chapter 16

• electric current

  rate of flow of electric charge

  si unit is ampere, or coulomb per second

  one coulomb is equal to the amount of charge when one ampere current flows for one second
• conventional current


  from positive of the dry cell to the component
• electron flow


  from the negative of the dry cell to the component
• electric circuit


  a closed path connected with metal wires usually made from copper

  caused by the flow of electrons

  scalar quantity
• ammeter


  connected in series

  ideal ammeter has zero resistance
• voltmeter


  connected in parallel
• electromotive force


  si unit is volt

  the electromotive force of an electrical source is the work done by the source in driving a unit charge around a complete circuit

  present even when no current is drawn from the source
• potential difference


  si unit is volt

  the potential difference across a component in a circuit is the work done per unit charge in driving charges through component

  pd across any component is zero in absence of a current
• electrical resistance


  when the electrons flow through a component, they constantly collide with ions of the components and with other electrons. these collisions oppose the flow of electrons such that the degree of the opposing force which the electric current experiences is the resistance of the component

  no object has full resistance to electric current

  the resistance of a component is the ratio of the potential difference across it to the current flowing through it
• ammeter


  connected in series

  measure the magnitude of electric current flowing through it
• voltmeter


  connected in parallel
• when dry cells are connected in series


  the negative terminal of one cell is connected to the positive terminal of the other cell

  the resultant or total emf is the sum of the individual of each cell
• when dry cells are connected in series


  the cells are arranged in two branches

  resultant or total emf is equal to that of a single cell
• electromotive force of an electric source is the work done by the source to drive a unit charge around a complete circuit
• potential difference of a component in a circuit is the work done to drive per unit charge in driving charges through the component
• resistance


  scalar quantity

  si unit is ohm

  every object has resistance, only thing different is its degree of resistance

  no object has zero resistance in a circuit
• resistor


  to control the amount of current flowing through the circuit

  can be a fixed or variable resistors
• variable resistors


  rheostat

  potentiometer
• resistance of objects based on length of wire

  the longer the length, the greater the resistance

  the shorter the length, the smaller the resistance
• resistance of objects based on thickness of wire

  the thicker the wire, the smaller the resistance

  the thinner the wire, the greater the resistance
• ohmic resistor


  increases linearly with temperature
• non-ohmic conductors

  do not increase linearly with temperature

  examples are diodes and thermistors