DC Theory Flashcards
Describe the temperature coefficient of resistance
This is the rate of change of resistance with respect to temperature. Hence it is the derivative of resistance with respect to temperature.
Explain omhic and non ohmic materials
Ohmic materials follow ohms law directly so there is a linear relationship between voltage resistance and current.
Non ohmic materials such as semiconductors and gases have a non linear relation between voltage, resistance and current.
Explain how current is divided between parallel resistors
I₁= I × R₁/R₁+R₂
The current through a resistor equals the total current multiplied by the value of that resistor divided by the sum of all the resistors.
Explain voltage across individual resistors in a series circuit.
V₁ = I × R₁
Voltage is calculated by the current multiplied by the value of the individual resistor. The voltages across the individual resistors in a series circuit will always add up to the source voltage. Hence
V = (I ×R₁)+(I × R₂)+(I×R₃)
Describe a potentiometer
Is a device for splitting electric potential.
For smaller currents you will have a sliding contact on a semiconductor material.
For larger current applications there is a wire coil wrapped around a non conducting cylinder. The voltage can be tapped of at any point. This creates two resistances in parallel.
Both of these types work on the principal that resistance and voltage drop are proportional to the length of a conductor of even thickness.
Explain Kirchoff’s current law
The sum of all currents going into a node is equal to the sum of all currents leaving a node. Regardless of how many branches there may be on the supply or load side.
Explain Kirchoff’s voltage law
The algebraic sum of all voltage drops in a system is equal to zero. This can also be expressed as the voltage drops across all resistors will add up to the source voltage.
Explain electrical energy
W= Pt
Electrical energy (unit: Joules) and it is equal to power (unit: watts) × time (unit: seconds).
