Electricity Flashcards
define and calculate current
current - the rate of flow of charge per second
I = Q/T
elementary charge
1.6 * 10^-19
net charge of lithium atom
0C
net charge of lithium 1+ ion
+ 1.6*10^-19
Kirchhoff’s first law
sum of currents entering a junction is equal to the sum of the currents leaving the junction
define mean drift velocity
average velocity that charged particle attains due to an applied electric field
calculate current from drift velocity
I = Anev
n - free electron density v - drift velocity I - current in amps A - cross-sectional area of the conductor e - charge of an electron
describe conductors in terms of charge carrier density
a material with a very high free electron density
metals have 10^29 electrons per m^3
describe semi conductors in terms of charge carrier density
a material with a intermediate free electron density but the electron density will change when conditions change e.g. temperature
describe insulators in terms of charge carrier density
a material with a very low free electron density
define pd and states its unit
the work done by a component on each charge to decrease their energy
measured in volts (V)
define E.M.F. and states its units
the work done by a power source on each charge to increase their energy measured in volts (V)
define voltage and state its units
measurement of the electrical potential energy per unit charge between two electric fields
describe the difference between E.M.F. and P.D. in terms of energy transfer
EMF sources increases the energy of the charges, P.D. decreases the energy of each charge
calculate the work done by a component in a circuit
W = QV
state and re-arrange an equation using conservation of energy to calculate the velocity of electrons accelerated by an electric field in a vacuum
eV = 0.5 * m * v^2
re-arrange for v. I cant write it here
explain why drift velocity in a circuit is considerably less than this velocity
collision with atoms in an atoms electric fields provide a force opposite to the motion of electrons, decreasing the acceleration and therefore decreasing the final velocity achieved by electrons
define and calculate resistance
the ratio of potential difference to current (measured in ohms)
R = V/I
state ohms law
Potential difference is directly proportional to current when the potential difference is across a fixed resistance
(temperature can affect this)
describe an experiment to investigate the I-V characteristics of a component
- Adjust the variable resistor between its maximum and minimum settings, taking 7 measurements of potential difference and current.
- Turn off the circuit when not taking measurements to ensure temperature does not change which would affect the I-V characteristics.
- Plot a graph of current vs potential difference (the resistance is the reciprocal of the gradient of that graph)
describe the key feature of an LDR
as the LDR absorbs photons, more free electrons are released, increasing the charge carrier density of the LDR. Therefore using I = Anev, if n increases, the current must also increase if light intensity increases. If the potential difference supplied is fixed and current increases, its resistance must have decreased as current and resistance are inversely proportional for fixed potential difference.
calculate the resistivity of a material
ρ = RA/L
describe an experiment to determine resistivity of a metal
- Using a micrometre, measure the diameter at 3 different parts of the wire and calculate an average cross sectional area.
- Measure the resistance at different lengths using an Ohmmeter.
- Plot a graph of resistance vs length, determine the gradient and multiply by the area to get resistivity
R = (ρ/L) x L
describe how resistivity varies with temperature
as temperature increases, the atoms in the conductor vibrate with a larger amplitude. Therefore each electron is involved with more collisions with the atoms, decreasing current and therefore resistance increases.