Electricity

Question 1

What is Mains Electricity?

Answer 1

• Electricity generated by the power stations and transported around the country through the national grid
• It is an alternating current (a.c) supply.
• In UK, domestic electricity supply is an alternating current which has a frequency of 50Hz and voltage of 230V
• Frequency= 50Hz meaning the current changes back and forth 50 times every second.

Question 2

Wires in mains circuits: Live wire

Answer 2

• Live wire provides the path along which the electrical energy from the power station travels.
• This wire is brown insulated
  Carry alternating voltage of -230V and - 230V. Making the current flow backwards and forwards through the circuit.
• Will give you a shock if touched
• Only one that needs a fuse.

Question 3

Wires in mains circuits: Neutral wire

Answer 3

• Completes the circuit and is kept at 0V
• exit point
• Blue insulated

Question 4

Wires in mains circuits: Earth wire

Answer 4

• it is a safety wire that connects the metal body/casing of an appliance to earth
• It is there to protect you if the appliance develops a fault. (if live wire breaks and touches metal casing it becomes live asw so the earth wire which is connected to casing will allow a low resistance pathway to the earth for the current.) therefore not electrocuting you when you touch it.
• It provides a path for current to escape without passing through user.
• if the appliance has no earth wire and the live wire comes lose a current could flow through the user, causing an electric shock.
• Yellow-green insulated

Question 5

What is a ring main circuit?

Answer 5

These circuits provide a way of allowing several appliances in different parts of the same room to be connected to the mains using the minimum amount of wiring.

Question 6

When plugging a wire check the following:

Answer 6

1. Wires are connected to the correct terminals, using colour code on the plug diagram. (live wire, earth, neutral).
2. The cable grip is held firmly by the grip
3. The correct fuse is fitted

Question 7

Fuses

Answer 7

• Prevents the device and user from getting a shock and reduces the risk of a live wire. Also prevents wire from catching fire and saving appliances. It does this by: If current is too high it runs the risk of killing us, so the wire will overheat and melts to blow breaking the circuit.
• It is connected to the live wire.
• a thin piece of wire
• A plug is usually fitted with either a 3A, 5A or 13A fuse (UK). The value tells you the amount of current needed to blow the fuse. It must be greater than the normal current required through the appliance but as close to it as possible so that the fuse will blow as soon as current gets too high.
• Once the fault has been corrected - blown fuse must be replaced with a new one of the same size then the appliance can be used again.

Question 8

Circuit breakers

Answer 8

• This is used in your consumer unit (also known as fuse box)(but it has nothing to do with fuses) are often in the form of trip switches.
• If too large a current flows in a circuit a switch will automatically open making the circuit incomplete.
• The fault has been corrected - the switch is reset by pressing the reset button.
• There is no need for the switch or circuit breaker to be replaced

Question 9

Switches

Answer 9

• This is connected in the live wire for safety
• With the switch open and connected to the live wire the current cannot reach the appliance, so user is safe from electrical shock.
• If the switch is open and connected to the neutral wire the current can reach the faulty appliance, user is not safe from electric shocks.

Question 10

Double insulation

Answer 10

• Some appliances do not have an earth wire. this is because their outer casing is made of plastic. Plastic is a non metal and acts as an extra layer of insulation around wires.
• Which prevents any current from flowing to the user even if the live wire comes loose inside the appliance.

Question 11

Heating effect of current

Answer 11

• Heating in wires and appliances is caused by the resistance and current passing through them.
• The wiring in a house is designed to let current pass through it easily.
• Wires do not become warm when appliances are being used. Wires have a low resistance and lower heating effect.

Filament bulbs:
- When current passes through the very thin metal wire (filament) of a traditional light bulb it becomes very hot and glows white. The bulb transfers electrical energy > light> heat energy.

Other common appliances that make use of heating effect of electricity are: Kettles, electric ovens, toasters, electric fires, and hairdryers.
- They want wires (more usually heating elements) to become warm.
- The wires of heating elements are designed to have a high resistance so that as current passes through them energy is transferred and the element heats up.

Question 12

Power

Answer 12

Energy transferred per unit time. (P)
Power (p) = Current (I) x Voltage (V)

Power (p) = Energy transferred (E in joules) / Time (s= seconds.

The amount of energy an appliance transfers depends on:
1. How long the appliance is switched on for
2. The power of the appliance

Question 13

Alternating current or ac

Answer 13

The current which flows forward and backwards. it comes from mains electricity of a house and generator.
drawn with current on y axis time on x axis and up and down graph.

Question 14

Direct current or dc

Answer 14

Is the current that flows in one direction, comes from a cell or battery.
Drawn with current or voltage on y axis and time on x axis with a straight line extending from y axis.

Question 15

Hazards of electricity

Answer 15

1. Damaged insulation: If someone touches an exposed piece of wire, they could be subjected to a lethal shock.
2. Overheating of cables: Passing too much current through a small 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.
3. Damp conditions: If moisture comes into contact with live wires, the moisture could conduct electricity within a device (which could cause a fire) or posing an electrocution risk.
4. excess current from overloading plugs, extension leads, single and multiple sockets when using a mains supply: If plugs become overloaded due to plugging in too many components the heat created can cause fires.

Question 16

Electric current

Answer 16

Amount of charge passing a point per second
Charge (Q) = Current (I) x Time (t)
Charge measured in coulombs.
1 ampere is 1 coulomb per second.
- Direction of current is same to the direction of moving of positive charges.
- direction of current is opposite to the direction of moving of the negative charges

Question 17

Conductors

Answer 17

Conductor is a metal that allows charge (usually electrons) to flow through easily, this is because it has free-moving electrons. E.g metals have a lattice structure with delocalised electrons which can move about and carry charge. metals are good conductors of electricity because current is the rate of flow of charged particles.free electrons move to the positive end of the wire.

best conductors Worst conductor
Silver, copper, iron, carbon, water, air, glass, rubber

Question 18

Insulators

Answer 18

Insulator is a material that does not allow the flow of charge through them very easily, this is because it has no free-moving electrons. Electrons are held tightly in the atom and unable to move
Electrons do not flow easily through plastics, rubbers, glasses or woods so they are poor conductors of electricity.
graphite is conductor

Question 19

Series circuit

Answer 19

A simple loop circuit with no branches or junctions there is only one path fro the current to flow.
- a single switch placed anywhere in the circuit can turn off or on all the bulbs. If any components liek a bulb breaks there is a gap in the circuit and everything will stop working.
- Current has the same value in all parts of the circuit. The size of the current depends on the voltage supplied to it and the number and type of the other components of a circuit.
- Voltage/ energy supplied by the cell is ‘shared’ between all the bulbs.
Resistance: the component with the higehst resistance will use the highest share of the voltage to push the current through it. Rtot= r1+r2+r3
total resistance = sum of all resistances.
e.g decorative lights are often wired in series. Each bulb only needs a low voltage , so even when the mains supply is shared out between them each bulb still gets enough energy to produce light. However, if the filament breaks then all the other bulbs will go out.

Question 19

Parallel circuits

Answer 19

Is a circuit with branches or junctions, providing more than one path for the current to flow through.
- switches can be placed in different parts of the circuit to switch each bulb on and off and the other bulbs will not be effected.
- current is shared in each branch because the number of electrons that flow into a junction each second must be equal to the number leaving each second. The currents entering a junction must always be equal to those that leave.
The lights in your home are wired in parallel this is why you can switch lights on and off separately and the brightness remains unaffected. If a bulb breaks the others will still work.
- each branch has same voltage. if more bulbs are added in parallel they all keep the same brightness.
- Voltage remains the same across each branch and is equal to the voltage across the cell or battery.

Question 19

Measuring current

Answer 19

• Ammeter is used to measure the current in a circuit.
  It must connect in series
  good ammeter has a very low resistance and allow more charges to flow through it.
  Conventional current is the flow of current from a positive terminal to a negative terminal
  Conventional free-moving electrons flow from negative terminal to a positive terminal of a cell.

Question 20

Voltage

Answer 20

transfer of energy per unit charge that passes through a component or a circuit. also known as potential different.

Energy transferred (e) = charge (q) x voltage (v)
1 volt = a joule per coulomb 1J/C
- voltmeter is used to measure voltage across battery cell and components.
- a good voltmeter should have a very high resistance to block current flow through it
- voltmeters are connected across with a component or batter will measure the energy given to each coulomb of charge that passes through it.

Question 21

Resistance

Answer 21

The opposition to the flow of current.
voltage = current x resistance
v = I (amps) x R (ohms)
- Resistor the electrical energy is transferred to heat when the charge is passing through it. If the circuit has no resistor the current will be too high and the bulb breaks.
- variable resistors, it is possible to alter their resistance and hence the size of current flowing through it. if resistance is increased the current will be smaller and light will glow less brightly or not at all
if it is decreased current will increase asw and will glow more brightly

Question 22

Diode

Answer 22

A component that only allows current to flow in one direction. These are used in rectifier circuits that convert alternating current into direct currents

Question 23

LED - light emitting diode

Answer 23

LED glows when current flows through in one direction

Question 24

Thermistor

Answer 24

these change resistance in a circuit depending on the temperature. Its resistance decreases when temperature increases increasing current causing the bulb to glow brightly and vice versa. Uses of thermistors: 1. fire alarms 2. freezers 3. computers

Question 25

Light dependent resistor (LDR)

Answer 25

is a detector of light intensity. it's resistance increases when it is dark (low light intensity) which means more current flows through and vice versa. Uses of LDR: 1. Photographic-exposure 2. Automatic lighting control 3. Burglar alarms.

Question 26

Electric charge

Answer 26

There are two kinds of electric charge: positive and negative. when two charges are brought close together there will be a force between them. Like charges + + or - - repel opposite charges + - attract. Measured in coulombs (C)

Question 27

Where does electric charge come from?

Answer 27

Everything is made of atoms. Atoms have equal numbers of protons (positively charged) and electrons (negatively charged) so there is a net charge of 0. when atoms lose electrons = proton > electron number so net charge is positive. positive ion. When atoms gain electrons electron number > proton number so net charge of the atom is negative so it is a negative ion atom.

Question 28

Electrostatic phenomena

Answer 28

Rub the plastic ruler on a cloth, the ruler will become charged. If it is held close to some small uncharged pieces of paper some electrons on the paper will be attracted to the edges closest to the ruler. There will be an attraction between these negative parts of the paper and the positive ruler. when you come your hair the friction between the comb and hair cause each of them to become oppositely charged to then attract each other. A negatively charged ballon attracts the positive charges in the stream of water. The stream of water bends towards the ballon because the attractive forces are greater than the repulsive forces between like charges. Lightning during a thunderstorm: The attractive forces between the negative charges at the bottom of the clouds and the positive charges on the grounds causes negative charges (thunder/lightening) to travel from the cloud to the ground. This creates an electrical current. The electrical transfers to light, heat, and sound energies.

Question 29

How can a insulator be charged by friction?

Answer 29

1. When the uncharged rod plastic rod is rubbed with an uncharged cloth. 2. the friction causes the electron transfer from the rod to the cloth. 3. Plastic rod losses the electrons to become positively charged. Cloth gains electrons to become negatively charged. 4. This causes the cloth to be negatively charged and rod is positively charged.

Question 30

Uses of static electricity: Electrostatic paint spraying

Answer 30

As the drops of paint emerge from the spray gun. They are charged. As the drops all carry the same charge they repel each other and spread out forming a thin layer of spray paint. The metal bicycle frame has a wire attached to an electrical supply giving the frame the opposite charge. The paint drops are therefore attracted to the surface of the frame. *there is an added benefit that paint is atrracted into places such as corners and behind the frame that would normally be hard to reach.*

Question 31

Uses of static electricity: Photocopiers

Answer 31

1) positive charges are applied to a rotating drum coated with selenium, a light sensitive metal 2) A bright light shines on the document to be copied. The white areas reflect light onto the drum while the dark areas (text and images) absorb the light. Where the light strikes the drum the positive charge is lost. 3) negatively charged tones, a fine black powder is attracted to the remaining positive areas on the drum creating an image of the document. 4) A sheet of paper with a positive charge is then pressed against the drum, picking up the toner particles. 5) Finally, heat melts the toner, permanently sticking to the paper, creating the copy.

Question 32

Dangers of static electricty

Answer 32

Friction with air causes aircraft to accumulate static electricity during flight. Upon landing, this built-up of charge can discharge a spark, posing a serious risk of explosion if it occurs during refueling. To prevent this hazard, aircraft are grounded with a conductor as soon as they land and before refueling begins, allowing static electricity to safely dissipate. Similarly fuel tankers transporting fuel on roads must also be grounded before transferring any fuel to prevent sparks that could ignite a fire or explosion.