Home > Preview
The flashcards below were created by user
ghoran
on FreezingBlue Flashcards.

an electric current is defined as
the rate at which electrically charged particles pass through a point in a circuit

the size of the current is measured in
coulombs per second or amperes (amps)

how many amperes is equal to 1 coulomb per second
1

in metallic conductors the charge carriers are
electrons ,

which way do electrons move in a circuit
from the negative terminal of the dc supply towards the positive charge . confusion can arise because current is normally shown as moving from the positive terminal towards the negative terminal . this is referred to as conventional current

all current arrows on wires and component symbols point in
the conventional current direction

the size of the current is defined mathematically by
 I = ΔQ/Δt
 where :
 I = current in amperes (A)
 Q = charge in coulombs (C)
 t = time in seconds
 Δ = change in ... (charge or current in this case)

what must exist to make a current flow
potential difference (p.d.)

a potential difference is defined as
the electrical energy transferred or converted per unit of charge passing between the two points

potential difference is measured in
joules per coulomb or volts

how many coulombs per second is equal to one volt
1

the size of the potential difference (p.d.) is defined mathematically by
 V = W/Q
 were :
 V = p.d. in volts (V)
 W = work (energy) in joules (J)
 Q = charge in coulombs (C)

a charge gains energy when it
passes through a cell

a charge releases the energy its gained as it
passes through the components in a circuit (e.g. a lamp or resistor) i.e. p.d. exists across the component . thus both a cell and component have a p.d. across them when charge flows in a circuit

charges faces opposition when they
flow around a circuit . this is called resistance and it is measured in ohms (Ω)

the potential difference needed to make a current flow in a circuit is dependant on the
resistance in the circuit . the bigger the resistance , the more p.d. is required to make a certain current flow

resistance is defined by the equation
 R = V/I
 where :
 I = current in amps (A)
 V = p.d. in volts (V)
 R = resistance in ohms (Ω)

milliamps
symbol 
quantity 

microamps
symbol 
quantity 
 ^{}A
 1x10^{6}Amps

kilohm
symbol 
quantity 
 K
 1x10^{3} ohms

megohms
symbol 
quantity 
 M
 1x10^{6} ohms

draw a diagram to show the metric prefix scale

the number of charge carriers is equal to
total charge/charge on charge carrier

example : in a conductor the charge carriers each have a charge of 1.6x10^{19} C
a) calculate the number of charge carriers passing a point in the conductor per second if the current is 4 microamps
 Q = It
 Q = 4x10^{6 }x 1 = 4x10^{6}C
 number of charge carriers is equal to the total charge divided by the charge on charge carrier
 number of charge carriers = 4x10^{6}/1.6x10^{19}
 number of charge carrier = 2.5x10x10^{13}

draw a diagram to show a circuit that can be be used to investigate how the potential difference across a component affects the current through it .
explain how to use the equipment to produce characteristic curves for the component
 the component under test is placed in the circuit as shown so that the circuit is complete when the switch is closed (note the diagram should have a switch on it)
 by varying the resistance using the variable resistor a range of current and p.d. values can be recorded for each change in resistance
 the battery is reversed and the variable resistor varied over the same range to produce a second set of readings
 a graph of current/voltage can be drawn using the results this is the characteristic curve for the component

draw a graph to show the characteristic curve for a resistor or wire both of which are ohmic conductors
 when current is plotted against the p.d. a straight line graph is obtained
 the positive part of the graph shows current flowing from positive to negative and the negative part of the graph shows the current reversed .
 the current and p.d. are directly proportional to each other (straight line through the origin) when the current flows in either direction . the conductor is said to follows ohms law

draw and explain a graph showing the characteristic curve for a diode which is a semiconductor
 in the case of the semiconductor diode , the shape of the curve obtained depends on the direction in which the current is flowing .
 when the diode is forward biased (arrow facing the direction of the conventional current) :
 between 0V and about 0.7V , the diode offers a large resistance to current
 between about 0.7V and 1V the resistance of the diode falls rapidly and a large current flows  this is shown by the steep rise in the graph
 when the diode is reversed biased (arrow facing opposite direction to conventional current) :
 the diode offers high resistance , so very little or no current flows
 at the breakdown voltage typically between 50 and 500 V , a large current flows
 most diodes cannot recover and are destroyed by the heating effect of the large current

draw and explain a graph showing the characteristic curve for a filament lamp which is a non ohmic conductor
when a filament lamp is connected in a circuit and the voltage is steadily increased the graph becomes less steep . the p.d. and voltage don't increase proportionally because the current heats the filament and so increases the resistance and therefor decreases the rate of increase of current with p.d. . the curve is symmetrical on either side of the origin showing that the lamp behaves in the same way for current flowing through it in either way .

all the characteristic curves can be produced automatically using a voltage sensor (V) and a current sensor (A) . these together with a data logger (D) capture data which is then fed into the computer for analysis . draw A typical set up and explain how characteristic graphs can be produced from the equipment
 please note that the filament lamp may be another component .
 the potential difference is varied across the component under investigation (wire , resistor , lamp , diode) using a potential divider and the current is recorded . the data logger software is then used to display the collected data in a tabular and graphical form

ohms law is a special case and only applies to certain components in certain conditions . ohms law states that
 the current in a conductor is directly proportional to the p.d. across it
 I is proportional to V
 provided that the temperature and other physical conditions remain the same

the current voltage graphs show clearly whether or not a component obeys ohms law .
therefor name the ohmic conductors we have seen
the resistor/wire is the only ohmic conductor while the semiconductor diode and the filament lamp are non ohmic conductors

two factors which affect the resistance of a conductor are its
length and cross sectional area

resistance is ..... to length so doubling length ..... the resistance

resistance is ..... proportional to area so doubling the cross sectional area ..... the resistance

don't confuse cross sectional area wit diameter
cross sectional area of a wire is equal to
pi x (d/2)^{2 }

doubling the diameter of the wire will ..... the resistance by .........

the resistivity of a material is given by :
 resistivity = AR/l
 where :
 A is the cross sectional area of the conductor in m^{2}
 R is the resistance of the conductor in ohms
 l is the length in m
 the resistivity is a constant of the material from which the conductor is made and measured in ohm metres

when converting mm^{2} to m^{2} divide by
10^{6}

draw a diagram to show how the resistivity of a material in the shape of a wire can be measured . and explain how to find the resistivity using the apparatus
 please note that the wire under test is taped to a metre rule .
 start by measuring the 100cm of the wire under test . tape the wire on to a metre rule , to avoid any kinks or twists . connect the wire to the circuit using crocodile clips .
 record , in a table , the p.d. displayed on the voltmeter and the current displayed on the ammeter for this length of wire .
 move the voltmeter connection along the wire in the range 100cm to 30cm and record the p.d. and current for each length
 calculate the resistance of wire for each recorded length using R = V/I
 measure the diameter of the wire several times over its length , using a micrometer , to determine a mean value for the diameter
 use the mean diameter to calculate the cross sectional area using A=pix(d/2)^{2}
 plot resistance (y axis) against length (x axis)
 since R = resitivity x l /A
 the graph is a straight line through the origin and resistivity can be found from the gradient . the gradient = R/l = resistivity/area so to find resistivity we have to find the gradient go the graph and times it by the cross sectional area
 essential notes // avoid large currents which will heat the wire and increase the resistance
 essential notes // a multimeter set on the ohms range could be used to measure the resistance directly , instead of using a battery , ammeter and voltmeter . however the ohms range usually has an uncertainty of + or  1 ohms

temperature always affects conduction , no matter whether the material is a conductor , and insulator or a semiconductor . in conductors the resistance increases as
the temperature increases

metal wires and resistors have
free electrons that move when a p.d. is applied , causing a current to flow . the metal also has vibrating positive ions . electrons collide with these ions . causing the wire to have resistance to current

explain what happens as the temperature of the wire increases
the positive ions and electrons asorb heat energy , causing the ions to vibrate with greater amplitude and the electrons to move faster . both of these effects result in a greater number of collisions between electrons and ions i.e. the resistance of the conductor increases . However the gradient of the graph isn't very steep showing that resistance doesn't change greatly with temperature

draw a graph to show the increase in resistance of a conductor with increased temperature

a thermistor is a device used for
temperature measurement and control

in the case of thermistors the resistance decreases significantly as
temperature increases

draw a graph to show how the resistance of thermistors varys with temperature

small increases in temperature produce large changes in resistance of the thermistor . explain why
 the thermistor is made from semiconductor material and therefor has few electrons to produce a current . as the temperature of the thermistor increases , the thermal energy is enough to release further electrons from the ions to make the material conductive , this means resistance decreases .
 essential notes// at higher temperatures the ions of the semiconductor vibrate more . This would normally cause the resistance to rise . However the release of conduction electrons is the dominant effect . this also explains the shape of the graph .

why is care needed when passing currents through thermistors
currents produce heat and this decreases the resistance of the thermistor , allowing more currents to flow . this further heats the thermistor producing further resistance changes and the process can continue until the component overheats and burns out or melts

if the temperature of a conductor is reduced so that it approaches absolute zero (0k or 273^{0}C) what happens to the electrical conductivity
it disappears completely . The material is said to have become a superconductor . Its resitivity has dropped to zero and an electric current can pass through without transferring any energy to the conductor . the temperature at which the material becomes superconducting is known as the critical temperature T_{C }

the critical temperatures for metal superconductors are typically
close to absolute zero , 1 to 4k . ceramic superconductors now exist that have critical temperatures as high as 125K (148^{0}C)

superconductors have important uses for example
carrying electrical power without losses and constructing very strong electromagnets

draw a graph to show resistivity against temperature for a high temperature superconductor

superconductors are materials
that acquire zero resistance when they are cooled below a critical temperature

what are the rules for series circuits
 potential difference is shared between various components . so the voltages round a series circuit always add up to the equal the source voltage
 current is the same everywhere
 the total resistance is the sum of all the resistances
 cell voltages add up

what are the rules for parallel circuits
 p.d. is the same across all components
 current is shared between branches
 the total resistance of any number of resistors is given by 1/R_{T }= 1/R_{1 }+ 1/R_{2 }+ 1/R_{3}

Draw diagrams to illustrate how three 10 ohm resistors can be connected in four different ways . calculate the total resistance of each network of resistors

to make current flow , a
p.d. must exist

the p.d. is the
amount of electrical energy that must be transferred to the charge and is measured in joules per coulomb , or volts

the charge releases the gained energy as it
passes through components in the circuit (e.g. lamp , motor , resistor etc) . all the potential energy lost by the charge is ultimately changed into heat

energy is measured in
joules

the energy converted to heat is given by :
 energy change (work done) W = VIt
 where :
 W = energy change in joules (J)
 V = p.d. in volts (V)
 I = current in amperes (A)
 t = time in seconds (s)

power is the
rate of change of change of energy and is measured in joules per second (Js^{1}) or watts (W)

power is given by :
 P = IV
 where :
 P = power in watts (W) or (Js^{1})

by substituting V=IR into P=VI We can arrive at an alternative equation :

by substituting I = V/R into P=VI we can arrive at alternative equation
P=V^{2}/R

the equation P=I^{2}R is important because
it shows the heating effect is proportional to the square of the current . Therefor doubling the current will produce four times the rating of heating

the power dissipated in a resistor R carrying current I is P . if the resistance is doubled and the current halved , what power is now dissipated
original power is given by P=I^{2}R but I_{1} = (I/2)^{2} ^{ }R_{1}= 2R . Now power is given by P=I^{2}/4X2R = I^{2}XR/2 = 1/2 P

in all circuits , what is conserved
electric charge i.e. all charge which arrives at a point must leave it

current is a flow of charge , so this can be stated as follows . At any point in a circuit where conductors join , the total current towards the point must equal
 the total current flowing away from the point . or the algebraic sum of currents at a junction is zero . this is known as Kirchhoff's first law .

in circuits , energy differences are expressed as
potential differences and measured in volts .

why do filament lamps blow
 filament lamps are more likely to blow (the filament breaks) when you first switch them on .
 when you first switch a bulb on , the filament has a lower resistance because its cold . This means that the initial current flowing through the filament will be larger than the normal current , so the filament is more likely to burn out at this time .
 the filament also heats up very quickly from cold to its operating temperature when it's switched on . the rapid temperature change could cause the filament wire to break to

