When a battery is connected to the circuit, the current in this circuit is made to flow from the positive to negative terminal 2.1 We will discuss voltage sources in more detail later on. When the switch is open the current can not flow - as it can not jump from one side of the switch to the other in normal circumstances. When the switch is closed it behaves as a piece of wire and the current flows unimpeded. There is no general rule as how they look like, but often there are two metal strips on the two sides, and we put them in the circuit by joining circuit wires to these stripes. A measure how hard it is for current to flow through a given resistor is called its resistance .
Resistance is measured in units called ohms .
If the resistance is large, then the current flows with great difficulty. If it is small, then the current flows very easily and it may reach very large values. Resistors often look like tiny painted cans with two wires, one at each end. The two wires are identical and it does not matter which way they are connected in the circuit. Wire wound resistors are used where the resistor has to dissipate a lot of heat.