Principle and Working:
Ohm’s Law: The opposition to current flow is called resistance and is measured in ohms, (Ω).
Resistance is defined as follows. Resistance = Voltage / current = V/I
The resistance will be measured in ohms when the voltage is in volts and the current is in amps.
Kirchoff’s current law (KCL or Point law): In any electrical network, the algebraic sum of currents at any junction (or node) is always zero. i.e Σ I = 0, – at a node.
I1− I2 – I3 = 0
Kirchoff’s voltage law (KVL or Mesh law): In any closed path, the algebraic sum of all the emf’s in that path and voltage drops in all resistances in the same path is zero. i.e Σ emf + Σ IR =0, – around a closed path
For (Loop1) V − I1R2 –I2R2 = 0
For (Loop2) I2R2 – I3R3 = 0
Sign conventions: • All currents carrying (or flowing) towards a junction are assumed positive while all currents leaving a junction are assumed negative.
• The rise in voltage is considered as positive and fall in potential is considered as negative.
Series effect of resistances: When several resistors, eg. R1, R2, and R3 are connected in series, the same current, I, passes through each resistor, the applied voltage, V, is equal to the sum of the voltages across the separate resistors :
Parallel effect of resistances: When two resistors, eg. R1 and R2, are connected in parallel, as in above figure, the voltage across each resistor is the same, V, the total current, I, is equal to the sum of the currents in the separate resistors.
Voltage Dividers: Two fixed resistors can be used to obtain a lower voltage from a fixed voltage supply. The voltage across each resistor is in the same ratio as their resistances.
Thermistors: This is a resistor whose resistance decreases considerably when its temperature rises. It can therefore convert changes in temperature into changes in electric current.
Light Dependent Resistor (LDR): The resistance of a light dependent resistor decreases as the illumination on it increases. It can therefore convert changes in light intensity into changes in electric current.
