3.1 - Electrical Quantities

Question 1

Tell me about the electric charge

Answer 1

Some particles have an electric charge. For example, the electron has a negative charge.

In SI units, electric charge is measured in coulombs

Question 2

What’s the amount of charge on s single electron

Answer 2

-1.6 x 10^19 C

Question 3

How many electrons is is 1 coulomb

Answer 3

6. 25 x 10^18

6. 25x10^18 x 1.6 x 10^-19 = 1 C

Question 4

How must total charge be conserved

Answer 4

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

Question 5

What’s current

Answer 5

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

Question 6

When does electric current occur

Answer 6

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

Question 7

What are most electric currents made from

Answer 7

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.

Question 8

Which way is conventional current

Answer 8

From positive to negative (from left to right) but in reality it goes from negative to positive as electrons are negative

Question 9

What’s the SI unit for electric current

Answer 9

Ampere, A

Question 10

How can current be calculated

Answer 10

Current = charge passing a point/ time for that charge to pass

I = Q/t

Question 11

What is 1 ampere equal to

Answer 11

One ampere is the movement of one coulomb of charge per second

Question 12

How do we calculate charge

Answer 12

Q = I x t

Question 13

How can we observe charge flow

Answer 13

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

Question 14

What are ionic charge characters

Answer 14

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.

Question 15

What’s the charge on a proton

Answer 15

It’s the same magnitude as an electron but positive

1.6 x 10^-19

Question 16

Question - what would be the charg on an iron ion (III), Fe^3+

Answer 16

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

Question 17

Define charge

Answer 17

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

Question 18

What is one coulomb

Answer 18

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

Question 19

Define the electric current

Answer 19

It’s the rate of flow of charge

Question 20

What is 1 ampere

Answer 20

It’s the movement of one coulomb of charge per second

Question 21

How can an electrical circuit be useful

Answer 21

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

Question 22

Defin voltage

Answer 22

It’s a measure of the amount a component transfers per unit of charge passing through it

Question 23

How can voltage be calculated

Answer 23

Voltage = energy transferred / charge passing

V = E/Q

Question 24

What’s an electromotive force

Answer 24

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

Question 25

What’s potential difference

Answer 25

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

Question 26

How is potential difference calculated

Answer 26

Pd = energy transferred (referred to as work done this time rather than energy when calculating emf or voltage) / charge passing

V= W/Q

Question 27

What’s the electronvolt

Answer 27

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

Question 28

How can we measure voltage across a component

Answer 28

With a voltmeter connected in parallel

Question 29

Tell me about using electrical models

Answer 29

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

Question 30

What’s one way to model voltage

Answer 30

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

Question 31

Tell me how modelling voltage on a ski area works

Answer 31

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

Question 32

Define voltage

Answer 32

It’s a measure of the amount of energy a component transfers per unit of charge passing through it

Calculated V = E/Q

Question 33

What’s the electromotive force or emf

Answer 33

It’s the supply voltage

Question 34

What’s potential difference

Answer 34

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

Question 35

What’s the electronvolt

Answer 35

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

Question 36

What could the emf do to current

Answer 36

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

Question 37

What can resistance be considered as

Answer 37

Considered to be the opposition to the flow of current within a conductor

Question 38

How can we calculate resistance at a given moment

Answer 38

Resistance = potential difference / current

R = V / I

Question 39

What’s resistance measred in

Answer 39

Ohms, weird symbol

Question 40

What’s an ohmic conductor

Answer 40

A component through which the current is proportional to the voltage driving it is referred to as an ohmic conductor - it follows ohms law

Question 41

What’s the resistance for an ohmic conductor

Answer 41

The answer to calculation of resistance would be the same for all voltages as current would change accordingly (providing temperature is constant)

Question 42

How can we investigate I - V relationship, practical

Answer 42

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

Question 43

What’s part of the specification of any component

Answer 43

A graph of its I-V characteristics

Question 44

How does the I-V graph change with resistors

Answer 44

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

Question 45

What’s the I-V graph for a filament bulb

Answer 45

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

Question 46

What’s the I-V graph like for a diode

Answer 46

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

Question 47

What’s the I-V graph for a thermistor like

Answer 47

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 ❤️❤️)

Question 48

Define resistance

Answer 48

Resistance is the opposition to the flow of electrical current, it can be calculated via r = v/ I

Question 49

What’s ohms law

Answer 49

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

Question 50

What’s part of the specification of any component

Answer 50

It’s a graph of its current-voltage characteristics

Question 51

What is resistance a result of

Hint: think of collisions

Answer 51

Resistance is the result of collisions between charge carriers and atoms in the currents path.

Question 52

What does resistance depend upon

Answer 52

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

Question 53

What’s resistivity

Answer 53

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

Question 54

How do we calculate the resistivity of a material

Answer 54

Resistance = (resistivity x sample length) / cross sectional area

R = pl/ A

Values vary grestly between materials, and is also dependant on temperature

Question 55

How can you investigate resistivity

Answer 55

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

Question 56

Define resistivity

Answer 56

The resistivity of a material is defined as the same value as the resistance between opposite faces of a cubic metre of the material

Question 57

In order to conduct electricity, what do solids need

Answer 57

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

Question 58

How do we get these free electrons

Answer 58

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

Question 59

Tell me about the overall position of charge in metal

Answer 59

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

Question 60

How does drift velocity occur

Answer 60

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

Question 61

When does conduction in metals happen

Answer 61

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

Question 62

Fun

FACT
FACT FACT FACT FACT

Answer 62

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

Question 63

Define current with an equation

Answer 63

I = triangle Q / triangle T

Question 64

What’s the drift velocity equation / TRANSPORT EQUATION

Answer 64

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

Question 65

What equation is drift velocity / transport equation derived from

Answer 65

I = Q/T

Question 66

How can we investigate conduction velocity of coloured ions practical

Answer 66

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

Question 67

How does temperature affect resistance

Answer 67

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

Question 68

What does a higher current do to resistance

Answer 68

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

Question 69

Why does the IV graph for a filament lamp begin to flatten out

Answer 69

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

Question 70

When does resistivity of semiconductors fall

Answer 70

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

Question 71

What happens when there’s a slight decrease in n for METALS

Answer 71

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

Question 72

What’s drift velocity

Answer 72

The slow overall movement of the charges in a current is called their drift velocity

Question 73

What’s the transport equation

Answer 73

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

Question 74

Tell me about conduction is semi conductors

Answer 74

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

Question 75

What happens in solid materials with free electrons being liberated

Answer 75

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

Question 76

What’s the VALENCE band

Answer 76

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

Question 77

How does an electron go to the conduction band/ what is it

Answer 77

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

Question 78

What does the gaps between energy bands between energy levels show

Answer 78

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

Question 79

How does number of delocalised electrons compare with semiconductors and metals and how does it relate to current

Answer 79

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

Question 80

Why does current increase as temperature increases in semiconductor

Answer 80

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

Question 81

How does /(do?) insulators conduct electricity

Answer 81

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

Question 82

Why does resistance increase with temp

Answer 82

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

Question 83

What happens when you cool a circuit to reduce resistance - tell me about super-conduction

Answer 83

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

Question 84

When are superconductors useful

Answer 84

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

Question 85

What’s 0 Kelvin equal to

Answer 85

0 K is equal to absolute zero,

Lowest temperature possible as this represents a temperature where the particles have 0 internal energy

Question 86

What’s 0K in degrees C

Answer 86

0K = -273.15 degrees c

Usually ignore 0.15 lol

Question 87

What’s 0 degrees C in kelvins

Answer 87

0 degrees C = 273.15 K

Usually ignore 0.15 lol

Question 88

Define a semiconductor

Answer 88

Semiconductors have a lower resistivity than insulators, but higher than conductors - usually only have small numbers of delocalised electrons that are free to conduct

Question 89

Define the valence band

Answer 89

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

Question 90

Define the conduction band

Answer 90

The conduction band is a range of energies in a solid material can have which delocalised them to move more freely through the solid

Question 91

Define the critical temperature

Answer 91

The critical temperature for a material is that below which it’s resistivity instantly drops to zero

Question 92

What are conduction holes

Answer 92

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

Question 93

Tell me a tip about explaining conduction by holes

Answer 93

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

Question 94

How is a semiconductor diode made / explain the I-V characteristics of it

Answer 94

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

Question 95

Tell me about light dependant resistors (LDRs)

Answer 95

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

Question 96

How do thermistors work

Answer 96

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