9. Energy, power, resistance Flashcards
(93 cards)
1
Q
What are the purpose of circuit symbols?
A
- To represent components used in electrical circuits
2
Q
What are the rules for drawing circuit diagrams?
A
- Drawn carefully, with arrows pointing in the correct directions
- Wires should be shown as straight lines in junctions draw at 90 angles to each other
- Do not leave gaps between the wires
3
Q
Define a ‘battery’ in physics
A
- Two or more cells connected end-to-end in serious
4
Q
Why is it important to use circuit symbols?
A
- Widely recognised
- Ensure polarity is represented correctly (especially with components such as diodes)
5
Q
Describe the circuit symbol of a switch
A
6
Q
Describe the circuit symbol of a cell
A
7
Q
Describe the circuit symbol for a voltmeter
A
8
Q
Describe the circuit symbol for an ammeter
A
9
Q
Describe the circuit symbol for a thermistor
A
10
Q
Describe a circuit symbol for a battery
A
- Longer terminal represents positive terminal
- When using a power supply a small plus sign is often placed next to the positive terminal
11
Q
Describe a circuit symbol for a resistor
A
12
Q
Describe a circuit symbol for an LDR
A
13
Q
Describe the circuit symbol for an LED
A
14
Q
Describe the circuit symbol for a capacitor
A
15
Q
Describe a circuit csymbo for a variable resistor
A
16
Q
Describe the circuit symbol for a lamp
A
17
Q
Describe the circuit symbol for a fues
A
18
Q
Define potential difference
A
- Energy transfer from electrical energy to other forms, per unit charge
19
Q
What is potential difference measured in?
A
- Volts
20
Q
Define one volt
A
- One volt is the potential difference across a component when 1 J of energy is transferred per unit charge (C) passing through the component. (1 V = 1 J C^-1)
21
Q
What is a voltmeter used to measure?
A
- A voltmeter is used to measure the potential difference between two points in a circuit.
- Therefore, they are connected in parallel to the circuit.
22
Q
Describe an ideal voltmeter
A
- An ideal voltmeter would have an infinite resistance to ensure that no current passes through the voltmeter itself.
23
Q
What is potential difference used to measure/describe?
A
- When work is done BY charge carriers
- which LOSES energy as they pass through the components in the circuit
24
Q
When is the term potential difference used?
A
- To describe when charge particles lose energy in a component
25
Describe the relationship between potential difference and energy per coulomb
* The greater the p.d. the more energy per coulomb is transferred from energy into other forms as the charges move through the component
26
Define electromotive force
* Energy transferred from chemical energy (or another form) to electrical energy per unit charge)
27
What is electromotive force used to/measure describe?
* When work is done ON the charge carries
* Which GAINS energy as they pass through a cell or a power supply
28
When is the term electromotive force used?
* when charge carrier gain energy
29
Describe the relationship between e.m.f and energy per coulomb
* The greater the e.m.f the more energy per coulomb has been transferred (often from form of chemical energy in a cell) into electrical energy
30
How can charge carriers gain energy? (other sources of e.m.f)
* From cells, batteries, and power sources.
* They can also gain energy from solar cells(light), dynamos(movement), and thermocouples(heat)
31
State symbol equation for potential difference
V = W / Q
32
State the word equation for potential difference
potential difference = energy transferred / charge
33
State the symbol equation for electromotive force
e =W / Q
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State the word equation for electromotive force
e.m.f. = energy transferred / charge
35
What is an electron gun?
* An electrical device used to produce a narrow beam of electrons
* These electrons can be used to ionise particles by adding or removing electrons from atoms and can have precisely determined kinetic energy
36
Where are electron guns used?
* Electron microscopes, mass spectrometers, oscilloscopes
37
Describe how an electron gun works
* All electron guns need a source of electrons
* Small metal filament is heated by a electrical current
* Electrons in this piece of wire gain KE
* Some gain enough KE to escape from the furnace of metal - this process is called thermionic emission (emission of electrons though the action of heat)
* If heated filament is placed in a vacuum and a high p.d. is applied between filament and an anode, the filament acts as a cathode
* Freed electrons accelerate towards the anode, gain KE - if the anode has a small hole in it then electrons in line with this hole can pass through it, creating beam of electrons of a specific KE
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* AS electrons acceralted towards the anode the gain KE
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The work done on a single electron travelling from cathode to anode is ewualt to eV, where e is elementarily charge and V is accelerating PD
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Law of conversation of energy
work done on election = gain in KE
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eV = 1/2mv^2
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Assumes electrons have negligible KE at cathode
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* Changing accelerating p.d. changes the KE of the electrons within the beam
* Therefore, the greater the p.d. the more energy is transferred to the electrons, so the faster they move
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* particle acceleartors pg144
45
Define a resistor
* Electrical components that have a known resistance
* They resist the flow of charge carries through it
46
Describe the relationship between energy and resistance
* It takes energy to push electrons through a component, the higher the resistance the more energy it takes
47
How would you determine resistance of a component?
* By measuring the current in the component and the p.d. across the component
48
Define resistance
* The ratio between V and I
49
State the symbol equation for resistance
R = V/I
50
State the word equation for resistance
resistance of component R = p.d across component / current in component
51
State the units of resistance
* Ohm
52
Define the ohm
* Resistance of a component when a p.d. of IV is produced per ampere of current
* 1 Ohm = 1 VA^-1
53
Describe Ohm's law
* For a metallic conductor kept at a constant temp, the current in the wire is directly proportional to the p.d. across its ends
* i.e. when p.d. across wire doubles, current is also doubled
54
Describe the relationship between current and resistance
* As current changes, temp of wire increases as a result of heating caused by current
* As wire gets hotter, resistance increases
55
Explain why resistance increases as temp increases
* When temp of wire increases, positive ions have more internal energy and vibrate with greater amplitude about their mean positions
* Frequency of collisions between charger carries (free electrons in metal) and positive of ions increases so charge carries do more work (transfer more energy as they travel through wire)
56
Explain why tungsten is used in filaments
* Incredibly dense and robust
* High m.p. (highest of all elements at over 3000 Celsius)
57
State the other uses of tungsten
* Radiation shielding
* Make penetrating tips for high-speed military projectiles
58
What do I-V characteristics show?
* The relationship between electrical current in a component and potential difference across it for any electrical component
59
collecting data for I-v
60
IV- characeritscs for a resistor
* Fixed resistors are designed to ensure their resistance is constant, regardless of changes in temp as current varies
* Potentail difference across resistor is directly proportional to current in resistor
* As a result it obeys Ohm's law and so can be described as an ohmic conductor
* resistance of resistor is constant
* resistor behaves same way regardless of polarity
* low resistance steeper graph
* most wires and other metallic connectors behave in same way as resistor - thought of as resistors with very low resistance
61
I-v of filament lamp
* potentail difference is not directly proprotial to the current though the resistor
* dose not obey oh,'s law is describe as a non- ohmic copmoentn
* resistance of filament lamp is not constant
* filament lamp bejaves in the same way regardless of polarity
at the resistance of filament increases as pd across it increase
* increase in resistance is caused by wire getting so hot it glows
* as current increases so does the rate of flow of charge though filament. - more electrons per second pass through it so more collisions occur between electrons and positive metal ions per second
when electrons collide iwht ions they transfer energy to ions, causing ions to vibrate more, increases temp,
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How are LEDs different from a filament lamp>
* elecTraciy energy is transferred directly into light, so they do not get hot and are more efficient and draw much less power
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* diode only allows current in one particle directions
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* bc LEds are so efficient they are sometimes used as simple indicator to show the direction of current through a particular part of the circuit - light up when there is current showing that part of the circlet icl I've
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iv charactersicti for a diode
potentail difference across a diode is not directly proportional to current
* diode does not obey ohm's law and decstibre as a non - ohmic component
8 resistance of diode is not constant
* diodes behaviour depends on polarity
* in negative quadrant of graph, resistance of diode is high (infinite), as p.d. is in reverse direction, diode does not conduct
In positive quadrant, as p.d. increases the resistance Strats to drop (0.7V threshold p.d.) Above this value ressotamce drops sharply, above this diode has little ressistnac e
* different LEDS have different values for their threshold p.d related to colour they emit
66
Define the term thermistor
* Temperature- sensing components
67
* Some semiconductor components have a negative temp coefficient, meaning that their resistance drops as the temp increases
* Effect can be explained in terms of number density of the charge carries within the material from which the component is made
* in some semiconductors as the temp increases the number dniety of the charge carries also increases
68
What is a thermistor
* An electrical component made from a semi conductor with negative temp coefficient, as the temp increases, resistance drops
* makes them useful in temperature sensing circuits
69
When are thermistors used
* IN simple thermometers
* In thermostats to control heating and air conditioning units
* to monitor the temp of components inside electrical devices so they can power down before overheating damages htem
* to measure temp in electrical devices
* to monitor engine temps to ensure engine does not overheat
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thermistors experiment
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IV characteristics of a thermistor
* thermistors are non ohmic
* iv characteristic features similar to filament
* the current increases, temp increases, but unlike the lamp, temp increase Leeds to a drop in resistance because number density of charge carries increases
* confmired by comparing r = v / I at various points on the graph
* increase in temp leads to increase in no density of free electrons mean the resistance of thermistor decreases as temp increases
72
Light dependant resistors (LDRs) are small, non-ohmic components made from semiconductors.
* change their resistance depending on light intensity
* typical ldd is made from a semiconductor in which the no density of charge carries changes depending on the intensity of the incident light
* in dark conditions the ldd has high resistance, no of free electrons inside semiconductor is very low so the resistance is very high
* when light shines onto an ldd the number denisy of charge carries increases dramatically leading to a rapid decrease in resistance of this comopentn
When the light intensity incident on the resistor is increased, the resistance of the LDR decreases
73
investigating LDRS
*
74
three factors that affect resistance (apart form temp)
* material of wiere
* length of wire
* cross sectional area
75
he resistivity, ρ, of a material is a physical property of the material. used to describe the electrical property of a material
e.g. different components made from copper may have different resistances (lengths or cross sectional area differ, but have same unique resistive
It is the same for any
the material with its area and length. Resistivity is given using the formula 𝜌𝜌 = 𝐿𝐿 , where R is
the resistance of the object, A is the cross-sectional area of the object, and L is the length of the object. Resistivity is measured in Ohm-meters (Ωm), and is small for metals and much larger for insulating materials such as glass.
76
resisntace and length
for any given current the increases length of the wire will increase p.d. across it
doubling the length doubles the p.d. so the resistance must have double
resistance of wire is directly proporital to its length
77
resistance and cross sectional area
when cross sectional area increases, resistance drops
for any given p.d double the cross sectional area with halve current so half the resisntace
the resistance is inversely proprtional to its cross sectional area
78
R = pL/A
* equation used for a known constant temp
79
* resistive of a material at a given temp is the product of the resistance of a component made of the material and its cross sectional area divided by its length
* the resistance of a material varies with temp in the same way as resistance of most components varies with tem, as material gets hotter, resitivy increases
80
The resistivity of a material varies with temperature. For metals, when the temperature is increased, the fixed metal ions will vibrate at a greater frequency and amplitude. This increases the number of collisions of electrons with the ions, increasing the resistance. For semiconductors, the number density of charge carriers increases with increasing temperature, so the resistance of the material decreases. As the resistivity is a constant linking the shape of the material with its resistance, if the resistance of the material increases or decreases with changing temperature, the value for resistivity of the material will be affected in the same way.
81
echniques to determine resistivity
The resistivity of a wire at a specified temperature can be determined experimentally. First, the cross sectional area of the wire is recorded, by taking multiple readings with Vernier callipers at different points along the wire, and taking an average. Then, a circuit is set up using a recorded length of the wire, with a voltmeter connected in parallel and an ammeter in series. The values for p.d. and current can be recorded to determine the resistance of the wire, and used along with the length and cross sectional area to determine the resistivity of the material the wire is made from.
* determined by multiplying the gradient of graph by cross sectional area of wire
82
different materials have widely different values for resistivoty
good conductors like metals have resistivity order of 10^-8
insulators have a value of the order 10^16
83
the rate of energy transfer of each electrical component is called electrical power
depends on current in component, measure in amperes and p.d.
P = VI
84
When there is a current through a component, electrical energy is transferred to other forms, such as light in a filament bulb. Electrical power is defined as the rate of energy transfer, and is measured in watts (W) or Js-1. Using the definition of power as work done per unit time,
85
p = w /t
p = vq/t
p = vi
p = I^2R p = V^2/R
W = vIt
86
electric meter
accurately records the transfer of energy from national grid to the house
87
cost to run depends on two factors
* power of the device, how long the device is used for
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kilowatt hour
Si unit for energy is joule - however one joule is tiny compared to scale of energy transferred to homes
* electrically bills use a derived unit (kilowatt-hour kWh)
* defined as the energy transferred by a device with the power of 1kW operating time of 1 hour
* equivalent to 3.6MJ
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SI units: energy transferred (J) = power of device (W) x time for which device is used (s)
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kWh: energy transferred (kWh) = power of device (kW) x time for which the device is used (h)
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typical cost are around 6-15p per kWh
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