Charge + Current Flashcards
current
rate of flow of charge
elementary charge
the charge on 1 proton (1.6 * 10^-19)
electric current in a wire
lattice of positive ions with free electrons.
reasons for a larger current
- greater number of electrons moving past a given point each second
- same number of electrons moving faster through the metal
conventional current
positive to negative
what are electrolytes
liquids that carry an electric current with anode (positive electrode) and cathode (negative electrode). the movement of the ions to the electrodes is a flow of charge so there’s an electric current.
kirchoffs 1 law
for any point in an electric circuit. sum of currents into that point is equal to the sum of currents out of that point/
number density in conductors
higher the no. densoty the greater the number of free electrons so the better the electrical conductor.
definition for a volt
the pd across a component when 1j of energy is transferred per unit charge passing through the component.
emf
the energy transferred from chemical energy to electrical energy per unit charge.
when work is done ON the charge carriers. charges gain energy as they pass through a cell.
emf equation
e = w/q
how do electron guns work
small metal filament heated, electrons gain enough kinetic energy to escape from the surface of the metal which is thermionic emission. when filament placed in vacuum and high pd applied between it and an anode, filament becomes a cathode. electrons accelerate towards anode and become a beam. zapppp.
electron gun equation
wd on electron = gain in KE
eV = 1/2mv^2
ohm definition
the resistance of a component when a pd of 1v is produced per ampere of current.
ohms law
for a metallic conductor kept at a constant temperature the current in the wire is directly proportional to the pd.
tempereature and resistance
as temp increases the positive ions inside the wire have more internal energy, they vibrate more, have more collisions, and the charge carriers do more work so resistance increases.
diodes and ohms law
diodes are non-ohmic + resistance of a diode is not constant
what affects resistance in a wire
- material of the wire
- length of the wire
- cross-sectional area of the wire
unit for resistivity
ohm metre
define resistivity
resisitivity of a material at a given temperature is the product of the resistance of a component made of the material and cross sectional area divided by length.
resistivity and temperature
as temperature increases resistivity increases
how to calculate resistivity from graph
is graph is R against L then gradient is p/A and times with cross-sectional area of wire
what is a negative temperature coefficient
the resistance drops as the temperature increases e.g. thermistor
filament lamp resistance
as current increases, electrons transfer energy to the positive ions which raises the temperature causing an increase in resistance
thermistor resistance
increase in temperature leads to increase in number density of the free electrons so resistance decreases.
LDR resistance
made from a semiconductor in which the number density of charge carriers depends on the intensity of the incident light. dark conditions - high resistance. light - no. density increases so resistance decreases.
define power
the rate of the energy tansferred
power equations
P = VI
P= I^2 R
P = V^2/R
costs depends on
- power of the device + how long it’s used for
