2.3.1 - 2.3.5 - Electrical power and mains electricity Flashcards
What is power?
The rate of energy transfer or the amount of energy transferred per second
What does a power of a device depend on?
The power of a device depends on:
The voltage (potential difference) of the device
The current of the device
What is the equation for power?
Power = current x voltage
Why are fuses selected for circuits?
A fuse is a safety device designed to cut off the flow of electricity to an appliance if the current becomes too large (due to a fault or a surge).
Fuses usually consist of a glass cylinder which contains a thin metal wire
If the current in the wire becomes too large:
The wire heats up and melts
This causes the wire to break, breaking the circuit and stopping the current
This makes sure that more current doesn’t keep flowing through the circuit and causing more damage to the equipment, or, causing a fire
What is the equation for current?
Current = Power / Voltage
The fuse should always have a current rating that is higher than the current needed by the appliance, without being too high
If the fuse current rating is low, it will break the circuit even when an acceptable current is flowing through
If the fuse current rating is too high, it will not be breaking the circuit in enough time before damage occurs
Why should fuses have a higher current rating?
The fuse should always have a current rating that is higher than the current needed by the appliance, without being too high
Because of this, the rule of thumb is to always choose the next size up
If the fuse current rating is low, it will break the circuit even when an acceptable current is flowing through
If the fuse current rating is too high, it will not be breaking the circuit in enough time before damage occurs
What is the equation for energy transfer?
Energy transfer = power x time
or
Energy transfer = Current x potential difference x tim
What common hazards do electrics cause?
Damaged Insulation – if someone touches an exposed piece of wire, they could be subjected to a lethal shock
Overheating of cables – passing too much current through too small a wire (or leaving a long length of wire tightly coiled) can lead to the wire overheating. This could cause a fire or melt the insulations, exposing live wires
Damp conditions – if moisture comes into contact with live wires, the moisture could conduct electricity either causing a short circuit within a device (which could cause a fire) or posing an electrocution risk
What saftey features are built into domestic appliances?
Double insulation
Earthing
Fuses
Circuit breakers
How does Insulation and Double insulation ensure saftey in domestic appliances?
The conducting part of a wire is usually made of copper or some other metal
If this comes into contact with a person, this poses a risk of electrocution
For this reason, wires are covered with an insulating material, such as rubber
Some appliances do not have metal cases and so there is no risk of them becoming electrified
Such appliances are said to be double insulated, as they have two layers of insulation:
Insulation around the wires themselves
A non-metallic case that acts as a second layer of insulation
Double insulated appliances do not require an earth wire or have been designed so that the earth wire cannot touch the metal casing
How does earthing ensure saftey in domestic appliances?
Many electrical appliances have metal cases
This poses a potential safety hazard:
If a live wire (inside the appliance) came into contact with the case, the case would become electrified and anyone who touched it would risk being electrocuted
The earth wire is an additional safety wire that can reduce this risk
If this happens:
The earth wire provides a low resistance path to the earth
It causes a surge of current in the earth wire and hence also in the live wire
The high current through the fuse causes it to melt and break
This cuts off the supply of electricity to the appliance, making it safe
How do circut breakers ensure saftey in domestic appliances?
A circuit breaker consists of an automatic electromagnet switch that breaks the circuit if the current exceeds a certain value
This has a major advantage over a fuse because:
It doesn’t melt and break, hence it can be reset and used again
It works much faster
For these reasons, circuit breakers are used in mains electricity in homes
Sometimes they are misleadingly named “Fuse boxes”
How does electricity create heat?
When electricity passes through a component, such as a resistor, some of the electrical energy is turned into heat therefore increasing its temperature
This is because energy is transferred as a result of collisions between:
Electrons flowing in the conductor, and
The lattice of atoms within the metal conductor
Metals are made up of a lattice of ions
As the electrons pass through the metal lattice they collide with ions
The ions resist the flow of the electrons
How is this heating effect utilised?
This heating effect is utilised in many appliances, including:
Electric heaters
Electric ovens
Electric hob
Toasters
Kettles
What is Direct current?
Direct Current
A direct current (d.c.) is defined as
A current that is steady, constantly flowing in the same direction in a circuit, from positive to negative
The potential difference across a cell in a d.c. circuit travels in one direction only
This means the current is only positive or only negative
A d.c. power supply has a fixed positive terminal and a fixed negative terminal
Electric cells, or batteries, produce direct current (d.c.)
What is alternating current?
An alternating current (a.c.) is defined as
A current that continuously changes its direction, going back and forth around a circuit
An a.c. power supply has two identical terminals that switches between positive and negative
The current is therefore defined as positive or negative, depending on which direction it is flowing at that time
The frequency of an alternating current is the number of times the current changes direction back and forth each second
What are the differences between d.c and a.c?
