Charges

Particles can carry a  positive or negative charge. This charge is a  fundamental property of the particle and can not be changed. The charge q is measured in Couloumb [C]. A larger charge means stronger attractive or repulsive forces between charged particles.  

There are four fundamental forces in nature:

• gravitational force
• strong force
• weak force
• electromagnetic force

The electromagnetic force is responsible for the interactions between charged particles. It explains why:

• Protons have a positive charge (+e=1.6 x 10^-19 C)
• Electrons have a negative charge (-e=-1.6 x 10^-19 C)
• Neutrons are neutral (q=0)

Opposite charges attract, while like charges repel.

Charge Quantization

Electric charge is quantized, meaning it always appears in integer multiples of the elementary charge: e=1,6x10^-19 C

The charge of an electron is -e=-1.6x10^-19 C. No particle has a charge smaller than this basic unit.

Couloumb's law

All charged particles exert a force on each other, called the electrostatic force (Couloumb force). The direction of this force aligns with the line connecting two charges:

• Like charges repel and the force is directed outward
• Opposite charges attract and the force is directed inward

The magnitude of the Couloumb force is proportional to both charges and inversive proportional to the square of their distance:

 F=k x (q1xq2)/r^2

Where q1 and q2 are the charges, r is the distance between them, and k is Coloumb’s constant.

The equation shows that the force increases with larger charges and decreases as the distance increases.

Potential difference (voltage)

Potential difference (V) arises from attraction between positive and negative charges. When charges are separated, they create an electric potential – a kind of „electric pressure“ that drives them back together.

Imagine a positive particle between two poles. It starts at the positive pole and accelerated towards the negative pole due to electric attraction. It gains energy as it starts moving from rest. 

Potential difference is a measure of how much energy a charged particle gains by traveling from one pole to another.

Voltage is defined as: V=W/q

• V is the voltage (in volts, V)
• W is the energy (in joules, J)
• Q is the charge (in Coloumbs, C)

A higher voltage means a stronger push on charges.

Voltage sources

A voltage source, such as a battery or power socket, maintains the potential difference by continuously separating charges.

Electric Current

Electric current (I) is the flow of charge per unit time through a conductor:

I=q/t

Where I is the current (in amperes A), q is the charge (in coloumbs C), and t is time (in seconds s)

In metals, the charge carriers are electrons, which move from the negative pole to the positive one of a circuit.

Ohm‘s law and resistance

Resistance  determines how much a material opposes the flow of current.

Ohm‘s law states that the potential difference across a metallic conductor is proportional to the current through it, provided the physical conditions do not change:

V=I x R

Where V is the voltage, I is the current, and R is the resistance (in Ohm)

Graphical interpretation

 A linear graph (voltage vs. current) indicates a constant resistance (the gradient).

A steeper slope means higher resistance. For instance, copper has lower resistance than aluminum, making it a better conductor.

Current in circuits

Series circuits:

• current entering a component is the same as the current leaving the component: I=I1=I2=I3…
• Total voltage is the sum of individual voltages across each component: V=V1+V2+V3…

Parallel circuits:

• Total current is the sum of individual currents: I=I1+I2+I3+…
•  Voltage is same across all components: V=V1=V2=V3…