CHAPTER TWO Flashcards
What is voltage a measure of?
Joules of energy received per unit charge passing through
In which direction does CONVENTIONAL current flow?
positive to negative
In which direction do charge carriers actually flow around a circuit?
negative to positive terminal
the opposite to conventional current
What is the charge of one electron?
1.6*10^-19 coulombs
How many electrons are in one coulomb of charge?
1/charge of one electron
=6.25*10^18 electrons
What resistance do ammeters have?
A NEGLIGIBLE one (so that they do not affect current flowing through a circuit)
What do voltmeters measure?
How much ENERGY charge carriers have (the voltage in a circuit)
What resistance do voltmeters have?
A VERY HIGH resistance so that when current splits at a junction only a tiny amount of charge goes to the voltmeter, having little affect on the circuit
How are ammeters and voltmeters connected?
voltmeters=parallel
ammeters=series
What are the units of charge?
coulombs, C
What is the formula for charge?
current*time
Q=I*T
Why is potential difference called POTENTIAL difference?
The charge carriers gain POTENTIAL electrical energy which they then transfer as they move through the circuit
When resistors are connected in series what happens to the pd and the current?
The current is the same through each resistor
The potential differences through each add up to the total EMF
What is the total resistance when resistors are connected in series?
The sum of all resistances of the resistors
When resistors are connected in parallel what happens to the PD and current?
All component have the same PD, equal to the EMF
The branch currents add up to the total current (as the current splits across branches, depending on the resistance)
What is the total resistance when resistors are connected in parallel?
Always less than the smallest resistance
1/Rt=1/R1+1/R2+1/R3+etc
What is the symbol for conductance?
G
What is conductance?
How well a component in a circuit conducts electricity
The reciprocal of resistance
What are the formulas for conductance?
G=I/V
G=1/R
What is the SI unit of conductance?
Siemens, S
What happens to conductance when resistors are connected in parallel and in series?
(the opposite of resistance)
series: 1/Gt=1/G1+1/G2 etc
parallel: Gt= G1+G2 etc
What is ohm’s law?
V=IR
The resistance must be independent of the voltage and current.
What is ohmic behavior?
When a component follows ohm’s law, V=IR
Why is a lightbulb not an ohmic conductor?
Because when the voltage increases the charges carriers get more energy and more energy is transferred to the metal atoms they hit. The atoms vibrate more and make it more difficult for charge carriers to flow through, increasing the resistance. This means voltage is not independent of resistance.
What will the VI graph of an ohmic conductor look like?
A straight line
Where would there be potential difference across a circuit?
Across the power supply
Across a resistor
What does an increased voltage lead to?
Increased current
What does an increased resistance lead to?
Decreased current
What is the difference between EMF and PD?
EMF is energy given (to charge carriers) per unit charge
PD is energy transferred (by charge carriers)per unit charge
What is the formula for EMF?
energy supplied/charge
What is one volt?
one joule per coulomb
What is one amp?
one coulomb per second
What is potential difference?
The work done to drive coulomb of charge through a component
=energy transferred per unit charge
What is power?
Energy transferred per second
P=E/t OR P=IV
How can EMF be measured?
When no current flows (as otherwise EMF will be lost to internal resistance in the battery)
You can connect just a voltmeter across the battery as these have such a high resistance that current flowing is almost negligible.
Describe a diode
- Only let current flow through one way
- Non metallic, made of semi-conducting materials
- Do not always obey ohm’s law
- Can be used to convert AC to DC
What is the formula for energy?
energy=voltagecharge
E=VQ
energy=powertime
E=PT
What is the formula for EMF?
E = IR +Ir
Currentresistance)+(currentinternal resistance
what does EMF stand for?
electromotive force
How would you measure and find the EMF and internal resistance?
Make a circuit with a voltmeter across a battery with an ammeter and variable resistor in
Change the resistance and measure the current and voltage
Plot a V against I graph
This gives V=-rI+E
Meaning the gradient is negative internal resistance and EMF is h Y-intercept
What is a potential divider circuit?
Two or more resistors are in series with each other, with a source of fixed OD between them
The PD is divided between the resistors
Usually one of the resistors is some kind of sensor (eg thermistor or LDR)
What is the potential divider equation?
V1=V0+R1/R1+R2
What is resistance measured in?
Ohms (volts per ampere)
What is the symbol for resistivity?
ρ (rho)
What is the formula for resistivity?
(resistance*cross sectional length)/length
ρ=RA/L
What is the symbol for conductivity?
σ (sigma)
What is the formula for conductivity?
(the opposite of resistivity)
σ=GL/A
What is resistivity measured in?
Ωm (ohm metres)
What is conductivity measured in?
Ωm^-1
What are the differences between resistance and conductance, and resistivity and conductivity?
Resitance and conductance are OBJECT properties, resistivity and conductivity are MATERIAL properties
What is the range of a sensor?
The difference between the maximum and minimum output voltages.
What is the sensitivity of a sensor?
change in output voltage/change in physical property
What is the resolution of a sensor?
The smallest detectable change in physical property
smallest change in output/sensitivity
How would you increase the range of a potential divider sensor circuit?
By decreasing the resistance of the fixed resistor
What effect do low light levels have on LDRs?
They have a higher resistance–>A higher voltage output (which can be used to switch on other circuits)
What gives the best sensitivity in a potential divider sensor circuit?
Having the fixed resistor at the median average of the variable resistor (sensor)
better range=better sensitivity
What is a calibration curve (for a sensor circuit)?
A graph of output voltage against physical property, by extrapolating the graph you can see what value of physical property would produce the output voltage you have
What is a linear response in a sensor?
The output voltage is proportional to the changes in physical properties (making a straight line graph rather than a calibration curve)
What are the important qualities of a sensor?
- Good resolution
- appropriate sensitivity
- rapid response time
- no or small unsystematic random errors, possibly reduced by averaging
- no or small systematic or zero error
