P5: Electricity in the home

Current

Flow of electrons

Type of current from a cell

DC

Why is the current from a cell DC?

• Moves in 1 direction

• Leaves cell from 1 end, goes around circuit + returns to cell at the another end

Direct current (DC)

Current only moves in 1 direction

What type of supply is mains electricity?

Ac supply

Alternating current (AC)

Current constantly changes direction

Benefit of using AC

Easy to use transformer to increase or decrease PD

When are transformers used?

When electricity is transferred from power stations to homes

Frequency of AC (domestic electricity supply) in UK

50 Hz

What is meant by frequency of AC is 50 Hz

AC switches direction 50 times per second

PD of AC in UK

230 V

What does an oscilloscope show?

Pattern of electrical current

What type of current does this show and why?

AC

• PD rising + falling (shows current going back + forwards)

What does the height of peak show?

Max PD (230 V)

What type of current does this show and why?

DC

• PD doesn’t change

How to use the oscilloscope trace to calculate frequency of AC

• From 1 peak to the next, current has changed direction + changed back again

• Time from 1 peak to the next peak = 1 complete cycle

• 1 / time taken = freq

Frequency

Number of cycles AC changes direction in 1s

• Rate at which AC reverses direction

Calculate the frequency of this

50 Hz

• 0.03 s - 0.01 s = 0.02 s

• 1 / 0.02 s = 50 Hz

DC vs AC

DC

1. PD doesn’t change

2. Current flows in 1 direction

AC

1. PD reverses (positive - negative)

2. Current constantly changes direction

How are most electrical appliances connected to the mains electricity?

Using three-core cable

What are the wires in a three-core cable made of and why?

Copper

• Good conductor of electricity

What are the coatings of wires in a three-core cable made of and why?

Plastic

• Doesn’t conduct electricity

Why is the insulation covering each wire colour coded?

For easy identification

3 types of wires in a three-core cable

• Live wire

• Neutral wire

• Earth wire

Color of each wire in a three-core cable

1. Live wire = brown

2. Neutral wire = blue

3. Earth wire = green + yellow stripes

Live wire

Carries the alternating PD from the supply

• At 230 V

Neutral wire

Completes the circuit (w live wire)

• 0V (compared to live wire)

What is the earth wire for?

Safety wire

Earth wire

• Safety wire

• To stop the appliance becoming live

What is the live wire connected to?

A fuse in the plug

Voltage of the live and neutral wire

• Live: 230 V

• Neutral: 0V (earth potential)

PD between live and neutral wire

230V

Explain this circuit wired to the main supply

1. Live wire- carried 230V electricity from generators, connected to fuse

2. ET as electrical current passes thru appliance

3. Neutral wire- completes circuit

Why is the live wire dangerous + fatal if touched, even if off (switch is open)?

1. First part of live wire running to switch still has PD of 230V

2. PD of earth is 0V

3. Person touched live wire → current flows thru person, into the earth + they would be electrocuted

Why are appliances with a metal case dangerous?

1. If live wire comes loose + touches metal case → case becomes live (gets PD of 230V)

2. Person touches live case → fatal electric shock

Why are most appliances, especially those with metal cases, connected to an earth wire?

1. Metal case attached to earth wire

2. Earth wire connected into ground w a metal rod

3. If case becomes live → huge current flows to Earth

4. Fuse melts + shuts off current

5. Prevents person from getting electric shock from touching the case

Power

Rate at which energy is transferred

1 W =

Energy transferred at 1 Joule per second

10 V =

10 J of energy transferred per coulomb of charge passing thru component

1 A

1 coulomb of charge flowing per second

Power equation (using voltage)

P = VI

Power equation (using resistance)

P = I²R

What do everyday electrical appliances do?

Transfer electrical energy into other forms of energy (thermal, kinetic)

What do blenders + fans transfer electrical energy into?

Kinetic energy of electric motors

• Main purpose of these appliances is movement

What do irons + kettles transfer electrical energy into?

Thermal energy

• Both designed to get hot

What categories can appliances be divided into?

Those that transfer EE into:

1. KE

2. TE

What do hair dryers transfer electrical energy into?

1. KE of electric motors

2. TE (heating element heats air)

What do washing machines transfer electrical energy into?

1. KE of electric motors (turns drum)

2. TE (heating element heats water)

Which appliances have a higher power rating?

Those that are designed to generate TE

• Over those designed to generate KE

What does the amount of energy an appliance transfers depends on?

1. How long the appliance is switched on for

2. Power of the appliance

Energy transferred equation (power)

E = P t

Energy transferred equation (charge)

E = Q V

Work is done when…

Charge flows in a circuit

How does charge flowing thru a resistor make it hot?

1. ET to resistor

2. TE store of resistor increases

3. Resistor becomes hotter

4. TE transferred to surroundings

Why do all electrical appliances waste energy?

Heating effect of current in wires

Why do electric appliances with an electric motor waste energy?

Friction betw moving parts

Max efficiency

100% (1)

Efficiency equation

Output power / input power x 100

Domestic energy meter

Measures how much energy is supplied

What happens when electrical charge flows thru an appliance?

Electrical energy transferred by appliance to another energy store

Simple answer: why does a resistor become hotter when charge flows thru it?

ET to resistor

Fuse

Safety device that breaks the circuit if a fault in an appliance causes too much current to flow

What does a fuse contain?

Piece of metal wire that melts easily

How to choose a fuse?

Fuse current rating is slightly higher than current thru appliance

What happens if fuse size is smaller or a lot larger than the normal current thru the appliance?

1. Smaller: wire inside will melt as soon as the appliance starts

2. Larger: current will continue to increase → leads to overheating

What wire does a 2 core cable not have?

Earth wire

What happens if the current thru a fuse becomes too big?

Melts + cuts of current to live wire

How are appliances with a metal case earthed?

Case attached to earth wire in cable

How does earthing an appliance increase safety?

Stops metal case becoming live if live wire breaks + touches case

Do appliances with plastic cases need to be earthed- why or why not?

No

• They are double insulated so only connected to live + neutral wire

Cables of different thicknesses are used for?

Diff purposes

The larger the current to be carried…

The thicker the cable must be

Examples of DC supply

• Cells

• Battery

Frequency equation

1/time taken for 1 cycle

When does a earth wire carry and not carry a current?

• Not carry: normally

• Carry: if fault occurs in appliance → so current will flow to ground

Energy transfers in battery powered torch

1. Battery converts chemical to electrical energy

2. Bulb converts EE to light

3. + waste energy (heating)

Energy transfers in battery powered motor

1. Battery converts chemical to electrical energy

2. Motor converts EE to KE

3. + waste energy (heating due to friction

National grid

A system of cables + transformers linking power stations to consumer

Issue with getting electricity to homes

Energy always lost in power cables due to resistance of wires

Purpose of national grid

To link power stations to consumers so they have access to electricity

Bigger the distance between power stations + homes…

Greater the energy lost

How can you reduce energy loss due to resistance of wires?

1. Build power stations near homes

2. Use transformers

What is transferred from power stations to consumers using the National Grid?

Electrical power

2 types of transformers

• Step up

• Step down

Step up transformers

Increase the PD from the power station to the transmission cables

Step down transformers

Decrease, to a much lower value, the PD for domestic use.

How do step up transformers help?

Reduce energy lost in transmission cables

Do step up + down transformers increase or decrease PD?

• Up: increase

• Down: decrease

How do transformers work?

1. Electricity fm power station passes thru step up transformers (increase PD)

2. High PD = less energy lost in power cables

3. Electricity passes thru step down transformers (reduce PD, 230 V)

To what PD do step down transformers reduce PD to?

230V

Why is it efficient that national grids transfer electricity at high potentials?

1. High PD → low current

2. Lower current = lesser energy wasted as heat

Why does PD need to be decreased between transmission lines + houses?

1. Lower PD safer for domestic use

2. + reduces chance of electrocution

Where do step up transformers increase PD from?

Power stations to transmission cables

Good conductors of electricity

Metals

Why are metals good conductors of electricity?

Electrons can easily flow thru them

Good insulators

• Plastic

• Glass

Insulators

Don’t conduct electricity

Why can’t insulators conduct electricity?

Electrons can’t move thru them

What happens when insulating materials are rubbed against each other?

They become electrically (statically) charged

Why do insulators become electrically charged when rubbed against each other? (static electricity)

1. Negatively charged electrons rubbed from 1 material to another

2. Material that gains electrons becomes negatively charged

3. Material that loses electrons becomes equally positively charge