Electric Circuits. Series and Parallel Circuits. Short Circuits

In these revision notes for Electric Circuits. Series and Parallel Circuits. Short Circuits, we cover the following key points:

• What is an electrical circuit?
• What are the symbols of electric circuit? Why do we use them?
• What are some of the circuit components? Which of them are necessary to make a circuit operate regularly?
• What are the sources of electricity? What is their operating principle?
• How do we combine the cells? What are the advantages and drawbacks each combination does provide?
• What kind of circuits are there? How do we calculate the quantities involved in these circuits?
• What are short circuits? What do they cause?
• How to protect electrical appliances from excessive current?

Electric Circuits. Series and Parallel Circuits. Short Circuits Revision Notes

An electrical circuit is a closed-loop or path, which forms a network of electrical components, where electrons are able to flow. This path is made by conducting wires - which allow electrons to easily flow through them - and is powered by a source such as a battery.

An electrical circuit must necessarily include the following components, without which the circuit cannot operate:

• A power (electricity) source (batteries or plugs)
• Conducting wires which act as pathways for free electrons
• One or more electrical components that consume electricity

Scientists have agreed to use simplified symbols in order to represent the circuit elements.

A cell is a source of electricity that converts chemical energy into electrical energy. There are three main types of cells:

a. Wet (voltaic) cells

A wet cell is made up by a copper and a zinc plate immersed inside a weak acid solution. The acid solution is called electrolyte and it is used to conduct electricity, while the plates are called electrodes. The free electrons emerge from the zinc plate (anode) and move to the copper plate (cathode) when a conducting wire connects the two electrodes. As a result, an electric current is produced.

A wet cell produces 1V of potential difference in the circuit. Positive and negative ions are the carriers of electricity inside the electrolyte while electrons are carriers of electricity in the rest of the circuit.

b. Dry cells

A dry cell consists of a zinc cylindrical can (anode) and a carbon rod at the centre (cathode). A chemical paste here substitutes the effect of acidic solution used in the wet cells.

The operating principle of dry cells is the same as in wet cells. Dry cells produce a potential difference of 1.5 V.

c. Accumulators

Accumulators are also known as "storage batteries." They are used in cars, laptops, flashlights etc. Accumulators are groups of wet cells enclosed inside plastic containers. Lead electrodes replace zinc and copper electrodes used in the other two types of cells. Each cell of accumulators produce 2V of potential difference.

Accumulators are rechargeable sources of electricity.

Cells - just like resistors - can be combined in series and in parallel. Such combinations are made for practical purposes or for necessity.

When the positive terminal of one cell is placed in contact with the negative terminal of another cell, we say they are connected in series. A series combination of cells offers the advantage of the increase in electromotive force. However, there is a drawback when connecting two cells is series: their life is not so long as they have to work at full capacity.

When two cells are placed side by side in a circuit, we say they are connected in parallel. A parallel combination of cells has an advantage to the series combination: the batteries last longer as they help each other to do the required work. This results in a slower consumption of each battery. However, a parallel setup of cells has a serious drawback: the potential difference is the same as if there was a single cell in the circuit.

The electric current chooses the easiest path to flow. This path is not meant to be the shortest one. When we bypass a resistor through an extra conducting wire added in that part of the circuit, this action brings the formation of a short circuit, which is a circuit with a very low resistance (just the resistance of the conductor and that of the source), and which brings an increase of the current in the circuit. In other words, a short circuit "cancels" the effect of the resistance it bypasses.

To protect the appliances from such unexpected situations, producers usually install in series to the appliances some inexpensive devices called fuses. A good fuse holds a maximum current up to 0.5 A higher than the operating current of the appliance.