Charge & Current (DP IB Physics: SL)

• Charge is measured in coulombs (C). One coulomb is defined as:

The charge carried by an electric current of one ampere in one second 

• Charge is a scalar quantity
  • Electrons have a negative charge 
  • Protons have a positive charge 
• Like charges repel each other and attract opposite ones
• In neutral (i.e. uncharged) atoms and objects the number of electrons and the number of protons are equal
• The movement of electrons determines the charge of an atom or object
• In charged atoms or objects there is a net amount of either positive or negative charge
  • An object that gains electrons has an overall negative charge
  • An object that loses electrons has an overall positive charge

• In conductors, some electrons are free to move between atoms

Delocalised electrons in the outer shell of the atoms in a conductor can move between atoms. When a neutral atom loses electrons it becomes a positive ion

• When several differently charged but identical objects are brought together briefly and then separated they end up with the same charge
  • Calculate the total charge present at the point of contact
  • Then divide by the number of objects brought together
  • This will give the value of the charge present on each object after separation

• The electric current through a conductive material is defined as follows:

The rate of flow of electric charge past a cross-section of material 

• Current is measured in amperes (A)
  • The ampere is a fundamental unit of the SI system

• Current is calculated as follows:

• Where:
  • I = current in amperes (A)
  • Δq = net charge flowing past a cross-section of material in coulombs (C)
  • Δt = time interval in seconds (s)

• The conventional direction of current is from positive to negative
  • Electrons flow opposite to the conventional direction of current

By definition, conventional current always goes from positive to negative (even through electrons go the other way)

Step 1: Write down the known quantities 

• • Δq = 20 μC = 20 × 10^–6 C
  • Δt = 60 ms = 60 × 10^–3 s

Note the conversions:

• • The amount of charge must be converted from micro coulombs (μC) into coulombs (C)
  • The time must be converted from milliseconds (ms) into seconds (s)

Step 2: Write down the equation for the electric current I

Step 3: Substitute the numbers into the equation 

I = 3.3 × 10^–4 A