Electric Charge - The electric charge is basic property of matter carried by some elementary particles. Electric charge, which can be positive or negative, occurs in discrete natural units and is neither created nor destroyed.
• Benjamin Franklin named the two types of charges as Positive and Negative.
• Two objects that have an excess of one type of charge exert a force of repulsion on each other when relatively close together.
• Two objects that have excess opposite charges, one positively charged and the other negatively charged, attract each other when relatively near.
- Become positive in charge
Materials that gain a positive (+) electrical charge (or tend to give up electrons)• Dry human skin
• Rabbit fur
• Human hair
• Cat fur
- There are very few materials that do not tend to readily attract or give up electrons when brought in contact or rubbed with other materials.
- Materials that are relatively neutral- Example - Cotton, Steel
• Hard rubber
• Nickel, Copper
• Brass, Silver
• Gold, Platinum
• Styrene (Styrofoam)
• Saran Wrap
• Polyethylene (like Scotch Tape)
• Vinyl (PVC)
Surface Density of Charge- Surface density of charge is defined as the amount of charge per unit area on the surface of conductor.
The surface density of charge at a point on the surface of conductor depends upon the shape of conductor and presence of other conductors or insulators near the conductor.
The surface density of charge at any part of the conductor is inversely proportional to the radius of curvature of the surface of that part.
This is why surface density of charge is maximum at the pointed parts of the conductor.
Some common conductor materials are –
• dirty water
Insulators are materials that have just the opposite effect on the flow of electrons. They do not let electrons flow very easily from one atom to another. Insulators are materials whose atoms have tightly bound electrons. These electrons are not free to roam around and be shared by neighboring atoms.
Some common insulator materials are-
• (dry) cotton
• (dry) paper
• (dry) wood
• pure water
Some Common Facts About Conductors and Insulators
• In conductive materials, the outer electrons in each atom can easily come or go, and are called free electrons.
• In insulating materials, the outer electrons are not so free to move.
• All metals are electrically conductive.
• Dynamic electricity, or electric current, is the uniform motion of electrons through a conductor.
• Static electricity is an unmoving (if on an insulator), accumulated charge formed by either an excess or deficiency of electrons in an object. It is typically formed by charge separation by contact and separation of dissimilar materials.
• For electrons to flow continuously (indefinitely) through a conductor, there must be a complete, unbroken path for them to move both into and out of that conductor.
Electric field - Electric field is defined as the electric force per unit charge. The direction of the field is taken to be the direction of the force it would exert on a positive test charge. The electric field is radially outward from a positive charge and radially in toward a negative point charge. The concept of an electric field was introduced by Michael Faraday.
The electric field is a vector field with SI units of newtons per coulomb (N C−1) or, equivalently, volts per metre (V m−1). The SI base units of the electric field are kg⋅m⋅s−3⋅A−1. The strength or magnitude of the field at a given point is defined as the force that would be exerted on a positive test charge of 1 coulomb placed at that point; the direction of the field is given by the direction of that force. Electric fields contain electrical energy with energy density proportional to the square of the field amplitude. The electric field is to charge as gravitational acceleration is to mass and force density is to volume.
Electric Field of Hollow Conductor- The electric field inside a charged hollow conductor is zero. Charge supplied to such a conductor remains on its surface only. This explains why a hollow conductor acts as an electrostatic shield. It is for this reason that it is easier to sit in a car or bus during lightening.
Electric Potential- The electric potential (a scalar quantity denoted by φ, φE or V and also called the electric field potential or the electrostatic potential). The electric potential at a point is equal to the electric potential energy (measured in joules) of any charged particle at that location divided by the charge (measured in coulombs) of the particle.
Capacitance – The capacitance of a conductor is defined as the charge needed to increase the potential of the conductor by 1 unit. If potential of a conductor is increased by V, when a charge Q is given to it; then capacity of the conductor is Q/V. The unit of electrical capacitance is Faraday, symbol; F.
Why Do Batteries Discharge More Quickly in Cold Weather?
The electric current generated by a battery is produced when a connection is made between its positive and negative terminals. When the terminals are connected, a chemical reaction is initiated that generates electrons to supply the current of the battery. Lowering the temperature causes chemical reactions to proceed more slowly, so if a battery is used at a low temperature then less current is produced than at a higher temperature. As the batteries run down they quickly reach the point where they cannot deliver enough current to keep up with the demand. If the battery is warmed up again it will operate normally.