Electric Circuits

Question 1

What is the definition of charge?

Answer 1

• charge is the fundamental property of some particles. it is the cause of the electromagnetic force and it is a basis aspect of describing electrical effect
• Charge is measured in coulombs, C. One coulomb is the quantity of charge that passes a point in a conductor per second when one ampere of current is flowing the conductor
• it is impossible to create or destroy charge - the total charge must always be conserved

Question 2

what is the charge of a electron?

Answer 2

the amount of charge in a single electron in these units is -1.6 x 10^-19C

Question 3

How is electrical current defined?

Answer 3

electrical current is the rate of flow of charge

Question 4

What is 1 coulomb of charge equivalent to?

Answer 4

that you would have one coulomb of negative charge if you collected together 6.25 x 10^18 electrons
-total charge = Q=ne , 6.25 x 10^18 x 1.6 x 10^19 = 1C

Question 5

what is electrical current?

Answer 5

• if electrical charge moves this is referred to as electric current
• and the strict definition of electric current is the rate of movement of charge as it is usually a physics movement of billions of tiny charged particles such charge movements are often said to flow
• electric current occurs when a charged particle which is free to move, experiences an electric force, if it can move it ill be accelerated by the force
• this movement of charge forms the electric current
• most electric circuits are mode from metal wiring in which there are electrons that are free to move these conduction electrons then for a current

Question 6

in which direction do electrons flow around a cell?

Answer 6

electric force experience by negative conduction electrons so they move through the metal - they are attracted to the positive anode of the cell

Question 7

how do you calculate current?

Answer 7

• current (A) = charge passing a point (C)/time for that charge to pass (s)
  I =ΔQ/Δt
  -thus one ampere (1A) is the movement of one coulomb (1C) of charge per second (1s)

Question 8

How do you calculate charge?

Answer 8

• ΔQ=IΔt

remember as QuIt

Question 9

how can you observe charge flow?

Answer 9

• using a hanging ball that will conduct electricity, this suspended ball can carry small numbers of electrons across a high voltage gap, and this current is measure using galvanometer
• this high voltage set up across the air gap between two metal plates encourages negative electrons to want 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 plate to another each time
• we measure this small movement of charge on a very sensitive ammeter
• if we time the period of oscillations of the shuttling ball and the tiny current, we can calculate how many electrons would pas across on each journey of the ball
• if the ball is shuttling too fast to be timed by eye, then we can use a stroboscope to measure the frequency of oscillation
• Note that on the ball’s return journey it will be positively charge having lost an excess electrons when contacting the positive plate, however positive charge moving in the opposite direction still constitutes a current, and it will be the same rate - same current - because the ball speed is constant

Question 10

What is Ionic Charge?

Answer 10

• if the circuit is more unusual, there may be other charged particles, charge carriers, which can move to form the electric current
• e.g. in the electrolytic processing of bauxite ore to produce aluminium metal, the bauxite is dissolved in cryolite - another aluminium compound - and this solution then has free aluminium ions (charge carriers) that can move through the liquid as an electric current
• These ions are positively charged, and will move because of the electric force towards the negative cathode
• this is still an electric circuit that must obey the rule of conservation of charge, and in which we can measure the current as the rate of flow of charge
• as the charge on an electron is a fixed negative amount, we can easily calculate the charge on any ion
• this would be important in a situation where ions ere moving as the charge carriers in an electric current e.g. in electrolysis

Question 11

what is the charge on a proton?

Answer 11

the charge on a proton is the same magnitude as that on an electron but is positive = 1.6 x 10^-19

Question 12

What is voltage?

Answer 12

voltage is a measure of the amount of energy a component transfers per unit of charge passing through it

Question 13

how do you calculate voltage?

Answer 13

voltage (V) = energy transferred (J)/charge passing (C)

-V =E/Q

Question 14

What is Electromotive force?

Answer 14

• 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 one joules (1J) of energy per coulomb of charge (1C) that passes through it has an emf of 1 volt (1V)

Question 15

How do you calculate emf?

Answer 15

• emf (V) = energy transferred (J)/charge passing (C)

ε = E/Q

Question 16

what is potential difference?

Answer 16

• for a component which is using electrical energy in a circuit and transferring this energy into other forms, the correct term or the voltage is potential difference or pd.
• If a component uses one joule (1J) of energy per coulomb of charge (1C) that passes through it, it has a pd of 1 volt (1V).
• the energy being used by the component could e referred to as work done, W

Question 17

How do you calculate potential difference?

Answer 17

pd (V) = energy transferred (J)/charge passing (C)

-V=W/Q

Question 18

What is the electronvolt?

Answer 18

the electronvolt, eV is a unit of energy that is generally used with sub-atomic particles its definition comes form the equation defining voltage

• V=E/Q
• If an electron is accelerated by a potential difference of 1V, the energy it will gain is:
• E=Ve –> 1 x (1.6 x 10 ^-19) = 1.6 x 10^19J
• the amount of energy an electron gains by passing through a voltage of 1V is an electronvolt
• So 1eV= 1.6 x 10^-19J

Question 19

What is a Electrical models?

Answer 19

• a model is a way of thinking about an idea or phenomenon in order to help us understand it better
• electricity has many aspects that 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 is important to be able to evaluate the strengths and weaknesses of any model, in order to ensure that you do not rely too heavily on it

Question 20

How can Voltage been Modeled?

Answer 20

• one model that could be used to try and understand the transfers of energy in an electric circuit could be to think of electric circuit as a ski area
• the ski lift takes skiers to the top of the slope - it gives them potential energy like a battery gives electrons electrical energy. from there, they can slide down a number of possible routes e.g. separate, parallel, loops in a circuit , doing tricks off obstacles could be the same as them passing through and giving energy to a component
• emf of the chair lift and pdf of each obstacle on the course
• although there are skiiers and snowboarders we only refer to skiers, this is a weakness of the model as both would represent charge carriers flowing in our circuit, but usually electric circuits have only one type of charge carrier - the electron

Question 21

What is the definition of Resistance?

Answer 21

is the opposition to the flow of electrical current

Question 22

What is Ohm’s Law?

Answer 22

• a component through which the current is proportional to the voltage driving it is referred to as an ohmic conductor, as it follows Ohm’s law. The proportional relationship can be used to find the resistance of the component
• for an ohmic conductor, the answer to the calculation of resistance would be the same for all voltages and their corresponding current values (providing the temperature remains constant)

Question 23

How do we investigate I-V relationship?

Answer 23

• We can do an experiment to investigate whether or not a component follows Ohm’s law
• the component under test is the resistor, and we could replace this resistor to allow testing of other components
• Use various values of supply emf and measure the potential difference across and current through the resistor for each one
• this should include reversing the terminals on the power supply in order to measure the effects of negative pds across the resistor
• In the case of a resistor, negative pds correspond to negative currents - a voltage creating an electric force in the opposite direction in the wires will attract electrons in the opposite direction around the circuit
• you can calculate the resistance of the resistor from the graph, first calculate the gradient of the straight line –> m=ΔI/ΔV
• This calculation is the reciprocal of the one for resistance given above, thus, you can find the resistance here by taking the reciprocal of the gradient
• R = 1/m
• Note that using the gradient is only effective for an ohmic conductor. if the line is not straight, the resistance changes and can be calculated using Ohm’s law at specific V/I value on the line

Question 24

What are current-voltage characteristics?

Answer 24

• in designing an electric circuit, we need to know how components will react when the pd across them changes, in order to ensure that the circuit performs in intended function under all circumstances
• Part of the specification of any component is a graph of its I-V characteristic
• simple resistors, and a metal at constant temperature would produce the same straight-line result
• the only difference would be that the gradient will be different in each case, as it corresponds to the specific resistance of the resistor or wire under consideration

Question 25

What does an I-V graph look like for filament bulb?

Answer 25

• at small voltages current is proportional to voltage, as shown by the straight-line graph potion of the graph through the origin
• at higher voltages a large current is driven through the lamp filament wire, heating it up causing greater resistance
• (looks like an elongated s)

Question 26

What does an I-V graph look like for diode?

Answer 26

• diode only conducts in the forward direction, and so there is zero current for negative voltages
• it also requires a minimum driving i the forward direction
• the threshold voltage is typically around 0.6V
• (straight line at zero for negative voltages until threshold voltage then increases)

Question 27

What does am I-V graph look like for a thermistor?

Answer 27

a thermistor is designed to alter its resistance with temperature, in a reverse manner to a filament bulb

• the gradient of the line increases with the heating effect of the increasing current
• the gradient represents the reciprocal of the resistance. larger gradient value means lower resistance
• the is a result of its manufacture from semiconductor materials, whose atoms release conduction electrons as the temperature rises

Question 28

What is resistance the result of?

Answer 28

collisions between charge carriers and atoms in the current’s path

• this effect will vary depending on the density of charge carriers and the density of fixed atoms, as well as the strength of the forces between them
• thus different materials with identical dimensions will have differing resistance

Question 29

What is resistivity?

Answer 29

• Ωm
• the resistivity of a material is defined as the same value as the resistance between opposites faces of a cubic metre of the material
• all samples of the same material, regardless of their shape and size will have the same resistivity, whilst their resistance may be very difference

Question 30

How do you calculate resistivity?

Answer 30

resistance (Ω) = resistivity (Ωm) x sample length (m)/cross-sectional area(m^2)

R=pl/A

Question 31

How do you investigate resistivity?

Answer 31

• use a micrometer 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 we take mean diameter measurements
• For several different lengths of wire the wire’s resistance should be measured using the volt meter and ammeter using R=V/I
• the resistance will be small, so care must be taken to ensure currents are safely low. R=pl/a
• The equation involving resistivity means that we could calculate a value for it by re-arranging the equation and taking one of the results and making the calculation
• However, it is always more reliable to produce a straight-line graph experimental results and calculate our answer from the gradient
• y-axis = resistance and x-axis = lengthto give us the gradient p/A to work out resistivity

Question 32

What is drift velocity?

Answer 32

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

Question 33

How can solids conduct electricity?

Answer 33

need to have electrons that are delocalised fro the solid’s atoms so, that they can move through the solid causing an overall movement of charge - a current

Question 34

How do metals conduct electricity?

Answer 34

• the structure of metals has a regular lattice of metal atoms, these are bonded together trough the sharing of electrons, which act as if they were associated with more than one atom
• many of the atoms also have an outer electron that is not needed for bonding between atoms
• these free electrons have a random motion, which changes as they collide with atoms or other electrons, but on average overall position of all the charge in the metal is stationary
• However, 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 negative electrons towards the positive end of the field.
• this slow overall movement of the electron as is call drift velocity

Question 35

how does the random thermal motion of electrons compare to drift velocity?

Answer 35

the random thermal motion of the free electrons will be at speeds of thousands of kilometres per second, whereas the drift velocity during conduction is usually only millimetres per seconds

Question 36

How do you calculate drift Velocity?

Answer 36

current (A) = charge density - number of electrons per m^3 (m^3) x Cross-sectional area (m^2) x drift velocity (ms^-1) x charge on electrons (C)

-I=nAve –> transport equation

Question 37

How do you derive drift velocity?

Answer 37

• the value of current in a metal can be calculated from the movement of electrons –> I=ΔQ/Δt
• if we consider the cylinder shaded as the length of the wire that the charges move through in a time Δt
• then we need to calculate how much charge flows through it in that time
• there are ‘n’ electrons per cubic metre of this metal, and the wire has a cross-sectional area A. their movement is at a drift velocity, v. and the distance this takes them along the wire in that time is Δx so: Δx=vΔt
• the total charge will be the number of electrons multiplied by the charge on each, e
• the number of electrons will be their density, n, multiplied by the volume of the cylinder, V, that they travel through in Δt
• ΔQ=n x V x e =n x AvΔt x e
• I=ΔQ/Δt = nAvΔte/Δt
• I=nAve

Question 38

How do you investigate conduction velocity of coloured ions?

Answer 38

• In this experiment you 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 (VII) will each dissolve, producing positive blue copper ions and negative purple ions of manganate (VII)
• Conducting a pd of 30V across the wet filter paper will cause the ions to flow slowly across the wet filter paper will cause the ions to flow slowly across it, in opposite directions, and their velocity can be measure using a ruler and stop clock
• Expect a velocity of about 1mm per minute

Question 39

How does resistance link to drift velocity?

Answer 39

• we have seen that whilst the natural speed of electrons within a metal is very high, their progress through a metal’s lattice of fixed atoms is slow
• they are constantly colliding with those atoms, this is electrical resistance, the frequency of collisions is determined in part by the temperature as this affects the vibration of the fixed atoms
• The higher the metal’s temperature, the more its atoms vibrate, and this means more collisions, further slowing the drift velocity of the electron
• However, a higher current will cause more collisions, as more electrons move faster through the metal, structure, this makes the metal atoms vibrate more - effectively increasing the temperature

Question 40

How doe resistivity link to drift velocity?

Answer 40

• resistivity varies with temperature for metals and semiconductors
• the resistivity of semiconductors was seen to fall as the temperature rises
• resistivity of metals is much lower than for semiconductors –> The negative temperature coefficients of resistivity for semiconductors shows that their resistivity falls as the temperature goes up
• the reasons for this is that the value for n in the transport equation will be higher at higher temperatures, So the fact that the resistivity goes down is actually just a consequence of the fact that these materials, like silicon, conductor better at higher temperatures

-a slight decrease in n for metals at higher temperature is due to 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 vibration

Question 41

What is a semiconductor?

Answer 41

-semiconductors have lower resistivity that insulators, but higher than conductors, They usually only have small number of delocalised electrons that are free to conduct

Question 42

How does conduction occur in semiconductors?

Answer 42

-semiconductors are generally solid materials that only have small numbers of delocalised electrons that are free to conduct
-free atoms have a series of discrete energy levels in which we can find find their electrons, depending on the energy the electrons have received, If the electron received enough energy it will leave atoms although leaving behind an ion
-In solid material where there are many,many atoms close together, the allowed energy levels become much wider, forming energy bands, The electrons can have a large range of energies and still be within the same band
-There are energy amount which are forbidden for the electrons, but it is a very different situation from the highly limited energy levels of the isolated atom’s electron
As these energy bands are bands are created by the collective grouping of the solid’s atoms, the bands are attributed to the semiconductor as a whole rather than to individual atoms
-Valance band –> electrons with this amount of energy remain tied to atoms
- Conduction band –> those that gain energy to jump up to conduction band become delocalised and can move through semiconductors as part of a current

Question 43

What is the valence band?

Answer 43

the valence ais a range of energy amounts that electrons in a solid material can have which keeps them close to one particular atom

Question 44

What is a conduction band

Answer 44

the conduction band is a range of energy amounts that electrons in a solid material can have which delocalises them to move more freely through the solid

Question 45

How does the delocalised electrons in a semiconductor compared delocalised electrons in a metal?

Answer 45

• the number of delocalised electrons in a semiconductor is low compared with metals, and so the current they will carry is therefore lower than metals for the same applied voltage
• At higher temperatures they have more conduction electrons, as more electrons are elevated into the conduction band
• There will be a temperature-related reduction in current due to increase collisions with fixed atoms,but the increase in available conduction electrons far out weighs this. The overall effect is that a semiconductor will carry more current as the temperature goes up - its resistivity effectively drop as the temperature rises

Question 46

What are conduction ‘holes’?

Answer 46

• When an electron enters the conduction band and moves away, this leaves the atom with an effective positive charge. The empty space the electrons has left is referred to as a (positive) hole
• If an electron form 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 positive direction of an applied voltage, the 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 is electrolysis, where positively and negatively charged particles are moving in opposite direction at the same time
• note that the 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

Question 47

What as the I-V characteristics of a semi-conductor diode?

Answer 47

• a diode is made by joining together different types of semiconductors, which morally create 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 very 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 and force the conduction process in the opposite direction

Question 48

How does a Light dependent resistor (LDR) work?

Answer 48

• Light dependent 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 up to the conduction band, increasing the number of conduction electrons
• The effect of this is to make the LDR conduct better - it has a lower resistance

Question 49

How does a thermistors work?

Answer 49

• Thermistor work in exactly the same as way, except that their resistance depends on thermal energy from the surroundings
• The most common type of thermistors are referred to as negative temperature coefficient thermistors
• These use thermal energy to boost their electrons into the conduction energy band, meaning their resistance falls as the temperature rises

Question 50

What is an Insulators?

Answer 50

• electrical insulators can be thought of as materials i which the energy gap between the valence band and the 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

Question 51

What is critical temperature?

Answer 51

-the critical temperature for a material is that below which its resistivity instantly drops to zero

Question 52

What is Superconductivity?

Answer 52

• Resistance increases with higher temperatures, because the higher level of internal energy in the material causes more vibration of the fixed ions and these collide more often with charge carriers to reduce their speed of movement through the material, Reducing the temperature therefore reduces resistance, allowing greater current flow
• Cooling to ever lower temperature continues this trend, until something quite unexpected happens
• Below a certain critical temperature the resistance suddenly drops to zero, this is called superconductivity
• this critical temperature varies with material, but for most metals it will be below -243°C
• Complex ceramic superconductors have been created that have temperatures at which they conduct without any resistance as high as -135°C

Question 53

What are the uses of Superconductors?

Answer 53

• especially useful i application where a large current is needed
• as a large current would normally waste too much energy or damage the surrounding with heat dissipated
• For example, the strong magnet needed in a particle accelerator will often be superconducting electromagnets, cooled to very low temperatures to maintain their superconductivity. The electro magnets in the Large Hadron collider operate at about 1.9K
• Mag-lev trains

Question 54

What is the temperature of absolute zero?

Answer 54

0K = -273.15°C

Question 55

What is the effect that allows superconductors to levitate magnets?

Answer 55

• they levitate magnets as they do not accept penetration by magnetic field
• This is called the Meisser effect

Question 56

What has to stay the same for the current?

Answer 56

the total amount of charge within a circuit cannot increase or decrease when the circuit is functioning

Question 57

How is a Ammeter placed in a circuit?

Answer 57

-Currents are measures of charge flow through a component, so they must be measured in the same line as the component i.e. must always be placed in series next to the component

Question 58

What is current like in a series circuit?

Answer 58

any group of components that follow in series in a circuit, with no junctions in the circuit must have the same current through them all

Question 59

What is the current like in a parallel circuit?

Answer 59

• whenever a current encounters a junction in a circuit, the charge can only go one way or the other so the current must split
• The proportion that travel along that path, If the path has high resistance, the current is less likely to go that way, However, the total along the branches must add up to the original total current

Question 60

How is a Voltmeter put into a circuit?

Answer 60

-voltages are measures of energy change as charge passes through a component, so they must be measure from one side to the other, i.e. the voltmeter must always be placed in parallel across the component

Question 61

What is Voltage like in a series circuit?

Answer 61

-Amy group of emfs that follow in series in a circuit, with no junctions in the circuit, will have a total emf that is the sum of their individual values, accounting for the direction of their positive and negative sides

Question 62

what is voltage like in parallel circuits?

Answer 62

-if the total voltage across any branch is known, and other branch in parallel will be identical

Question 63

What is resistance like in series?

Answer 63

-any group of resistances that follow in series in a circuit, with no junctions in the circuit will have a total resistance that is the sum of their individual values

Question 64

How do you derive Resistance in series?

Answer 64

• potential difference = Vtotal = V1 + V2 + V3
• and V=IR
• IRtotal = IR1 + IR2 + IR3
• Rtotal = R1 + R2 + R3

Question 65

What is resistance like in parallel?

Answer 65

• the total resistance of a group of a resistors in parallel can be calculated from the equation
• 1/Rtotal = 1/R1 + 1/R2 +1/R3

( once you get you answer in a fraction invert it for the answer)

-Any combination of resistors in parallel will have a total resistance that is smaller than the smallest individual resistance in the group

Question 66

How do you derive resistors in parallel?

Answer 66

current = I1 + I2 =I3

• and I=V/R
• V/Rtotal = V/R1 + V/R2 + V/R3
• 1/Rtotal = 1/R1 + 1/R2 + 1/R3

Question 67

How do you calculate resistors branch in combinations?

Answer 67

• use the rules for series and parallel resistors separately then combine the sub totals

Question 68

How do you investigate circuit rules?

Answer 68

• Make a complex circuit with various components in series and parallel combinations
• using only ammeter and voltmeter, your partner should be able to verify the circuit rules explained in this section
• If the resistance are large, the ammeter mat need to be a multimeter set to read milliamp currents
• Join various resistors in various combinations of series and parallel connections
• Your partner should calculate what they expect overall resistance of the combination to be
• Check whether they have calculated correctly by measuring the overall resistance of the resistor combination using an ohmeter

Question 69

Who was Gustav Kirchhoff?

Answer 69

• Gustav Kirchhoff was a Prussian physicist working in the middle of the nineteenth century
• He made important contributions to several areas of physics, and there are also laws named after him in spectroscopy and thermochemistry
• the electrical circuit rules presented here sometimes carry his name

Question 70

What was the Electric current rule?

Answer 70

-electric current rule : the algebraic sum of the current entering a junction is equal to zero
-In order to conserve electric charge, the sum of all the currents arriving at any point (such as a junction) in a circuit is equal to the sum of all the current leaving that point
ΣI=0

Question 71

What is the Electric Voltage rule?

Answer 71

• In order to conserve electrical energy around any closed loop in a circuit, the sum of emfs is equal to the sum of the pds around that loop
• Note that a loop being considered might not be the whole of a circuit - the only requirement is that the loop is complete
  -Note also that the potentiate differences referred to here are defined as being the products of component currents and resistance, as per Ohm’s law
  -So, if we isolate a particular closed loop, it may have potential differences with positive values, and with negative values when the current flows in the opposite direction
  Σε=ΣV

Question 72

How is the voltage rule used with calculated pd’s?

Answer 72

• potential differences are the product of currents passing through resistances, in order to use up electrical energy, Thus, each pd could be calculated from Ohm’s law as V=IR so this becomes
• Σε=ΣIR

Question 73

How is voltage split up in a circuit?

Answer 73

in proportion to the resistances of the components in the circuit loop

Question 74

How can Ohm’s law explain how voltage is split in proportion to the resistance?

Answer 74

-for two different resistors R1 and R2
- I1=V1/R1 and I2=V2/R2
they both have the same current therefore I1=I2
- therefore V1/R1=V2/R2
-therefore V1/V2 = R1/R2
-So the voltage is split in the same proportion as the ratio of the resistances for any value of resistance

Question 75

What does it mean that the voltage is proportional to the resistances of components in a circuit?

Answer 75

• this means that for a known emf supply voltage we can carefully chose resistances to share the voltage, and provide a specific value of pd on one component
• if we then also remember that all parallel branches in a circuit must have the same voltage then any branch we st up in parallel with that specific pf will also have the same voltage
• thus we can set up a circuit to provide an exactly chosen value of voltage to the parallel branch
• Using variable resistors, this set up can then be use to provide whatever voltage we choose, this can be particularly useful if our emf source of a fixed value like a battery which can only operate at say 6V

Question 76

What is the potential divider equation?

Answer 76

• As the pd in a potential divider circuit is split in proportion to the resistance of the components we can calculate the pd across them mathematically
• Vout = Vin x R2/(R1 + R2)

where Vout is the second voltage and Vin is the emf supplied

Question 77

How does a potentiometer work?

Answer 77

• The idea of using a potential divider to supply a variable voltage as required has been incoroportated into an electrical component called the potentiometer
• A movement contact can slide over a fixed length of resistance wire, adjust the length that is on either side of this wiper
• The two lengths of wire are the two resistors in our potential divider circuit, adjusting their relative lengths alters their relative resistances and hence the voltage across them
• if we want to power a separate circuit with a variable voltage, we connect it across one side of the potentiometer
• where we would connect the right-hand side connections across R2 to our main circuit, The block marked R1 and R2 and a continuous piece of resistance wire, which will be split into two resistances by the position of the wiper,
• Adjusting the wiper up and down allows us to tap off any desired voltage up to the maximum of Vsupply

Question 78

How does a potential dividers in sensor circuits?

Answer 78

• If one of the resistors in a potential divider is a sensor component (such as a thermistor or LDR) then we can use its changing resistance to control an external
• The resistance of a LDR increases if the ambient light level drops ( darker surroundings make its resistance go up). In brighter conditions, the LDR has a very low resistance, so it takes a very small proportion of the 12V supplied by the battery
• This will make the lamp very dim, probably to the extent that it will appear off, it is not needed if the surroundings are bright, As conditions become darker, the LDR resistance goes up the bulb in parallel is supplied with increased proportion this increase in voltage make the lamp brighter as the surroundings become darker

Question 79

How can the voltage switch on an electronic components be used?

Answer 79

• many eletornic components have a switch on voltage of 5V. These could be used with a sensor in a potential divider circuit, and would only switch on when the sensor’s resistance reached a level where its pf was 5V, Altering the resistor in this circuit would then control the sensor level at which the electronic component was switch

Question 80

What is internal resistance?

Answer 80

the resistance of an emf source is called its internal resistance

• in most situations, the internal resistance of an emf source will not affect the performance of a circuit, it may not be noticeable
• However, as the power lost - energy wasted as heat through a component increases with the current through it there can be significant power loss when large currents are used
• The net effect of this energy usage by the emf source itself will be for it to supply a smaller voltage to the rest of the circuit which could hamper performance
• A large current through the internal resistance could also damage the emf source as the result of ohmic (resistance) heating

Question 81

What is the effect of having an internal resistance?

Answer 81

the effect of having internal resistance is that an emf will never be able to fully supply its notional maximum voltage
-There will always be a small drop in voltage over the internal resistance and this drop will be bigger with a higher current

Question 82

How do you calculate and derive internal resistance?

Answer 82

• The pd over the internal resistance, r, can be found from Ohm’s law:
• Vinternal= Ir
• this pd is some time referred to as ‘lost volts’ as the circuit appears to have fewer volts in use by the load than the emf should be supplying, A measurement of voltage across the terminals of a cell powering a circuit would not measure, the emf, it would measure the emf minus the lost volts, This ‘terminal pd’ would then be:
• Vterminal = ε - Ir
• As all parallel branches have the same voltage across them, the terminal d would also equal the pd across the load resistance
• Vterminal = Vload = ε - Ir
• By considering energy conservation, we can draw up an equation for the circuit that incorporates the internal resistance
• Σε =ΣV
• ε = Vload + Vinternal
• ε = Vload + Ir =IR + Ir
• ε = V + Ir

Question 83

How can you investigate internal resistance?

Answer 83

• We can find the internal resistance for a source of emf such as a cell experimentally.
• Using a circuit involving only a Emf source with a voltmeter across it, ammeter and a variable resistor (to change the current through the circuit) and at each current value we record the voltmeter reading
• using the equation ε - Ir = V which can be rearranged to V = -rI + ε
• compare this with the equation for a straight line y=mx + c shows us that a graph plotted with V on the y-axis and I on the x-axis we will have a gradient equal to the magnitude to the internal resistance r. ( it will come out as negative (r) the y- intercept C represents the voltmeter reading when no current flows, if no current flows there is no internal resistance, so no lost volts so the voltmeter will read the full value of ε

Question 84

What is electrical Work Done?

Answer 84

• Work done has the symbol ‘W’ and as the equation defining potential difference includes a term for the amount of energy transferred, E, these two will be the same.
• W=E
• the equation for pd, in a rearranged for gives us:
• W = V x Q
• and as Q= It
• W= V x I x t
• W = VIt

Question 85

What is electrical power?

Answer 85

• Power, P, is the rate of transfer of energy, or the rate of doing work. Inn an electrical circuit, the energy is dissipated by a component. The mathematical definition is:
• P=E/t OR P=W/t
• incorporating the equation for work in a circuit:
• P=VIt/t
• P=VI

Question 86

How can you work of Power dissipated by a resistor?

Answer 86

• we can write variations of the electrical equation, substituting in terms from Ohm’s law so that we can calculate the power dissipated by resistors in a circuit
• P=VI AND V=IR
• if we only know current and resistance:
• P=IR x I = I^2 x R
• Or, if we only know pd and resistance:
• P = V x V/R = V^2/R

Question 87

What is efficiency?

Answer 87

the effectiveness of a machine at converting energy usefully is called efficiently

Question 88

How do you calculate efficiency in a electrical circuit?

Answer 88

-efficiency = useful energy output/total energy output x 100

• efficiency = useful power output/ total power input x 100
• the answer will be between 0 and 1, it is common to convert this to a percentage value

Question 89

How do you investigate efficiency?

Answer 89

• we can find the efficiency of an electric motor by measuring the electrical energy it uses, and by comparing this with its useful energy output in lifting a mass against its weight to give the mass gravitational potential energy
• You switch on the motor to find the time (t) it take to lift the known mass (m) through a fixed height (Δh). During the process you measure the average pd (V) across the motor and current through it (I) using an ammeter and voltmeter
• You use these measurements to calculate the total energy input and the useful output for use in the efficiency equation
• useful energy output = gpe gained = mgΔh
• total energy input = electrical energy used = VIt
• motor efficiency = useful energy output/ total energy output = mgΔh/VIt

Question 90

What are the uses for a diode?

Answer 90

-Using diodes to prevent damage to circuits –>
Diodes are normally used to prevent damage to other polarised components in circuits: eg a diode can protect against current flowing the wrong way if the battery is put in back to front.
-An important use of diodes is to prevent circuit damage due to back electromotive force (known as EMF). This is a momentary change in the direction of the flow of electricity when components such as motors, solenoids or relays are switched off. The diode is connected in parallel to the component, which generates the back EMF in reverse bias to the ‘normal’ direction of flow of electricity in the circuit. A diode used in this way is called a clamping diode.

Question 91

what is a LED?

Answer 91

A light-emitting diode (LED) is a special kind of diode that glows when electricity passes through it. Most LEDs are made from a semi-conducting material called gallium arsenide phosphide.
LEDs can be bought in a range of colours. They can also be bought in forms that will switch between two colours (bi-colour), three colours (tri-colour) or emit infra-red light.
In common with all diodes, the LED will only allow current to pass in one direction. The cathode is normally indicated by a flat side on the casing and the anode is normally indicated by a slightly longer leg. The current required to power an LED is usually around 20 mA.