Current Electricity

• Current flows from the positive terminal of the cell to the negative terminal.
• Direct current (d.c.) is a current of constant magnitude flowing in one direction.
• A cell basically consists of a vessel containing two conducting rods, called electrodes, at some separation, placed or immersed in a solution, called electrolyte.
• Kinds of cells:

  1. Primary cells
  2. Secondary cells
• Primary cells:

  • Provide current as a result of an irreversible chemical reaction.
  • Discarded after use when the entire chemical energy in them has been converted into electrical energy.
  • They cannot be recharged.
  • Can only provide a weak current.
  • Light and cheap.
  • Examples:- simple voltaic cell, Leclanche cell, Daniel cell, dry cell
• Secondary cells:

  • Also provide current as a result of a chemical reaction but in this cell the chemical reaction is reversible.
  • Can be recharged after use by connecting to a direct current source.
  • While charging, electrical energy gets converted into chemical energy and gets stored in the cell.
  • These cells are also called storage cells.
  • Can provide low and high currents.
  • It is heavy and expensive.
  • Examples:- Lead accumulator and Ni-Fe accumulator.
• The rate of flow of charge gives the magnitude of current.
• Current is the rate of flow of charge across a cross-section normal to the direction of flow of current.
• Current = charge/time

  Current is a scalar quantity
• The convectional current is in a direction opposite to the direction of motion of electrons.
• S.I. unit of current is ampere (A)
• 1 milli-ampere (mA) = 10^-3 ampere (A)

  1 micro-ampere = 10^-6 ampere
• Types of current sources:

  1. Alternating current (a.c.) sources such as mains in our homes
  2. Direct current (d.c.) sources such as a cell or battery
• A key is used to put the current on and off in a circuit. It may either be a plug key, a switch, or a tapping key.
• A resistance wire is generally made from an alloy called manganin and it has a fixed resistance depending upon its length and thickness. It is used as a standard resistance.
• A rheostat is a device by which resistance in a circuit can be varied continuously. It is used to adjust the magnitude of current in a circuit by changing the length of the resistance wire included in the circuit.i The main use of a rheostat is as a smoothly varying resistance.
• An ammeter is used to measure the magnitude of current flowing in the circuit.
• A voltmeter is used to measure the potential difference between two points of a circuit.
• A galvanometer is used when we want to detect the presence of a very weak current or to know the direction of a current in a circuit.
• Load is the appliance connected in a circuit.
• When two conductors are joined by a metallic wire, free electrons flow from a conductor having a higher concentration of electrons to the conductor having a lower concentration of electrons.
• The movement of electrons stops when both conductors have an eual concentration of electrons.
• The direction of the flow of electrons is determined by a quantity called the electric potential of the coductor.
• A conductor having an excess of electrons is said to be at a negative potential while a conductor having a deficit of electrons is said to be at a positive potential.
• The direction of flow of electrons is said to be from higher potential to lower potential i.e. from negative potential to positive potential.
• Potential is a scalar quantity.
• S.I. unit of potential is joule/coulomb or volt (V).
• Potential = Work / Charge
• The potential difference between two conductors is equal to the work done in transferring a unit positive charge from one conductor to another.
• Potential difference is a scalar quantity.
• Potential difference = Work done / Charge
• Potential difference between two points is said to be 1 volt if work done in transferring 1 coulomb of charge from one point to another is 1 joule.
• The obstruction offered to the flow of current by a conductor is called its electrical resistance.
• Ohm's law states that if a current flows through a wire when the potential difference across the ends of the wire is constant, the resistance offered by the wire to the flow of current is the ratio of potential difference across it to the current flowing in it.
• Resistance = Potential difference/Current
• Resistance is a scalar quantity.
• S.I. unit of resistance is ohm.
• The resistance of a conductor is said to be 1 ohm if a current of 1 ampere flows through it when the potential difference across its ends is 1 volt.
• Factors affecting resistance:

  1. Material of wire - good conductors offer less resistance
  2. Length of wire - a long wire offers more resistance than a short wire
  3. Area of cross-section of wire - a thick wire offers less resistance
  4. Temperature of wire - the resistance of a wire increase with the increase in its temperature.