Electricity Flashcards
what is current
the rate of flow of charge
define potential difference
the energy transferred between two points from electrical energy to other forms per unit charge
how can current be measured
connect an ammeter component in series
what is Kirchhoff’s first law
the conservation of charge. The sum of currents entering a junction equals the sum of currents leaving
what is meant by mean drift velocity
the average velocity of electrons as they move through a wire
whats the average electron density of conductors, semiconductors and insulators
conductors- 10 ^29 m^-3
semiconductors- 10^19 m^-3
insulators- much less than 10^9 m^-3
how can voltage be measured
connect a voltmetre in parallel
define electromotive force (EMF)
the work done on charge carriers per unit charge measured across a cell/battery. the energy is transferred from chemical to electrical
define resistance
a property of components calculated as voltage/current through the component
how is the total resistance in a parallel circuit found
the reciprocal of the total resistance equals the sum of the reciprocal resistances of each resistor
1/ R(total) = 1/R(1) + 1/R(2) …
what is an ohmic conductor
a conductor that obeys ohm’s law. this means a current-voltage graph would be linear. this can change at high voltages, it stops being linear so it’s no longer ohmic
what is a diode
a semiconductor. it only allows current to flow in one direction
what is reverse bias and forward bias
reverse bias is the direction that a diode stops current from flowing. forward bias is the direction that current can flow through a diode
what is the breakdown voltage
the voltage required for current to travel in reverse bias
what is the threshold voltage
the voltage required for current to travel in forward bias (through a diode)
why are filament bulbs, non-ohmic conductors
as current flows through the bulb it produces heat. increasing temperature increases resistance limiting the current that can flow through. therefore an increase in the voltage doesn’t produce the same increase in current
define resistivity
a property of a material. its the product of the resistance and the cross-sectional area divided by the length
what is a thermistor?
resistors that change resistance depending on their temperature
whats an NTC thermistor
a negative temperature coefficient thermistor. increasing its temperature decreases resistance
experiment to determine the resistivity of a wire
- find the length and cross-sectional area
- set up an ammeter and voltmeter in a circuit
- use a low current and find the resistance for different wire lengths
- plot a resistance-length graph and find the gradient
- multiply the gradient by the area to get resistivity
what happens to power in a circuit
when the current flow in a component with resistance power is dissipated
what is Kirchhoff’s second law?
the total electromotive force in a series circuit equals the sum of the P.d across each component. it’s the conservation of energy
how can you find the total resistance in a series circuit
the sum of the resistances of each resistor
what is the internal resistance of a cell
it reduces the voltage output before the current reaches the circuit.
experiment to find a cell’s internal resistance
- connect the cell to two wires and use a voltmeter to find the voltage (this is the electromotive force)
- connect a resistor of known resistance and measure the voltage across it
- use an ammeter to find the current in the circuit
- calculate the internal resistance with an equation
what is a potential divider circuit?
it uses the resistance from two resistors to change the potential difference across components. the greater the P.d of the first resistor the smaller the P.d across the component. it’s used when the P.d it too high for the intended use
what is a variable resistor
it’s a resistor that can change resistance.
what is a light-dependent resistor?
its resistance changes with the light intensity. greater light intensity causes lower resistance because light causes the release of electrons
define voltage
the P.d produced when one coulomb flows and releases one joule of energy
what is Ohm’s law
the P.d is directly proportional to the current in a component if the temperature remains constant
define coulomb
the amount of charge passing a point from a current of 1 amp flowing for 1 second
which side of a cell (its symbol in a circuit) is positive and negative
the longer line is positive and the shorter one is negative
what is conventional current
it’s used to describe the current in a circuit. it travels from the positive terminal to the negative terminal of a cell. this is imaginary because electrons flow in the opposite direction.
what does the symbol ‘n’ mean
the number density. it’s the number of free electrons per unit volume.
how does an electron gun work
thermionic emission: a small metal filament is heated with a current. the electrons gain enough energy to escape the surface.
these electrons are accelerated towards an anode
why does resistance increase with temperature
as temp increases the positive ions increase in internal energy. this causes them to oscillate more. this increases the rate of collisions with electrons so the electrons have to do more work.
how does resistivity change with temperature for a conductor
resistivity increases with temperature. this is because there are more collisions between the electrons and oscillating ions
how does resistivity change with temperature for a semiconductor
resistivity decreases with temperature. as temp increases, more electrons are released so there number density increases. this makes it a better conductor
why does the resistance of an NTC thermistor decrease with temperature
as the temp increases the number density of electrons increases so the resistance decreases
uses of thermistors
thermometers,
thermostats to control heating,
monitor the temp in electrical devices,
what is a kilowatt-hour
its a unit of energy used to measure large amounts of energy(joules). it’s the energy transferred by a 1kW device in 1 hour
how can you analyse a circuit if there are multiple cells in parallel
for each parallel part treat it as its own loop then combine the currents depending on the direction you labelled them and the junctions
what causes internal resistance
the metal casing and materials in the cell. some energy is transferred to heat energy. this is wasted energy. this only happens when a current is flowing
whats the terminal p.d of a cell
the p.d across the terminals of a cell when current is flowing
