ELECTRICITY

• Electricity starts with electrons, a particle that carries

negative charge

• An atom consists of a dense nucleus of positively charged protons and electrically neutral neutrons, surrounded by a much larger electron cloud consisting of negatively charged electrons.

ELECTRICITY: Current and voltage

• Using a pump we can push water through a pipe around a closed circuit (top drawing). The rate at which it flows depends on the pressure difference between the entrance to the pipe (left) and the exit from the pipe (right).

• The greater the pressure difference, the greater the flow. In addition, given a certain pressure difference, a fatter pipe will carry more water, and a longer one will resist the flow and carry less.

All this mirrors the behavior of the electric fluid.

• In our homes and appliances electricity usually flows in metal wires, most often of copper. Electrons in a metal can jump from atom to atom, and that way carry negative charge around the circuit.

• Like a fluid, they are driven by a kind of electric pressure, known as voltage, because it is measured in units known as volts. An electric battery produces (by a chemical process) a voltage difference V between its two ends, and therefore acts like a pump (bottom drawing).

• The electric current I flows from high voltage to low voltage and is measured in units known as Amperes

ELECTRICITY: Resistance

• Anything that opposes the flow of current in a circuit is said

to offer resistance. We measure resistance in ohms ( Ω ).

• All electrical components offer some resistance to the flow of electricity.

• Resistors are components specially made to resist the flow of electricity.

ELECTRICITY: Resistors in series

• In series circuits, resistors are joined together end to end in

a single line.

• The resistors do not have to be in a straight line.

• The same current flows through all the resistors.

• The equivalent resistance of resistors connected in series is the sum of their resistances.

ELECTRICITY: Resistors in parallel

• In parallel circuits, both ends of all resistors are joined together.

• Resistors can be connected such that they branch out from a single point (known as a node), and join up again somewhere else in the ciruit. This is known as a parallel connection

Figure 1. Example of a circuit containing three resistors connected in parallel

Circuit containing resistors in parallel, equivalent to Figure 1