4.1.1: Charge and current

Electric current is the rate of flow of charge. It is measured in amperes (amps) and is the amount of charge passing through a given point per unit of time (e.g one coulomb (unit of charge) is 1 amp second (As)).

You can use the equation Q = It for questions involving charge, current, and time.

To measure electric current we use an ammeter. They are placed in series so must have a very low resistance as a high resistance will greatly affect the value of current measured.

Electric charge is a physical property, a measure of how charged something (e. a particle) is - a charge carrier is simply a particle that has electric charge. They can be electrons (usually in metals) or ions (usually in electrolytes), for example. Charge can either be positive or negative, opposite charges attract (negative and positive) and the same charges repel each other (e.g positive and positive, or negative and negative). A charge of -1, or +3 that you may see on the periodic table, for example, is known as a relative charge. It means that element has a charge of -1e or +3e. e is a constant charge (it is the elementary charge, 1.6x10^-19C).

As mentioned above, charge is measured in coulombs.

A coulomb is the amount of electrical charge flowing past a point in one second when there is a current of one amp.

Electrons are negatively charged. It therefore follows that if an object gains electrons, it becomes more negatively charged and if an object loses electrons it becomes more positively charged (as removing negative charge is like misusing a minus, you plus it). You can determine the net charge on an object using the equation Q=ne (n= number of electrons). From this equation we can see that charge is quantised as it can only take values of e (the elementary charge). You cannot get an object with a charge of 0.5e as that would result in it having gained/lost 0.5 electrons and this is not possible as electrons are fundamental particles.

So, how do charged particles move through the medium they are in?

Well, in metals most electrons remain fixed to their atoms. However, there are a number of free/delocalised electrons. The positive ions do not move but the free electrons are free to move around the metal. When a negative charge is applied at one end/positive charge at the other, these free electrons will flow from the more negative pole to the more positive pole. This is because the are negatively charged and so repel the negative end. This induces a current as it is a flow of charged particles. To increase the current we must increase the rate of flow of charge. This can be done by increasing the amount of electrons tat cross a given point per second (e.g greater cross-sectional area), or by making the same number of electrons move faster (introduce a bigger negative pole).

Just a tiny bit more in this section…

Long ago before we knew about electrons and stuff, we thought things flowed in the positive to negative direction. Well, now we know that electrons flow from negative to positive. If something is referring to conventional current/flow, it means positive to negative. If something refers to electron flow, it means negative to positive.

Also, electrolytes are just liquids that can carry an electrical current (therefore, they must have charged particles inside them somewhere). Electrolytes are either ionic solutions or molten ionic compounds. In electrolytes current is a flow of IONS (there are no free electrons in electrolytes). A good example is NaCl (tale salt, sodium chloride) as it contains the cations (positive ions) Na⁺ and anions (negatively charged ions), Cl^2-. The anions (negative ions) are attracted to the positive electrode (the anode) and the cations (positive ions) are attracted to the negative electrode (the cathode). This movement of ions is a flow of charge meaning there is a current.