Charge & Current

• Electric Current:

  • Electric current is defined as the rate of flow charge
  • In other words, the size of an electric current is the amount of charge passing through a component per second
• • The wires in an electric circuit are made of metal, because metal is a good conductor of electric current
  • In the wires, the current is a flow of electrons
• In metal wires, the current is a flow of electrons. This image shows the electrons flowing through a lattice of metal ions:

  
• Measuring Current:

  • The amount of current flowing through a component is measured using an ammeter
  • Ammeters should always be connected in series with the part of the circuit you wish to measure the current through
• An ammeter can be used to measure the current around a circuit and is always connected in series:

  
• Calculating Electric Current,

  • The current, charge and time are related by the equation:

  
• • Where:
  • I = current, measured in amperes (or amps, A)
  • Q = charge, measured in coulombs (C)
  • t = time, measured in seconds (s)
• Charge, current, time formula triangle:

  
• Current in a Loop:

  • A current will flow in a circuit if:
  • The circuit includes a source of potential difference
  • The circuit is closed - i.e. there are no gaps in the circuit
• • Sources of potential difference include:
  • A cell
  • Batteries (multiple cells)
  • Electrical generator
  • In a circuit that is a closed-loop, such as a series circuit, the current is the same value at any point
  • This is because the number of electrons per second that passes through one part of the circuit is the same number that passes through any other part
• • This means that all components in a closed-loop have the same current
• The current is the same at each point in a closed-loop:

  
• Current at a Junction:

  • At a junction in a circuit (where two or more wires meet) the current is conserved
  • This means the amount of current flowing into the junction is equal to the amount of current flowing out of it
  • This is because charge is conserved
• The current coming into a junction must equal the current coming out of a junction:

  
• • In electrical wires, the current is a flow of electrons
  • Electrons are negatively charged; they flow away from the negative terminal of a cell towards the positive terminal
  • Conventional current is defined as the flow of positive charge from the positive terminal of a cell to the negative terminal
  • This is the opposite to the direction of electron flow, as conventional current was described before electric current was really understood
• By definition, conventional current always goes from positive to negative (even through electrons go the other way):