3-Sensing Flashcards
What are ions?
Electrically charged atoms or molecules in which the number of electrons is different to the number of protons.
List 2 materials where ions make up the structure:
- Metals
- Ionic compounds
What produces an electric current?
A flow of charged particles
What is electric charge measured in?
Coulombs (C)
What is the equation of charge?
Current X Time
What is the total charge flowing across a section?
Charge X Amount of electrons
What is current measured in?
Amperes (A)
What is Kirchhoff’s first law?
Current at a junction must add up
When current divides at a junction, what must happen?
It must add up.
What is 1 Amp equivelant to?
1 coulomb per second
What does the total charge into a junction equal?
The total charge out of the junction.
What is the potential difference between 2 points?
The potential energy difference for every unit of charge
What is the equation of work done?
Charge X Voltage
What is dissipation
When energy is ‘wasted’
When calculating the amount of energy dissipated in a given amount of time, what 4 equations do you have to use?
Charge = Current X Time Work done (energy dissipated) = Charge X P.D or Power = Current X P.D Power = WD /time
What is power?
The amount of energy transferred in a given amount of time
What is the equation of power?
Energy transferred / Time
What is the 2nd equation of power?
Power = Current X Voltage
What is the 3rd equation of power?
Power =Current^2 X Resistance
What is the equation of conductance?
Current / Voltage
What is Conductance measured in?
Siemens
What is the equation of Resistance?
Voltage / Current
What is resistance measured in?
Ohms
What is 1/Resistance equal to?
Conductance
What is 1/Conductance equal to?
Resistance
How do you calculate the total conductance of a parallel circuit?
Adding the conductances of the separate components (G1, G2, etc).
What equation do you need to use to find out the total conductance?
1/R = 1/R1 +1/R2 …
How do you find the total resistance in a series circuit?
Adding the resistance of each component
In parallel, how do you calculate the total conductance ?
Add up all the individual conductances from each component
When does Ohm’s law apply to conductors?
If the temperature remains constant
As the current in a resistance increases, what happens to the temperature?
It increases as the current dissipates energy into the resistance, raising the resistance.
If the potential difference in a conductor increases, what will happen to the current?
It will increase aswell, raising the temperature further, making the resistance rise and the conductance fall
If the component is ohmic, what will the I-V line be?
A straight line
If the component is non-ohmic, what will happen to the resistance and conductance as the P.D changes?
They will change
What are the 3 factors that the resistance of a conductor depends on?
The length, the area and the type of material used.
What happens to the resistance of a conductor as the Length increases?
It increases
What is the relationship between the area and the resistance of a conductor?
As the area increases the resistance decreases
What is the equation that links Length, Area and resistivity together?
Resistance = (Length X Resistivity) / Area
What is the variable used for resistivity?
Rho
What is the variable used for conductivity?
Sigma
What does the electrical resistivity depend on?
The material of the conductor
What is the S.I. unit of resistivity?
Ohm m
How is the electrical conductivity related to the resistivity?
Conductivity is the reciprocal of the resistivity
What are the S.I. units of conductivity?
(Ohm metres)-1 or Sm-1
How is conductance related to resistance?
Conductance is the reciprocal of the resistance
Why are metals good electrical conductors?
Large amount of free electrons
What do insulators lack?
Mobile charges
What do semiconductors have conductivities between?
Conductors and insulators, due to having few mobile charges
Is the number density of free electrons greater in semiconductors or metals?
In metals
When the temperature of a metal is raised, what happens to the conductivity?
It decreases
Why does the conductivity of a metal decrease when the temperature is raised?
The vibration of positive ions impedes the movement of free electrons
When the temperature of a semiconductor is raised, what happens to the conductivity?
It increases
When the temperature of a semiconductor is raised, why does the conductivity increase?
More and more atoms recieve enough energy for electrons to be released
What 2 things do you need to calculate to find the resistivity or conductivity of a specimen?
Either the resistance or the conductance
How would you calculate the resistivity or conductivity of a specimen?
- Measure the length and thickness of the specimen
- Measure the P.D across different lengths of a long wire with a constant current.
- The graph of V against L would be a straight line
- The gradient would give the resistivity of the specimen and hence the conductivty
In series, what happens to the current?
It is the same everywhere
In series, what happens to the potential difference?
The P.D of each component adds up
V=V1 +V2
In series, what is V1 / V2 equal to?
R1 / R2
What happens to resistance in series?
It adds up to create a total reistance
What is a fixed resistor?
A resistor who’s resistance does not change with temperature or voltage.
If you have a sensing circuit with a fixed resistor and a thermistor, what will happen as the temperature increases?
-The P.D across the thermistor will decrease, so the P.D will increase across the fixed resistor.
What does the P.D across each component in series add to give?
The input voltage
Name 2 components who’s resistance changes with the environment:
- Light dependent resistor
- Thermistor
Why does the resistance drop as you increase the temperature of a thermistor?
The internal energy of the thermistor increases, which frees more mobile charges. so the resistance drops.
What happens to the resistance of a LDR when the light intensity increases?
The resistance decreases
What does Kirchhoff’s first law describe?
What happened to current at a junction
What is Kirchhoff’s second law?
As you move around any complete circuit loop, the sum of all e.m.f.s will be equal to the sum of all p.d. drops
What does E.M.F. stand for?
Electro-motive force
What symbol is E.M.F. given?
E
What is the E.M.F. in simple terms?
The push the battery gives to charges
What is the E.M.F?
The energy given to each coulomb of charge by the source
What causes internal resistance?
In any source of E.M.F, some energy is dissipated within the source itself. Resistance inside the source
What is the P.D. across the internal resistance sometimes called?
Lost volts
What does the e.m.f. equal?
The loss volts across the internal resistance + p.d. across R
In terms of e.m.f. what does R equal?
Ir +IR
What is r?
The internal resistance
In terms of e.m.f. what does V equal?
V=-rl + E
What can V=-rl + E form?
Y=mx + c
In the equation y = mx +c, what is the y intercept?
The e.m.f.
In the equation y = mx + c, what is the gradient?
-r
Why does the potential difference across a battery fall when current is drawn? The larger the current drawn, the smaller the measured p.d.
𝝴 = Ir + V. The larger the current, the greater the value of Ir, which is a measure of the “lost volts”, the potential difference across the internal resistance.If Ir is larger, V, the p.d. across the battery must get less as the EMF 𝝴 is constant.
Why does the resistance increase when the current increases?
With a larger current, there are more frequent and more energetic collisions between the free electrons that make up the current and the vibrating ions of the metal lattice. This leads to larger amplitude vibrations of these ions, corresponding to a higher temperature. Free electrons are more likely to hit vigorously vibrating ions, limiting the rate at which the electrons move through the metal, corresponding to a higher resistance.
When the temperature is increased, the resistance of a semiconductor falls. Why does this happen?
Higher temperature leads to a larger number of atoms having enough energy for one of their outer electrons to break free. Greater density of free electrons corresponds to lower resistance.
Why do we want an ammeter to have as low internal resistance as possible?
Low resistance means that the overall resistance of the circuit is not affected much by the insertion of the ammeter. Potential difference across the ammeter is low.
Why do we want a voltmeter to have as high internal resistance as possible?
High resistance means that little current is diverted through the voltmeter / equivalent resistance of voltmeter in parallel with the component it is connected across is approximately that of the component.
What does e.m.f mean?
The total voltage available
What does the ‘total voltage available’ equal?
The ‘voltage in circuit’ + the ‘voltage in battery’
What is the equation of e.m.f.?
e.m.f. = IR + Ir
What happens when you reverse a cell?
It reduces the p.d., which reduces the current flow, so more uncertainty
