Today’s lesson. Electric current flow Electric current flows from the POSITIVE terminal of a power...

Today’s lesson

• Understand the term resistance• Resistance = voltage / current• Recall and use: V = IR• The gradient of a Voltage current graph is

known as the conductance

Electric current flow

Electric current flows from the POSITIVE terminal of a power supply around a circuit to the NEGATIVE terminal.

The longer thinner line of the symbol for a cell is the positive terminal.

In the circuit above the diode is aligned so that it allows current to flow through the radio.

ResistanceResistance is the opposition that an electrical device has to the flow of electrical current.

All devices have some resistance. A resistor is a device that has a particular resistance.

Now fill in the resistance sheet

a resistor

circuit symbol for a resistor

Measuring Resistance

The resistance of a component can be found by measuring the current through, and voltage across, the component.

Circuit used for measuring the resistance of an indicator lamp

Current-voltage graphsThese are used to show how the current through a component varies with the applied voltage.

The circuit opposite could be used to obtain a current-voltage graph of a variable resistor.

Typical results:

Now draw a graph with the data given

Voltage (V) 0.00 0.30 0.60 0.90 1.20 1.50

Current (A) 0.00 0.04 0.08 0.12 0.16 0.20

The current-voltage graph of a wire or a fixed resistor at a constant temperature

The graph is a straight line through the origin.

The wire or resistor obeys Ohm’s law which states that that the current is proportional to the voltage at a constant temperature.

I

V

Copy and complete:

An electric ________ will only flow around a circuit if there are no ______ in the circuit.

All components have __________. The greater the resistance the ________ is the current for the same applied voltage. Resistance is measured in ______.

A current – voltage graph for a ________ is a straight line through the _______. This shows that the current through the resistor is ___________ to the applied voltage.

resistance currentresistor ohmsgaps originsmaller

WORD SELECTION:

proportional

resistance

current

resistor

ohms

gaps

origin

smaller

proportional

11 of 46 © Boardworks Ltd 2007

Resistance is a measure of how hard it is for electrons to move in an electrical circuit.

The connection between current, voltage and resistance was discovered in 1827 by Georg Ohm, a German physics and maths teacher.

The formula R = V/I is known as Ohm’s Law. It was such an important discovery in electricity that the unit of resistance is called the ohm. This unit is represented by the symbol W.

The irresistible Georg Ohm


What are the units of voltage, current and resistance?

Ohm’s law is usually written as:

Voltage is measured in volts (V). Current is measured in amps (A). Resistance is measured in ohms ().

This formula can also be written as:

voltage = current x resistance V = I x R

resistance = R =

voltagecurrent V I

What is the formula/equation for Ohm’s law?

Ohm’s Law

• V = IR

V

RI X


What does Ohm’s Law show?

What do the different arrangements of Ohm’s law show about the links between current, voltage and resistance?

V = I x R

R = V / I

I = V / R

This version of Ohm’s Law shows that as the voltage increases, the current increases. The voltage and current are proportional, while the resistance remains constant.

The voltage and current are proportional, so the resistance of a material is constant, as long as the temperature does not change.

For a low resistance material, more current is allowed to flow for a given voltage. For a high resistance material, less current will flow at the same voltage.


A formula triangle helps you to rearrange a formula. The formula triangle for V = IR is shown below.

Cover up the quantity that you have to work out and this gives the formula needed.

So to find current (I), cover up I…

…which gives the formula…

I =VR

x

Resistance formula triangle


A filament bulb has a current of 0.2 A running through it, with a potential difference of 5 V across it.

V = IR

Calculating the resistance of a bulb

What is the resistance of the filament in the bulb?

= 5 V 0.2 A

R = V I

= 25

Question 1

Calculate the resistance of a lamp if a voltage of 12V causes a current of 3A to flow through the lamp.

resistance = voltage current

= 12V / 3Aresistance = 4 ohms (4Ω)

Question 2

Calculate the resistance of a heater if a voltage of 230V causes a current of 200mA to flow through the heater.

resistance = voltage current

= 230V / 200mA= 230V / 0.200Aresistance = 1150 Ω

Question 3

Calculate the voltage across a resistance of 40Ω when a current of 5A is flowing.

V = I x R= 5A x 40Ωvoltage = 200V

Question 4

Calculate the current flowing through a wire of resistance of 8Ω when a voltage of 12V is connected to the wire.

current = voltageresistance

= 12V / 8Ωcurrent = 1.5A

Now complete the Ohm’s Law questions


Resistance calculations


The resistance of a wire depends on several factors:

material

length

thickness

temperature

For example, a copper wire has a lower resistance than a nichrome wire of the same size. Copper’s excellent ability to conduct electricity means it is often used in wiring.

What experiments could be used to test how length and thickness affect resistance?

What affects resistance?


Investigating resistance and length


How does length affect resistance?

When the length of the wire increases, resistance increases.

The table shows the resistance of different lengths of wire.

Why would increasing the length of the wire result in a increase in resistance?

length (cm)

5

10

20

voltage (V)

current (A)

resistance ( )W

2.6

2.9

3.0

5.0

4.3

2.1

0.52

0.67

1.48

What general pattern do these results show?


Why does length affect resistance?

The effect of length of a wire on resistance can be understood by looking at the atomic structure.

Resistance is caused by electrons colliding with metal ions. When the length of the wire is increased, the electrons have to travel further. So the chance of collisions will increase, causing the resistance to increase.


Investigating resistance and thickness


How does thickness affect resistance?

When the thickness of the wire increases, resistance decreases.

The table shows the resistance of different thicknesses of wire.

Why would increasing the thickness of the wire result in a decrease in resistance?

thickness (mm)

1

2

4

voltage (V)

current (A)

resistance ( )W

3.0

2.9

2.6

2.1

4.3

5.0

1.48

0.67

0.52

What general pattern do these results show?


Why does thickness affect resistance?

Increasing the thickness of a wire increases the surface area that the electrons can flow through.

In thick materials the charge carrying particles are able to move through the conductor more easily, reducing resistance.

This decreases the chance of collisions with metal ions.


How does temperature affect resistance?

If a component obeys Ohm’s law, then decreasing the temperature decreases the resistance of the material.

A superconductor is a material that conducts electricity with very little resistance at very low temperatures.

This is because the positively charged ions in the metal do not vibrate as fast, and the electrons can flow more easily.

Superconductors can be used to make very fast circuits and to make magnets levitate.

Do Now

• Question paper 3 November 2006

• Question paper 6 June 2004

• Stick in notes on factors affecting resistance.

November 2006 paper 3

June 2004 paper 6

Today’s lesson

• Understand the term resistance• Know how the four factors affect resistance

and their mathematical relationships needed for paper 6 questions – question November 2004

• How to calculate resistance in series• How to calculate resistance in parallel

November 2004


Resistance – true or false?


total resistance in series = R1 + R2

There is only one path for the current to travel, which means that it flows through the resistors one after the other.

When two (or more) resistors are connected in series, the combined resistance is higher than the individual resistors.

4 2

Total resistance

= R1 + R2

= 4 + 2

= 6

How is resistance affected in a series circuit?

This has the same effect as using a longer piece of wire.


What is the total resistance for each of these circuits?

Total resistance = R1 + R2

= 6 + 34

= 40 6 34

Calculating resistance in series

15 5

Total resistance = R1 + R2

= 15 + 5

= 20


Total resistance

2

4

= 1.33

When two (or more) resistors are connected in parallel, the current splits at the branches and does not go through each resistor.

= R1 x R2

R1 + R2

= 4 x 2 4 + 2

How is resistance affected in a parallel circuit?

This has the same effect as using a thicker piece of wire.

This means the total combined resistance is less than any of the individual resistors.

1 11total resistance in parallel

R1 R2

= +


What is the total resistance for each of these circuits?

= 3.4 6

8

Total resistance = R1 x R2

R1 + R2

= 8 x 6 8 + 6

Calculating resistance in parallel

5

5

= 2.5

Total resistance = R1 x R2

R1 + R2

= 5 x 5 5 + 5


Resistors in a circuit

Complete use page 181symbol component symbol component

A

filament bulb

ammeter

diode

heater

thermistor

cell LDR

resistor

Resistance of a lamp

• As the current in a lamp increases, it gets hotter, and its resistance increases. Why?

http://www.absorblearning.com/advancedphysics/demo/units/020102.html


• As the current in a lamp increases, it gets hotter, and its resistance increases.

Copy please.


AV

Resistance = voltage/current R = V/I

Vary the voltage and current using a variable resistor (rheostat). Plot a graph of resistance against current

Filament lamp

The resistance of a filament lamp increases as the temperature of the filament increases.

Reversing the voltage (negative values on the graph) reverses the direction of the electric current but does not change the shape of the curve.

The lamp does not obey Ohm’s law

I

V

Diode

Current only flows through a diode in one direction. (Shown by the arrow on its symbol)

The diode has a very high resistance in the reverse direction.

A light emitting diode (LED) emits light when electric current flows.

I

V

Thermistor

The resistance of a thermistor decreases as the temperature increases.

The higher temperature line therefore has a greater slope than the lower temperature case.

I

V

HOT

COLD

Light dependent resistor (LDR)

The resistance of a light-dependant resistor decreases as light intensity increases.

The bright light line therefore has a greater slope than the dim light case.

I

V

BRIGHT

DIM

Choose appropriate words to fill in the gaps below:

The resistance of a filament lamp _________ when the lamp comes on and the filament rises in ___________.

A ______ only allows electric current to flow one way. The allowed direction is shown by the _______ on its circuit symbol.

The ________ of a thermistor decreases if its temperature is increased. The resistance of a LDR _________ if the _______level is increased.

resistancedecreasesarrow diode lightincreases

WORD SELECTION:

temperature

resistance

decreases

arrow

diode

light

increasestemperature

Online SimulationsSimple demonstration of the effect of a variable resistor - Freezeway.com Battery-Resistor Circuit - PhET - Look inside a resistor to see how it works. Increase the battery voltage to make more electrons flow though the resistor. Increase the resistance to block the flow of electrons. Watch the current and resistor temperature change Light bulb being controlled by a variable resistorElectric circuits with resistors - series & parallel with meters - netfirms Variable resistor with an ammeter & a voltmeterResistance measurement demo - Molecular Expressions Ohm's Law - PhET - See how the equation form of Ohm's law relates to a simple circuit. Adjust the voltage and resistance, and see the current change according to Ohm's law. The sizes of the symbols in the equation change to match the circuit diagram. Ohm's Law - Fendt

Simple V-I circuit with a light bulb - Freezeway.com Resistance in a Wire - PhET - Learn about the physics of resistance in a wire. Change its resistivity, length, and area to see how they affect the wire's resistance. The sizes of the symbols in the equation change along with the diagram of a wire. Resistance Wire Simulation - by KT - Designed for the GCSE Investigation but can also be used to show the affect of source resistance and to show power supply maximum power. Introduction . Worksheets for GCSE investigation Conductivity - PhET - Experiment with conductivity in metals, plastics and photoconductors. See why metals conduct and plastics don't, and why some materials conduct only when you shine a flashlight on them. BBC AQA GCSE Bitesize Revision: Calculating resistance Changing resistance Filament lamp with IV curve Thermistors & LDRs

http://www.freezeray.com/flashFiles/rheostat.htm

http://www.freezeray.com/flashFiles/rheostat.htm

http://phet.colorado.edu/new/simulations/sims.php?sim=BatteryResistor_Circuit

http://www.ngsir.netfirms.com/englishhtm/Circuit.htm

http://micro.magnet.fsu.edu/electromag/java/ohmslaw/

http://phet.colorado.edu/new/simulations/sims.php?sim=Ohms_Law

http://www.walter-fendt.de/ph14e/ohmslaw.htm

http://www.freezeray.com/flashFiles/voltageCurrent.htm

http://phet.colorado.edu/new/simulations/sims.php?sim=Resistance_in_a_Wire

http://www.ktaggart.co.uk/physics/Simulations/EJS/ResistanceWire.html

http://www.ktaggart.co.uk/physics/GCSE/ResistanceWireSimulationIntroduction.doc

http://www.ktaggart.co.uk/physics/GCSE/ResistanceWireSimulationWorksheets.doc

http://phet.colorado.edu/new/simulations/sims.php?sim=Conductivity

http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/electricity/resistancerev1.shtml




AnswersV I R

20 V 4 A

5 A 40 Ω

300 V 0.20 50 Ω

8 V 500 mA

3 kV 150 Ω

4 mA 30 kΩ

5 Ω

200 V

6 A

16 Ω

20 A

120 V

Complete:

Resistance

Measures how difficult it is for current to flow. Measured in Ohms (Ω)

VA


Resistance

Measures how difficult it is for current to flow. Measured in Ohms (Ω)

VA


☺Can you copy this

please?

QuestionCalculate the voltage of a battery if it supplies 300 joules of energy to 50C of charge.

voltage = energy ÷ charge= 300 J

50 C

battery voltage = 6V

Complete:Voltage Energy Charge

12V 480J

500J 25C

6V 20C

230V 69kJ

Answers

300C

120J

20V

40C

Filament lamp

The resistance of a filament lamp increases as the temperature of the filament increases.

The lamp does not obey Ohm’s law

I

V

Can you copy this please?

Diode

Current only flows through a diode in one direction. (Shown by the arrow on its symbol)

The diode has a very high resistance in the reverse direction.

A light emitting diode (LED) emits light when electric current flows.

I

V

Can you copy this please?

Today’s lesson

• Understand the term power• Recall and use: P =VI• Recall and use E = VIt

Power

The amount of energy used by a device per second, measured in Watts (Joules per second)

VA

Power = voltage x current P = VI

Power

The amount of energy used by a device per second, measured in Watts (Joules per second)

VA


☺Can you copy this

please?

Power of a lamp

AV


Measure the power of the lamp at it’s operating voltage. What is the electrical energy being turned into?

Example

• A 200 W television is plugged into the 110V mains. What is the current in the television?

PIV X

Example

• A 200 W television is plugged into the 110V mains. What is the current in the television?

• I = P/V = 200/110 = 1.8A

PIV X

Example

• A kettle uses 240V and 8A. What is its power?

PIV X

Example


• P = VI = 240x8 = 1920W (=1.9kW)

PIV X

Remember

Power is the amount of energy used by a device per second, measured in Watts (Joules per second)

VA


Total energy

So the total energy transformed by a lamp is the power (J/s) times the time the lamp is on for in seconds,

E = VItE = energy transformed (J)V = Voltage (also called p.d.)I = current (A)t = time (s)

Example


• P = VI = 240x8 = 1920W (=1.9kW)• How much energy does the kettle use in 5

minutes?

PIV X

Example


• P = VI = 240x8 = 1920W (=1.9kW)• How much energy does the kettle use in 5

minutes?• E = VIt = 240x8x300 = 576000 J

PIV X

Today’s lesson. Electric current flow Electric current flows from the POSITIVE terminal of a power...

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Transcript of Today’s lesson. Electric current flow Electric current flows from the POSITIVE terminal of a power...