Resistance

The current through a component depends on what voltage we put across it and also on a property of the component we call its resistance. The greater the resistance then the bigger the voltage we need to make a certain current flow.

Resistance (Ohms) = Voltage (V) / Current (A)           R = V / I

The resistance of many components can vary, e.g. the resistance of a bulb gets bigger when the bulb gets hotter.

A resistor is a component that is designed to have a fixed resistance. When resistance is fixed then voltage and current are proportional.

There are a number of resistive components with interesting properties.

Fixed Resistor	Variable Resistor	Bulb	Thermistor	L.D.R.	Diode
Resistance is designed to be fixed.	Resistance varies when a knob is turned. There are a lot of these in your home.	Resistance increases with temperature, as it does for all metals.	Resistance decreases with temperature. We shall study semiconductors later.	Light dependant resistor. Resistance decreases when light shines on it.	High resistance one way round, low resistance the other.

Ammeters are assumed to have zero resistance. When we put them in series with other components we assume they do not add to the total resistance and therefore change the current they are supposed to be measuring.

Voltmeters are assumed to have infinite resistance. When we put them in parallel with other components we assume they do not change the resistance of the circuit and therefore the voltage they are supposed to be measuring.

There is a property called Conductance G (Siemens) which is just the inverse of resistance, i.e. G = 1 / R

Conductance (S) = Current (A) / Voltage (V)       or      G = I / V