Electricity Flashcards
Remember the stuff for physics (25 cards)
Ohms law
‘For metals at a constant temperature, the current in the metal is proportional to the potential difference across it’
Electon Charge
e = -1.6 x 10^-19 C Charge = Number of charge carriers x Charge on one carrier
Current
‘the rate of flow of positive charge’ (A)
Potential difference
The work done per coulomb of charge passing a point (V)
The energy transferred from the charge
EMF
The energy transferred to each coulomb of charge at a source
The energy transferred to the charge
Emf = Current x Internal resistance + Current x terminal resistance (V)
Resistance
The ratio of potential difference to current accros a component (Ohms)
Power
The rate of change of Work done
Electrical work
Work Done = Potential Difference x Current x time
Resistivity
Resistivity = (resistance x Cross-Sectional area) / Length of wire (Ohm Meteres)
Current in series
On a single branch (or loop) of a circuit, the current is the same all the way around
Current in parallel
At a junction (or branching) of a circuit, the current is shared inversely proportional to the resistance of each branch.
Potential difference in Series
Components connected in series along a branch (or loop) will share the supplied potential difference in proportion to their resistances (i.e. more p.d. across components with higher R)
Potential difference in Parallel
In parallel, each branch (or loop) has the same potential difference supplied across it.
Resistance in series
Rtotal = R1 + R2 + …
Resistance in parallel
1/Rtotal = 1/R1 + 1/R2 + …
Drift Velocity
Speed at which a charge carrier moves through a conductor when a current flows. (ms-1)
Speed in metals: 0.1 mms-1
Charge carrier density
“The number of charge carriers per unit volume” (m-3)
Semiconductors
Have a lower amount of free electrons, so have higher resistances.
When temperature increases in a thermistor, the atoms in the material release more electrons, due to an increase in thermal energy of the electrons, causing a decrease in resistance. This causes an increase in current. There is also an increse in the amplitude of the lattice Ion vibrations, increaseing the resistance but the current increase outweighs this.
In a Light Dependant Resistor (LDR) Light decreases the resistance by releasing more electrons and increasing the amplitude of the lattice ion vibrations.
Conductors
Have a large amount of free electrons, so have low resistances.
When temperature increases in a conductor, the amplitude of the lattice ion vibrations increases, causing an increase in resistance. This causes a decrease in current.
Insulators
Have a very low amount of free electrons, so have high resistances.
When temperature increases in an insulator, the amplitude of the lattice ion vibrations increase, causing an increase in resistance. This causes a decrease in current
Internal resistance
The resistance inherent to an emf source or power supply. Causes a dissipation of energy inside the source, and so there is some p.d. wasted or ‘lost’
Internal resistance = ‘Lost volts’/Current (Ohms)
‘lost volts’
The p.d. dissipated (as heat) inside a source or power supply due to its internal resistance, r.
‘Lost Volts’ = Current x Internal resistance (V)
Terminal potential difference
The p.d. available to a circuit, once some has been lost due to the internal resistance of the source. This is the p.d. that is measured across the terminals of a power source (Ohms)
Potential divider
A circuit that consists of two (or more) resistors (R1, R2, …) in series that share a potential difference between them. Since the current through each resistor is the same:
VTOT / RTOT = V1 / R1 = V2 / R2 = V3 / R3 …
