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.
describe the key features of a thermistor
as temperature increases, more charge carriers are released in the material, increasing the electron density. 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.
calculate the power dissipated by a component
P = IV P = I^2R P = v^2/R
calculate the work done by a component using power
P = W/T
re arrange to get:
W = PT
W = IVt
calculate the cost of running components using the unit of kilowatt hours for energy
1KWH = 13p
Power x Hours x 0.13
Kirchhoff’s second law
sum of the EMFs around a series loop is equal to the sum of the PDs.
calculate the total resistance of resistors in series
add resistors to find total
calculate the total resistance of resistors in parallel
reciprocal of resistance to find total
EMF source in series
same direction: add
opposite direction: subtract
Identical EMF sources in parallel
Same EMF as one on its own
Different EMF sources in parallel
Average of the EMFs
internal resistance
a model for work done to decrease the energy in each charge occurring inside a power source
Terminal PD
the remaining energy in each charge when they have left the terminals of the battery (already decreased from internal resistance)
calculate terminal PD
E = IR E = I(R + r)
describe an experiment to measure EMF and internal resistance
- keeping the power source on a fixed setting, varying the external variable resistor and take 7 readings of terminal voltage and current.
- ensure the circuit is turned off between readings to ensure any temperature change is minimised.
- Plot a graph of terminal voltage against current where the gradient x - 1 is the internal resistance and the y - intercept is the EMF
describe a potential divider circuit
a potential divider is created when 2 or more components are connected in series so that any charge that passes through one component must also pass through the other
describe the impact on the ammeter and voltmeter readings due to temperature of the thermistor increasing
as temperature increases, the resistance of the thermistor will decrease so as the circuit EMF is constant, ammeter reading of current will increase. If the current increases and the resistance of the fixed resistance is constant, according to Ohms law, the voltmeter reading of potential difference must increase.
explain how a potential divider circuit that can be used as a sensor circuit work
as light intensity decreases, the resistance of the LDR increases, increasing the total circuit resistance. Therefore the circuit current decreases as EMF is fixed.
If the current decreases and the resistance of the fixed resistance is constant, according to Ohms law, the potential difference across the fixed resistor must decrease and then using Kirchhoff’s second law if EMF is fixed, the potential difference across the lightbulb must increase.
