Current | AP Physics C Electricity and Magnetism - Grade 9 PDF Download

Current

• The current refers to the amount of charge that flows through a point in a circuit per second.
• It is helpful to envision current as the amount of charge passing through a specific point in a circuit in one second.
• The relationship between charge, current, and time is defined by the following equation:

• Where the symbols:
  • The symbol Q represents charge, measured in coulombs (C).
  • The symbol I represents current, measured in amperes (A).
• You can rearrange this equation with the help of the formula triangle:
  

Direct & Alternating Current

• When two oppositely charged conductors are connected together (by a length of wire), charge will flow between the two conductors.
  
• The movement of charge is referred to as electric current. 
  • A higher rate of charge movement corresponds to a stronger electric current.

Direct Current and Alternating Current

• Current can be direct current (dc) or alternating current (ac).
  • In terms of calculations, they can be treated in the same way.

Direct Current (dc)

• Direct current is produced when using dry cells and batteries (and sometimes generators, although these are usually AC)
  • The electrons flow in one direction only, from the negative terminal to the positive terminal

Alternating Current (AC)

• Alternating current typically comes from mains electricity and generators
• It is needed for use in transformers in the National Grid (covered later in this topic)
  • The direction of electron flow changes direction regularly
  • A typical frequency for the reversal of AC current in mains electricity is 50 Hz

Measuring Current

• Current is measured using a device called an ammeter.
• An ammeter must be connected in series with the part of the circuit you intend to measure the current through.
  • Ammeters gauge the amount of charge moving through them per unit time, necessitating that they are in series to measure all the charge.

Types of Ammeters

• Ammeters can be categorized as either Digital (featuring an electronic readout) or Analogue (with a needle and scale).
• Digital ammeters display readings electronically, while analogue ammeters use a needle and scale for measurement.

Reading Analog and Digital Meters

• Always approach the meter from a position directly perpendicular to the scale.

Digital Ammeters

• Digital ammeters are capable of measuring very small currents, often in milliamperes (mA) or microamperes (µA).
• Digital displays provide precise values, making them easy to use. However, they may exhibit slight fluctuations between readings.
• Users should ensure zero error by verifying that the reading is zero before commencing an experiment. If not, the 'zero' value must be subtracted from the final results.

Electrical Conduction in Metals

In metals, electrons move freely, causing current

Conventional Current

• Electrons are negatively charged
• This means that the electrons flow from negative to positive
• Conventional current, however, is still defined as going from positive to negative