Electricity Flashcards
current
rate of flow of charge
potential difference
work done per unit charge
resistance
voltage / current
equation linking current, charge and time
Current = change in charge / change in time
Equation in terms of voltage, work done and charge
Voltage = work done / charge
conventional current
flow of positive charge from the positive terminal of a cell to the negative terminal
why is conventional current the opposite to electron flow
conventional current was described before electric current was understood
what is current measured in
Amps
how do you measure current
using an ammeter, connected in series
how do you measure PD
using a voltmeter, connected in parallel
what is PD measured in
volts
what is resistance
the opposition to current
the higher the resistance ….
the lower the current
why are wires commonly made of copper
it has a low electrical resistance ( it is a good conductor)
what is resistance measured in
Ohms
what is Ohms law
voltage is proportional to current under constant physical conditions (temperature)
constant temperature =
constant resistance
how do you set up a circuit to investigate the relationship between PD across an electrical component and the current
set up a circuit with a cell, and a variable resistor and a voltmeter and another electrical component
what does the IV graph of a fixed resistor look like
a straight line through the origin
how can you tell if an electric component obeys Ohms law
if its IV graph is a straight line through the origin
what are some ohmic electrical components
fixed resistor and wire
what are some non-ohmic electrical components
diode, filament lamp bulb, thermistor
what does a filament lamp do
it transfers electrical energy into light and heat as the current flows through it
what happens as the current flowing in a filament lamp increases
As the current flowing increases, the temperature also increases. This causes an increase in the movement of the lattice/ions. Therefore there are more frequent collisions between the electrons and the positive metal ions so the resistance increases.
higher current =
higher temp = higher resistance
what does the IV graph for a filament lamp look like
a curve throughout
what do semiconductor diodes do
they act as one way gates, preventing the current from flowing back through the circuit
what are semiconductor diodes useful in
converting ac to dc in circuits
forward bias
in the direction of the arrow on the symbol
in what direction do semiconductor diodes work
in forward bias
how can you tell which end is the forward bias end
the component has different coloured ring at the forward bias end
what is the resistance of the diode in reverse bias
infinite
what is the resistance of diodes in forward bias
very low resistance
what does the IV graph of a semiconductor diode look like
straight line through the negative end and then steep uphill slightly curved
total resistance in series =
R1 + R2 + R3
total resistance in parallel
1/R = 1/R1 + 1/R2 + 1/R3
What happens to the total resistance if more resistors are added in parallel
Resistance decreases
Current in series
The same for all components
Current in parallel
Split between the different branches
Total current into a junction =
Total current out of a junction
What does the amount of current in each branch depend on
The resistance of components in the branch. More resistance = less current
Voltage in series
Shared between components but depends on their resistance. More resistance = more voltage (USE RATIOS)
Voltage in parallel
Equal
Voltage in cells connected in series
total voltage between end of the chain of cells is the sum of PD across each cell
Voltage in cells connected in parallel
Total voltage across arrangement is the same as for 1 cell
Conservation of Charge
Charge is never used up or lost at a circuit
Kirchoff’s first law
Sum of current entering junction = Sum of current leaving the junction
Conservation of energy
Energy is never used up or lost in a circuit
Kirchoff’s Second law
The total emf in a closed circuit = sum of PD’s across each component
in parallel circuits :
The sum of the voltages in each closed circuit loop is equal to the total e.m.f of the power supply
how does voltage/ current flow in a circuit
From the long end of a cell to short end
PD
Work done per unit charge
Equation linking power, energy and time
Power = Energy / Time
Equation linking power, work done and time
Power = Work done / Time
Equation linking power, current and voltage
Power = Current x Voltage
Equation linking power, current and resistance
Power = Current^2 x Resistance
Equation linking power, voltage and resistance
Power = Voltage ^2 / Resistance
Equation linking energy, current, voltage and time
Energy = current x voltage x time
Purpose of potential dividers
Get a variable PD / a constant PD
Choose a specific PD
Split PD between 2 components
Voltage out when using potential dividers =
R2 / R1 + R2 x Voltage in ( when the voltmeter is on R2)
Why are variable resistors used in potential dividers
to vary the voltage out and cause external components to switch on or off
What happens to the resistance of an LDR when the light intensity increases
Resistance decreases
What happens to the resistance of an LDR when the light intensity decreases
Resistance increases
What happens to the resistance of a thermistor when temperature increases
Resistance decreases
What happens to the resistance of a thermistor when temperature decreases
Resistance increases
What happens to voltage out in a potential divider circuit when resistance increases
Voltage out also increases as voltage out and resistance in this type of circuit are proportional
Why is their electrical heating in wires
As the free electrons move through a metal wire, they collide with ions which get in their way.
This means they will transfer some/al their kinetic energy on collision.
This causes heating
What does resistance depend on
Length of the wire
Cross sectional area of the wire
Resistivity of the material
Resistance =
Resistivity x Length / Cross sectional area
What is the units of resistivity
ohmic metres
What is the units of the area
m^2
What happens to resistance if the length of the wire doubles
the resistance also doubles
What happens to the resistance if the thickness of the wire doubles
The resistance will half
What is resistivity
Property that describes how much a material opposes the flow of electric current through it
Increase in resistivity =
Increase in resistance
Why is copper used for wires
It has low resistivity so current flows through easily
If the cross sectional area is a circle, what is the relationship between area and diameter
Area is directly proportional to diameter^2
What happens to the area and resistance if the diameter doubles
area x 4
resistance x 1/4
What happens to the speed of atoms if temperature increases
Atoms move faster
What happens to resistance of a metallic conductor which obeys Ohm’s law if temperature increases
resistance increases
What happens to resistance of a metallic conductor which obeys Ohm’s law if temperature decreases
Resistance decreases
Thermistor
A non-ohmic conductor and sensory resistor whose resistance varies with temperature
What happens to the resistance of a thermistor as temperature increases
Resistance decreases
What are thermistors used in
Ovens, fire alarms and thermometers
What happens if a material is cooled below the critical temp
Its resistivity disappears completely and is now a superconductor
Superconductor
A material with no resistance below a critical temp
Critical temp
The temp at which a material becomes superconductive
What are superconductors useful for the productions of
Strong magnetic fields
Reduction of energy loss in the transmission of electrical power
Describe how to determine the resistivity of constant wire (from the gradient of a graph)
Measure the thickness of the constantan wire using the micrometer in at least 3 places and find the mean diameter ‘d’.
Set up the apparatus with an ammeter and a voltmeter parallel to the wire
Attach the crocodile clips so that the length of wire between the crocodile clips, L = 1.000m measured on the metre rule.
Set the voltage, V, to a suitable value.
Record the current and record values for L, I, and V in the table.
Repeat the procedure for L = 0.900, 0.800, 0.700, 0.600, 0.500, 0.400 and 0.300m.
Repeat experiment for each length so you have a total of 3 values for I for each length L.
Calculate average current for each length.
Calculate the resistance R = V/I in Ω for each length and record values in your table.
Plot a graph of the mean R against L.
Draw the best straight line of fit through the points and find the gradient (the graph should be a straight line through the origin).
Calculate the cross-sectional area of the wire A = πd2/4 in m2.
From the gradient of your graph calculate the resistivity of constantant. The accepted value is 4.9 x 10-7 Ωm
What are the control variables in the resistivity experiment
Voltage in through the wire
Cross sectional area of the wire
What is the independant variable in the resistivity experiment
Length of the wire
What is the dependant variable in the resistivity experiment
Current through the wire
Why should we only use small currents in the resistivity experiment
To keep the temperature constant since it affects the resistivity and resistance
Why should we use low voltages in the resistivity practical
If a high voltage was used, the wire would become very hot and would be dangerous to touch
What is the gradient of the graph in the resistivity practical
resistivity / area
What happens as charge passes through a power supply
it gains electrical energy
What is EMF
The amount of chemical energy converted to electrical energy per coloumb of charge when passing through a power supply
equation linking EMF, energy transferred to electrical energy and charge
EMF = E / Q
How is EMF measured
By connecting a high resistance voltmeter (so there is no/little current) around the terminals of a cell in an open circuit.
Terminal PD
PD across terminals of a cell
If there is no internal resistance, terminal PD =
EMF
Benefits of superconductors in electrical transmission over long distances
there is 0 resistance which means there is a reduced power loss
Difficulty of superconductors in electrical transmission over long distances
Very low temperature is needed to achieve the critical temperature.
It must be kept at the critical temperature
Why is the terminal PD always lower than the EMF
due to internal resistance
Lost volts
Work done per unit charge to overcome internal resistance.
OR
Voltage lost in the cell due to internal resistance
lost volts (v) =
EMF - V
EMF =
current x ( resistance of load x internal resistance)
IR =
terminal PD
Ir =
lost volts
Internal resistance
resistance of the materials within the battery
Describe the procedure for the EMF and internal resistance RP
Set up the circuit as shown in the diagram.
Set the variable resistor at its maximum value.
With the switch open, record the reading, V, on the voltmeter.
Close the switch and take the readings of the terminal p.d., V, on the voltmeter and current, I, on the ammeter.
Adjust the variable resistor to obtain pairs of readings of V and I, over the widest possible range. Do this in eight approximately equal increments of the current.
Open the switch after each pair of readings. Only close it for sufficient time to take each pair of readings.
Rearrange the equation: ε = V+I r into the form y=mx+c and plot a suitable graph to determine the internal resistance being careful of the ‘sign’ of the gradient.
What is the aim of the EMF and internal resistance RP
Investigate the relationship between emf and internal resistance, by measuring variation of I + V using a variable resistor
Independant variable of the EMF and internal resistance RP
Voltage and current
Dependant variable of the EMF and internal resistance RP
resistance
Control variable of the EMF and internal resistance RP
EMF of the cell
Internal resistance of the cell
