DC Circuits Flashcards
Current
Rate of flow of electric charge past a point
Ohm’s Law
A conductor obeys Ohm’s Law if the current through it is directly proportional to the potential difference across its ends (for constant physical conditions e.g. temp)
The microscopic explanation of non-ohmic behaviour (e.g. why a filament lamp’s resistance increases with temperature, which increases with current).
- As the temperature increases, the amplitude of vibrations of the lattice ions increases, impeding the flow of charge carriers (drift velocity decreases).
- Impurities interrupt the structure of the lattice and impede the flow of charge carriers (again, drift velocity decreases).
LDRs/Thermistors Explanation
The light/thermal energy frees more electrons so the number of charge carriers per unit volume increases, and so the current increases.
Definition of Resistance (from Ohm’s Law)
The resistance of a component is the ratio of the potential difference across its ends to the current flowing through it.
Conduction and Valence Bands (of Electrons)
Electrons in the conduction band a free to move through the material, whereas electrons in the valence band are bound to atoms.
Electromotive Force (e.m.f.)
The total work done to move a unit charge around a complete circuit.
Potential Difference (p.d.)
The energy lost per unit charge as charges pass through a component.
Lost Volts
The difference between the e.m.f. and the terminal p.d. in a circuit (equal to the voltage across the internal resistance).
Kirchhoff’s First Law (and the corresponding conservation law)
The sum of currents entering a junction is equal to the sum of currents leaving that junction. This is equivalent to the conservation of charge.
Kirchhoff’s Second Law
The sum of e.m.f.s in a given loop is equal to the sum of p.d.s within the same loop. This is equivalent to the conservation of energy (within a loop).
Potential Divider
A potential divider is a combination of resistors in series across a voltage source. (The potential of the voltage source is provided in the ratio of resistances, so one can obtain a chosen potential difference across another component connected in parallel to one of the series resistors.)
