5.1 Charge and voltage

Objective 5.1.1: Describe how to give an electrically neutral object made of Perspex or PVC an electric charge.
Objective 5.1.2: Explain how to tell the difference between a positive charge and a negative charge.
Objective 5.1.3: Explain the role electrons play when an object is charged and discharged.
Objective 5.1.4: Describe two ways of discharging an object that has been charged.
Objective 5.1.5: Explain what voltage sources are used in daily life.
Objective 5.1.6: Perform calculations using the formula $Q = I imes t$ .
Objective 5.1.7: Perform calculations involving the elementary charge of the electron and the proton.

Static Electricity Phenomena: Everyday examples include hearing a soft crackling sound or seeing sparks jumping across when taking off a fleece sweater on a dry winter day. These sparks are discharges of static electricity.
Safety Note: Under normal conditions, these discharges are not dangerous, though the sensation of current running through the body can be unpleasant.
The Process of Charging by Rubbing: * An object can become electrically or statically charged through friction (rubbing). * PVC Example: Rubbing a PVC tube with a woollen cloth causes the tube to become charged. Evidence of this charge includes the tube attracting paper shreds or a fine stream of water. * Perspex Example: Rubbing a Perspex rod firmly with a silk cloth also causes the rod to become charged.
Indicators of a Charged Object: * Attraction: Charged objects attract other objects, such as polystyrene balls, paper fragments, or dust. * Sparks: Electricity may jump between objects in the form of sparks. * Sensory Feedback: Charged discharges can be heard (crackling), seen (sparks), or felt (a mild shock).
Environmental Factors: * A charged object typically loses its charge quickly. * Humidity plays a critical role; higher water vapor content in the air accelerates the rate of discharge. * Experiments involving static electricity are most successful when the air is very dry.

Two Types of Charge: Physicists have identified two distinct types of electric charge, designated as positive ( $plus$ ) and negative ( $minus$ ).
Material-Specific Charges: * Perspex: A Perspex rod rubbed with a silk cloth acquires a positive charge. * PVC: A PVC tube rubbed with a woollen cloth acquires a negative charge.
Laws of Attraction and Repulsion: * Repulsion: Objects with the same charge repel each other (e.g., two positive Perspex rods or two negative PVC pipes). * Attraction: Objects with different charges attract each other (e.g., a positive Perspex rod and a negative PVC pipe).

Electrical Neutrality: An uncharged (neutral) object contains exactly the same amount of positive charge and negative charge, resulting in no observable net charge.
Electrons: These are small, negatively charged particles. Charging an object involves these particles "jumping" between a cloth and an object. * Negative Charging: If electrons move from the cloth to the object, the object gains a surplus of electrons and becomes negatively charged. The cloth, having lost electrons, becomes positively charged. * Positive Charging: If electrons move from the object to the cloth, the object loses electrons and becomes positively charged. The cloth gains electrons and becomes negatively charged.
Protons: These are particles responsible for positive charge. Unlike electrons, protons are fixed in their positions within a solid and cannot move from one object to another.
Fundamental Rule of Charging: When an object is rubbed, it is always the negative electrons that move; protons never move between objects during this process.

Voltage Definition: Voltage is a measure of the electrical potential difference between two points. For example, if Ball A is negatively charged and Ball B is positively charged, there is a voltage between A and B.
The Mechanism of Current: When a conducting connection is made between two objects with a voltage difference (e.g., from Ball A to Ball B), electrons move from the negative side to the positive side. This flow of electrons constitutes an electric current.
Duration of Current: In static discharge scenarios, the current flows only for a very short duration (a fraction of a second) because the voltage disappears as soon as the charges equalize.
Sparks and High Voltage: * Sparks can jump if there is a sufficiently high voltage between a charged object and its surroundings. * Automotive Example: The exterior of a car can build up a static voltage of up to $3000\,V$ while driving. Getting out and touching the car can result in a shock as the car discharges through the body. * Safety: These shocks are harmless because the total current is small and the flow duration is instantaneous.
Practical Limitations of Static Electricity: Static electricity generators (such as the large one found in the Teylers Museum in Haarlem) can generate high voltages, but they discharge too quickly for practical utility.

Requirements for Practical Use: Daily applications require a voltage source that can provide a constant voltage and produce a current for an extended period.
Common Sources: * Dynamos. * Batteries.

Electron Flow in Currents: When a current of $1\,A$ (ampere) flows through a wire, approximately $6.2 \times 10^{18}$ electrons move across any given cross-section of that wire every second.
The Coulomb (C): * The unit of electrical charge is the coulomb, symbolized by $C$ . * Named after the French researcher Charles-Augustin Coulomb ( $1736-1806$ ). * Definition: $1\,coulomb$ is equal to the charge transported by a current of $1\,ampere$ in $1\,second$ . * Electron Equivalent: $1\,C$ is equal to the combined charge of $6.2 \times 10^{18}$ electrons.
The Mathematical Formula: The total charge ( $Q$ ) flowing through a wire is the product of the current ( $I$ ) and the time ( $t$ ): * $Q = I imes t$ * Where: * $Q$ is the charge in coulombs ( $C$ ). * $I$ is the current in amps ( $A$ ). * $t$ is the time in seconds ( $s$ ).