Electricity is a flow of electrical charge caused by an electrical potential difference.
Syllabus reference R3.2.6Reactivity 3.2.6 - A primary (voltaic) cell is an electrochemical cell that converts energy from spontaneous redox reactions to electrical energy.
- Explain the direction of electron flow from anode to cathode in the external circuit, and ion movement across the salt bridge.
Guidance
- Construction of primary cells should include: half-cells containing metal/metal ion, anode, cathode, electric circuit, salt bridge.
Tools and links
- Reactivity 1.3 - Electrical energy can be derived from the combustion of fossil fuels or from electrochemical reactions. What are the similarities and differences in these reactions?
The nature of electricity
Direct current (d.c.) electricity is a flow of electrical charge from a region of negative potential to a region of positive potential. The negative electrons are attracted by the positive charge. This force of attraction is called the electromotive force (E.M.F.), and is measured as a voltage. It is also called the potential difference between the negative and positive terminals, or 'ends' of the voltage source.
The amount of current that can flow depends on how much 'push' or energy it's given by the voltage, as well as the resistance of the circuit. The relationship is a very simple one, called 'Ohms Law':
Voltage = Current x Resistance
V = IR
The resistance of any given circuit is effectively constant, hence the current is directly proportional to the voltage applied.
Conventional current and real current
In physics, the convention is to describe a flow of electrical charge in the opposite direction to chemistry, i.e. from + to -.
For this reason the physics version of current is often called 'conventional current', it flows from positive to negative. This arose historically because the laws of physics, as regards electricity, emerged before the discovery of the electron. In chemistry we know that electrons actually make up the current, so we've got it right!
Electrons and electrical charge
Electrons are tiny particles that carry a negative electrical charge. Although we simplify matters by stating that the charge on an electron is 1 minus, this is not an actual value for electrical charge.
Charge is measured in Coulombs. The actual charge on an electron is about 1.602177 x 10-19 Coulombs. This value is given in the physical constants section of the chemistry databooklet.
Hence, a flow of electrons along a conducting wire constitutes an electrical current, as the electrons carry electrical charge.
If 1 Coulomb of electrical charge flows through a wire per second, this constitutes an electrical current of 1 Amp.
1 Amp = 1 Coulomb per second
It should be noted that the actual charge on an electron is very small and very large number of electrons would be needed to provide a current of 1 Amp (approximately 6.24 x 1018 electrons).
Ions and electrical charge
Ions are also charged particles and therefore they can provide an electrical current. This happens in electrolytes, solutions containing ions, where the ions are able to flow from one potential to another. Negative ions flow towards regions of positive potential and vice-versa.
In electrochemical cells, the movement of ions is essential to complete the electrical circuit and prevent charge build-up.
In the salt bridge, negative ions flow towards the positive half-cell (cathode) and positive ions flow towards the negative half-cell (anode).
According to convention, the anode is the electrode at which oxidation occurs, and the cathode is the electrode at which reduction occurs.