Chapter 11- Electricity

Course Content • Definition of Electricity • Circuit Diagrams

– Series and Parallel Circuits

• Calculating total resistances • Measurement of Electricity

– Ammeters and Voltmeters – Ohm’s Law

• Safety Devices and Hazards • Wiring the Plug

What is Electricity

• An atom is a fundamental unit of matter • made up of

• protons (with a positive charge) • neutrons (neutral – no charge) • electrons (with a negative charge)

What is electricity? • Everything is made of atoms which contain POSITIVE particles called PROTONS and NEGATIVE particles called ELECTRONS.

Proton (+)

Neutron

Electron (-)

What is Electricity?

• Electricity – the flow of electrons through a circuit i.e. the flow of electrical charges

• The path along which the electric current moves is called the ELECTRIC CIRCUIT

• The device which causes the flow of electrons through a circuit is called a CELL or POWER SOURCE (Battery)

What is Electricity? • Electrons move round a circuit from negative

terminal to the positive terminal of the cell, producing an electric current. (note: like charges repel, opposites attract)

• The rate of flow of electric charges is called ELECTRIC CURRENT

connecting wire electric cell

filament

flow of electrons

Electric Current

• Conventional Current flow – electric current flows from positive terminal to negative terminal

• Alternating Current (AC) – electric current flows in a clockwise direction and then changes direction to flow anti-clockwise

• Direct Current – electric current flows only in one direction

Electric Current • Electric current is given the symbol, I • Can also be defined as - the amount of charge

flowing per second • Electric Current is measured in Amps (A) • Thus, Current in Amps (A) =

tQI =

Charge transferred in coulombs (C)

time in seconds (s) Current in Amps (A)

Electric Current

•This can be rearranged as

• or

ItQ =

IQt =

Electric Current • The SI Unit for Electric Current is Ampere (A) • Electric Current is measured using an

Ammeter

Ammeter • It must be connected in series in the circuit. • Positive side of ammeter must be connected

nearest to the positive terminal of the battery (electric cell), and vice versa.

Electric Circuit Electric circuit :- is a continuous and closed path of an electric current. A schematic diagram of an electric circuit comprising of a cell, electric bulb, ammeter and plug key.

A +

-

+ - bulb

cell

ammeter

plug key

Electric Circuits • Electric circuits are

made up of electrical components

• These components must be joined together without any gap in between to form a closed circuit.

connecting wires

electric cell

circuit board light bulb

Note:

Components refer to the light bulb, wires, battery

Electric Circuits • Incomplete circuits are called open circuits.

connecting wire is

missing no source of

electrical energy

Both the circuits in the diagram are incomplete, hence they are known as “open circuits”.

An electric current flows only when there is:

• a source of electrical energy and

• a closed circuit connecting wires

electric cell

circuit board light bulb

How to draw Circuit Diagrams Component Symbol Component Symbol

An electric cell Battery

Light bulb (lamp) Switch

Switch (open)

Switch (closed)

Connecting wires (not joined)

Connecting wires (joined)

+ +

Symbols are used to represent the various electrical components in circuits.

Summary: Electrical Charge

• Electric charge is given the symbol

Q • Electrons are the charge carriers that

flow in an electrical circuit from the negative to positive terminals.

Summary: Electrical Charge

• Charge is measured in

Coulombs • which is given the symbol

C

Summary: Electrical Charge

• The charge on a proton is

1.6 x 10-19C

which is the same size as the charge on an electron.

Summary: What is electricity?

So electricity is… movement of electric charge round a

circuit. We call this electric current.

Summary: Charge, Current & Time

• Electric current is given the symbol

I • Electric current is the movement of

negative charges (electrons) in a circuit

Summary: Charge, Current & Time

• Electric Current is the amount of charge flowing per second and is given the unit

Amps (A)

Summary: Charge, Current & Time

•If current is charge flowing per second then

tQI =

time in seconds (s)

Current in Amps (A)

Charge transferred in coulombs (C)

so a current of 1 A is 1 C of charge transferred in 1 s.

Summary: Circuit Diagrams Component Symbol Component Symbol

An electric cell Battery

Light bulb (lamp) Switch

Switch (open)

Switch (closed)

Connecting wires (not joined)

Connecting wires (joined)

+ +

Symbols are used to represent the various electrical components in circuits.

Summary: Circuit Diagrams

Different types of circuit

• There are different ways in which you can connect cells and electrical components (such as lamps) to create a circuit:

series parallel a mixture of both

Series Circuit • A series circuit connects the components one after

the other

• A single loop is formed

• A break in any part of a series circuit stops the flow of current in the whole circuit.

Parallel Circuit • A parallel circuit divides into two or more branches.

• The current divides and flows through each parallel branch.

• If a component breaks or is removed, the other components remain on.

Voltage • An electric cell gives energy to the electrons and pushes

them round a circuit. Voltage is a measure of how much energy the electrons receive.

• Different voltages are supplied by different cells and batteries.

12 V Car Battery

1.5 V Dry Cell

9 V Dry Cell

How to Measure Voltage?

• The SI unit for voltage is volt (V).

• A voltmeter is an instrument used for measuring voltages.

Voltmeter

• Voltmeters must be connected in parallel to the circuit.

• The positive side of voltmeter is connected to the positive

terminal of the cell, and vice versa.

Recall: Ammeter!!! • It must be connected in series in the circuit. • Positive side of ammeter must be connected nearest to the

positive terminal of the battery (electric cell), and vice versa.

measuring current

Electric current is measured in amps (A) using

an ammeter connected in series in the circuit.

A

measuring current

A A

This is how we draw an ammeter in a circuit.

SERIES CIRCUIT PARALLEL CIRCUIT

measuring voltage

The ‘electrical push’ which the cell gives to the current

is called the voltage. It is measured in volts (V) on a

voltmeter

V

measuring voltage

V

This is how we draw a voltmeter in a circuit.

SERIES CIRCUIT PARALLEL CIRCUIT

V

measuring current SERIES CIRCUIT

PARALLEL CIRCUIT

• current is the same at all points in the circuit.

2A 2A

2A

• current is shared between the components

2A 2A

1A

1A

fill in the missing ammeter readings.

?

?

4A

4A

4A

3A ?

?

1A

?

3A

1A

1A

V

measuring voltage

V V

V

series circuit

1.5V

• voltage is shared between the components

1.5V

3V

• voltage is the same in all parts of the circuit.

3V

parallel circuit

3V

3V

measuring current & voltage complete the missing current and voltage readings.

V V

6V 4A

A

A

a)

measuring current & voltage

V

V

6V 4A A

A

A

b)

answers

3V 3V

6V

4A 4A 6V

6V

6V 4A 4A

2A

2A

4A

a) b)

The circuit is no longer complete, therefore current can not flow

The voltage decreases because the resistance increases. The current flow in each bulb is the same.

Resistance

• When an electric current flows through a circuit, there will be some resistance that opposes it. (similar to friction)

• It can be measured by dividing voltage by the current.

R = V I

R = Resistance

V = Voltage

I = Current

Ohm’s Law

I = V / R I = Current (Amperes) (amps) V = Voltage (Volts) R = Resistance (ohms)

Ohm’s law states that : ‘The current flowing through a conductor is directly proportional to the potential difference between its ends provided its temperature remains constant.’

Resistance • Good conductors of electricity have LOW

RESISTANCE. (Eg. Metal objects) – Electricity is able to flow through them very

easily • Poor conductors of electricity have HIGH

RESISTANCE. (Eg. Wood, cloth) – Electricity is not able to flow through them

easily • The SI unit for resistance is ohm (Ω)

Example 1 • An electric rice cooker operates at 240 V and

uses a current of 8 A. What is the resistance of the rice cooker?

Voltage (V) = 240 V

Current (I) = 8 A

R = V I

R = 240

8

= 30 Ω

Resistors in Series

• When resistors are connected

in series, the resistance will

add up and increase.

• R = R1 + R2 + R3… 2 Ω 3 Ω

The total resistance for this circuit is:

2 Ω + 3 Ω = 5 Ω

• When resistors are connected in parallel, the resistance will decrease

• This is because they will provide alternate routes for the current to flow.

Resistors in Parallel

Resistors in Parallel

6 Ω

6 Ω

1 R

= 1 R

1 R

1 R

+ + 1 2 3

What is the resistance of the

circuit?

1 R

= 1 6

1 6

+

1 R

= 2 6

1 3

= R = 3Ω The final resistance is

smaller

Summary: Series and Parallel Circuits

Voltage, Current and Resistance Vs

I3

I2

I1

R3

+ -

R2 R1

V1 V2 V3

What type of circuit is this?

Vs

I3

I2

I1

R3

+ -

R2 R1

V1 V2 V3

One electrical path from negative to positive therefore series.

Vs

I3

I2

I1

R3

+ -

R2 R1

V1 V2 V3

What is the relationship between the three currents?

321 III ==The current is the same at each point.

Vs

I3

I2

I1

R3

+ -

R2 R1

V1 V2 V3

What is the relationship between the four voltages?

321 VVVVs ++=They add to equal the supply voltage.

Vs

I3

I2

I1

R3

+ -

R2 R1

V1 V2 V3

How do you find total resistance in series?

321 RRRRtotal ++=Add each resistance together.

Disadvantages of Series Circuits?

• When one component fails the whole circuit fails.

• The current is the same at all points and the

voltage is divided between the bulbs. The more bulbs added the dimmer each one is.

What type of circuit is this?

IT IT

Vs

I3

I2

I1

R3

+ -

R2

R1

V1

V2

V3

More than one electrical path – components connected on different branches therefore parallel.

IT IT

Vs

I3

I2

I1

R3

+ -

R2

R1

V1

V2

V3

What is the relationship between the four currents?

321 IIIIT ++=

The four currents add to give the total current.

IT IT

Vs

I3

I2

I1

R3

+ -

R2

R1 V1

V2

V3

What is the relationship between the four voltages?

321 VVVVS ===

Each voltage is equal to the supply voltage.

IT IT

Vs

I3

I2

I1

R3

+ -

R2

R1 V1

V2

V3

321

1111RRRRT

++=

IT IT

Vs

I3

I2

I1

R3

+ -

R2

R1 V1

V2

V3

The resistance in parallel?

• If more resistors are connected in parallel the total resistance will always decrease

• This is because there are more

branches through which the electricity can flow.

Advantages of the Parallel Circuit?

• When one bulb fails the rest of the circuit continues to work.

• The more components, the lower the resistance. The total current drawn increases.

• Voltage in each branch is the same as the supply voltage therefore bulbs in parallel will each be as bright as a single bulb.

NOTE!! Hazards of electricity +

Safety Devices + Wiring of an Electric Plug

= Self Study