ELECTRICITY

Part of Physics Achievement Standard 2.6 – Demonstrate Understanding of Electricity and Magnetism

Physical Relationships

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Current. Symbol: I unit: A (amps) Current is “movement of charge.” The moving

charges could be electrons or sometimes ions.

1 Amp is 1 Coulomb of charge passing a point in 1 second.

This concept can be extended to calculate a current:

𝐼 =𝑞

𝑡

q is the amount of charge,

t is the time.

Question: How many electrons are there in a coulomb?

The direction of current?

Physics is stuffed.

Before any one had discovered electrons, they decided that they would be positive.

Whoops! This means that current is the direction that positive charges are moving.

This is known as “conventional current”

When we mean actual electron flow, we say “electron current”.

So current is the movement of positive charges.

A Current rule

Just like energy, charge cannot be created or destroyed.

This means the total current into a point is equal to the total current out of a point.

This explains the series and parallel rules about current that you should have learnt last year.

Remember:

Total Current In = Total Current Out

Conductors and Insulators

What is the difference between conductors and insulators?

Why do some materials conduct electricity and some materials not?

In a material, most of the electrons are attached to a specific atom.

However, all materials have a certain number of free electrons.

If there are a lot of free electrons, the material is a good conductor. If there are almost no free electrons, the material is a good insulator.

Voltage. a.k.a. Potential difference

The symbol for voltage is V, and the unit is Volts (V).

The voltage measures the amount of energy per unit of charge.

Its kind of like… When a charge q has electric potential

energy Ep at a point then the voltage V is given by

𝑉 =𝐸𝑝

𝑞 What is a volt?

Voltage is always measured between two points. Its like a measure of the difference in the energy.

A Rule for voltages

As a charge travels around a circuit loop, all of its electric potential energy must be used up.

When it reaches the other end of the battery/power pack, its energy is ZERO.

This is why components will blow if we don’t think ahead and make sure the component can handle the amount of energy we are giving it.

Components. (Memorise these symbols)

Cell Lamp

Battery

Resistor

Variable Resistor

Ammeter

Voltmeter

Diode

Fuse

Switch

LED

LDR

Thermistor

Series and Parallel

Components are connected one after the other, so the current has no choice where to go.

Ammeters should always be connected in series.

Why? We want to measure how

much charge is passing through.

Series: Parallel

Components are connected in such a way that the current has a choice which way to go.

Voltmeters should always be connected in parallel.

Why? Remember the term

potential difference!

Resistance.

Symbol: R Unit: Ohms (Ω)

For many conductors, we find that the voltage across them is proportional to the current through them: V ∝ I.

For example, when we triple the voltage across them we find that the current through them also triples.

Let’s introduce a constant of proportionality, and call it resistance, R.

Ohm’s Law Ohm’s Law is actually a graph showing that voltage is proportional to current.

If the V-I graph has a straight line through (0,0), it is called an Ohmic Resistor.

V

I

V = R I

Any conductor that has a proportionality that is not a straight line through (0,0) is called a non-Ohmic resistor.

V

I

V

I

Combining Resistances

Series.

The Current travels through all components.

The Voltage must add to the correct value for the loop or the circuit.

Parallel.

The Current must go down

either one path or the other.

The Voltage across components in parallel is equal.

We want to work towards replacing all our resistors with 1 resistor that would provide the equivalent resistance.

Some equations…

For resistances in series:

For resistances in parallel:

Note: For resistances in parallel, the total resistance will be less than the smallest individual resistance. Why is this?

Some Common Resistances…

Internal resistance: Anything that supplies a voltage

uses some form of conducting material to get the energy from inside it to the outside. For example, a power pack would use wires, a battery uses a small conductive ribbon.

This material has a certain amount of resistance, which is known as the Internal Resistance of the component.

The internal resistance will also limit the maximum current that can be supplied, which is a good safety aspect.

Skin Resistance. One way to think of insulators

is that they have resistances so high that the electric field cannot cause electrons to flow.

The resistance of skin is quite high, which means that any shocks we get will be a significant voltage and or current.

Wet skin is much more conductive than dry skin. Sweaty skin is even worse, due to the ions in the sweat.

This is why you should never put a fork in a toaster.

The Voltage Divider

A voltage divider uses two resistors to produce an output voltage lower than the input voltage.

Problems:

The output voltage decreases when current is drawn from the output.

Some of the current that is supplied will be wasted.

The Potentiometer

The potentiometer is a variable resistance device that overcomes some of the above difficulties with voltage dividers.

To obtain a particular voltage from the divider, we can simply vary the resistance ratio till the desired output voltage is obtained.

Power

When current flows through a resistor, there is a potential difference across the resistor.

This means that some of the energy is being used up in the resistor.

We measure this difference in energy as a voltage.

The amount of energy that is lost per second is a measure of the power.

Power = Change in Energy

TIme

𝑬𝑷 = 𝒒𝑽 𝑷 =𝒒𝑽

𝒕

𝑷 =𝒒

𝒕× 𝑽 𝑰 =

𝒒

𝒕

𝑷 = 𝑰𝑽

What is the unit for power?

You have 30 minutes…

You choose which order you do the following 3 activities. Each must be done.

• You have been given three 10Ω resistors. Draw and calculate all of the different resistances you can achieve using different combinations of these.

• Household mains supply voltage is 230 V, yet transmission voltages in power lines can be 1000 times as high or more. Write a paragraph discussing this difference.

• Discuss whether a light bulb is an ohmic or a non-ohmic resistor.

Diodes

A Diode is a electronic component that does not obey Ohm’s Law.

The Voltage-Current graph of a Diode looks like this:

Bias A Diode is constructed so that it has a Bias:

If it is connected the right way around, current will flow once the voltage is higher than the cut-off voltage.

If it is connected the incorrect way, then it will act as an insulator until the point of Zener Breakdown occurs. This will normally wreck the diode.

Note that diodes indicate the direction of connection with reference to conventional current.

Light Emitting Diodes (LEDs)

LEDs are diodes which

emit light when a current

passes through it.

They have a number of advantages over filament lamps.

Typically they are cheaper to make,

require less electrical power to run and…

less easily damaged. Plus you can make an awesome TV ->

Sensors

In physics, we frequently need to measure temperature, light intensity, sound intensity, force, position and so on.

Often this is done by turning the input into a voltage.

This is because voltage can easily be measured.

Example: In almost all cars, the petrol tank level is measured by having sliding contact of a potentiometer floating at the level. The output voltage can then be calibrated to give a reading in litres rather than volts.

• Light Dependent Resistors (LDRs)

• The circuit shows a voltage divider in which the top component is an LDR.

• At a high light level, the LDR’s resistance is low.

• At a low light level, the resistance is high.

• What could these be used for?

More types of common sensors:

• Thermistors

• This circuit shows a voltage divider in which the top component is a thermistor.

• At a high temperature, the thermistor’s resistance is low.

• At a low temperature, the resistance is high.

• What could these be used for?

More types of common sensors:

Fields

What is a Field?

A region of space where every point can be assigned a vector which indicates the motion of an appropriate test particle.

Types of fields that we have already seen…

Gravitational fields

These are attractive fields that act upon mass.

We are going to focus on electric fields.

Electric fields are regions of space where electrically charged particles feel a force.

Fields of the earth:

The Earth’s Gravitational Field

Why is a gravitational field different to all other types of fields?

Electrostatic Forces

Electrostatic forces exist between all charged particles.

These forces are actually the reason I can’t put my hand through a table. Why?

This is why we say that gravity is the weakest of all forces.

To show the electrostatic forces, we draw field lines.

Field lines are drawn to indicate the direction that a small positive charge would move.

Examples:

Like charges repel.

Unlike charges attract.

A spherical (point) charge.

The field lines around a point charge are radial:

The strength of the electric field is the amount of force, in Newtons, that would act on a +1 Coulomb test charge.

(In other words, the amount of force per unit charge.)

The equation:

𝐸 =𝐹

𝑞

Electric Field Strength (E) equals Force (F) divided by charge (q)

Force is measured in… Newtons (N)

Charge is measured in… Coulombs (C)

Therefore Electric Field Strength must be measured in…

Newtons per Coulomb (NC-1)

Your turn…

Draw the Electric Field that exists between the following two charged plates.

+

-

+ + + + + + + +

- - - - - - - -

Electric Potential Energy

Electric potential energy is gained by a charge when it is moved against an electric force.

The work done to move a charge against an electric force is found by:

Work = Force × Distance

𝑊 = 𝐹𝑑

But we know in an electric field: 𝐹 = 𝐸𝑞

Therefore 𝑊 = 𝐸𝑞𝑑

So the amount of work done to move each charge in an electric field is

𝑊

𝑞=

𝐸𝑞𝑑

𝑞

The work done to move a charge is commonly called Voltage. This gives

𝑉 = 𝐸𝑑 or rearrange to find the Electric Field Strength:

𝐸 =𝑉

𝑑

This equation gives the Electric Field Strength between two charged plates a distance d apart at a voltage V. Therefore another unit for E is… Vm-1