chapter 2 Flashcards
advantages of microsensors
unobstructive and convenient
how do charged particles move?
by other charged particles attracting and repelling them
define potential difference
the difference in electric potential between 2 points in a circuit
what is p.d measured in?
volts
what is 1 volt defined as?
1 joule of energy per coulomb of charge = 1 JC^-1
give 3 examples of potential difference
1) a dry cell: the chemical reaction drives electrons to one pole leaving positive charge at the other. the difference in distribution of charges creates a p.d between the poles
2) a storm cloud: strong air currents rub ice crystals against one another separating charges. the pd between top and bottom of the cloud can be millions of volts
3) a nerve cell: ‘pumps’ in the walls of the cell drive sodium ions outside the cell. This difference leads to a pd of about 70mV between inside and outside. Changes in this pd provide the electrical signals for our nerves
what makes potential difference?
uneven distributions of charge
how is a coulomb moved in a circuit?
the coulomb is pushed by electric forces generated by the battery. these forces grip it wherever it is in the circuit and drive it round, transferring electrical energy from the battery
why don’t charges get used up?
because they exist in the wires of a circuit at all times, but they are not moving. When a battery is connected to a circuit, it causes the charges to move from the positive terminal towards the negative terminal
why does current increase when voltage increases?
1 volt will give each coulomb of charge an energy of 1 joule. The charge will then travel around the circuit but only spends that energy when it comes across resistance in the circuit. so when voltage is increased, each charge receives more joules of energy to spend and travels around the circuit faster, so current increases as it is the rate of flow of charge
what happens when resistance increases in a circuit?
current decreases because it takes longer for the charges to spend their energy
equation for current, charge and time
current I = charge Q / time t
equation for pd
v= energy E / charge Q
what is power and its equation?
power is the rate of transfer of energy, the rate of work done, power = energy E/ time t p= W/ t
energy =
charge Q x voltageV
current =
number of ions arriving per second x charge
I= NQ
what is the mass of an electron?
9.1 x 10^ -31 kg
what is the charge on an electron?
1.6 x 10^-19 C
in 1 coulomb of charge, how many electrons are there?
6.2 x 10^18
what does 1 amp = in terms of electrons per time
6.2 x 10^18 electrons passing per second
why are electrons speed fast but their velocity is slow?
because, electrons move randomly due to the positively charged ions which they collide with and are attracted to and they collide with each other, so are constantly changing direction meaning their velocity is slow
what does an ammeter measure?
the amount of coulombs passing per second in amps- measuring the current in amps
if the ammeter has no resistance, how does this affect the circuit?
it doesn’t because you want as little voltage to be dropped as possible while conducting current so it doesn’t affect the circuit
what does a voltmeter measure?
the amount of energy each coulomb of charge has before the component and after
why do you want voltmeters to have high resistance?
to take a tiny amount of current to measure the difference in energy so it won’t affect the circuit
what does e stand for?
charge on 1 electron
1 amp=
1 Cs^-1
current and charge are related because?
current is the rate of flow of charge measure in coulombs per second= Amps so 1 A= 1C/ s
what is resistance?
The resistance of an electrical component is a measure of its opposition to current.
what is the equation for resistance?
R= V/I
what is conductance?
the inverse of resistance= it is the measure of how good an electrical conductor a component is
what is conductance measured in?
ohms ^-1 or siemans, S
what is the equation for conductance?
G= I/V = 1/ R
if a component has a resistance of 1 ohm and a pd of 1 v, what is the current flowing through it?
1A
if V=IR what does P=?
P= I^2 R and P= V^2/ R
what is power dissipation?
the rate that component converts electrical energy into other types of energy
in I/ V graphs what does a shallower gradient mean?
greater resistance
in a I/V graph what does a curve represent?
resistance is changing
what is ohm’s law?
provided the temperature is constant, the current through an ohmic conductor is directly proportional to the pd across it
what is ohmic behaviour?
ohm’s law says that the conductance and resistance of a given component is constant. So if the conductance and resistance are constant, independent of the current or pd, the conductor obeys ohm’s law and is said to be ‘ohmic’
examples of ohmic conductors
- wires made of copper, iron, silver
- metallic conductors
examples of non-ohmic conductors
- silicon
- germanium
- diodes
- light bulbs and LEDs
- ionised gases
what does the graph look like for an ohmic conductor?
linear
what does the graph look like for an non-ohmic conductor?
curved
why does electrical resistance increase if temperature increases?
because resistance is due to electrons colliding with ions and each other so as temperature increases there are move collisions as electrons have more energy and are moving faster and so and increase in resistance. lattice ions vibrate which obstructs the elctrons as they flow
why does a filament lamp have a curved I/V graph?
it starts steep but gets shallower as the voltage rises. The filament in a lamp is a coiled up length of metal wire and it gets hot. current flowing through the lamp increases its temperature. the resistance of a metal increases as the temperature increases
what is a thermistor?
a resistor with a resistance that depends on its temperature so can be used as temperature sensors. as the temperature does up, its resistance decreases
why does a rise in temperature for a thermistor decrease the resistance?
because it gives more electrons energy to escape from their atoms. This means that there are more charge carriers available, so the resistance is lower
what is the equation for sensitivity?
change in output pd/ change in physical property
eg the sensitivity of an oscilloscope display might be stated in mm per volt
what is internal resistance?
in a battery, chemical energy is used to make electrons move. As they move they collide with atoms inside the battery, so batteries have resistance. Internal resistance is what makes batteries and cells warm up when they are used
what is e.m.f?
electromotive force. it is the amount of electrical energy the battery produces for each coulomb of charge. measured in volts
what is the load resistance?
the total resistance of all the components in the external circuit
what is the potential difference across the load resistance?
it is the energy transferred when one 1 coulomb of charge flows through the load resistance. called the terminal p.d (V)
when would the terminal pd be the same as the emf?
if there was no internal resistance
what does the conservation of energy tell us?
energy/ C supplied by the source= energy/ C used in load resistance + energy/ C wasted in internal resistance
what are the equations for internal resistance?
E= IR +Ir E= V + Ir E= V + v
a car battery needs to deliver very high current so?
it needs to have a low internal resistance generally less than 1 ohm
why do high voltage power supplies have high internal resistance ?
because if they’re accidentally short-circuited only a very small current can flow so is much safer
properties of diodes:
- semi -conducting materials
- don’t obey ohm’s law
- allow current to flow in one direction only
- they can be used to convert alternating current to a direct current
what is meant by ‘lost volts’?
the energy wasted per coulomb overcoming the internal resistance
what does the gradient show in a V against I graph?
- internal resistance = -r
the total current entering a junction =
the total current leaving it
what is the difference between emf and potential difference?
energy transferred to a charge is e.m.f. and energy transferred from a charge is potential difference
the total e.m.f around a series circuit=
sum of the p.d.s across each component
properties of series circuits:
1) same current at all points of the circuit
2) emf split between components so : E = V1 + V2 + V3
3) resistance adds to equal total resistance because :
E = V1 + V2 + V3
so IRt = IR1 + IR2 + IR3
IRt = I (R1 +R2 +R3)
Rt= R1 + R2 + R3
properties of parallel circuits:
1) current is split at each junction so : I = I1 + I2 +I3
2) same p.d. across all components- in each loop the emf equals the sum of individual p.d.s
3) V/ R total = V/R 1 + V/ R2 + V/ R3
cancelling the Vs gives: 1/R1 + 1/R2 + 1/R3 or G = G1 + G2 + G3
resistance is always less than the smallest resistor in the circuit
what is a potential divider
a circuit with a voltage source and 2 or more resistors in series with eachother
what is the equation for current in a potential divider circuit?
I0 = V0/ R1 + R2
what is the equation for V1 and V2 in a potential divider circuit?
V1 = V0 R1/ R1 + R2
V2 = VO R2 / R1 +R2
V1 / V2 =
R1 / R2
What is a potential divider circuit mainly used for?
calibrating voltmeters, which have a very high resistance
what would happen if you put a voltmeter of a relatively low resistance across R?
you’ve effectively got 2 resistors in parallel, which will have a total resistance less than R. So V1 will be less than you’ve calculated
what is a LDR?
Light dependent resistor : high resistance in the dark but a lower resistance in the light
what is an NTC thermistor?
high resistance at low temps and low resistance at high temps
what does P= V^2 / R say and why?
it says that lamps operating at the same pd will need a lower resistance to provide a greater power because if you have lower resistance then you have more current so more energy per second as more electrons passing through so more coulombs per second and power is the rate of energy transferred per second
what does P= I^2 R say and why?
says that lamps to operate at the same current will need a larger resistance to provide more power because high resistance means higher p.d. so more energy per unit charge is transferred to the lamp so more energy per second
cost of electricity=
power in kw x time in hours x cost per unit
1 kwh =
1000 w for 3600s = 3.6 MJ
how do you reduce the effect of the internal resistance?
need to get a small current and a high voltage
what does an oscilloscope show?
frequency- voltage against time
define frequency
number of waves per second
how do you reduce the uncertainty in an oscilloscope?
by increasing the number of cycles
define resolution
smallest change in input (physical property) that can be detected- depends on what you’re using to measure the voltage and the sensitivity. The resolution is related to the precision with which the measurement can be made
define response
how quickly can it change depending on change in the physical property
what does a photovoltaic cell do?
used in solar panels- converts light energy into electrical energy. voltage decrease when light intensity decreases
in a photodiode how does voltage change with light intensity
voltage increases when light intensity increases
what’s an infra red phototransistor?
used as a detector of radiant heat and is connected as a potential divider in the circuit- as heat intensity increases, voltage increases
what does a microphone do?
produces a changing voltage (a ‘signal’) when activated by a sound input, so the multimeter is set to an A.C. range
what’s a picoscope?
a digital oscilloscope
what is a position sensor?
a simple potentiometer can be used to detect position or movement. datalogger is more accurate and can take more readings
what is a thermocouple?
consists of two junctions of dissimilar metals e.g. copper and constantan. If there is a temperature difference between the 2 junctions, an e.m.f. is generated in the circuit which can be detected by a sensitive voltmeter
what’s a hall probe?
detects and measure steady magnetic fields
