Edexcel A Level Physics:复习笔记3.1 Electric Current

Electric Current

• Electric current is defined as:

The rate of flow of charge

• It is measured in units of amperes (A) or amps
• The charge, current and time are related by the equation:

• There are several examples of electric currents, including in household wiring and electrical appliances

Electric Charge

• Electric charge is a property of some particles
  • For example, protons have a positive charge and electrons have a negative charge
• Charge has the unit Coulombs, C
• In electric circuits, electrons are usually the charge carriers
  • They have a charge of 1.6 × 10−19 C
• Charge, Q, can be calculated using the equation

Q = ne

• Where:
  • Q = charge (C)
  • n = number of electrons
  • e = electron charge (C)

• If 1 electron has a charge of 1.6 × 10−19 C
  • Then, 1 C of charge contains 6.25 × 1018 electrons

• Charge is sometimes written as ΔQ which means 'change in charge'
  • Similarly, time is written as Δt means 'change in time'

   

• When two oppositely charged conductors are connected together (by a length of wire), charge will flow between the two conductors, causing a current
• Therefore, rearranging for current, I gives the equation:

Charge can flow between two conductors. The direction of conventional current in a metal is from positive to negative

Direction of Flow of Charge

• 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 the 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)

Measuring Current

• Current is measured using an ammeter
• Ammeters should always be connected in series with the part of the circuit the current is to be measured through
  • This is because the current is the same in all components connected in series

An ammeter can be used to measure the current around a circuit and always connected in series

ANSWER:   B

Step 1: Write out the equation relating current, charge and time

Q = It

Step 2: Rule out any obviously incorrect options

• • Option A does not contain charge or time, so can be ruled out

Step 3: Try the rest of the options to determine the correct answer

• • Consider option B:

I = 4 / 500 = 8 × 10–3 = 8 mA

• • Consider option C:

I = 8 / 100 = 80 × 10–3 = 80 mA

• • Consider option D:

I = 1 / 8 = 125 × 10–3 = 125 mA

• • Therefore, the correct answer is B