electricity Flashcards
property of matter that causes electric field
charge
charge scalar or vector
scalar
value of fundamental charge
1.6x10^-19
what is fundamental charge
smallest amount of charge that exists in ordinary matter - protons and electrons have this same charge but with opposite sign
electric field
region of space where an electric force is experienced
field strength (E)
the force experienced by a small + charge placed at a point in the field
potential of a sphere
the amount of work done per unit charge to bring a small positive charge from infinity to the surface of the sphere
units of potential
JC^-1 (same as volt)
coulomb’s law
The force experienced by two point charges is directly proportional to the product of their charge and inversely proportional to the square of their separation
law for the force between masses in a) an electric field and b) a gravitational field
a) Coulomb’s law
b) Newton’s law
what makes smth a good conductor
allows a large current for a given pd
ohm’s law
the current through an ohmic conductor is directly proportional to the pd across it provided that the temperature remains constant
why does temperature affect resistance
because an increase in temperature means increased lattice vibrations resulting in more collisions between electrons and the lattice
why does increasing the temperature of a semiconductor lead to lower resistance
liberates more electrons
primary cell
cell that cannot be recharged
emf of a cell layman’s and IB
- the energy provided by a cell or battery per coulomb of charge passing through
- the work done taking unit charge from one terminal to the other (low potential to high potential)
how many kg in a tonne
1000
electric potential energy for a sphere of charge
V =Qk/r^2
how many electrons per second to produce 1 amp of current?
6.2x10^18
internal resistance
the resistance of the components inside a battery.
terminal potential difference what is it and relationship to emf
pd across the terminals of the battery
equal to emf, but if a current flows it will be less than emf
what does emf stand for
electromotive force
electrical power
the rate at which energy is changed from one form to another
kirchhoff’s first law
current into junction = current out of junction
kirchhoff’s second law
around any closed circuit, the sum of the emfs = the sum of the pds
ohm’s law equation
V = IR
ideal ammeter vs ideal voltmeter
Ideal voltmeter has infinite resistance
Ideal ammeter has zero resistance.
field strength unit
NC^-1
EPE (electric potential energy) equation
EPE = Vq
conventional current flow vs electron flow
conventional current moves from high to low potential (ie +ve to -ve) however electrons flow the other way as attracted to +ve
convert joules to electron volt (eV)
divide by fundamental charge
what is an electronvolt
energy gained by an electron accelerated through a pd of 1V
emf
energy converted from chemical to electrical per unit charge
electrical power
the rate at which energy is changed from one form to another
power delivered in a perfect battery
energy converted from chemical to electrical per unit time
power dissipated in resistor
amount of electrical energy converted to heat per unit time
electric kettle
transfers the heat produced when current flows through a wire element to the water inside the kettle
magnetic flux density (B)
quantity used to measure how string magnetic field is
definition of ampere
one ampere is defined as the current that would cause a force of 2x10^-7N/m between two long parallel conductors separated by 1m in a vacuum
what does it mean if a lamp is labelled ‘3 volts, 0.6 watts’
This means that if the bulb is
connected to 3V then 0.6W of power
is dissipated
electric cell
device that uses the energy stored in chemicals to arrange charges in such a way that a p.d. is created which can be used to cause a current to flow in a conductor
how is chemical energy = electrical PE
chemical energy = energy associated with molecules
molecules = charged bodies arranged in such a way that they don’t fly apart
when is the pd across r (internal resistance) equal to the EMF
when resistance R of external circuit = 0
when is the pd across the terminals of the cell equal to the emf?
When I is zero there will be no pd across r (internal resistance) , so V = E
pd across a resistor
amount of electrical energy converted to
heat per unit charge so if energy is
conserved
why might resistance be greater if power dissipated in the resistor is greater than recommended maximum
When current passes through the resistor heat is dissipated causing the temperature of the resistor to increase. Increased temperature will increase theresistance.
the equation for k when you have a situation with charges not in a vacuum,
k= 1/4πε
direction of movement of POSITIVE charge
Positive charge will always move from high to low potential
resistivity
the resistance of 1 m3 of substance.
why does the ideal voltmeter have high resistance
so it doesn’t draw any current
magnetic field lines direction
always north to south
state, in terms of electrons, the difference between a conductor and an insulator
a conductor contains ‘free’ electrons and insulators do
not
resistance mark scheme definition
the ratio of potential difference across a device/load/resistor to current in the device/load/resistor
difference in thermal capacity gas heated at constant pressure vs gas heated at constant volume
the thermal capacity for gases at constant volume is less, because all the thermal energy supplied goes to increasing the internal energy, so the increase in temperature in the constant volume case is greater
why must there be an electric field inside a conductor
Because in order to have a current, electrons must be moving along the wire -> so a force (electric force) must be acting on them
what is the force on a free electron inside a conductor
electric field strength x elementary charge
whats a test charge
point charge which has no effect on the field in which it is placed
what do electric cells do
convert chemical energy into electrical potential energy
what is a magnetic field
a region of space where a small magnetic dipole experiences a turning force
why are magnetic poles called north-seeking and south-seeking
because they point in those directions if suspended
unit of magnetic flux density (B)
tesla(T)
charge x voltage
energy
