Electricity Flashcards
What is the voltage
Energy supplied to electrons
Potential difference official definition
Energy transferred/work done per unit charge V = W / Q
How many electrons in a coloumb
6.25 x 10^18
What is a coloumb a unit of
Charge
What is the charge of an electron
1.6 x 10^-19
How much energy is supplied to each coloumb
Voltage of the circuit
What does the voltmeter measure
Energy lost by each coloumb of charge
What direction does convential current go
Positive to negative
What direction do electrons flow
Negative to positive
Q, E and V in a formula
V = E/Q
What is the energy/work done per unit charge
Voltage
Relationship between emf and current
Higher emf, electrons flow faster, higher current
I, Q and T
I = Q/T or Q = IT
What is the current
Rate of flow of charge in coloumb per second
V / I =
Constant, resistance
What is ohms law
For an ohmic conductor, current is directly proportional to the potential difference across it, given that physical conditions (temperature) is kept constant
Graph of ohmic conductor
V-I graph and I-V graph are the same, goes through origin, straight line, positive gradient
What is ohms law formula
V = IR
What is resistance
How hard is it for the current to flow, ohms
What energy changes take place in the resistor
Elecrical energy to heat
What is power
How much energy is consumed (converted) or delivered every second
Units for power
JS^-1 or Watts (W)
P, I, R, and V
P = VI, V = IR, P = I^2R
Formula for power lost in resistor
I^2R
Resistor symbol
Rectangle with wire coming out of each side (wire doesn’t go through box)
Gradient of a V-I graph
Resistance
V-I graph, straight line, positive constant gradient, goes through origin, positive and negative points, what component
Resistors, thermistors, LDR
Features of resistors
Ohmic, not metal, semiconductors
V-I graph, sideways s, more vertical at ends, more horizontal at origin, goes through origin, what component is it
Lamp, filament bulb
Resistance of a filament bulb
Increases with I
Why does R increase with I for bulbs
Filament heats up, ions from metal move more, electrons collide with ions more, R increases
Thermistors symbol
Resistor symbol with positive, straight line going through with horizontal cap on the end
LDR symbol
Resistor symbol, circle around it, two arrows pointing from top left
Application of thermistors
Temperature sensor, can trigger an event to occur once the temperature drops or reaches a certain value - automatically turn on heating
Features of thermistors and LDR’s
Contant R, ohmic at constant temp/light levels, higher temp/more light = lower R
Diode symbol
Sideways isoceles going into vertical line with horizontal lines coming out of it
Diode V-I graph
Negative or positive voltage, positive I, backwards curved l shape, levels out before I axis
Features of a diodes resistance
Very high in one direction, very low in other direction
Factors of resistance
Length (R = kL), Cross section (R = k/A), Resistivity (R = kP)
What is the weird slanted P
Resistivity
Formulas involving resistivity, resistance, length and cross-sectional area
R = (PL) / A, P = (RA) / L
Resistivity at a constant temp is…
Constant
Resistivity of a good conductor
Small
Resistivity of a good insulator
Large
How does resistivity change as the temp increases
Increases
Why does resistivity increase as the temp increases
Atoms vibrate more, more collision between electons and atoms, harder for electrons to pass through, harder for current to pass
Example of a semi-conductor
Thermistor
Semi-conductor temp and resistance relationship and explanation
Higher temp = more charge carriers available = lower resistance
Semiconductor resistance-temperature graph
Decreasing exponential
What are superconductors
Materials with potential for very small (0) resistance
What is the critical temperature of a superconductor
Temperature when the resistance drops to 0
A-C graph, B is a point on C, vertical line at B, then increases exponentially, what is A
Resistance
A-C graph, B is a point on C, vertical line at B, then increases exponentially, what is B
Critical temperature
A-C graph, B is a point on C, vertical line at B, then increases exponentially, what is C
Temperature
A-C graph, B is a point on C, vertical line at B, then increases exponentially, what is the graph of
Superconductors
Application of thermistors
Triggers event when temperature drops below a certain value (automatically triggers heating)
Where are superconductors used
Strong electromagnetics such as the large hadron collider or MRI scanners, magnetic fields, transmission of electrical power, reduces energy lost
Using superconductors in strong magnetic fields
Does not require a constant power source, could be used in maglev trains (no friction) or in certain medical applications
Why are superconductors used in electromagnets
large currents are needed
How superconductors are used in electromagnets
Coils are cooled below the critical temperature using liquid helium
When can a current flow without a power source
In superconductors, a power supply is needed to start it off then the current flows by itself
What benefits would cheap, room temperature superconductors have
Would save energy and money
Time, power, energy formula
Power = Energy / time
What is the terminal p.d
Voltage through the circuit
What is the EMF
Total voltage that can be supplied when the current is 0
Equation for total p.d and emf
EMF = terminal p.d + the lost volts
What are the lost volts
Voltage lost in the internal resistor - pd across r
Actual definition of EMF
Energy that is transferred from chemical to electrical energy in the cell per coloumb
Actual definition of terminal pd
Energy transferred from electrical energy to any other type in the external circuit
Actual definition of lost volts
Energy wasted by cell per coulomb of charge
EMF formula involving current and resistance
E = IR + Ir, E = I(R + r)
How to extrapolate the data to find the EMF in the required practical
Plot a V-I graph, Y-intercept is EMF, gradient is r, V = -Ir + E, y = mx + c
Correlation between current and the lost volts
Higher current means more volts lost
What does emf stand for
Electromotive force
What is the base unit for energy
Joules
What is 1 Joule equal to
Work done by a force of 1 Newton acting on a distance of 1 metre
What is a small unit of energy
eV
What is a large unit of energy
kWh
Can energy be created or destroyed
No
What is energy transferrerd equal to
Work done
What is a potential divider
A circuit with several resistors in series used to produce a specific fraction of the source pd - either a constant or variable pd supplied
In a potential divider ciruit, what share of the pd does the smaller resistor get
The smaller share
What is a potentiometer
A type of varaible resistor
What is the forward bias of a semiconductor diode
Direction in which it allows current to flow
Threshold voltage of a semiconductor diode
Minimum voltage needed to allow the current to flow
Assumption made about ammeters
Have 0 resistance so will not affect the measurement of current in a circuit
Assumption made about voltmeters
Infinite resistance so no current can flow through them
What is the value of the resistivity
Resistance through a material of length 1m and cross sectional area 1m^2
Is resistivity dependent on environmental factors such as temperature
Yes
Most known superconductors have a critical temp of close to…
0 K (-273 celcius)
How to add resistances in series
Rt = R1 + R2 + R3 …
How to add resistances in parallel
1 / Rt = (1 / R1) + (1 / R2) + (1 / R3) …
What does VIt equal
Energy
Current through a series circuit
Constant throughout
pd through a series circuit
Total sum of voltages across all elements is equal to the supply p.d.
Current through a prallel circuit
Sum of current in each parallel set of branches is equal to the total current
pd through a parallel circuit
Potential difference across each branch is the same
What happens to voltage when identical cells are joined in a parallel circuit
Total voltage = voltage of one cell
Total voltage = voltage of one cell (in a parallel circuit) explanation
Current is split equally between branches so overall pd is the same as if the total current was flowing through a single cell
What is always conserved in DC circuits
Charge and energy
Kirchoff’s first law
Total current flowing into a junction is equal to the current flowing out of that junction (so no charge is lost at any point in the circuit)
Kirchoff’s second law
Sum of all voltages in a series circuit is equal to the battery voltage
How can you make a potential divider supply a variable potential difference
Using a variable resistor
How does an LDR’s resistance change with light intensity
Decreases as light intensity increases
What is a battery’s internal resistance caused by
Electrons colliding inside battery
What does the emf formula suggest the definition is
Energy transferred by a cell per coulomb of charge
What is the emf a sum of
Terminal pd and lost volts; E = V + v
How to measure the emf of a battery
Measuring the voltage across a cell using a voltmeter when there is no current running through cell (open circuit)
What is an NTC
Negative temperature coefficient thermistor
What is a negative temperature coefficient thermistor
Thermistor who’s resistance decreases as temperature increases
Application of potential divider
Triggering certain events; if light intensity falls, resistance across LDR increases so current decreases and resistance across a second resistor in series decreases, so pd would increase as light intensity decreased and circuit would go on to switch on light bulb because threshold voltage has been met
E / Q =
e
