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AQA As Level: Electricity

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D. O'Keeffe

on 8 August 2017

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Transcript of AQA As Level: Electricity

Charge, current and potential difference
Learning objectives

E) Define current, voltage and resistance

C) Apply and use V = W/Q, I = Q/t and R = V/I

What is resistivity?
AQA As Level: Electricity
Why is electricity useful?
Current in metals
Charge is a fundamental property of matter

What are the charges of a neutron, proton, electron?

Electric current is the rate of flow of charged particles

Sketch the diagram below and annotate using the notes
Discuss with a partner
Using charge (Q) and time (t) can you create an equation for electric current
Create a definition for electric current
Explain what happens to the free electrons when a metal wire is connected to the positive and negative terminals of a battery

Answers:

I = ∆Q/∆t units. Amps (A) or Coulombs per second (C/s)

Current is the rate of flow of charge

When a wire is conected to the positive and negative terminals of a battery a potential difference (voltage) is created. This causes an electric current

Using the second part annotate on your diagram information about potential difference

Keywords: energy, positive/negative terminal, work
Potential difference
Potential difference measures the amount of energy carried by the charged particles

Potential difference (V) = Work done(J)/Charge(C

V = W/Q
What does this mean?
Current tells us how much charge is passing through a point every second

Potential difference tells us how much energy each unit of Charge carries
What are the units?
Questions
1) There is a current of 10A passing through a lamp for 1 hour. Calculate how much charge flows through the lamp

2) Calculate the current in a circuit when a charge of 180C passes through a point in a circuit in 2 minutes.

3) Calculate the potential difference when each unit of charge has 0.5J of energy

4) Calculate the work done by 10C in a potential difference of 50V

Ext: Define electrical resistance
Homework
Define electrical resistance and describe how it is related to potential difference and current
Taboo
Electricity
Current
Charge
Potential difference
Electrical resistance
Change in Charge
Current/Voltage characteristics
Learning objectives:
E) State Ohms Law
C) Investigate the relationship between V and I for a filament lamp
A) Determine the relationship between V and I for an ohmic conductor, a semiconductor diode and a filamen
Keywords: Ohmic material, diode
What did you find out about resistance?
R = V/I
The SI unit of electrical resistance is the ohm (Ω),
Ohms Law
V = IR
The potential difference across an ideal conductor (an ohmic material) is directly proportional to the current through it.
Why might this not be the case for all materials?
I/V Graph
V = IR
Investigation
Using the equipment available you need to investigate the hypothesis

'As the p.d increases, the current increases'

Success criteria:
Choose your range of independent variable values
Do repeats
Plot a graph I/V
Draw a curve of best fit
I/V graph for a filament lamp
Diodes only let current flow in the ‘forward’ bias because if it flows in the ‘reverse’ direction the diode offers a very high resistance.

Diode
Task
1) State whether your filament bulb obeyed ohms law, explain why/why not.

2) Sketch an I/V graph for a thermistor (As temperature increases, resistance decreases)

3) Sketch an IV graph the line for an LDR in low light intensity and in high light intensity. (LDRs release more electrons in greater light intensity)

4) Explain why the IV graph for a diode is zero at any negative p.d


Homework
E) Define resistivity of a material
C) Select and use the equation p = RA/L
B) Describe how the resistivities of metals and semiconductors are affected by temperature
A) Explain how the resistance of a pure metal wire and of a negative temperature coefficient (NTC) thermistor is affected by temperature

Keywords: resistivity, conductors, thermistor
What causes resistance in a wire?
Resistance is due to collisions between free electrons and the metal lattice
Resistance is proportional to
length
R α length

Resistance is inversely
proportional to cross sectional
area
R α 1/A

Task 1: Rearrange the equation to make resistivity the subject and define resistivity
Task 2: What is the unit of resistivity?
Task 3: Find the resistivity of a wire of radius 0.15 mm and length 3.2 m with a resistance of 14 ohms

Extension, what would happen to the resistance if the length was x5 and
The radius was x2?

Resistivity
How does temperature affect resistance?

Task 1: p = RA/L Resistivity of a material is defined as the resistance of a 1 metre length with a 1 m2 cross sectional area.

Task 2: Ωm

Task 3: (14 x π x 0.00015^2)/3.2

Extension, what would the resistance be if the length was x5 and
The radius was x2?

Other than length, area and intrinsic resistivity temperature also affects resistance
For metal conductors increasing the temperature increases the vibrations of the metal atoms in the lattice

This increases the interaction between the electrons and the metal atoms increasing the resistance.

In a thermistor increasing the temperature increases the number of free electrons. This will decrease the resistance.
When might an increasing temperature decrease the resistance of a circuit?
Super conductivity
Resistance transfers electrical energy to thermal energy. How can we reduce this?
If mercury is cooled below 4.2 K, it loses all electric resistance - it becomes a superconductor

The critical temperature for a superconductor is the temperature at which the electrical resistivity of the substance drops to zero.
Applications: Very strong electromagnets, power cables
Series and parralel circuits
C) State the rules for resistance in series and parrallel circuits.
B) Describe what happens to p.d and current in series and parrallel circuits.
A) Apply the rules for resistance to exam style questions
Keywords: Series, parrallel
Why does this happen?
http://www.physics-chemistry-interactive-flash-animation.com/electricity_electromagnetism_interactive/components_circuits_association-series_parallel.htm
Questions
1) What happens to current in:
a) A series circuit
b) A parrallel circuit

2) What happens to voltage in
a) A series circuit
b) A parrallel circuit

3) Explain why the battery will be depleted faster by 2 components in a parrallel than a series circuit

Ext: Describe potential applications of parralllel circuits
Resistance rules
Series:
Parrallel:
Energy in a circuit
C: Calculate total for current and p.d in circuits
B: Apply and us equations for power and energy of a circuit
A: Apply the conservation of energy to kirchoffs first law.
Keywords: Power, energy, kirchoffs laws.
Recap
A circuit with a 2V cell contains three 2 ohm resistors in series. Followed by three 2 ohm resistors in parrallel. What is the total resistance?
Cells in series and parrallel
In each case what is the total potential difference?
When cells are connected in series the total potential difference supplied to the circuit is the individual potential differences added together.

Vtotal = V1 + V2 + ... Vn
When identical cells are in parallel with each other the total potential difference is equal to the potential difference of just one of the cells.

Vtotal = V1 = V2 = Vn

So if three 2V cells are connected in parallel with each other the potential difference supplied to the circuit is 2V.
Questions
Conservation of charge

The total charge flowing into a junction of wires must equal the total charge flowing out of the junction”.

Kirchoff’s first law


The sum of the currents flowing into a junction of wires must equal the sum of the currents flowing away from the junction of wires”.
1) What is the total p.d for three 4V cells in series

2) What is the total p.d for the same cells in parrallel

3) Using 'joules per coloumb' explain the rule for total p.d of cells in parrallel
Current in series circuits.

When you put an ammeter into a series circuit the current is the same wherever you put the ammeter.


Current in parallel circuits.

The total current flowing from the cell towards the branches in the circuit must always equal the current flowing through each component in the branches of the circuit when they are added together.



If the components have different resistances then the current through each component may be different but when you add them together they must add up to the total amount of current leaving the cell.
Questions
1) Describe what happens to the current at a junction of paths of equal resistance. Use 'Coulombs per second'

2) For the below circuit with a p.d across the cell of 10v and two resistors 2 ohms each

Calculate the total resistance
Calculate the current through each

Potential difference in series and parallel circuits

Kirchoffs second law


The sum of the Emf’s in any closed loop in a circuit must be equal to the sum of the potential differences in the closed loop in the circuit”.
Potential difference in a series circuit.

The total potential difference supplied by the cell is divided up between the components. If the components all have the same resistance they will have equal amounts of potential difference across them.

If the resistance are not equal they may have different amounts of potential difference across them but when added up they must always equal the p.d. supplied by the cell.
Potential difference in parallel circuits.

The potential difference supplied by the cell is the same potential difference as that across each component in the parallel circuit.
Revision Mindmap
Rules for p.d and current
Cells in series and parrallel
Current in series and parrallel
P.d in series and parrallel.
Circuits Continued
C: Calculate total for current and p.d in circuits
B: Apply and use equations for power and energy of a circuit
A: Apply the conservation of energy to kirchoffs first law.
Starter
State:

1) The total p.d of three 4V cells in parrallel

2) What happens to a 3A current entering a junction with 6 paths of equal resistance

3) The rule for p.d (splits/stays the same) in series and parrallel circuits


Presentation
You are explaining:

what happens to current/voltage when resistors are in series.
What happens to current/voltage when resistors are in parallel.
The rate of heat transfer.
Success criteria:

Use of keywords correctly.
Discussion of derivations
Answer 3 questions from the class.

Summary Practice Questions
Energy in a circuit
C: State the units and symbols for power and energy in a circuit
B: Derrive the equations for energy and power
A: Explain why a high current is needed for a starter motor in a motor car.
Keywords: Energy, power
Energy and Power
P = E/t

Energy (E) is measured in joules

Power (P) is measure in watts (joules/second)
Using the SI units for current and p.d derive

An equation for energy using only I,V and t
An equation for power using only I and V
An equation for power using only I and R (Hint use ohms law)
Equations:
E = IVt

J = C/s x J/C x s

P = E/t

P = IV

V = IR

P = I R
2
Practice Questions
Test Reflection
Task 1:

Go through your test and see how you did
Correct all wrong answers with red pen (leave question 6b)
Circle any questions you are still unsure of.

Task 2:

On the back page in bullet points write down

a) Your strengths (which questions you answered well and why.
b) Your improvements (which questions did you not answer correctly and what did you need to know to do so.
Intervention: For any students who did not reach their target grade.

30 minutes this Friday.
Homework: Active reading on EMF
6B.

On lined paper using the mark scheme re-attempt this question
Peer Assess using the mark scheme
Electromotive Force
E) State what is meant by internal resistance
C) Apply and use the equations for electromotive force
B) Explain the need for a low emf in a car battery
Keywords: internal resistance, emf
Internal Resistance
An electrical cell is made from either metal or chemicals, which themselves have some resistance. This resistance is called the internal resistance of the cell (r).

Some of this energy from a battery is used to overcome the internal resistor.

Potential difference is used up by the internal resistor “r” is Ir

This means there is still some P.d left to be used by the resistor outside the battery.

P.d used up by internal resistor
AND the outside resistor is equal to EMF, “ξ” given by the battery
ξ = IR + Ir
ξ = I(R + r)



Where:
V = pd across the external circuit / terminal pd (V)
ξ = emf of the cell (V)
I = current through the cell (A)
r = value of the internal resistance (Ω)
R = value of the external resistence (Ω)

Explain what we need to calculate to work out the internal resistance
Example

What is the terminal p.d. for a cell of emf 2V and internal resistance 1Ω when it is connected to a 9Ω resistor?
ξ = IR + Ir

ξ = I(R+r)

2 = I(9+1)
I = 2/10 = 0.2A

V = IR = 0.2 x 9 = 1.8V

So this 2V emf cell actually supplies 1.8V to the external circuit.

Calculate internal resistance
Measure the terminal pd 'V' when the current is zero,
Close the switch
Vary the current using the variable resistor
Measure the terminal pd
Plot a graph of V vs I

V = ξ - Ir
V = - Ir + ξ
y = -mx + c



Emf is the Y axis intercept, r is the gradient

Calculate internal resistance 2
Connect a voltmeter alone to a cell to measure the Emf.
Connect, one at a time a total of six lamps to the cell in parallel
Measure the terminal pd and the current each time
Plot a graph of V vs I

V = ξ - Ir
V = - Ir + ξ

y = -mx + c

In pairs prepare to explain a method and what equipment you would use.
Alternative Method
Just pretend the internal resistance is one of the normal resistors in the circuit.

RT = 10Ω

I = Emf/RT = 2/10 = 0.2A

Voltage across the 9Ω resistor =

V = IR = 0.2 x 9 = 1.8V

So this 2V emf cell actually supplies 1.8V to the external circuit.

Emf = IR + Ir
Emf = I(R+r)
= I (3+2)
I = 2.4A (this is the current through the cell
The current through the branch = 1.2A

Potential Dividers
Two or more resistors in series with each other.

Source of fixed pd between them
(ie 1.5 v cell)

Pd of the source is divided between components in circuit

Theory
If the resistors are the same value, how do you think the pd will be divided?

If the resistors are different, how will this effect the division of potential?

Remember V=IR and current stays the same.

Predict how the pd across V1 and V2 will change as you change the resistance across R1

You need to conduct an experiment to prove your predictions and write it up, convincing me of your findings.
Ext derive the equation:
Potential Divider Investigation
Can you find V ?
out
Investigation
Potential divider circuits are used for street lights and heats
They work using an LDR or thermistor to change the resistance. These have to be chosen at a specific resistance to activate at a certain pd.
Title: Potential Divider Experiment
1) Using your results draw a graph of V2 against R1

2) A street light will only switch on once it is receiving 5V. Use your graph to estimate the resistance an LDR in the circuit as R1 would need to be to provide this p.d

3) How could a thermistor be used to turn on a heater at a certain temperature in a potential divider circuit.
Exam question
Electromotive Force Investigation
Catchphrase
tive
Homework SA
Using Lab books
Calculating Internal resistance experimentally
As E = IR + Ir (V = - Ir + E) , if you plot a graph of terminal pd, V, against current, I, the gradient of the graph will be equal to the internal resistance of the cell. (negative because the graph slopes down)

By recording values of current and terminal pd as the external resistance changes you can plot the graph and find the internal resistance and the emf of the cell.

If there is more than one cell in series the internal resistances of the cells must be added.
Internal Resistance
What is the terminal p.d. for a cell of emf 2V and internal resistance 1 ohm when it is connected to a 9 ohm resistor?
Pretend the internal resistance is one of the normal resistors in the circuit. Draw it in the circuit diagram next to the cell so that all the current that goes through the cell also goes through the resistor.

To find V, the terminal pd (or the voltage available to the external circuit), calculate the current, I, for the whole circuit:

VT and RT are the voltage and resistance for the whole circuit, including external and internal resistance.

Therefore, the 9ohm; resistor gets V = IR = 0.2 x 9 = 1.8V

So this 2V emf cell actually supplies 1.8V to the external circuit.
Competency: Following written instructions

Complete the practical task using only the instructions provided.
1) Set up the circuit as shown in the diagram

2) Measure the p.d across your cell unconnected to anything else

3) Reconnect your circuit. Change the variable resistor to measure 5 values of potential difference across your cell and the current in the circuit

4) Record this data in an appropriate table

5) Use your data to create an I/V graph and calculate the EMF and internal resistance of your cell.
Alternating currents and the oscilloscope
C: State what is ment by root mean squared current and p.d
B: Calculate peak to peak values for sinusoidal waveforms
A: Calculate average power using rms p.d and current
An alternating current is constantly changing direction. It is normally sinusoidal.


The frequency of an alternating current supply, f, is the number of cycles completed per second.Measured in Hertz (Hz).

The period, T, of an alternating supply is the time taken to complete one cycle.

The peak values of current, Io, and voltage,Vo, are the maximum values, equivalent to the amplitude of a wave. The peak-to-peak value, is double the peak value.
What values of I and V do you use for calculations seeing as they are always changing?
RMS Values
RMS values are the d.c. equivalent of an a.c. value.
Where V0 and I0 are the peak values.

Note: rms values are less than the peak values of voltage and current.
How do we calculate these values?
Using an Oscilloscope
The two controls on an oscilloscope we are interested in are;

Y-gain in volts/division
, 1 v/div.
Time base in time/division
, e.g. 0.002 s/div.

Input connected to ground (zero volts) and timebase switched off, oscilloscope trace is a spot in the middle of the screen.
3 volts from a dc battery (Y gain set to 1V/div, timebase off), the oscilloscope trace is a spot which moves 3 squares (divisions) up.
Recap
Japan has the lowest mains voltage 100V. Calculate VO for japan.

Describe with a diagram how to calculate rms Voltage
Input connected to ground (zero volts) and timebase switched on, trace appears as a horizontal line in the middle of the screen.


What will it look like at 3V DC?
3 volts from a dc battery (Y gain set to 1V/div, timebase on), trace appears as a horizontal line 3 squares above the middle.
3Volts AC time base off?
3 volts from an ac power supply (Y gain set to 1V/div, timebase off), trace appears as a vertical line 6 squares long.
3V AC time bas switched on?
3 volts from an ac power supply (Y gain set to 1V/div, timebase on 0.002 s/div), sine like trace appears.
The peak voltage (amplitude) is 3V.

The time period (T) is 8 squares x 0.002 = 0.016 s.

We can calculate the frequency (f) in hertz (Hz) using the equation below;

f = 1/T

f = 1 divided by 0.016 = 62.5 Hz
Overview
Waves Overview
http://filestore.aqa.org.uk/subjects/specifications/alevel/AQA-2450-W-SP-14.PDF
http://filestore.aqa.org.uk/subjects/AQA-PHYA2-QP-JUN14.PDF
Electricity Revision
Plan
1) Overview of the topic
2) Just a minute topics
3) Improv narration
4) PPQS
Your turn...
What quantities and formula do you need to know for electricity?
Practice Questions
Using your specification design questions for your peers
What is current?
Keywords: charge carriers, collisions, direction, ampere, coulomb
Resistance is a measure of how difficult it is for a current to pass through a component
Ohm’s law is a special case where potential difference is proportional to current
Electricity PPQs
SA
Other Factors
Temperature and Resistance
What would the graph look like for a metal wire?
A metal is said to have a positive temperature coefficient
A thermistor has a negative temperature coefficient
Superconductivity
Practice Qs
Required Practical
Measuring the Resistance of a Wire
Homework

Reading: page 214-16 Creat 1 side of A4 notes

Required practical.

Complete an evaluation of required practical 1.
It must describe
Accuracy
Reliability
Uncertainty
Practical Write up
In your lab books have complete:

A full table of results
A graph as detailed in the instructions
A risk assessment (table) that details the level, precaution and action for risks in this experiment.
A subtitle 'Uncertainty'
Uncertainty
A new concept at A level.The uncertainty of a measurement is an expression of the spread of values which are likely to include the accepted value.

Absolute uncertainty
Percentage uncertainty
Uncertainty of a gradient
Power and EMF
Electromotive Force
Electromotive force ξ is the electrical energy per unit charge produced by the source.

What would this look like as an equation?
What would it be measured in?

Why might some of the energy per unit charge not be supplied to the rest of a circuit?
Recall the equations of power
Competency: Follow written instructions
Aim: Follow your own method to calculate the internal resistance of a power source
Find and print a method for calculating the internal energy of a power source.

You will need to be following this method in class

Referencing. You must have the source and date accessed e.g

Source: http://www.s-cool.co.uk/a-level/physics/resistance/revise-it/internal-resistance-emf-and-potential-difference date accessed: 16/11/2016
Aim: To find out what resistance an LDR would need to activate a light at 5V
Aim:
1. Set up a potential divider circuit
2. Explain how you will use the circuit to find out what resistance an LDR would need to activate a light at 5V
3. Draw a sensible table for your results
Just a minute topics
Current and charge
Potential difference and power
Resistance
Circuit rules
Electromotive force
Potential divider
Catchphrase
tive
If this is the answer what is the question?
Current
Charge carriers
Charge
coulomb
ampere
ions
semiconductor
Insulator
Work done
Power
potential difference
Ohm's Law
Reisistance
Diode
Series
Parallel
Internal resistance
Electromotive force
http://www.classtools.net/widgets/fruit_machine_6/35GgM.htm
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