electricityelectricity unit 1 physics. introduction – electrons & matter all material is made...

ELECTRICITYUnit 1 Physics

INTRODUCTION – ELECTRONS & MATTER

All material is made up of atoms – with Protons, Neutrons & Electrons.

The negatively charged electrons revolve around the outside of the atom.

When large numbers of electrons move through a conductor, electric current is able to flow.


As different elements have different numbers of electrons in their outer shell, some elements are better conductors of electricity than others.

Atoms with an outer shell that are not full are better conductors than atoms with a stable and complete outer shell, they seek electrons/atoms to bond with.


We can categorise elements with these properties as:

Conductors Insulators Semiconductors


Some common conductors / semiconductors / insulators

INTRODUCTION – WHAT IS ELECTRICITY?

Video: Electricity (Hila Science Videos)

https://www.youtube.com/watch?v=D2monVkCkX4




INTRODUCTION – WHAT IS ELECTRICITY?

Video: Electricity (Bozeman Science Video)

https://www.youtube.com/watch?v=fDvZOp9Oqro



CONVENTIONAL CURRENT VS ELECTRON CURRENT

We can represent the current flow in a circuit two ways:

Conventional current

Electron Current


Electron Current: A flow of current in a wire is a flow of negatively charged electrons. That is, the electrons flow from the negative terminal to the positive in an electric circuit.


However…..Before electrons were discovered, the conventional current theory was used (and is a still widely used and accepted method).Conventional Current, the direction of flow is positive to negative.

DIRECT CURRENT VS

ALTERNATING CURRENT

Direct Current: Refers to circuits where the flow of charge is in one direction only.eg. Power Pack (chargers for electronic goods), Batteries

Alternating Current: Refers to circuits where the charge carriers move backward and forward periodically.eg. The electricity that comes out of a power point.

WHAT IS CURRENT?HOW IS IT MEASURED?

Electric Current is the measure of the rate of charge around a circuit.

Expressed by: where I is the current (Ampere, Amps, A) Q is the amount of charge flowing

(Coulombs, C) t is the time (seconds)

So, 1 Amp = 1 coulomb passing a point in a second.


Eg1. Find the current flow in a circuit, if 50 coulombs passes a point in 10 seconds?

Eg2. Find the charge passing through a circuit, if 8 Amps flows through in 2 minutes?


Eg3. How long must a current of 50mA flow to make 3 Coulombs of charge pass a point in a circuit?

What is this value in minutes?

CURRENT, CHARGE & ELECTRONS

Recall: Electric Current is the measure of the rate of charge around a circuit.

I is the current (Ampere, Amps, A) Q is the amount of charge flowing (Coulombs, C)

t is the time (seconds)So, 1 Amp = 1 coulomb passing a point in a second.

1 coulomb of charge = the amount of charge carried by electrons.

Each electron carries a charge of Coulombs Coulombs

As electrons are negatively charged, their charge is Coulombs

NOW DO: TEXT BOOK PROBLEMS

QUESTIONS 1 - 12

ELECTRICAL POTENTIAL ENERGY

So we know that energy is transferred around a circuit by a flow of electrons.

Electrical Potential Energy is the potential energy of these charge carriers (electrons) due to their position in a circuit and is measured in Joules.

As electrons are forced away from the negative terminal to the positive terminal of an energy source (recall: electron flow notation), potential energy is converted into other forms of energy, so electrical potential energy drops over time.

As electrons move away from the negative terminal, their Electrical Potential Energy is converted into other forms (eg. Thermal Energy Heat).

Notice that batteries, mobile phone, computer etc.. get hot as you use them? This can be explained as the circuit losing Electrical Potential Energy as it’s converted to Thermal energy.

The amount of electrical potential energy lost by each coulomb of charge in a given part of a circuit is called the Potential Difference or Voltage Drop, measured in Volts.

POTENTIAL DIFFERENCEVOLTAGE


One Volt is defined as the voltage drop that occurs when 1 coulomb of charge transforms one Joule of energy.

So we can say that the Voltage drop (V) across a device in a circuit is equal to the amount of energy (E) transformed divided by the amount of charge (Q) that has passed through the device.

Voltage (V) measured in Volts (V)

Energy (E) measured in Joules (J)

Charge (Q) measured in Coulombs

(C)


Which can be rearranged and represented in the following ways:




(C)


Example 1: Use the formulas to solve the following problems:

𝑉=𝐸𝑄

=106.2

=1.61𝑉

𝑉=𝐸𝑄

𝑄=𝐸𝑉

𝐸=𝑄𝑉

𝑉=𝐸𝑄

=1

2.2×10−3=454.55𝑉

𝐸=𝑄𝑉= (6×10−3 )×500=3 𝐽𝐸=𝑄𝑉=10×240=2400 𝐽𝑄=

𝐸𝑉

=819

=9𝐶


Example 2: What is the Potential Difference across a component if 2000 Joules of heat is generated when a current of 10A flows for 15 seconds?

So, there has been a drop (or loss) of 13.33V across the component, due to heat.

MEASURINGVOLTAGE IN A CIRCUIT

We can measure the voltage in a

circuit using a voltmeter.

These are connected in parallel with the circuit, as pictured below (we will learn more about series and

parallel circuits later)

MEASURINGCURRENT IN A CIRCUIT

We can measure the electric current in a circuit using an

ammeter.

These are connected in series with the circuit, as pictured

below.


For some ammeters we can connect the ammeter to the circuit via 3 different terminals and as such, there are three

different scales to display the current reading to cover a range of readings, small or large.

The Black terminal is connected toward the negative terminal of the power supply and the

red terminal is connected to the positive terminal of the power supply.

mA

CONNECTING VOLTMETER AND AMMETER’S TO A CIRCUIT

For both voltmeter and ammeters, the black terminal is connected toward the negative terminal of the power supply and the red

terminal is connected to the positive terminal of the power supply.

This is the same as

this

Ammeter Setup

CONNECTING VOLTMETER AND AMMETER’S TO A CIRCUIT

For both voltmeter and ammeters, the black terminal is connected toward the negative terminal of the power supply and the red

terminal is connected to the positive terminal of the power supply.

This is the same as

this

Voltmeter Setup


Example: If the ammeter is connected to a circuit and gives the following reading

(pictured left) – what is the current in the circuit if:

• The 5 A terminal is connected?

• The 50 mA terminal is connected?

• The 500 mA terminal is connected?

mA

3.5 A

35 mA

350 mA

MEASURINGVOLTAGE IN A CIRCUIT

Example: If the voltmeter is connected to a circuit and gives the following reading

(pictured left) – what is the voltage in the circuit if:

• The 3 V terminal is connected?



mA

1.4 V

7 V

14 V


QUESTIONS 14, 15, 16, 17, 18

How do batteries work?

http://www.qrg.northwestern.edu/projects/vss/docs/power/2-how-do-batteries-work.html

http://engineering.mit.edu/ask/how-does-battery-work









AC vs DC

https://www.youtube.com/watch?v=xyQfrzBfnDU



POWER DELIVERED BY A CIRCUIT

Power is the rate of doing work, or the rate at which energy is transformed from one form to another.

The rate at which energy is transformed in a circuit, determines its effect on the circuit.

eg. The brightness of a light in a circuit is determined by the rate that electrical potential energy is transformed into other forms of energy (heat, light etc.)A circuit with a large amount of power is going light a globe brighter than that with lower power input.


Power is equal to the amount of energy transformed per second.In simpler terms – Power is the rate of energy use.

This equation tells us that:1 Watt of Power = 1 Joule of energy transformed per

second.

Power (P) measured in Watts (W)


Time (t) measured in Seconds (s)






Current (I) measured in Amps (A)

SIMPLE REPRESENTATION OF SOME OF THE FORMULA’S USED SO FAR







(C)


eg1. What is the power rating of a torch if acurrent of 0.5A flows through it when thereis a voltage drop of 9V across the lamp?





eg2. What is the power rating of a heater if acurrent of 10A flows through it when thereis a voltage drop of 120V across the heatingelement?





eg3. A lamp has a power rating of 80 W.What current flows through the lamp ifthere is a voltage drop of 120V across the globe? Express your answer in mA.





eg4. Calculate the voltage of an iPad if it israted 4 W and draws a current of 500mA.





QUESTIONS 19 - 25

BATTERIES & CELLS Cells can be represented in a circuit by the symbols:

Batteries are one or more electrochemical cells, which are placed together and packed, and such can be represented in a circuit by the symbols:

Variable Power Supplies arerepresented by the symbol:

BATTERIES & CELLS

Batteries:

https://www.youtube.com/watch?v=CX84l5ZZHVg

How they work/recharge – the car battery:

https://www.youtube.com/watch?v=4IgHj2Uim_0






EMF ELECTROMOTIVE FORCE

• Batteries and Power Packs (like computer chargers, phone chargers etc), provide energy to a circuit.

• These devices are said to provide an Electromotive Force (EMF).

• EMF is a measure of energy supplied to the circuit for each coulomb of charge passing through the power supply.

• EMF is represented by the symbol ε and is measured in Volts.

EMF ELECTROMOTIVE FORCEThe amount of energy (E) supplied to the charge passing through a power supply (Battery Cells or Plug-In power packs) is equal to the amount of energy passing given to each coulomb, or EMF (ε), multiplied by the amount of charge passing through the power supply.

This formula is used to determine the rate that the source of the EMF provides energy to the circuit.

Energy (E) measured in Joules

(J)

EMF () measured in Volts (V)

Charge (Q) measured in

Coulombs (C)

Power (P) measured in Watts

(W)

Current (I) measured in Amps

(A)


eg1. A 12V car battery has a current of 3A passing through it.At what rate is it providing energy to the car’s circuit?

The in this equation is the same as V.

Recall: 𝑃=𝑉𝐼


eg2. What is the EMF of a battery in a circuit that provides 18 J of energy and 2 C of charge?

The in this equation is the same as V.

Recall:

𝑉=𝐸𝑄


QUESTIONS 14 , 26, 27

RESISTANCE

The Resistance (R) of a device is a measure of how difficult it is for a current to pass through it.

The higher the value of the resistance, the harder it is for the current to pass through the device.

Resistance is measured in Ohms, given by the symbol Ω

Circuit Representations of resistors:

RESISTANCE

The Resistance (R) of a substance is the ratio of the Voltage Drop (V) across it, to the current (I) flowing through it. Given by the equation:

Resistance (R) measured in Ohms (Ω)



RESISTANCE

eg1. Find the resistance of the resistor in the following circuit if a current of 2A flows through it.

Ω




RESISTANCE

eg2. Find the resistance of the resistor in the following circuit if a current of 20mA flows through it.

Ω




RESISTANCE

eg3. Find the Voltage across the resistor in the following circuit if a current of 50mA flows through it.




RESISTANCE

eg4. Find the Current flow in the following circuit. Express your answer in mA.





QUESTION 28

OHM’S LAW

Recall Ohm’s Law:

• When we plot the graph of Voltage (V) vs Current (I), obeying Ohm’s Law, this gives us a linear graph.

• The resistance is given by the gradient of the line.

• This tells us that under constant physical conditions (ie. Constant temperature), the resistance is constant for all currents that pass through it.

• We say that a component/device that produces this graph is called Ohmic device – if not, we say it’s non-Ohmic.


QUESTION 31

RESISTANCE The Ohm is named in honour of Georg Simon Ohm (1787-1854).

Ohm was a German Physicist who investigated the effects of different materials in electric circuits. His studies found:

The resistance of a conductor relies on 4 factors:

1) The length (l) of the conductor – The longer the length, the greater the resistance

2) The cross sectional area (A) – The greater the Area, the easier current flows through – so the smaller the resistance will be.

3) The type of material that its made from – some materials are better conductors of electricity than others – the property that contributes to its resistance is its resistivity ()

4) Temperature of the wire

RESISTANCE

The resistance of a conductor relies on factors:

Length (l), measured in meters (m)

Cross sectional area (A), measured in metres squared )

Resistivity (), measured in ohm metres (m)

We can use these to find the resistance that a material provides:

Temperature effects the resistance, however we are ignoring it in our calculations in the text. See page 39 table 2.1 for further.

RESISTANCE

eg1. Use the table below to find the resistance of a copper wire at temperature 18˚, with length 5m, cross-sectional Area of 1

Length (l) measured in metres (m)

Cross-Sectional Area (A) measured in

Resistivity () measured in Ohm metres

(Ωm)

RESISTANCE

eg2. Use the table below to find the resistance of a silver wire at temperature 100˚, with length 50m, cross-sectional Area of 1




(Ωm)

RESISTANCE eg3. Use the table below to find the resistance of a carbon wire at temp 18˚, with length 25m, diameter of 2 Give your answer in to 1 decimal place.




(Ωm)

Note: Cross-Sectional Area of a wire =

A=


QUESTION 30

Note: Cross-Sectional Area of a wire =

RESISTORS Resistors are generally made from the semiconductor Carbon.

Variable Resistors

Potentiometer or “Pot”

Trim Pot

RESISTORS Resistors have 4 - 6 bands of colour on them.We can read the value of the resistance on each resistor by reading and interpreting their colour bands.

We will learn how to read 4-colour band resistors.

RESISTORS eg1. Find the resistance of a resistor with

the colours (in order): Blue, Green, Red,

Silver.6 5 ±𝟏𝟎%

So measuring the resistor would show a resistance between 6500-650 to

6500+650, so between5850 – 7150


the colours (in order): Brown, Black,

Orange, Gold.1 0 ±𝟓%

So measuring the resistor would show a resistance between 10000-500 to

10000+500, so between9500 - 10500


the colours (in order): Red, Black, Brown,

Silver.2 0 ±10%

So measuring the resistor would show a

resistance between 200-20 to 200+20, so between

180 - 220

NOW DO: TEXT BOOK PROBLEM

QUESTION 29


QUESTION 29

NOW DO: Q32-34

(AND ANY OTHER QUESTIONS STILL REMAINING FROM CHAPTER 2 ON YOUR

WORK RECORD)

SERIES VS PARALLEL CIRCUITS

Series Parallel

SERIES CIRCUITS

ResistanceThe total resistance is equal to the sum of all individual resistors.

CurrentThe current flowing in all parts of the circuit is the same, found using Ohm’s Law:

SERIES CIRCUITS

eg1. What is the total resistance of the circuit?

What is the Current in the circuit?

10Ω10Ω

10Ω

9V

SERIES CIRCUITS



40Ω100Ω

220Ω

12V

15VThis Law is an application of the conservation of electrical energy around a circuit.

It can be stated as:

In any closed loop of a circuit, the sum of the voltage drops

must equal the sum of the EMF’s in that loop.

The following slides show how this impacts the voltage throughout a series circuit…

KIRCHOFF’S SECOND LAW

Voltage Drop

The voltage drops across each resistor.

The sum of the voltage drops is equal to the applied voltage.

15V

5V

10V

eg1. How can we find the voltage drop across each resistor?

Find the value of in the circuit.

Use this to calculate the current:

Then use Ohm’s law to find the voltage drop across each resistor

0V

I = 0.5A

Voltage Drop

9V

5V

7.5V

eg2. Find the voltage drop across each resistor.

Find the value of in the circuit.

Use this to calculate the current:

0V

I = 0.05A

Then use Ohm’s law to find the voltage drop across each resistor

9V

a) Current is the same throughout the circuit,

b) Voltage drop across

c) Voltage drop across 100V-60V = 40V

d)


CHAPTER THREE

QUESTION 3-9

KIRCHOFF’S CURRENT LAW Kirchoff’s Current Law (Kirchoff’s First Law) tells us that electric charge in a circuit is conserved. This means that current flow in and out of junctions in the circuit is equal.

Total Current In = Total Current Out

ie. For the junction of wires pictured, Kirchoff’s Law tells us that:

eg1. If ∴ 𝐼𝑑=1.5 𝐴

KIRCHOFF’S CURRENT LAW eg2. Using this law analyse the junction of wires pictured.

For the wire marked ;

• What amount of current runs through this branch?

• Is the current going in or out of the junction?

Total Current In = Total Current Out Current IN = 1 A + 4 A = 5 A Current OUT = 1.3 A + 2.5 A = 3.8 A

Unknown = 5 A – 3.8 A = 1.2 A OUT

eg3. In the pictured circuit, find the current at point a, b, c, d, e

Total Current In = Total Current Out

a Junction A: 15.3 = 7.9 + aa = 15.3 - 7.9 = 7.4 mA

b Junction B: 7.9 + 7.4 = bb = 15.3 mA

c Junction C: 15.3 = 2.1 + cc = 15.3 – 2.1 = 13.2 mA

d Junction D: 13.2 = 6.5 + dd = 13.2 – 6.5 = 6.7 mA

e = 2.1 mA

f Junction E: (6.5 +6.7) + 2.1 = ff = 15.3 mA

PARALLEL CIRCUITS Resistance The total resistance found by the formulas -

For 2 resistors: For any number of resistors:

VoltageThe voltage flowing in each branch of the circuit is equal to the EMF of the battery – the voltage drop across each

resistor is the same.

CurrentTotal current is equal to the sum of all of the currents in

each branch. Ohm’s Law can be applied to resistors in each branch to

find the current in each branch.

PARALLEL CIRCUITS



10Ω10Ω

10V In each branch:

PARALLEL CIRCUITS eg2. What is the total resistance of the circuit?


50Ω200Ω

10V In each branch:

PARALLEL CIRCUITS eg3. What is the total resistance of the circuit?


3kΩ1kΩ

24V In each branch:


CHAPTER THREE

QUESTION 2, 10-16

electricityelectricity unit 1 physics. introduction – electrons & matter all material is made...

Documents

current flow

flow of current

electrons flow

flow of electrons

electric current

charge electrons

conventional current

physics slide