Objectives:

  • Explain what electrical current is;
  • You need to be able to select and use the equation \Delta Q = I \Delta t ;
  • You need to know the units used for Charge, Current and time;
  • Describe how an ammeter may be used to measure the current in a circuit;
  • Understand that for electrical charge to flow through a closed circuit the circuit must include a source of potential difference.

What is electric current?

An electric current generally is the flow of electrons through a wire.  Electrons are negatively charged and are free to flow between atoms in a wire, there are vast numbers of these electrons in any given volume of a metal and collectively they contribute towards a current in a wire.

The ions in a wire (or other solid structures) are fixed and so cannot flow. The electrons, when given energy to move all flow in one directions. Figure 1 displays these ions and electrons.
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The current flows from the positive terminal of a battery to the negative as shown:

Screen Shot 2017-07-29 at 22.24.24.png

Electric current?

An electric charge is a physical property of matter, both electrons and protons have an elementary charge of -1 and +1 respectively.

Electrons are the particles that flow throughout a circuit, the flow of electrons is also known as a current, the more electrons that flow in a given unit of time, the greater the current (and vice versa). If the electrons are stationary there is no current as they would take an infinitesimal amount of time to travel anywhere, therefore an electrical current is linked to time, the more electrons that flow in a given amount of time the more current there is. It is the charge that an electron has that gives the flow of electrons a current. We can link the three quantities Q, I and t together to form the equation;

\Delta Q = I \Delta t

where;
is the charge measured in Coulombs, C
I
is the current measured in Amperes or Amps, A
t
is the time measured in seconds, s

Here you can see that current is equal to the amount of charge in a certain time frame. Current, as you know is the flow of electrons, well it is actually defined as the number of electrons that pass a point in one second. By its definition we know that a current cannot flow if the electrons are stationary (as mentioned above), this means that any number of electrons would take an infinite amount of time to pass a point in a given second, if we substitute this into the equation we can see that the current is zero and thus works;

I = \frac{Q}{\infty} = 0 A

1 Coulomb is defined as the charge transported by a constant current of 1 Ampere in one second;

1 C = 1A \times 1s

Here are some questions for you to practice the use of the equation above with – Q=It questions.

 

How doe we measure current?

In any circuit, the current that flows through the wires or components can be measured using an ammeter, which needs to be placed in series to circuit components as shown;

Image result for ammeter in series

Image result for ammeter in series circuit

In simple terms, an ammeter works using the equation I = \frac{\Delta Q}{\Delta t} to measure the number of coulombs of charge flowing through the ammeter per second.

Standard classroom ammeters may measure the current to 0.01A, however some digital multimeters (devices that can be used to measure a variety of quantities)  have the ability to measure up to 1μA (1\times 10^{-6} A.

More questions using \Delta Q = I \Delta t link.


Further reading:

  • More information, just another link
  • You could investigate how large charges build up in thunder clouds and how lightning forms.
  • The Doctrine of Description: Gustav Kirchhoff, Classical Physics, and The “Purpose of All Science” in 19th-century Germany by Kalil T. Swain Oldham (A preview can be found here)
  • Research older circuit symbols and compare them with those of today, commenting on which they prefer and why.
  • Children of Light: How Electrification Changed Britain Forever by Gavin Weightman (Can be found on Amazon).