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Electrical circuits: 2.1 Important definitions in electricity science and exactly what is an electric current ?and an electrical circuit?

Doc Brown's Physics exam study revision notes? What is an electric circuit and what is an electric current?  How do you draw an electric circuit?   How do you interpret a circuit diagram?  Do you know your circuit symbols?   What is the difference between a series circuit and a parallel circuit? Can you interpret what happens when a circuit is switched on?

INDEX for physics notes on drawing and interpreting simple electrical circuits


See also Part 1.1 Important definitions, descriptions, formulae and units used in electrical science which overlaps with Part 2.1

2.1 Important definitions in electricity science and what exactly is an electric current and an electrical circuit?

Circuit and symbols

The diagram circuit 01 (right) is the simplest sort of electrical circuit that can do anything useful e.g. lighting a bulb (symbol ) using a single cell battery (symbol ).

The switch is closed ('on', symbol ) to complete the electrical circuit in which all the components must be connected together with an electrical conductor such as a copper wire.

This is one of the simplest circuit diagrams you can draw - so get used to them asap!

Circuit 01 is a simple closed loop and the current will be the same at any point in the circuit - the circuit of conducting material e.g. copper wire and components, must be .complete for an electrical current to flow.

Any break in the circuit e.g. a switch in the off position, will stop the electric current flowing.

Lots more on circuit symbols in the next section and is just the wire connections!


Measuring CURRENT flow

An ammeter (symbol ) is included to measure the current - the rate of flow of electrical charge - usually negative electrons.

The unit of current is called the ampere, symbol A.

The flow of electric charge is usually the flow of the tiny negative particles we call electrons.

A current of electric charge can only flow round a complete circuit - as the diagram - no gaps in the wires! AND there must be source () of potential difference (p.d.) like a cell or battery to drive the electrons around.

Note the current flow convention from +ve to -ve (opposite to actual electron flow)


Measuring POTENTIAL DIFFERENCE

It is the electrons (the 'charge') that transfer the electrical energy from a 'higher potential' to a 'lower potential'.

The unit of potential difference (p.d.) is the volt, symbol V e.g. a simple single torch battery might give a p.d. of 1.5 V, a car battery might deliver 12 V from six 2 V cells wired one after the other in series - more on wiring in series later.

It is the potential difference that drives the negatively charged electrons round a circuit and if you increase the p.d. then you push more electrons along in a given time i.e. you increase the current.

It is the potential difference ('voltage') that 'pushes' the electrical charge (-ve electrons) around the circuit.

If the p.d. is > 0 V, current flows in one direction, if the p.d. is <0 V, the current flows in the opposite direction!, and if the p.d. = 0 V, no current flows!

The everyday term 'voltage' is strictly speaking not correct, in an exam try to use 'potential difference' once and then use the abbreviation 'p.d.' after that.

Circuit diagrams must be drawn with the correct symbols for the components, and normally, wires are drawn as straight lines and the switch closed ('on') to complete the circuit - so it looks as if it works!

You should be able to follow the wire from one end ('terminal') of the power supply to the other and passing through any components in the circuit.

Circuit 29 (right) is essentially the same as circuit 01 above with a resistor (symbol ).

A resistor is a two terminal component that resists the flow of electric charge - reduces the current.

It is often a thin wire relative to the width of the wire used for the rest of the circuit. This thin resistance wire can convert electrical energy into heat and light (filament bulb), heat (heating element) or just light (LED lamp).

Note that a voltmeter is always wired in parallel across a component e.g. the resistor R.


What is a RESISTANCE?

A resistance is any component that restricts the flow of charge i.e. it opposes the current flow of electricity.

The unit of resistance is the ohm, symbol Ω.

The current flowing through a resistor depends on two factors:

(i) for a given fixed resistance, the larger the potential difference, the larger the current,

(ii) for a given fixed potential difference, the greater the resistance of a resistor, the lower the current.

For more details see section 3 of electricity and magnetism notes

Ohm's Law, experimental investigations of resistance, simple graphs & calculations

where we will introduce how to wire up and use a voltmeter.

Every cell (battery) has a positive (+) and negative (-) terminal and by convention the current flows from the positive terminal round to the negative terminal (clockwise here).


Note 1: Current convention and chemistry!

This electrical current convention may be a problem in chemistry because the electrons actually flow in the opposite direction to the convention current! That is, anticlockwise in circuit 29 - it is logical that negative electrons flow from negative to positive.

It is important you understand this because in chemistry you study electrolysis and need to know what the electrons are doing! The reason for this clash is the current convention was adopted before scientists knew about electrons!)


Note 2: Alternating current (ac) and direct current (dc)  (for future reference)

With an alternating current (ac), the current changes direction in a cycle e.g. 5O Hz and the potential difference goes through a cycle +/- V.

With a direct current (dc) there is no reversal in current direction, it flows one way with a constant voltage (pd/V).

Oscilloscope traces comparing ac and dc current signals - showing the changing direction + <=> - oscillation of the alternating current p.d. and the constant p.d. of a direct current.

Note that some devices in the home work off a dc current - but the output from e.g. the transformer in your computer power supply, is rectified to convert it to a dc supply.

INDEX of notes on drawing and interpreting simple electrical circuits

See also Part 1.1 Important definitions, descriptions, formulae and units used in electrical science which overlaps with Part 2.1


Keywords, phrases and learning objectives for ? electricity

Know what is electric current is.

Know what an electrical circuit is and must be at least one complete loop of conducting material for an electric current to flow.

Know how to draw an electrical circuit diagram with the correct symbols for the components.

Know the convention current flows in the opposite direction to the flow of electrons/

Know the difference between an a.c. alternating current and d.c. direct current (picture them on a CRO screen).


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INDEX for notes on drawing and interpreting simple electrical circuits

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