OCR A Physics A-level Charge and Current PDF Notes

OCR A Physics A-level Topic 4.1: Charge and Current Notes www.pmt.education  Charge Electric current 𝑑𝑑𝑑𝑑 Electric current, I, is defined as the rate of flo...

OCR A Physics A-level Topic 4.1: Charge and Current Notes www.pmt.education  Charge Electric current 𝑑𝑑𝑑𝑑 Electric current, I, is defined as the rate of flow of charge 𝐼𝐼 =. The SI base unit for current is 𝑑𝑑𝑑𝑑 Amperes (A). The current in an electrical circuit can be measured using an ammeter, which is placed in series. Charge and coulombs Charge, Q, is a physical quantity, which can be either positive or negative. It is measured in coulombs (C), where 1 coulomb is defined as the flow of charge in a time of 1 second when the current is 1 ampere. It has the SI base units of Is. Like charges repel each other, whereas opposite charges attract each other. When we refer to the charge of ions and the components of atoms, we look at it as a quantised number – a proton has a charge of +1, and an electron has a charge of -1. However, these numbers represent multiples of the elementary charge, e, 1.6x10-19 C. The net charge of a particle is due to the gain or loss of electrons. In an atom, the number of protons equals the number of electrons, so the charges cancel each other out and the overall charge is neutral. Increasing the number of electrons will produce a negative ion. Removing electrons will produce a positive ion, as there are now more protons than electrons. The net charge on a particle Q is given by Q = ± ne, where n is the number of electrons added or remove, and also the quantised charge value for the particle. Charge carriers Electric current is the rate of flow of charge, but charge can be carried in several ways, depending on the material the current is passing through. The current in metals is carried by electrons. In a metal, there is a lattice of positive ions, surrounded by free electrons. The positive metal ions are fixed in place, but the electrons can move around, and so when one side of the metal is made positive, and the other side is made negative, the electrons will be attracted to the positive side, and move through the metal as electric current. Some liquids can conduct a charge. These conducting liquids are called electrolytes, and are commonly ionic solutions. This means they contain positive and negative ions. An example of this is water with salt, NaCl, dissolved in it. The salt splits in to Na+ cations and Cl- anions. When a pair of electrodes (the anode is the positive electrode and the cathode is the negative electrode) are placed in the solution, the cations will be attracted to the cathode, and the anions will be attracted to the anode. This produces an electrical current. Conventional current Conventional current was discovered and defined well before the discovery of the electron. It is the rate of flow of charge from the positive to the negative terminal, and this is how all electric currents are treated, regardless of the direction the charge carriers are moving in. In metals, the www.pmt.education electrons flow from negative to positive, so the electron flow is in the opposite direction to the conventional current. Kirchhoff’s first law Kirchhoff’s first law states for any point in an electrical circuit, the sum of the currents in to that point is equal to the sum of the currents coming out of that point. This law is a consequence of the conservation of charge. Charge is a fundamental physical property, which cannot be created or destroyed, so it must be conserved. Mean drift velocity When electrons move through a metal, they frequently collide with the positive metal ions, resulting in random movement. When a power supply is connected, the free electrons are attracted towards the positive terminal, but they still collide with the positive metal ions. The mean drift velocity, v, is defined as the average velocity of the electrons as they travel down the wire, colliding with positive metal ions. The number density, n, of a material represents the number of free electrons per unit volume. Conductors, such as metals, have very high number densities, around 1028 per m3. Insulators, such as plastics, have much smaller number densities, and semi-conductors like silicon have in- between values. When the value of n is lower, the electrons must travel faster to carry the same current. An additional formula for current can be determined. 𝑑𝑑𝑑𝑑 𝐼𝐼 = 𝑑𝑑𝑑𝑑 The total charge, Q in the wire is the product of the number of free electrons per unit volume, n, the elementary charge, e, and the volume, V: 𝑛𝑛𝑛𝑛𝑛𝑛 𝐼𝐼 = 𝑑𝑑𝑑𝑑 The volume of the wire is equal to its cross sectional area, A, multiplied by its length. The length of the wire divided by the time taken for the electrons to cross this distance is equal to the mean V drift velocity, v, so we can rewrite 𝑑𝑑𝑑𝑑 as Av. This gives us our final equation, 𝐼𝐼 = 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 We can rearrange this equation to find the mean drift velocity for electrons in a wire when the current, the cross sectional area of the wire, and the number density of the metal are known. www.pmt.education