3.1 - Electrical Quantities Flashcards
Tell me about the electric charge
Some particles have an electric charge. For example, the electron has a negative charge.
In SI units, electric charge is measured in coulombs
What’s the amount of charge on s single electron
-1.6 x 10^19 C
How many electrons is is 1 coulomb
- 25 x 10^18
6. 25x10^18 x 1.6 x 10^-19 = 1 C
How must total charge be conserved
The chargers on fundamental particles such as electrons are fixed properties of these particles, it is impossible to create or destroy charge - the total charge must always be conserved
What’s current
If electric charge moves, it’s referred to as an electric current
The strict definition of current is the rate of movement of charge
Or rate of flow of charge
We can say it flows as it’s a physical movement of billions of tiny charged particles
When does electric current occur
When a charged particle, which is free to move, experienced an electric force, if it can move it will be accelerated by the force. This movement of charge forms the electric current
Any source of electrical energy can create an electric force in order to produce a current
In a circuit, a cell causes the electric force, experienced by the negative conduction electrons so they move through the metal - they’re attracted to the positive snore of the cell
What are most electric currents made from
Most electric circuits are made from metal wiring in which there are electrons that are free to move
There conduction electrons then form the current.
Which way is conventional current
From positive to negative (from left to right) but in reality it goes from negative to positive as electrons are negative
What’s the SI unit for electric current
Ampere, A
How can current be calculated
Current = charge passing a point/ time for that charge to pass
I = Q/t
What is 1 ampere equal to
One ampere is the movement of one coulomb of charge per second
How do we calculate charge
Q = I x t
How can we observe charge flow
We can monitor small movements of charge, to see how they form a current, using a hanging ball that will conduct electricity
A suspended ball can carry small numbers of electrons across a high voltage gap, and tha current is measured using a spot galvanometer
The high voltage set up across the air gap between the metal plates encourages negative electrons to went to move towards the positive side
The hanging ball is painted with conducting paint and swings backwards and forwards across the gap, ferrying a small quantity of electrons from one metal plate to the other each time
We can measure the small movement of charge on a very sensitive ammeter, if we time the period of oscillation of the shuttling ball and the tiny current, we can calculate how many electrons pass across each journey - if the ball is moving too fast to be timed by the eye, we can use a STROBOSCOPE to measure the frequency of oscillations
What are ionic charge characters
If the circuit is more unusual, there may be other charged particles, charge carriers, which can move to from an electric current
Eg free aluminium ions (charge carriers) that can move through the liquid as an electric current - still observe the conservation of charge.
What’s the charge on a proton
It’s the same magnitude as an electron but positive
1.6 x 10^-19
Question - what would be the charg on an iron ion (III), Fe^3+
Three electrons have been lost, so the net charge is that of the ions three excess protons
3 x 1.6 x 10^19 C
Define charge
Charge is a fundamental property of some particles. It is the cause of the electromagnetic force, and it is a basic aspect of describing electrical effects
What is one coulomb
One coulomb is the quantity of charge that passes a point in a conductor per second when one ampere of current is flowing in the conductor, the amount of charge on a single electron is -1.6x10^-19
Define the electric current
It’s the rate of flow of charge
What is 1 ampere
It’s the movement of one coulomb of charge per second
How can an electrical circuit be useful
It acts as a means to transfer energy usefully, the circuit must have at least one component that can supply electrical energy, it will also have components that convert this electrical energy into other forms, and st least one of these forms of energy will be useful in its purpose
Defin voltage
It’s a measure of the amount a component transfers per unit of charge passing through it
How can voltage be calculated
Voltage = energy transferred / charge passing
V = E/Q
What’s an electromotive force
For a supply voltage, a component which is putting electrical energy into a circuit - the correct term for the voltage is electromotive force or emf
If a cell supplies 1 J per coulomb, it has an emf of 1 volt
What’s potential difference
For a component which is using electrical energy in a circuit and transferring this energy into other forms, the correct term for the voltage is potential difference or pd
How is potential difference calculated
Pd = energy transferred (referred to as work done this time rather than energy when calculating emf or voltage) / charge passing
V= W/Q
What’s the electronvolt
The electronvolt, eV is a unit of energy that is generally used with subatomic particle
Definition comes from the equation defining voltage V = E / W
If an electron is accelerated by a potential difference of 1V, the energy it will gain is
E = V x e
1 x 1.6 x 10^-19 j = 1 eV
How can we measure voltage across a component
With a voltmeter connected in parallel
Tell me about using electrical models
Electricity has many aspects thag are not visible to us in everyday life, and physicists often use models to explain some of these
All models will have limitations, so it’s important to be able to evaluate the strength and weaknesses of any model, in order to ensure that you do not rely too heavily on it
What’s one way to model voltage
This model could be used to try and understand the transfers of energy in an electric circuit
- thus model is to think of an electric circuit as a ski area
Tell me how modelling voltage on a ski area works
The skiers on the lift gain gpe, representing electrical energy - by the time a skier have gone down all the runs and obstacles they have lost all gpe again back to level at start
Illustrates the principle of conservation of energy in an electric circuit, if moving charge is given energy by a source of emf, it will return all that energy in its journey around the circuit, through the various pds. Around the circuit, the total of all the emfs will be the same as the total of all the pds - total energy supplied will be equal to total energy used
An obstacle could be s component and a cell is the ski lift
Skiers and snowboarders may be using the ski area but in reality electric circuits only have one type of charge carrier that flows - the electrons, this is a weakness
Define voltage
It’s a measure of the amount of energy a component transfers per unit of charge passing through it
Calculated V = E/Q
What’s the electromotive force or emf
It’s the supply voltage
What’s potential difference
For a component that is using electrical energy In a circuit and transferring this energy into other forms, the correct term for the voltage is potential difference or pd
What’s the electronvolt
It’s the amount of energy an electron gains by passing through a voltage of 1V
There can be a mega electronvolt 1 MeV (same as 1eV but x 10^-6
What could the emf do to current
The movement of electrons through a circuit is caused by the electric force that an emf generates - an emf could be said to drive the current around a circuit - more emf, bigger driving force, bigger current
Electric force is directly proportional to the current, in many cases this is true but in some sitriorions other factors become important and alter the relationship
What can resistance be considered as
Considered to be the opposition to the flow of current within a conductor
How can we calculate resistance at a given moment
Resistance = potential difference / current
R = V / I
What’s resistance measred in
Ohms, weird symbol
What’s an ohmic conductor
A component through which the current is proportional to the voltage driving it is referred to as an ohmic conductor - it follows ohms law
What’s the resistance for an ohmic conductor
The answer to calculation of resistance would be the same for all voltages as current would change accordingly (providing temperature is constant)
How can we investigate I - V relationship, practical
We can do an experiment to investigate wether or not a component follows ohms law - use various values of supply emf and measure the potential difference across and current through the component - should include reversing the terminals on the power supply in order to measure the effects of negative pds across the component - causing a negative current
Results would produce a graph that’s a straight line with pd agaisndt current, proportional to eachother for all values so the resistor is an ohmic conductor
The gradient is I / v so resistance is 1 / gradient which should be the same for each value
Using the gradient is only effective for an ohmic conductor - if line isn’t straight - resistance can be calculated using ohms law as each specific v / I value
What’s part of the specification of any component
A graph of its I-V characteristics
How does the I-V graph change with resistors
Every resistor should produce the same straight line result, the only difference would be the gradient - as it corresponds to a specific resistance of the resistor or wire under consideration
What’s the I-V graph for a filament bulb
At small voltages - current is proportional- there’s a straight line portion of the graph through the origin
At higher voltages, a larger current is driven through the lamp wire, this heats it up - at hotter temperatures, metals have higher resistance- causing the gradient to reduce at higher voltages
It’s an example of a non-ohmic conductor, the current through it affects its own temperature, higher current means a higher temperature, and controlling temperature is part of the definition of ohms law
What’s the I-V graph like for a diode
Diode only conducts in a forward direction, so there is 0 current for negative voltages - also requires a minimum driving voltage in the forwards direction, threshold voltage is typically around 0.6 V
What’s the I-V graph for a thermistor like
It’s designed to alter its resistance with temperature, in the reverse manner to a filament bulb
Resistance reduces with temperature, gradient of line increases with heating effect of the increasing current, - gradient represents the reciprocal of the resistance, larger gradient value means lower resistance
This is as a result of its manufacture from semi conductor materials , whose atoms release more conduction electrons as the temp rises (you will learn about this in more detail later ❤️❤️)
Define resistance
Resistance is the opposition to the flow of electrical current, it can be calculated via r = v/ I
What’s ohms law
Ohms law states that the current through a component is directly proportional to the voltage across it, providing the temperature remains the same
Often expressed with the equation
V = IR
What’s part of the specification of any component
It’s a graph of its current-voltage characteristics
What is resistance a result of
Hint: think of collisions
Resistance is the result of collisions between charge carriers and atoms in the currents path.
What does resistance depend upon
The effect will vary depending on the density of charge carriers and the density of fixed atoms, as well as the strength of forces between them
Thus, pieces of different materials with identical dimensions will have differing resistances
What’s resistivity
The general property of a material to resist the flow of electric current is called resistivity, which has the same symbol as density (funny p / d ) and SI units are ohm metres
Resistivity is a property of a material. All samples of the same material, regardless of their shape and size, will have the same resistivity, whilst their resistances may vary
How do we calculate the resistivity of a material
Resistance = (resistivity x sample length) / cross sectional area
R = pl/ A
Values vary grestly between materials, and is also dependant on temperature
How can you investigate resistivity
You can investigate the resistivity for a metal in the school laboratory using a simple circuit
We will need to use a micro meter screw gauge to measure the wires diameter. For improved accuracy,this is done in right angled pairs at several places along the length of the wire, and then take the mean diameter measurement.
For several different lengths of the wire, wires resistance should be measured using the voltmeter - ammeter method ( R = V/I) the resistance will be small, so care must be taken to ensure currents are safely low
The equation involving resistivity means that we could calculate a value for it by rearranging the equation and taking one of the results and making the calculation. However Always more reliable to produce a straight line graph of experimental Results and calculate answer from gradient, plot resistance on y axis against length of x axis will give the grwident resistivity / area and then multiply by area to calculate resistivity with more confidence in our conclusion
Define resistivity
The resistivity of a material is defined as the same value as the resistance between opposite faces of a cubic metre of the material
In order to conduct electricity, what do solids need
Solids need to have electrons that are delocalised from the solids atoms, so they can move through the solid causing an overall movement of charge - a current
How do we get these free electrons
The structure of metals has a regular lattice of metal atoms, these are bonded together through the sharing of electrons, acting as if they were associated with more
Tell me about the overall position of charge in metal
These free electrons are created and have a random motion, which changes as they collide with atoms or other electrons - but on average the overall position of all the charge in the metal is stationary
How does drift velocity occur
If a source of emf is connected across the metal, the electric field it sets up in the metal will have a tendency to push the negative electrons towards the positive end of the field. The slow overall movement of electrons is called their drift velocity
When does conduction in metals happen
It happens as the free electrons add an overall movement along the direction of the voltage across the conductor (towards the positive anode) to their random collisions and vibration
Fun
FACT
FACT FACT FACT FACT
Did you know for a metal, the random thermal motion of the free electrons will be at speeds of o
Thousands of kilometres per second, whereas drift velocity during conduction is usually only one millimetres per second
Define current with an equation
I = triangle Q / triangle T
What’s the drift velocity equation / TRANSPORT EQUATION
I = vAnq
Where v is the drift velocity
I is current
n is charge carrier density, electrons per cubic metre of this metal
A is cross sectional area of the wire
Q is the charge on each charged particle eg an electrons charge
What equation is drift velocity / transport equation derived from
I = Q/T
How can we investigate conduction velocity of coloured ions practical
U can observe the movement of coloured ions as charge carrier particles on a piece of filter paper soaked in ammonium hydroxide solution . A crystal of copper sulfate and one of potassium manganate will each dissolve, producing positive blue copper ions and negative purple manganate ions
Connecting a pd across wet filter paper will cause ions to slowly move across in opposite directions - velocity can be measured using a ruler and stopclock - expect about 1 mm per minute
How does temperature affect resistance
The frequency of lattice ions and flowing electrons is increased by higher temperatures, the more atoms vibrate - more collisions so this slows the drift velocity of equation
What does a higher current do to resistance
A higher current will cause more collisions, as more electrons move faster through the metal structure , making the atoms vibrate more - effectively increasing the temperature
Why does the IV graph for a filament lamp begin to flatten out
We can not explain why increased current
Leads to higher temps, leading to greater resistance, causing curve to flatten out when higher voltages try to drive higher currents
When does resistivity of semiconductors fall
Resistivity of semiconductors was seen to fall as the temperature rises
The negative temperature coefficient of resistivity for semiconductors snow that their resistivity falls as temperature goes up
Reason for this is that n in transport equation will be higher at higher temps. So the fact that the resistivity goes down is actually just a consequence of the fact that these materials like silicon conduct better at higher temps
What happens when there’s a slight decrease in n for METALS
A slight decrease in n for metals at higher temperatures is due to their thermal expansion, rather than any change in the number of available conduction electrons, it is not nearly as significant as the increase in collisions between metal atoms and conduction electrons caused by increased thermal vibrations
What’s drift velocity
The slow overall movement of the charges in a current is called their drift velocity
What’s the transport equation
The transport equation is I = nAvq
This defines electric current, I, from a fundamental basis. It is the product of charge carriers, n; the charge on those carriers q; cross sectional area of the conductor, A; and the drift velocity of the charge carriers in that conductor v
Tell me about conduction is semi conductors
Semiconductors are generally solid materials that only have a small number of delocalised electrons that are free to conduct - a typical example is silicon, one of the most abundant elements on earth
Free atoms have a series of discrete energy levels in which we can find their electrons , depending on the energy the electrons have recieved
If the electron receives enough energy it will leave the atom altogether, leaving behind an ion
What happens in solid materials with free electrons being liberated
Broski, in solid material materials, where there are many, many atoms close together, the allowed energy levels for electrons become much wider forming energy bands. The electrons can have a large range of energies and still be within the same band - opposed to free atoms which have given set values for each level
There are energy amounts which are forbidden for the electrons, but it’s a very differnt situation from the highly limited energy levels of an isolated atoms electrons. - as these energy levels are created by the collective grouping of the solids atoms, the bands are attributed to the semiconductor as a whole rather than to individual atoms
What’s the VALENCE band
There is an energy called the valence band, electrons with this energy remain tied to the atoms and do not form any part of the electric current
How does an electron go to the conduction band/ what is it
Electrons that gain energy from the valence band, jump up to the conduction band
Where editions become delocalised and can move through the semiconductor as part of the current
What does the gaps between energy bands between energy levels show
They explain why they are conductors or insulators
If there is a large energy gap, electrons will need to gain a lot of energy to leave their atoms and conduct a current
Insulator has larger gap between empty conduction band and full valence band
As opposed to almost full conduction band and valence band in metals
And an almost empty conduction band and almost full valence band I’m semi conductors
How does number of delocalised electrons compare with semiconductors and metals and how does it relate to current
The number of delocalised electrons in a semiconductor is low compared with metals, and so the current they carry will therefore be lower than metals for the same applied voltage
Why does current increase as temperature increases in semiconductor
At higher temperatures semiconductors they will have more conduction electrons, as more electrons are elevated into the conduction band
There will be a temperature related reduction in current due to increased collisions with fixed atoms, but the increase in available conduction electrons far outweighs this
The overall effect is that a semiconductor will carry more current as the temperature goes up - it’s resistivity effectively drops as the temperature rises
How does /(do?) insulators conduct electricity
Electrical insulators can be thought of as materials in which the energy gap between the valence band and conduction band is so large that there are virtually zero electrons available for conduction - there will therefore be no conduction holes either. A very large input of energy is required in order to make the material conduct. Often this results in melting, or other damage, before the Material becomes electrically conducting.
Eg glass starts conducting when more than 10 million volts per meter are applied
Why does resistance increase with temp
Resistance increases with higher temperatures, because of the higher level of internal energy in the material causes more vibration of the fixed ions and these collide more with charge carriers to reduce their speed of movement through the material, reducing the temperature therefore reduces resistance, allowing greater current to flow
What happens when you cool a circuit to reduce resistance - tell me about super-conduction
Cooling to ever low temps continues trend where resistance will also reduce, but below a certain CRITICAL temperature, the resistance suddenly drops to 0 !!! This is called superconductivity. The critical temp varies with material, but for most metals it will be below -243 degree C
Complex ceramic superconductors have been created that have temperatures at which they conduct without any resistance as high as -135 degrees C
When are superconductors useful
Superconductors are especially useful in applications where a large current is needed, as a large current would normally waste too much energy or damage the surroundings with the heat dissipated. Eg strong magnets needed in a particle accelerator will often be superconducting electromagnets, cooled to very low temps to maintain superconductivity
What’s 0 Kelvin equal to
0 K is equal to absolute zero,
Lowest temperature possible as this represents a temperature where the particles have 0 internal energy
What’s 0K in degrees C
0K = -273.15 degrees c
Usually ignore 0.15 lol
What’s 0 degrees C in kelvins
0 degrees C = 273.15 K
Usually ignore 0.15 lol
Define a semiconductor
Semiconductors have a lower resistivity than insulators, but higher than conductors - usually only have small numbers of delocalised electrons that are free to conduct
Define the valence band
The valence band is a range of energy amounts that electrons in a solid material can have which keeps them close to one particular atom
Define the conduction band
The conduction band is a range of energies in a solid material can have which delocalised them to move more freely through the solid
Define the critical temperature
The critical temperature for a material is that below which it’s resistivity instantly drops to zero
What are conduction holes
When an electron entrees the conduction band and moves away, this leaves the atom with an effective positive charge
The empty space the electron has left is referred to as a (positive) hole. If an electron from another atom moves to fill the hole, leaving its original atom with a hole, the hole has effectively moved. As the electrons will be attracted to jump in the positive direction of an applied voltage, hole will slowly appear to move in the opposite direction. A positive hole moving towards the negative cathode is effectively another charge carrier, contributing to the current flow in a semiconductor
It is analogous to the current flow in electrolysis, where positively and negatively charged particles are moving in opposite directions at the same time
Tell me a tip about explaining conduction by holes
Note that conduction by holes does not mean there is overall movement of the positive lattice ions
the holes are an absence of an electron, and it is this space that seemingly moves as electrons actually jump in the opposite direction
LOL BANTS
How is a semiconductor diode made / explain the I-V characteristics of it
A diode is made by joining together different types of semiconductors, which normally creates an energy barrier at the junction between them, blocking the movement of charge carriers (holes and electrons) across the barrier
This barrier can be overcome in the forward direction if a small forward voltage is applied
In the reverse direction only a few charge carriers can pass through at low voltages, they account for a tiny “leakage current” , once the reverse voltage becomes large enough, it can overcome the large reverse energy barrier force for the conduction process in the opposite direction
Search up a pic of the IV graph of a semiconductor diode
Tell me about light dependant resistors (LDRs)
Light dependant resistors have the property that their resistance depends on the light level around them
In brighter conditions, the LDR will have a lower resistance. LDRs are made from semiconductor material, and light landing on the material can boost electrons from the valence energy band to the conduction band, increasing the number of conduction electrons, the effect of this it to make the LDR conduct better - it has a lower resistance
How do thermistors work
Thermistors work the same as an LDR but their resistance instead depends upon thermal energy from the surroundings
The most common type of thermistors are referred to as negative temperature coefficient thermistors - they use thermal energy to boost their electrons into the conduction energy band, meaning their resistance falls as the temp rises
