ANALOGY BETWEEN A WATER CIRCUIT AND AN ELECTRIC CIRCUIT
Current electricity is often thought of as a fluid that flows in the way that water flows through pipes. In the above comparison, the circuit elements on the right are analogous to the elements in the fluid system or water circuit on the left. The pump in the water circuit on the left produces high pressure water on its output side. If the valve is off, no water can flow but the pump will cause the water pressure to rise inside the pipe leading up to the valve.
In the electric circuit, the voltage source (in this case a dry cell) is the equivalent of a pump. The wire leading from the voltage source to the switch is equivalent to the pipe in the water circuit. The switch in the electric circuit is off. It is equivalent to the valve in the water circuit. In the off position, the switch does not allow any current to flow - just as the off valve will not allow any water to flow in the water circuit. The dry cell can be thought to pump electrons from its right side (+) to its left side (-). This causes the wire on the - terminal of the battery to have an excess of electrons - causing electrical pressure. This is analogous to the high water pressure in the pipe on the left side of the pump.
We assume that the valve in the water circuit has two positions; flow allowed and no flow allowed. If the valve is turned to allow water flow, water will begin to flow through the entire circuit. The constricted pipe on the right side of the water circuit can be considered to limit the rate of flow of water. It provides resistance to the flow of water in the circuit. Anyone who has ever bent a garden hose to slow or stop the water flow will realize that the pressure before the kink is higher than the pressure after the kink. The restricted flow pipe works the same way. The water pressure at the bottom of the constricted pipe is lower than the water pressure at the top. In fact, if the valve is ideal (offering no resistance) the water pressure at the top of the resisting pipe is the same as the water pressure at the output of the pump. The job of the pump is to take the low pressure water on its right side and pump it to its left side, and keep pumping until the pressure reaches the target pressure of the pump.
In the electric circuit on the right, the voltage source 'pumps' electrons to its left, increasing the electrical pressure up to the switch. If the switch is moved to the on position, the current will flow through the resistor, losing electrical pressure (voltage) through the resistor in exactly the same way that water pressure is lost through a kinked hose. The pressure (voltage) at the bottom of the resistor is low. The dry cell then pumps the electrons through to its left, maintaining the high pressure there.