Electricity and Ohm’s Law Completing the Circuit The flow of electrons is called electric current...

Electricity and Ohm’s Law

Completing the Circuit

The flow of electrons is called electric current

The unit of electric current is the ampere or amp for short

The symbol for amp is A

An amp is the measure of how many electrons flow per second

Electricity



For electricity to flow in a circuit it must have:

1. A complete pathway for the electrons to flow through eg connected wires

2. An energy supplier eg a battery

3. An energy user eg a lamp

Electricity



Draw the 3 circuits

Under each one say why you think they will work or won’t work.

A B

C



Voltage (Potential Difference)

• A potential difference occurs when there are more electric charges in one location than in another

• The unit of potential difference is a volt

• The symbol for volt is V

• Example: excess electrons will be attracted to the positive plate because of the potential difference

+ + + + + + + + + + + + + +

- - - - - - - - - - - - - - - - - - -

e-

A More Relevant Example

• In a household electric outlet, the potential difference (voltage) between the two slots is 240 volts

• Because of a power plant many miles away, electrons are under “pressure” to move from one slot to the next

Electrical Circuits

• An electrical circuit is formed when electrons are given a path to move across a potential difference

• Generally, conducting (metal) wires are used to “complete” the circuit

Electrical Circuits

• A circuit like this where the electrons have only one path is called a series circuit.

Bomb Circuit

• Pretend a bulb is a bomb and a switch is a key to detonate it.

• Draw a circuit where the key will set off the bomb

What type of Circuit is it?

A series circuit

Emergency

• Now the evil powers of the world rewired the circuit so two keys would detonate the bomb

• Draw this circuit

What Type of Circuit is this?

A parallel circuit

Electrical Components

An electric current is a movement of charged particles.

Draw, name and give the main function of the following components

Name: cell Function: source of electrical energy + -

Name: Lamp Function: turns electrical energy to light

Name: resistor Function: to slow current flow

Name: fuse Function: to melt when short circuit occurs or too much power is drawn

Name Function:

Ammeters must be placed in circuits in

V Name Function:

Name Function:

Voltmeters must be placed in circuits in

ammeter to measure current

series

voltmeter To measure voltage

parallel

diode lets current flow in one

direction only

Copy out and complete the following

A

Draw the following in symbol form

3 cells joined in series

power pack

open switch

variable power pack

Electrical Components

An electric current is a movement of charged particles.

Draw, name and give the function of the following components

Name: Rheostat Function: to vary resistanceOr variable resistor

• Draw a series circuit with a 6volt cell 2 lamps and two ammeters

If both lamps are the same comment on their brightness

Draw voltmeters to measure the voltage of each lamp

If 4 amps was applied to the circuit, what would each ammeter read

lamp

+ - A

A

V

V

• Now make the circuit with the gear provided.• Use the multimeters to:

+ -

A1

Measure the voltage across each lampLamp 1 =Lamp 2 =

Measure the current at 2 different places in the circuitA1 =A2 =

V

A2

V

Voltage

• Voltage (also called potential difference) measures:1. The energy lost across a component2. Energy supplied by the power supply or battery

The unit of voltage, the volt (symbol V) represents the energy change each second for every amp of current

If a lamp uses 1.5 volts this means that for every amp of current 1.5 joules of energy is lost every second.

Voltage is measured with a voltmeter.Voltmeters must always be connected in parallel to the

component for which the voltage is being measured

Current in a Series Circuits

• In a series circuit the current flow is along the same path

• Current in a series circuit is the same at any point of a series circuit

• Current is measured by an ammeter and must always be added to a circuit in series

+ -

A1

A2

If 4 amps is supplied to the series circuitA1 = 4 ampsA2 = 4 amps

+ -

A1

A2

Voltage in a Series Circuit

• If there is only one component in a circuit the voltage across the power supply is equal to the voltage across the components

+-

V

V 6V

6V

• In a series circuit the voltage across the components add up to the supply voltage

Voltage in a Series Circuits

+-

V

V1 4V + 2V= 6V

2V

• In a series circuit the voltage across the components add up to the supply voltage, what is the voltage at V1?

• What does the voltage drop over each component depend on?

V4V

The resistance of the component which is related to the energy usage

Voltage in a Series Circuits• What’s an advantage of a series circuit?

• What’s a disadvantage of a series circuit?

• Where are series circuits used?

Home Wiring: Series Circuits

• If lights or appliances are connected in series they will not experience the same voltage

• Since most lights and appliances are designed to work at a specific voltage, they cannot be connected in series

• The resistance of lights or appliances connected in series is the sum of their resistances

Voltage

Lights

Switch

Fuse

+ -

6 V

Parallel Circuits

These are circuits where the lamps are in parallel with each other in separate branches

All the lamps glow with same brightness.

Why?

Because the voltage over each lamp is the same as the supply voltage

V

V

6 V

6 V

+ -

6 V

Current in Parallel Circuits

Current splits over each branch of a parallel circuit

A2

A1

A3

A4

If the current supplied to the parallel circuit at ammeter A1 = 6 amps then:A1 = ____ampsA2 = ____ ampsA3= ____ ampsA4= ____amps

6

6

33

Home Wiring: Parallel Circuits• A single switch can be

used to turn on multiple lights or appliances if they are connected in parallel

• The voltage is the same for each light or appliance, although the current increases with each new addition

• A fuse is designed to break if the current gets too large

Voltage Lights

Switch

Fuse

Series circuit

Parallel circuit

Current splits over each branch in a parallel circuit

VoltageCurrent

Voltage in a parallel circuit stays the same as the supply voltage

Current stays the same at all points in a series circuit

Voltage drops over each component in a series circuit

Starter – draw the grid then make a statement

How can two switches work lights in a hallway?

switch switch

Conventional Current

• Conventional current is the flow of positive charges from the positive terminal of a power supply to the negative terminal of a power supply –

• we will use conventional current when looking at the direction of current flow in a circuit

Conventional current (I)

Electron flow moves this way through the wire

This is important when looking at diodes!

Resistance• Resistance is anything that slows the

flow of charged particles• Resistance is measured in ohms

(symbol )

Investigating voltage – current relationshipsWe will investigate the R = V/I relationship in a circuit to see if resistance changes when voltage and current is changed at constant temperature.

Method

Set up a series circuit with a heat sync resistor (designed to dissipate heat) a variable voltage supply and an ammeter.

Vary the voltage supply, read current and fill in the chart below.Voltage

Current

Now plot your results on the provided graph paperVoltage on the ‘Y” axis and Current on the “X” axis

Investigating voltage – current relationshipsOnce you have plotted the points draw a straight line graph and find the gradient of the line

The gradient of the line gave us the r________ of the circuit which = ____ ( )?

Ohmic conductors

Because the resistance did not change when both current and voltage were changed the resistor is an ohmic conductor

This is because the temperature of the heat sync resistor remained constant

Non Ohmic Conductors (conductors whose temperature increases with voltage and current)Now plot the figures from a circuit with a lamp (a non Ohmic conductor) on page 167What do you notice?

Investigating voltage – current relationshipsThe gradient of the ohmic voltage vs current graph gave us the ________ of the circuit.

Ohmic conductors

Because the resistance did not change when both current and voltage were changed the resistor is an ohmic conductor

This is because the temperature of the heat sync resistor remained constant

V

I

Non Ohmic conductors

NonOhmic Conductors (conductors whose temperature and therefore resistance increases with increasing voltage and current)Examples of non ohmic conductors are lamps and resistors

V

I

Ohm’s Law• Voltage, current and resistance in an

electric circuit can be related using Ohm’s Law

• Ohm’s law states thatV = I x R

where – V is the voltage (in volts)– I is the current (in amps)– R is the resistance (in ohms)

V

I R

X

Task: Rearrange the formula (V= I x R) using ohms law to find the formula for:

RV

I = V = R x I R =IV

Using Ohms law Example • If a lightbulb in a 240volt

circuit is connected and has a resistance of 60 , what current is flowing through the bulb?

Answer:• Rearrange V = I x R

AI

VI

RV

I

460240

Example 1• If 4 amps of current are flowing

through a toaster connected to a 120 V outlet, what is the resistance of the toaster?

• Answer: Rearrange V = I x R

304

120

RAV

R

IV

R

12 V

Level one Questions – draw the following circuit

L1

L2 L3

With both switches closed the current in L1 is 2A1. Draw the symbol for the component that could measure the voltage across L2

2. What would the device measure?

3. Calculate the resistance of lamp L1 show your working and state the unit

4. Describe the brightness of lamp L2 compared with the brightness of lamp L1 and lamp L3

PowerPower

In an electrical circuit power (P) is In an electrical circuit power (P) is the rate at which electrical energy the rate at which electrical energy is :is :

a.a. Used by a component eg a lamp or Used by a component eg a lamp or ……

b.b. Supplied by a power source eg Supplied by a power source eg batterybattery

Power continued Power continued Power is measured in Watts (W) Power is measured in Watts (W)

One watt = 1 joule of energy used per secondOne watt = 1 joule of energy used per second

Larger amounts of energy use the unit Larger amounts of energy use the unit kilowatts kilowatts

One kilowattt = One kilowattt = 1000 watts

Examples

A 100 W lamp uses 100 joules of energy per second.

A motor which uses 2,500 joules of electrical energy each second has a power rating of 2,500W or 2.5 Kw

Starter – QuestionWhere are parallel circuits used and why?

Calculating PowerCalculating PowerPower can be calculated using the voltage and current in the circuit. Using the following formula.

P = V x I Where: P = power (Watts)

V = volts

I = current

Can you put these in a triangle to help remember them

P

V I

X

A

V

12V

5A

Calculate the following power supplied to the lamp

P = V I

= 12v x 5A

= 60 W

Use the following circuit to answer the questions

A3

A2

A4

A1

V 1. If each of the cells is rated at 2.5 volts what would the voltmeter read?2.5v + 2.5v + 2.5v = 7.5v

(A)

2. If each lamp had a resistance of 1.25 ohms calculate the total resistance in the circuit

1.251.25 + 1.25 + 1.25 + 1.25 + 1.25 = 3.75 = 3.75 (A) (A)

2A Ω 3.75V 7.5

I RV

I R I V using current workout 1st

3. How much power would the circuit use? (M/E)

(W) watts15 2A x V 7.5 P

VIP usingpower workout Now

8 VThe resistors are identical

I= 1.5A

4V

1. What is the voltage across the bulb?

4V

2. What is the current through the bulb?

3 Amps

Radio (100 W)

Fridge (200 W)TV (500 W)

Heater (1000 W)

Fuse 10A

240 V AC

a. What is the purpose of a fuse?

To break the circuit if too much current is drawn

b. If all the appliances are turned on at once will the fuse blow?

7.5A 240V

1800W I

V

P I I V P

0.416A

0.83A

2.08A

4.16A

No the fuse would not blow

Draw the circuit

Read Page 168

Complete questions – 3 , 4 a & 4b on page 169 These are important questions

R= 3

V3V

What is the current through the resistor?

V3V

A0.5 A

A car battery supplies a current of 200A to a starter motor at a voltage of 10V. What is the resistance of the starter motor

What is the resistance of a bulb if a 240V supply causes a current of 2A to flow through it when it is heated to constant temperature.

QUESTION THREE: ELECTRICITY

Amanda connected a light bulb in series to a variable power supply and an ammeter. A voltmeter was connected in parallel to the light bulb.

(a) Draw a circuit diagram of this set-up in the space below.

(b)Comment on the positions of the ammeter and voltmeter in the circuit.

0

5

10

15

0 0.5 1

Vo

lta

ge

(V)

Current (A)

Voltage vs Current

Voltage (V)

2 4 6 8 10 12

Current (I) 0.2 0.4 0.6 0.8 0.90.95

The power supply voltage was gradually increased and the readings on the voltmeter and ammeter were recorded in the chart below.

(c) Draw a graph of these results on the grid below.

(d) Use the equation V = I x R to calculate the resistance of the light bulb when it is operating on 6 Volts.

R =V/I = 6/0.6 = 10 Ohms

(e) Describe what happens to the resistance of the light bulb at higher voltages, and give a reason for this behaviour.

The resistance of the bulb increases at higher voltages. At higher voltages the temperature of the filament increases. As the temperature increases so does the resistance of the circuit.

(f) (i) Show that the light bulb uses 90J of electrical energy when it is operated at 10V for 10 seconds.

One of the two steps attempted.

P =V x I = 10 x 0.9 = 9W

E = P x t = 9 x 10 = 90J

(f) (ii)Comment on whether you think the light bulb willproduce 90J of light energy in 10 seconds at this voltage setting.

It will produce less than 90J of light energy. As energy is lost as heat energy

Task 1 : Use the wires and bulbs to make a series circuit with one bulb

+ _

Transformer on 6 volts

Task 1 : Use the wires and bulbs to make a series circuit with one bulb

Task 2 : Use the wires and bulbs to make a series circuit with two bulbs

Write down how the brightness of the bulb in Task 1 compares the brightness of the bulbs inTask 2

+ _

Transformer on 6 volts

Bulbs in series

When the current flowed through just one bulb the lamp was b________.

When another lamp was added both lamps went d________.

This showed us that the voltage dropped each time it went through a lamp.

Adding Voltmeters

Voltmeters are always added to a circuit in parallel

Task

Now make a circuit with one lamp and measure the voltage over it using the voltmeter (remember voltmeters always go in parallel as I showed you)

+ -

V1 Draw the diagram

The voltmeter reading

V1 = ______ volts

Read parallel and series circuits page ____ in text

+ -

V1 V2

Now draw the series circuit below

The 2 voltmeter readings are

V1 = ______ volts V2 = _____ volts


Write Heading in book

Current in Series circuits

+ -

A1

Now draw the series circuit below

Use an ammeter to measure the current at A1 and A2

A1 = ______ amps A2 = _____ amps

A2

+ -

A1

Now draw the series circuit below and copy the text

Complete the following

Current in a series circuit s____ the s_____ at all points

A2

Ammeters are always added in series (that is one behind each other) to all circuits

Starter : On the sheet of paper place your name

then draw a series circuit with a cell two lamps and a switch between the two lamps

Now mark it

+ -

1 mark each for

Lamp symbol

Lamp symbol

Switch symbol

Cell symbol

All wires connected

Give mark out of 5

+ -

A1

Current in a series circuit

If current at A1 = 3 Amps then the current at A2= 3 amps

Current in a series circuit stays the same at all places

A2

Ammeters are always added in series (that is one behind each other) in all circuits

If the current was changed so A1 = 4 Amps what would A2 = 4 Amps

+ -

V1 V2

Voltage in a Series circuit

If the battery supplies 6 volts to the circuit and the voltage drops evenly over both bulbs then the voltmeter readings are

V1 = ___ and V2 = _____

In a series circuit voltage drops over each lamp

6 V

+ -

6 V

Voltage in Parallel Circuits

These are circuits where the lamps are in parallel with each other in separate branches

All the lamps glow with same brightness because the voltage over each lamp in a parallel circuit is the same as the cell voltage

V1

V2

This means if the supply voltage is 6 volts then:V1= ____ voltsV2= ____ volts

Series circuit

Parallel circuit

Current splits over each branch in a parallel circuit

VoltageCurrent

Voltage in a parallel circuit stays the same as the supply voltage

Current stays the same at all points in a series circuit

Voltage drops over each component in a series circuit

Summary – remember this

Task 1 : Make a series circuit with 2 bulbs and a switch that turns both bulbs off.Draw the circuit diagram

Task 2 : Make a parallel circuit with two bulbs in it Draw the circuit diagram

Task 3 : Place a switch in the circuit that turns off one lamp at a timeplace a variable resistor in it to dim the lightsDraw the circuit diagram

Electric Current

• The flow of electrons is called electric current

• The unit of electric current is the ampere or amp for short

• The symbol for amp is A

• An amp is the measure of how much charge flows by per second

19/04/23

The structure of the atomThe structure of the atom

ELECTRON – negative,

mass nearly nothing

PROTON – positive,

same mass as neutron

(“1”)

NEUTRON – neutral,

same mass as proton

(“1”)

Electrical Circuits

• An electrical circuit is formed when electrons are given a path to move across a potential difference

• Generally, conducting (metal) wires are used to “complete” the circuit

Electrical Resistance

• Electrical resistance is the measure of the resistance of part of a circuit to the flow of electricity (current)

• Electrical resistance is measured in ohms• Ohms are often represented by the

symbol omega ()• Example: the filament of a 100 W

lightbulb presents a resistance of about 140 to the flow of electricity

The Water Pressure Model

• A helpful way of thinking about current, voltage and resistance is to compare electricity to the flow of water– voltage (i.e. potential difference) is similiar

to the water pressure– current is similar to the flow of water– resistance is similar to the diameter of the

pipe (e.g. a small pipe allows much less water to flow than a large pipe)

Ohm’s Law

• Voltage, current and resistance in an electric circuit can be related using Ohm’s Law

• Ohm’s law states that

V = I x Rwhere

– V is the voltage (in volts)

– I is the current (in amps)

– R is the resistance (in ohms)

Example 1

• If 4 amps of current are flowing through a toaster connected to a 120 V outlet, what is the resistance of the toaster?

• Answer: Rearrange V = I x R to get R = V/I

Thus, R = (120 V)/(4 A) = 30

Example 2

• If a lightbulb connected to an outlet has a resistance of 144 , what current is flowing through the bulb?

• Answer: Rearrange V = I x R to get I = V/R

Thus,

I = (120 V)/(144 )

= 0.83 A

Home Wiring: Parallel Circuits• A single switch can be

used to turn on multiple lights or appliances if they are connected in parallel

• The voltage is the same for each light or appliance, although the current increases with each new addition

• A fuse is designed to break if the current gets too large

Voltage Lights

Switch

Fuse

Home Wiring: Series Circuits• If a lights or appliances are

connected in series they will not experience the same voltage

• Since most lights and appliances are designed to work at a specific voltage, they cannot be connected in series

• The resistance of lights or appliances connected in series is the sum of their resistances

Voltage

Lights

Switch

Fuse

Example

• What is the total resistance of three 100 W light bulbs (each 144 ) in series?

• Answer: 144 + 144 = 432 • What is the total current through three 100 W light

bulbs?• Answer: I = (120 V)/(432 ) = 0.27 A• Note: this is significantly less than the 0.83 A that

will flow through a single bulb (i.e. the bulbs will be dimmer)

Electricity and Ohm’s Law Completing the Circuit The flow of electrons is called electric current...

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Transcript of Electricity and Ohm’s Law Completing the Circuit The flow of electrons is called electric current...