P5- Electricity In The Home Flashcards
What are the two types of electricity supplies?
Alternating current (ac) and direct current (dc)
Direction of current in ac
The current is constantly changing direction
How are alternating currents produced?
Alternating voltages in which the positive and negative ends keep alternating
Is mains supply ac or dc?
ac supply at around 230v
What is the frequency of the ac mains supply?
50 Hz
What direction is direct current?
Current that is always flowing in the same direction
How is direct current created?
Its created by a direct voltage
What colour is the live wire?
Brown
What colour is the neutral wire?
Blue
What colour is the earth wire?
Green and yellow
Live wire
The live wire provides the alternating potential difference from the mains supply
What type of current do cells and batteries supply?
Direct current (dc)
Neutral wire
The neutral wire completes the circuit and carries away current- electricity normally flows in through the live wire and out through the neutral wire. It is around 0V.
Earth wire
It is for protecting the wiring and for safety- it stops the appliance casing from becoming live. It doesn’t usually carry a current- only when there’s a fault. Its also at 0V.
Which type of wire can give you an electric shock?
Live wire
How can a live wire give you an electric shock?
Your body (just like the earth) is at 0 V. This means that if you touch the live wire, a large potential difference is produced across your body and a current flows through you. This causes a large electric shock which could injure or even kill you.
Can you still get an electric shock from a live wire even if a plug socket or a light switch is turned off.
Yes, there is still a danger of an electric shock. A current is not flowing but there is still a pd in the live wire.
Why can any connection between live and earth be dangerous?
If the link creates a low resistance path to earth, a huge current will flow, which could result in a fire
How is energy transferred to electrical appliances?
Kettles transfer energy electrically from the mains ac supply to the thermal energy store of the heating element inside the kettle.
Energy is transferred electrically from the battery of a handheld fan to the kinetic energy store of the fan’s motor.
What does the total energy transferred by an appliance depend on?
How long the appliance is on for and its power
What is the power of an appliance?
The energy that it transfers per second.(therefore the more energy it transfers in a given time, the higher its power.
Power equation (in electrical appliances)
Energy transferred / Time
3000 Joules of energy are transferred to a kettle in 2 mins. What is the power of the appliance?
2 mins = 120s
3000/120 = 25W
What is an appliances power rating?
The maximum safe power that they can operate at (maximum operating power)
How does a power rating help customers when buying electrical appliances?
The lower the power rating, the less electrcicity an appliance uses in a given time and so the cheaper it is to run
What is the national grid?
A giant system of cables and transformers that covers the UK and connects power stations to consumers.
When does demand for electricity increase?
When people get up in the morning, come home from school/work and when it gets dark and cold outside. Additionally popular events such as a sporting final on tv could also cause a peak in demand.
Why do power stations run well below their maximum power output?
So there’s spare capacity to cope with a high demand, even if there’s an unexpected shut down of another station
What does the national grid use to transmit the huge amount of power?
A high pd and a low current
Why does the national grid use a high pd and a low current?
A low current is used because with a high current you lose loads of energy as the wires heat up and energy is transferred to the thermal energy store of the surroundings.
Its much cheaper to boost the pd up really high and keep the current as low as possible
What do transformers do?
Step the pd up at one end (step up transformers) for efficient transmission
Bring the pd down to safe, usable levels (step down transformers)
What does a step up transfomer do?
The pd is increased
What does a step down transformer do?
It decreases the pd for domestic use
What is a build up of static electricity caused by?
Friction - When certain insulating materials are rubbed together negatuvely charged electrons will be scraped off one and dumped on the other. This will leave the materials electrically charged, with a positive static charge on one and an equal negative charge on the other.
What happens when a polythene rod is rubbed with a cloth duster?
Electrons are transferred from the cloth duster to the polythene rod leaving the rod negatively charged and the cloth positively charged
What happens when an acetate rod is rubbed with a cloth duster?
Electrons are transferred from the acetate rod to the cloth duster leaving the cloth duster negatively charged and the acetate rod positively charged
How does static electricity cause sparks?
When an electric charge builds on an object, the potential difference between the object and the earth (0V) increases.
If the pd gets large enough electrons can jump across the gap between a charged object and the earth - this is the spark.
What do like charges do?
Repel
What do opposite charges do?
Attract
What happens when two charged objects move apart?
The electrostatic forces of either attraction or repulsion get weaker
What is an electric field?
An electric field is created around any electrically charged object
The closer you are to the object the stronger the field
How can you show an electric field around an object?
Field lines
Electric field lines always go from…
Positive to negative
