Electricity

What is it and how does it work?

Some definitions to start with

•An electrical current is caused by the flow of negatively charged electrons in a conductor

•A conductor is a material that allows the electrical current to flow easily. Examples include copper and iron.

•An insulator does not allow electrical current to pass through it easily. Examples include air, wood, glass. Can you suggest a few others?

•Current is measured in Amps (I), also known as amperes, with an Ammeter

•The potential of the current is called the Voltage (V) and measured with a voltmeter. Also known as potential difference.

•A resister (R) is something that reduces or slows the current and is measured in Ohms (). Some conductors are more resistant that others. The greater the diameter and the shorter its length, the smaller the resistance

•A circuit is the path along which the electrical current can flow

•A switch can be used to break a current in a circuit

•Electrical current can be one of two kinds, AC and DC…

The difference between AC & DC

AC stands for alternating current and means that the flow of electrons can move in both directions. The electrical circuits in our homes are AC and

they alternate, or move back and forth, about fifty times every second. This type of current is

produced by generators similar to those in power stations. It is more efficient than DC but more

dangerous if you get an electric shock. AC shock causes a person’s muscles to contract and

remain that way so they may not be able to let go of the source of the shock.

DC stands for direct current and means that the current flows in one

direction only, from the negative terminal to the positive terminal. This

type of current is produced in batteries. Shocks from these currents produce a sudden jolt and can throw

a person across the room.

Remember ‘AC holds & DC throws’

Drawing a Circuit – the symbols

Wire

Battery+ -

Lamp

Ammeter A

Voltmeter V

Resistor

Open switch

Closed switch

Motor M

Drawing a Circuit in SeriesIn a series circuit, the current can follow only one path. The following series circuits have two lamps

and two switches joined to a single battery.

If both switches are closed the circuit is complete and both the lamps glow

If one or both of the switches are open then the circuit is broken and the current cannot flow around to make a complete circuit. Neither lamp glows

Drawing a Circuit in ParallelIn a parallel circuit, the current can follow more than

one path. The following parallel circuits have two lamps and two switches joined to a single battery.

As with the series circuit, if both switches are closed then both lamps glow

As with the series circuit, if both switches are open then neither lamps glow

Unlike the series circuit, when in parallel one switch open breaks only that one circuit. With the other switch closed a continuous circuit allows that lamp to glow. Our homes are on parallel circuits for this reason – as soon as we turn off one light switch we don’t loose the electricity to ALL our lights!!!!

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The battery

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Flow of electrons

Flow of electrons

So how does electricity flow through a circuit?

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Flow of electrons

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Copper wire conductor

The narrow tube represents tungsten in a lamp. The tungsten

creates a resistance to the flow of electrons.

The resistance in the wire causes the wire to heat

and produce light

So how does electricity flow through a circuit?

•To understand how a current travels through a conductor, and how it reacts or changes when it comes across a resistor can be explained using a water model.

•The following model will help you imagine what is happening to the electrical current by relating it to a water current.

•Remember that it is a model and does not fit in every way.

The water model

Think of the Volts being the pressure of the water

A narrow pipe can represent a Resistor

Think of the quantity of water crossing a point in the circuit as the Amps or Intensity

Pressure (P) before the resistor is greater than…

…Pressure (P) after the resistor

Therefore a potential difference is created across

the resistor

Understanding how volts and amps react within a circuit – Kirchoff’s Laws

•One of Kirchoff’s Laws states that the current flowing into the fork in a a parallel circuit is equal to the sum of the current in each of the possible choices (considering that the resistance is equal in both of the parallel circuits).

•This means that I1 = I2 + I3 as shown on the following diagram:

I1I2

I3I1

•Look at the following diagram and calculate the amps for I3.

Hopefully your answer was 3A

•If the current splits 4 times then the same rule applies.

•I1 = I2 + I3 + I4 + I5. What would this look like on a circuit diagram?

Understanding how volts and amps react within a circuit – Kirchoff’s Laws

•Another of Kirchoff’s Laws states that in a series circuit the voltage drop across a set of resistors must equal the input voltage of the battery.

•If there were three resistors this means that V0 = V1 + V2 + V3

V0 = 9V

V1 V2 V3

•If these three lamps were identical then the voltage would be evenly shared across them. This means that V1 = V2 = V3. In this case, what is the value of V1, V2 and V3?

Understanding how volts and amps react within a circuit – Kirchoff’s Laws

How does this law work when the lamps are in parallel?

For each of the two circuits, the law states that the voltage across the resistor(s) in the circuit must equal the input voltage of the battery…therefore…

V0 = 9VV1 = 9V

V2 = 9V

Understanding how volts, amps and resistors interact with each other – Ohm’s Law

•Ohm’s Law states that the voltage in a circuit is equal to the product of the current (amps) and the resistance

•This means that voltage = current or amps x resistance

•This is written as V = IR

•Here’s an example: if you are using a 9V battery in a lighting circuit and read only 0.05A in the circuit, how much resistance must there be in the circuit?

•The answer is: V = IR so, 9 = 0.05 x R or 9/0.05 = 180 Ohms

•Try this one: Given the following circuit, find the current that flows through it.

•V = IR so, 3 = I x 6 = 3/6 = 0.5 A

      

The End