P4 Flashcards
How can an insulating material become electrically charged?
If it’s rubbed with another insulating material. Electrons transfer from one material to another, leaving one positively charged, and the other negatively charged.
How can someone generate static electricity?
By rubbing a balloon, comb or strip of plastic against an insulating material such as a jumper.
How can synthetic clothing become charged?
Due to friction between the person’s body and the clothing when the clothes are put on. When the clothing is removed from the body, static sparks are sometimes produced.
How can a charged object be discharged?
By earthing it. When an object discharges, electrons are transferred from the charged object to earth.
If you became charged and then earthed, what could happen?
You could get an electrostatic shock, for example, you become charged by friction between the soles of your feet and the floor if you’re walking on an insulator such as carpet or vinyl, and if you then touch a water pipe, the charge is earthed and discharge occurs, giving a shock.
How can static electricity be a nuisance?
It can cause dirt and dust to be attracted to insulating materials such as television screens and computer monitors, and it can cause some materials to cling to your skin.
How can static electricity be very dangerous?
Flour mills and petrochemical factories have atmospheres that can contain extremely flammable gases or vapours, or high concentrations of oxygen. A discharge of static energy, i.e. a spark, can lead to an explosion. As well as this, static is dangerous in any situation where large amounts could flow through your body to earth, for example, lightning.
What will two insulating materials with the same charge do?
They will repel, for example, if a positively charged Perspex rod is held near to a suspended positively charged Perspex rod, the suspended rod will be repelled,. The same would happen if both had a negative charge.
What happens when electrons are rubbed off one material onto another?
Electric or static charge builds up. The material that receives the electrons becomes negatively charged due to an excess of electrons, and the material giving up the electrons becomes positively charged due to a loss of electrons.
What happens to both when a Perspex rod is rubbed with cloth?
The perspex rod gives up electrons and becomes positively charged. The cloth receives the electrons and becomes negatively charged
What happens to both when an Ebonite rod is rubbed with fur?
The ebonite rod receives receives electrons and becomes negatively charged. The fur gives up electrons and becomes positively charged.
What are atoms and molecules that have become charged called?
Ions.
What are atoms and molecules that have an excess of electrons?
Negative Ions
What are atoms and molecules which have lost electrons?
Positive Ions
How can the chances of receiving an electric shock be reduced?
By making sure appliances are correctly earthed, by using insulation mats effectively, and by wearing shoes with insulating soles.
Why do lorries that contain inflammable gases, liquids or powders need to be earthed before unloading?
Because friction can cause a build-up of charge. This charge could lead to a spark, which could then ignite the flammable substance.
How do anti-static sprays, liquids and cloths help to reduce the problems of static electricity?
They prevent the transfer of charge from one insulator to another. If there is no build-up of charge, there can’t be any discharge.
How is static electricity used in spray painting cars?
The paint gains electrons as it passes through the negatively charged nozzle of the gun, making the paint particles negatively charged so that they repel each other, forming a fine spray. This ensures that the paint is applied evenly. The car panel has lost electrons and so is positively charged, attracting the oppositely charged paint. As the paint sticks to the car the charges cancel so that the ar becomes neutrally charged and no more paint is attracted.
How can static energy remove smoke particles from chimneys?
With Electrostatic dust precipitators. Metal grids are installed in the chimney and are connected to a high potential difference (voltage). The dust becomes positively charged as it passes the grid, inducing a positive charge on the dust particles when they lose electrons. The dust particles are attracted to the negative charged plates, where they form large particles that fall back down the chimney when they’re heavy enough, or if the plates are stuck.
How can electricity be used to start the heart when it has stopped?
With a defibrillator. Two paddles are charged and are put in good electrical contact with the patient’s chest using gel. Taking care not to shock the operator, charge is then passed through the patient to make the heart contract.
What is a circuit?
A completed loop that allows an electrical current to flow. Electrons flow around the circuit from the negative electrode of the power source to the positive electrode. But this was only discovered recently, so circuit diagrams are drawn showing the current flowing from the positive to the negative electrode.
What is resistance, and how is it measured?
Resistance is a measure of how hard it is to get a current through a component in a circuit at a particular voltage (potential difference). Resistance is measured in ohms (Ω)
How can the current through a circuit be controlled?
By varying the resistance.
There are two types of resistor. What is a fixed resistor?
A fixed resistor has constant resistance. The bigger the resistance, the smaller the current that flows for a particular voltage.
There are two types of resistor. What is a variable resistor (or rheostat)?
A variable resistor/rheostat has a changeable resistance.
How can the resistance of the rheostat/variable resistor be changed?
By moving the slider, changing the length of wire between the contacts. Long wire= high resistance, low current. Short wire = low resistance, large current.
What does a long length of wire between the contacts in a rheostat/variable resistor mean?
high resistance, low current.
What does a short length of wire between the contacts in a rheostat/variable resistor mean?
low resistance, large current.
Complete this sentence in terms of current and voltage: For a given resistor….
For a given resistor, current increases as voltage increases, and vice versa.
Complete this sentence in terms of current and resistance: For a fixed voltage….
For a fixed voltage, current decreases as resistance increases, and vice versa.
What is the formula to calculate resistance?
Resistance (Ω) = Voltage (V) / Current (A)
What colour is a Live wire, and what does it do?
A live wire is brown, and it carries current to the appliance at a high voltage (230V)
What colour is a Neutral wire, and what does it do?
A neutral wire is blue, and it completes the circuit and carries current away from the appliance
What colour is an Earth wire, and what does it do?
An Earth wire is green and yellow, and it is a safety wire that stops the appliance becoming live.
Which appliances do/don’t have earth wires?
All appliances with outer metal cases (conductors) have an earth wire, so they are earthed. An earthed conductor can’t become live. Appliances with cases made of insulators don’t have an earth wire, though still have a fuse. They’re double insulated so they can’t become live.
Describe how electrical appliances with outer metal cases are earthed in order to protect the appliance and the user, with the earth wire and fuse working together.
A fault in the appliance causes the casing to become live. The circuit short-circuits (the path of the flow of charge changes) because the earth wire offers less resistance. The fuse wire melts, and the circuit is broken. The appliance and the user are protected.
What is a fuse?
A fuse is a short, thin piece of metal with a low melting point. It’s used to prevent cables or appliances from overheating. To work properly, the current rating of the fuse must be just above the normal current that runs through the appliance.
How does a fuse work?
If the current flowing through an appliance is below the current rating of the fuse, the appliance will work properly. However, if a fault occurs inside the appliance, the live wire will make contact with the neutral wire. The current flowing would then be higher than the current rating of the fuse due to lower resistance. This causes the fuse wire to get hotter and hotter until it melts and breaks the circuit. The current is then unable to flow, so there’s no danger of the flex overheating, which would result in a fire.
What does the power rating of a device tell you?
How quickly electrical energy is being charged (transferred) into another form within that device.
What is Power measured in?
Watts (W)
What do fuses and circuit breakers prevent?
Injury and death, as they stop appliances from becoming ‘live’. Fires, as they stop cables and flexes from overheating. Damage to the components of an appliance as they break the circuit if a higher than normal current flows through the appliance.
What is ultrasound?
Sound waves with frequencies above the upper limit of the human hearing range [i.e. above about 20,000 hertz (Hz)]
