Electricity Flashcards
Area under IT graph
Charge
Drift velocity equation:
I = nAve
Current = (no. if charge carriers per m3) x (cross sectional area) x (average drift speed of carriers) x (1.6 x10-19)
Kirchhoff’s current distribution rule?
At any junction in a circuit, the total current leaving the junction equals the total current entering the junction
How can conductors become charged?
If they are insulated from their surroundings (suspended by nylon threats/in a vacuum)
Resistance of an ammeter:
Negligible resistance/ very low in comparison to current so it doesn’t alter the current in the circuit, only used as a measuring tool.
Kirchhoff’s potential difference rule?
The sum of the potential differences in a circuit is equal to zero (principle of conservation of energy)
Charge, electrons relationship?
Charge: electrons x (1.6x10-19)
Potential difference definition?
The pd between two points in an electric circuit is numerically equal to the work done by an electric field per unit charge passing between the points.
1 ev in Joules?
1eV = 1.6x10-19 J
The work of one electron volt
Define a volt
= The potential difference between two points when the electrical energy converted (to other forms of energy) is one joule per coulomb of charge
Resistance of voltmeter?
Very high so the current doesn’t pass through the voltmeter
Ideal: infinite
IV for fixed resistor:
Straight line through origin
IV filament bulb and why?
Initial region straight line, curve in negative and positive sections of graph - increasing vibrations of lattice ions scatter electrons and transfers energy into thermal, increase of temperature- resistance increases (non-ohmic conductor)
What is resistance caused by?
Caused by collisions between the free charges and the lattice of ions which make up the conductor. (High current - more collisions - resistor gets hotter
What is ohms law?
Current through a conductor is directly proportional to the voltage across it, provided the temperature remains constant
Investigating resistance important:
- Magnitude of current kept small, so temperature doesn’t heat up and therefore would not be constant throughout experiment
- switch only on while taking readings otherwise temperature would build up
- high resistance voltmeter
Is there a delay in bulbs turning on? Do long wires take longer than shorter?
No - all the wires and components are full of electrons and as soon as a power supply is connected, a current starts flowing.
What’s the charge of an electron?
-1.6x10^-19
How many electrons in one coulomb?
6.25 x10^-18
What is terminal pd
The potential difference across the source terminals
What is the terminal pd/lost volts equation?
E = V + v = Ir + IR = I(r + R)
terminal pd = E - Ir = V
Lost volts
The energy expended in compelling the charge carriers to drift inside the battery itself when a battery drives a current through an external circuit
Ir = lost volts
How to measure emf?
Connect high resistance voltmeter directly across its terminals
(High R because less current will flow, therefore Ir (lost volts) will be smaller. E - Ir = v is nearly zero. So the voltage measured is almost the Emf
What is a potential divider?
Consists of two or more resistors connected in series across a pd source
(Used to supply required values of pd to a circuit from a fixed emf source - pd required is obtained by tapping a sliding contact along the wire)
Resistance relationship with length / area
- Proportional to length
- Inversely proportional to cross sectional area
R = k L/A
k is the content of proportionality - resistivity
How do LDRs work?
- Semi conductor material
Light landing on the material can boost electrons from valence energy band up to the conduction band, increasing number of conduction electrons ➡️ conducts with lower resistance
How do thermistors work?
Thermal energy landing on the material can boost electrons from valence energy band up to the conduction band, increasing number of conduction electrons ➡️ conducts with lower resistance
Why are insulators?
There is an energy gap between the valence and conduction band that is so big that there are almost zero electrons available for conduction
