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An Introduction To Electromagnetism

A high level overview of Ohm`s law, starting with the basics of charge. This article is aimed at A level students.

Date : 02/11/2021

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Sam

Uploaded by : Sam
Uploaded on : 02/11/2021
Subject : Physics

Electromagnetism

Before the 1850s, electricity and magnetism were considered unrelated. James Clerk Maxwell proved them to be linked in the single phenomenon of electromagnetism. Maxwell`s equations describe how electric and magnetic fields are generated by charges, currents, and changes of the fields.

ChargeElectromagnetism starts with charge. Charge is a physical property of subatomic particles, and is based on the number of protons and electrons in a piece of matter. Charge can be positive or negative, and we observe that opposite charges attract, and similar charges repel. The symbol Q often denotes charge.


Protons have a positive charge, and electrons have a negative charge. If a piece of matter has more electrons than protons, then overall it will be negatively charged.


Charge is measured in coulombs, named for French scientist Charles-Augustine de Coulomb. Protons and electrons have the same magnitude of charge, called the elementary charge (e). It is the smallest amount of charge that can exist freely.

1 e = 1.602 10 19 coulombs -1 coulomb = 6.24x1018 e


A proton has a charge of +e, and an electron has a charge of e. Charges smaller than e do exist, but not freely. A quark has a charge of e, but they always exist in combination, and the combination always equals e (+ or ).


The triboelectric effect is evidence that charge exists. It s a form of contact electrification that occurs when two surfaces are rubbed together and then separated. They become electrically charged, and when separated, the opposite charges attract each other. Think of the example of a balloon rubbing against hair. The balloon collects electrons from the hair, resulting in the balloon being negatively charged. Conversely the hair has become positively charged because it has lost electrons.

Conservation of Charge In a closed system, the sum of all charges must stay the same, regardless of the reactions that happen within the system.


This is also true for the decay of charged particles.

Insulators and Conductors Within a solid, the nucleus of the atoms are firmly fixed in place. In conductors, the electrons have the ability to move between atoms when subject to a force, for example an electric field. In insulators, the electrons are also unable to move between atoms they don t have the required energy.


When conductors become charged, the excess charges (normally electrons) within the material (normally a metal) repel each other, which means that the excess charge is located around the edges of the material. This charge can be passed on to another conductor by physical touch, which allows the electrons to flow between the two objects.


Conductors can also become charged by induction. When one charged conductor is brought into proximity with another grounded conductor, the repelled charges can flow into the ground , leaving the grounded conductor oppositely charged to the initial conductor. Removing the connection to ground results in the second conductor staying charged even if the initial conductor is removed.


Note that conductors become charged by losing or gaining electrons. When they become negatively charged, they have gained electrons. When they become positively charged, they have lost electrons. Protons can t move like electrons.

Coulomb s LawCoulomb s law defines the electrostatic force experienced by two charged particles a known distance apart. The force is proportional to the product of the magnitude of the charges, and inversely proportional to the square of the distance between them.

Fe = keq1q2 / r2


Where ke is the electrostatic constant (~9x109 Nm2C-2). If the force is positive, it s a repulsive force. If the force is negative, it s an attractive force. At close range, the electrostatic force is much stronger than the gravitational force.

Electricity Now that we understand charge, we can unpack the various parts of physics that make up electricity.


Electricity is the set of phenomena that occur when charges move. The presence of charges produces an electric field. An electric field exerts a force on other charges that may come into in the vicinity of the original charge. This force produces movement. This movement of charged particles is known as an electric current. An electric current produces a magnetic field, which we will explore in more detail when we look at Maxwell s equations.


Note that current doesn t actually flow rather, current is the flow of charged particles, typically electrons. Charge flows in a current. Physicists are often loose with language when discussing electricity, and current in particular. We often hear physicists describe the flow of current or the flow of charge. In both of these cases, it is correct to say the flow of charged particles .


When we study currents we typically look at how currents behave in circuits.

Electric Circuits and Ohm s LawThe study of electric circuits typically revolves around voltage, current, and resistance. Ohm s Law defines the relationship between voltage, current and resistance in a circuit.

Current As we already know, current is the flow of charged particles. More specifically, current is a measure of how much charge passes a given point in a circuit per unit time. Current (I) is measured in amps (or amperes, A):

1A = 1C / 1s


In a circuit, free electrons in the wires act as the charge carriers and therefore makeup the flow of charged particles. This is why electricity is instant when a switch is closed the charge carriers that make up the current already exist in the wires of the circuit. In fact, they re also already moving, but the movement of these electrons is random. The application of a voltage in a closed circuit gives these electrons a direction to move in, which is what produces a current.


When a current is present, electrons gradually drift towards positive. The movement is actually just the net overall movement, because the random motion of free electrons doesn t cease but now on average the movement is towards positive. The motion of electrons in a current is called drift. Because of the random motion, the drift speed of electrons is actually very slow in the order of 0.0001ms-1.


Current can be calculated from the number of electrons in the cross section of a wire, and how fast they re moving:

I = nAQv


Where n is the number of charge carriers, A is the cross sectional area of the wire, Q is the charge on an electron, and v is the drift speed.


An old historical convention states that conventional current direction is the direction that a positive charge carrier would move in a circuit. In fact, this is the opposite to what happens when electrons are flowing in a circuit. Electrons in a battery powered circuit move from the negative terminal to the positive terminal.

VoltageVoltage is the measure of the electrical potential difference between two points in a circuit. In other words, it s a measure of how much potential energy is available to do work on charges in a circuit.


We often hear about voltages in batteries. In a battery the voltage occurs because at the negative terminal there is an excess of electrons, so it is negatively charged, while at the positive terminal there is a shortage of electrons, so it is positively charged.


Voltage is measured in volts, which are defined as the work required to move a charge between two points.

1 V = 1 J / 1 C


We know that 1C is equal to ~6x1018 electrons, so 1 volt gives |1| coulomb of electrons 1 joule of energy. Voltage can exist without a current between the two terminals of a battery we can measure a voltage even without a complete circuit present. This is because voltage is a measure of potential.

Resistance Resistance is a measure of how much the flow of charge is impeded (or resisted) by an object, typically a component in a circuit. It s measured in ohms ( ). Resistance depends on an object s size, shape, and resistivity. Resistivity ( ) is a measure of how much a specific material resists current, measured in ohm-meters ( m).

R = l / A


Resistance is proportional to the length (l) and the resistivity of an object, but inversely proportional to the cross sectional area (A).


Other factors can affect resistance, for example heat. Heat increases resistance by increasing the resistivity of the material. When a conductor heats up, the atoms within the structure vibrate more, which makes it harder for electrons to flow between atoms.

Ohm s Law Ohm s Law states the relationship between current, voltage, and resistance.

V = IR


For a constant resistance, increasing the voltage increases the current, and vice versa. For a constant voltage, increasing the resistance decreases the current.


If a current encounters resistance, the electric potential difference decreases according to Ohm s law. We sometimes call this a voltage drop.


It s important to note that note not call conductors obey Ohm s Law these conductors are known as non-ohmic conductors. For a conductor to obey Ohm s law, voltage and current must be directly proportional across the component.


This is not true for all components, for example, a light bulb is non-ohmic. As the voltage across the filament in a light bulb increases, the current increases in a non-linear way. This is because it heats up (and produces light), which increases the resistance, therefore decreases the current.

This resource was uploaded by: Sam