DC Electricity Flashcards
What is the si units for amps
1cs^-1
What is the si units for Ohms
1VA^-1
What is electo motive force (emf)
Energy available per coulomb of charge
What is current? Si unit?
Rate of flow of charge 1Cs^-1
Resistance, voltage and current in series and parallel circuits
Series: The current is the same through each resistor. Total resistance is found by adding resistance of each resistor.
Parallel: The voltage is the same through each resistor. Total resistance is found by adding up the reciprocals of each resistance and then reciprocal of the total.
What can be used to measure resistance in a circuit?
Ohmeter
What is Ohm’s Law? When is it non-ohmic
For metals at a constant temperature, the current in the metal is proportional to the potential difference across it. R=V/I IF the graph of p.d against I is not a straight line then non-ohmic, like a filament lamo
Symbols for resistance and resistivity and units
R for resistance (ohms) and the symbol similar to density for resistivity (ohms metre)
How do you measure the resistivity of a metal?
Resistivity= RA/L.
Calculate cross sectional area.
Record different values for different lengths of wire used for voltage and current then use V=IR for resistance of each one.
Plot resistance against length.
Gradient gives R/L so then times gradient by the area.
What happens to resistance in metals and negative temperature coefficient thermistors with increases and decreases in temp?
Metals: Resistance increases with temperature due to free electrons in metals colliding with vibrations of lattive, atoms vibrate more with temp, so more collisions. So flow of electrons decreases so current decreases thus resistance must increase V=IR
Negative temperature coefficient thermistors: Resistance decreases with temperature increase.
What is a potential divider?
Two resistors connected in series with a supply. They are in series the same current flows through both resistors and so the pd of the supply is divided by them.
What is meant by an open circuit?
The power supply has no connection between its terminals. I=0
What s meant by a short circuit?
The power supply terminals are joined by a connection with no resistance. V=0
What is potential difference? Formula and si unit?
Measure of energy transfer between two points in an electric circuit. v=w/q si unit= 1JC^-1
Explain semi-conductors
Materials that contain far fewer free electrons than metals. Semi.C with a negative temp coefficient NTC have resistance that decreases with temperature. As gets hotter, more vibrations, more electrons released, so current increase and so r decrease,
Semi.C with PTC the resistance increases with temp
What is power? SI unit?
Rate of doing work, rate of transfer of energy. 1Js^-1
What is number density?
The number of conduction electrons per unit volume of a material.
What is internal resistance
Many power supplies have some internal resistance, so some of the energy supplied to each coulomb is lost due to heating within the power supply
