9.1 - 9.11 Flashcards
which side of a cell is positive
the positive terminal is the longer side of the symbol
define potential difference
the energy transferred from electrical energy to other forms per unit charge
define the volt
the P.d across a component when 1 joule of energy is transferred per 1 coulomb of charge passing through the component
how can voltage be measured
connecting a voltmeter in parallel. ideally, it should have infinite resistance so that no current passes through it.
define electromotive force (e.m.f)
the energy transferred from chemical (or other forms) to electrical energy per unit charge
how does an electron gun work?
it needs a source of electrons. usually, through thermionic emission. then it’s placed in a vacuum and a high P.d is applied across the filament and an anode. the electrons accelerate towards the anode gaining kinetic energy. if the anode has a hole in it some electron pass through it creating a beam of electrons with a specific kinetic energy
what is thermionic emission
the emission of electrons from the surface of a hot metal filament
define resistance
a property of components calculated by dividing the P.d across it by the current in it
define the ohm
the resistance of a component that has one Volt of P.d per ampere of current
what is Ohm’s law
the P.d across a conductor is directly proportional to the current as long as the temperature remains constant
how does temperature affect resistance?
as temperature increases, resistance increases. The positive ions have greater internal energy so they vibrate with a greater amplitude about their mean position. charge carriers then collide with them more frequently causing more work to be done
what is the I-V characteristic of resistors
a straight-line graph with a constant gradient that goes through the origin. this means it has a constant resistance
what is the I-V characteristic of filament lamps
it’s not a straight line but goes through the origin, it curves off horizontally. this means it has non-constant resistance that increases with P.d
what is a diode
a semiconductor component that only allows current to flow in one direction
what is the I-V characteristic of diodes
the current is zero for all negative P.d and a small part of the positive P.d. it curves upwards into a straight line. this means the resistance is not constant.
what is the threshold P.d relating to diodes
the minimum P.d at which a diode begins to conduct. the resistance begins to drop sharply beyond this point
what three factors affect resistance
the material,
the length of the wire,
cross-sectional area
what’s the difference between resistance and resistivity
resistance may be different for components all made from copper due to different lengths or areas. resistivity is an electrical property that is constant for a material at a given temperature
how is resistance affected by wire length
for any current, increasing the length of the wire increases the P.d across it. doubling the length doubles the P.d. V=IR, therefore resistance must have doubled. R∝L (directly proportional)
how is resistance affected by the cross-sectional area
for any P.d, doubling the cross-sectional area doubles the current. V=IR so the resistance must have halved. therefore R∝ 1/A (inversely proportional)
how can resistance be obtained from resistivity
by combining R∝L and R∝1/A you get R∝L/A. ρ (resistivity) is the constant of proportionality. therefore R= ρL/A
define resistivity
property of materials. its the resistance of a component made of the material multiplied by its cross-sectional area divided by its length. measured in Ωm
whats a negative temperature coefficient (NTC) thermistor
for an NTC thermistor, as the temperature increases the resistance decreases. this is because as the temperature increases the number density of charge carriers increases.
what is the I-V characteristic of thermistors
it’s not a straight line but goes through the origin, it curves off vertically. this means it has non-constant resistance that decreases with P.d
what is an LDR
light-dependent-resistor. its resistance decreases as light intensity increases. this is because the number density of charge carriers increases as light intensity increases.
define electrical power
the rate of energy transfer by a component.
how can you derive the formula for electrical power
power = Work done / time. Voltage = work done/charge
therefore power = (voltagecharge)/time. since current = charge/time therefore power = voltagecurrent
what factors affect the electrical energy transferred to a device
the power of the device,
its operating time,
what is a kilowatt-hour
a unit of energy used to deal with large numbers of joules. It’s the energy transferred to a device with a power of 1kW operating for 1 hour
