6 Electricity Flashcards
What does the longer line of a cell/battery represent?
the positive terminal
What does the shorter, wider line of a cell/battery represent?
the negative terminal
In a circuit, which way does conventional current flow?
from positive to negative
Which direction will electrons flow?
from negative to positive
Define current
the rate of flow of charged particles around a circuit
What are the units of current?
Amps
Should ammeters be connected is series or parallel?
series
Define potential difference
the amount of work done (energy transferred) per unit charge
State the unit of potential difference/voltage
Volts
Should voltmeters be connected in series or parallel?
parallel
How many electrons are there in one Coulomb?
6 x 10^18 electrons
What three methods could be used to help find the voltage split in series?
- use Ohm’s law
- use ratios
- use the potential divider equation
State Kirchloff’s second law
in a closed series circuit, the sum of the voltages of each component must equal the voltage across the power supply
Define resistance
voltage
———-
current
What can the voltage of a power supply also be known as?
the electromotive force (EMF)
What does a voltmeter measure?
the difference in the potential (energy) carried by each coulomb before and after a component
What is Kirchloff’s first law?
the total current entering a junction must equal the total current leaving the junction
What is Ohm’s law?
the current through a conductor is directly proportional to the potential difference across it, provided the physical conditions do not change
What is a component that obeys Ohm’s law known as?
an ohmic conductor
For an ohmic conductor, what will an I against V graph look like?
a straight line through the origin
For an ohmic conductor, what happens to resistance when potential difference is doubled?
nothing, resistance remains constant
What are components that do not follow Ohm’s law called?
non-ohmic conductors
State four examples of non-ohmic conductors
- filament bulb
- thermistor
- light dependent resistor (LDR)
- diode
For a metal, what happens when it heats up? Why?
- resistance increases
- the vibrations of the ion lattice will increase, meaning electrons will collide more frequently with the ion lattice
Why don’t insulators allow current to flow easily?
there a very few charge carriers (electrons) available
State three examples of semi-conductors
- thermistor
- light dependent resistor (LDR)
- diode
For a semi-conductor, what happens when it heats up? Why?
- resistance decreases
- more charge carriers become available
How does the resistance of a diode change with voltage?
- forward bias: initially high resistance (gives small current), at 0.6-0.7 V, resistance decreases rapidly (current increases)
- reverse bias: high resistance (almost zero current)
What four factors affect the resistance of a wire?
- temperature
- length
- cross-sectional area
- the material
What is the effect of increasing temperature on the resistance?
- the higher the temperature, the more the lattice vibrates, increasing the chance of electrons colliding
- therefore resistance increases
What is the effect of increasing the length of wire on the resistance?
- the longer the wire, the greater the number of atoms, therefore the greater the chance of collision
- therefore resistance increases
What effect does increasing the diameter of the wire have on the resistance?
- increasing the width of the diameter increases the amount of charge that can pass through
- therefore resistance decreases
What is resistivity?
a measure of how easily current can flow through a material
Is resistivity a property of the material or the sample?
resistivity is a property of the material
What is the equation used to calculate resistivity?
resistivity = resistance x cross-sectional area
———————————————-
length
What is a superconductor?
a material with zero resistance when it is at or below the critical temperature
State three uses for superconductors and their advantages
- power cables: transmit electricity without loosing any power
- electromagnets: high magnetic field strengths
- computers: high speeds
Define power
the rate of transfer of energy
What is power measured in?
Watts
Give an equation which connects work done, potential difference, current and time
W = VIt
What causes internal resistance inside a battery?
collisions between electrons and atoms inside the battery
What is the definition of e.m.f?
the energy transferred by a voltage source per unit charge
Define internal resistance
the internal resistance of a source is the loss of potential difference per unit current in the source when a current passes through the source
How can internal resistance be found from a graph of terminal-voltage against current?
negative of the gradient
How can e.m.f be found from a graph of terminal-voltage against current?
y-intercept
What happens to the EMF when identical cells are arranged in series?
their individual EMFs will add together
What happens to the EMF when identical cells are arranged in parallel?
their individual EMFs won’t add together
What are the disadvantages of arranging identical cells in series?
- their internal resistances will add together, reducing the size of the current
- if one cell breaks, the whole circuit stops working
What are the advantages if arranging identical cells in parallel?
- resistance is reduced allowing for larger current
- if one cell breaks the circuit continues to work
What is a potential divider?
a combination of resistors in series connected across a voltage source
What is the EMF supplied known as when dealing with potential dividers?
voltage in
Compare the ability for a variable resistor and a potentiometer to vary current
- variable resistor: can vary current but not as sensitively and cannot achieve 0A
- potentiometer: can vary current sensitively from 0A to a maximum
Compare the ability for a variable resistor and a potentiometer to vary potential difference
- variable resistor: can vary p.d. but not as sensitively and cannot achieve 0V
- potentiometer: can vary p.d. sensitively from 0V up to EMF
Which is easier to connect: a variable resistor or a potentiometer?
variable resistor
