cells
The current will split equally between identical cells
The charge only gains EMF from the cells it travels through – so the overall EMF in the circuit Doesn’t increase
87
How would you investigate internal resistance and EMF with a circuit
Pg91
Vary the current in the circuit by changing the value of the load resistance using the variable resistor
Measure the potential difference for several different values of current
Record your data for V and I and a table and plot the results in a graph of V against I
The gradient of this graph is -r
The intercept on the vertical axis is the EMF(check)
88
And the easier way to measure the EMF of a power source is bye
But a small current flows through the voltmeter so there must be some
This means you measure a value very slightly less than the
Connecting a high resistance voltmeter across its terminals
Lost volts
EMF
89
As charge flows through a circuit it doesn’t get
This means that whatever charge flows Into a junction Will
Used up or lost
Flow out again
90
Since current is the rate of flow of charge, it follows that whatever current flows into a junction is the same as the current
Flowing out of it
91
What is Kirchhoffs office first law
The total current entering a junction equals the total current leaving it
92
Energy is
Conserved
93
Energy transferred TO a charge is
And energy transferred FROM a charge is
EMF
Potential difference
94
In a closed loop what 2 quantities must be equal if energy is conserved (which it is)
Kirchoffs second law:
| The total EMF around a series circuit equals the sum of the P.d’s across each component
95
Series circuits
Same… At all points of the circuit (since there are no junctions)
EMF split between...(by kirchhoffs 2nd law)
The total resistance equals
Current
Components
R1+R2+R3
96
Parallel circuits
Current is… At each junction
Same… Across all components (3 separate loops – within each loop the EMF equals sum of individual P.d’s)
1/Total resistance=
Split
P.d
1/R1 + 1/R2 + 1R3
97
Use a potential divider to get a
Fraction of a source voltage
98
What’s a potential divider
A circuit with a voltage source and a couple of resistors in series
99
The potential difference across the voltage source (eg battery) in a potential divider is split in the
Ratio of the resistances
100
If you had a two ohm resistor and a 3 ohm resistor youd get… Of the PD across the two ohm resistor and… across the 3 ohm resistor
2/5
3/5
101
You can use potential dividers to supply a
Potential difference ,V(out), between zero and the potential difference across
