Electricity Flashcards

Question

What are the advantages and disadvantages of a series circuit?

Answer 1

A series circuit consists of a string of two or more components connected in a loop Advantages of a series circuit All of the components are controlled by a single switch Fewer wires are required Disadvantages of a series circuit The components cannot be controlled separately If one component breaks, all other components stop working

Answer 2

A parallel circuit consists of two or more components attached across different branches of the circuit Advantages of a parallel circuit The components can be individually controlled using their own switches If one component breaks, then the others will continue to function Disadvantages of a parallel circuit Many more wires are involved which can be more complicated to set up All branches have the same voltage as the supply making it more difficult to control the voltage across individual components Exam Tip You may have noticed that for a parallel circuit, all of the components can be controlled by a single switch - like a series circuit. Nevertheless, the exam board still considers this an advantage of series circuits. Note that the current does not always split equally in a parallel circuit – often there will be more current in some branches than in others. The current in each branch will only be identical if the resistance of the components along each branch are identical. However, the voltage across two components connected in parallel is always the same

Answer 3

When two or more resistors are connected in series, the total resistance is equal to the sum of their individual resistances For two resistors of resistance R1 and R2, the total resistance can be calculated using the equation: R = R1 + R2 Where: R is the total resistance, in ohms (Ω) Increasing the number of resistors increases the overall resistance The charge now has more resistors to pass through The total voltage is also the sum of the voltages across each of the ind

Answer 4

For components connected in series: the current is the same at all points and in each component the voltage of the power supply is shared between the components the total resistance is the sum of the resistances of each component For components connected in parallel: the current from the supply splits in the branches the voltage across each branch is the same the total resistance is less than that of each component

Answer 5

When the voltage V across a component is varied, the current I flowing through it may vary linearly or non-linearly The relationship between current and voltage of a component can be shown on an IV graph When the relationship between current and voltage is linear: the IV graph is a straight line which passes through the origin the resistance is constant When the relationship between current and voltage is non-linear: the IV graph that is not a straight line the resistance is not constant

Answer 6

Components with linear IV graphs include: fixed resistors (at constant temperature) wires (at constant temperature) Components with non-linear IV graphs include: filament lamps diodes LDRs thermistors

Answer 7

The relationship between current and voltage for a wire or fixed resistor is linear, or directly proportional, which means the IV graph is a straight line, so voltage and current increase (or decrease) by the same amount the slope of the graph is constant, so resistance is constant

Answer 8

The relationship between current and voltage for a filament lamp is non-linear, or not directly proportional, which means the IV graph is not a straight line, so voltage and current do not increase (or decrease) by the same amount the slope of the graph is not constant, so resistance changes The IV graph for a filament lamp shows as voltage increases the current increases at a proportionally slower rate the resistance increases; the flatter the slope, the higher the resistance As current through a filament lamp increases, the resistance increases because: the higher current causes the temperature of the filament to increase the higher temperature causes the atoms in the metal lattice of the filament to vibrate more this causes an increase in resistance as it becomes more difficult for free electrons (the current) to pass through since resistance opposes the current, this causes it to increase at a slower rate

Answer 9

A diode allows current to flow in one direction only This is called forward bias In the reverse direction, the diode has very high resistance, and therefore no current flows This is called reverse bias When the current is in the direction of the arrowhead symbol, this is forward bias On the IV graph, this is shown by a sharp increase in voltage and current on the right side of the graph This shows the resistance is very low When the diode is switched around, this is reverse bias On the IV graph, this is shown by a zero reading of current or voltage on the left side of the graph This shows the resistance is very high

Answer 10

In order to investigate the relationship between current and voltage different components, the following equipment is required: an ammeter - to measure the current through the component a voltmeter - to measure the voltage across the component a variable resistor - to vary the current through the circuit a power source - to provide a source of potential difference (voltage) wires - to connect the components together in a circuit The image below shows the circuits set up to obtain IV graphs for a filament lamp and a diode The current is the independent variable The variable resistor is used to change the current flowing through the filament lamp / diode The voltage is the dependent variable The voltmeter is used to measure the voltage across the filament lamp / diode Recording measurements of current and voltage as the current increases enables an IV graph to be plotted for each component

Answer 11

Resistance is the opposition to the flow of current The higher the resistance of a circuit the lower the current Resistors come in two types: Fixed resistors Variable resistors Fixed resistors have a resistance that remains constant Variable resistors can change the resistance by changing the length of wire that makes up the circuit A longer length of wire has more resistance than a shorter length of wire

Answer 12

Environmental conditions, such as temperature and light intensity, can influence the resistance of resistors, such as Thermistors Light-dependent resistors (LDRs)

Answer 13

The resistance of a thermistor depends on its temperature The resistance of a thermistor is high in cold conditions and low in hot conditions As the temperature increases the resistance of a thermistor decreases As the temperature decreases the resistance of a thermistor increases

Answer 14

The resistance of a light-dependent resistor (LDR) depends on the light intensity on it The resistance of an LDR is high in dark conditions and low in bright conditions As the light intensity increases the resistance of an LDR decreases As the light intensity decreases the resistance of an LDR increases

Answer 15

Lamps and light-emitting diodes (LEDs) illuminate (light up) when a current flows through them This makes them useful for indicating the presence of a current in a circuit Light-emitting diodes (LEDs) LEDs are a type of diode This means they only allow current to flow through them in one direction Therefore, in a circuit, an LED will only light up if it is placed in the correct direction The circuit symbol for an LED is as follows: Exam Tip Make sure you learn the various symbols mentioned on this page. Many of them are very similar with small differences denoting what they do: Two arrows pointing towards a symbol mean that it is light-dependent Two arrows pointing away mean that it is light-emitting Symbols are sometimes drawn with circles around them (e.g. the LDR). These circles are often optional (although not in the case of meters and bulbs).

Answer 16

Power is defined as The rate of energy transfer or the amount of energy transferred per second The electrical power of a device depends on: The voltage (potential difference) of the device The current of the device The power of an electrical component (or appliance) is given by the equation: P = IV Where: P = power, measured in Watts (W) I = current, measured in amperes (A) V = potential difference, measured in volts (V) The unit of power is the Watt (W), which is the same as a joule per second (J/s)

Answer 17

Exam Tip Remember: Power is just energy per second. Think of it this way will help you to remember the relationship between power and energy. You can remember the unit by the phrase: “Watt is the unit of power?”

Answer 18

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 containing 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

Answer 19

Fuses come in a variety of sizes, typically 3 A, 5 A and 13 A In order to select the right fuse for the job, the current through an appliance needs to be known If the electrical power of the appliance is known (along with mains voltage), the current can be calculated using the equation: I = P divided by V Where: I = current in amperes (A) P = power in watts (W) V = voltage in volts (V) The fuse should always have a current rating that is slightly higher than the current needed by the appliance Because of this, the rule of thumb is to always choose the next size up If the fuse current rating is too 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 break the circuit in enough time before damage occurs

Answer 20

Answer: Step 1: Consider a 3 A fuse A 3 A fuse would be too small The fuse would blow as soon as the appliance was switched on Step 2: Consider a 5 A fuse A 5 A fuse would be an appropriate choice It is the next size up from the current required Step 3: Consider a 13 A fuse A 13 A fuse would be too large It would allow an extra 10 amperes to pass through the appliance before it finally blew Remember there are two steps involved in selecting a correctly sized fuse for an appliance: 1. Calculating the current required using the electrical power equation 2. Selecting the next size up fuse

Answer 21

Work is done when charge flows through a circuit Work done is equal to the energy transferred The amount of energy transferred by electrical work in a component (or appliance) depends upon: The current, I The potential difference, V The amount of time the component is used for, t When charge flows through a resistor, for example, the energy transferred is what makes the resistor hot The energy transferred can be calculated using the equation: E = P x t Where: E = energy transferred in joules (J) P = power in watts (W) t = time in seconds (s) P = IV as explained in Electrical power & fuses So this equation can also be written as: E = I x V x t Where: I = current in amperes (A) V = potential difference in volts (V) When charge flows around a circuit for a given time, the energy supplied by the battery is equal to the energy transferred to all the components in the circuit

Answer 22

Answer: Step 1: Write down the known quantities Time, t = 1 minute = 60 s Current, I = 0.7 A Potential difference, V = 4 V Step 2: Write down the relevant equation E = I x V x t Step 3: Substitute in the values E = 0.7 x 4 x 60 E = 168 J Exam Tip 'Energy transferred' and 'work done' are often used interchangeably in equations, don't panic, they mean the same thing! Always remember that the time t in the above equations must always be converted into seconds.

Answer 23

Mains electricity is potentially lethal Potential differences as small as 50 V can pose a serious hazard to individuals

Answer 24

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 insulation, exposing live wires Damp conditions – if moisture comes into contact with live wires, it could conduct electricity either causing a short circuit within a device (which could cause a fire) or posing an electrocution risk To protect the user or the device, there are several safety features built into domestic appliances, including: Double insulation Earthing Fuses Circuit breakers

Answer 25

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 To improve electrical safety wires are covered with an insulating material, such as rubber Some appliances do not have metal cases, 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

Answer 26

Many electrical appliances have metal cases This poses a potential electrical 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

Answer 27

Fuses and circuit breakers are electrical safety devices designed to cut off the flow of electricity to an appliance if the current becomes too large (due to a fault or a surge) As explained in the Selecting fuses revision note a fuse consists of a glass cylinder containing a metal wire A circuit breaker consists of an automatic electromagnet switch that breaks the circuit if the current exceeds a certain value A circuit breaker has a major advantage over a fuse as an electrical safety device 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 as the most important electrical safety device Sometimes they are misleadingly named "Fuse boxes" Exam Tip For your exam, you must explain how insulation, double insulation, earthing, fuses and circuit breakers protect the device or user in different domestic appliances.

Answer 28

A current passing through a resistor (or wire) results in the electrical transfer of energy As explained in Charge & current, current is the rate of flow of charge The temperature of a resistor increases due to the collisions of the free electrons within the wire Some of the energy is dissipated into the surroundings by heating This heating effect is utilised in many domestic contexts, including: Electric heaters Electric ovens Electric hob Toasters Kettles Exam Tip Remember that a charge moving around an electrical circuit are an example of an electrical transfer pathway. If you are unsure of how to explain energy stores and transfers use the Energy stores & transfers revision note to help.

Answer 29

Mains electricity can be supplied by alternating current (a.c.) or direct current (d.c.) from a cell or battery

Answer 30

A direct current (d.c.) is defined as A steady current, 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 The current travels from the positive terminal to the negative terminal A d.c. power supply has a fixed positive terminal and a fixed negative terminal Electric cells, or batteries, produce direct current (d.c.)

Answer 31

An alternating current (a.c.) is defined as A current that continuously changes its direction, going back and forth around a circuit An alternating current power supply has two identical terminals that change from positive to negative and back again The alternating current always travels from the positive terminal to the negative terminal Therefore, the current changes direction as the polarity of the terminals changes The frequency of an alternating current is the number of times the current changes direction back and forth each second In the UK, mains electricity is an alternating current with a frequency of 50 Hz and a potential difference of around 230 V

Answer 32

Exam Tip If asked to explain the difference between alternating and direct current, sketching and labelling the graphs above can earn you full marks. All the circuits you have studied so far are d.c. circuits. Don't be put off by an exam question if you are asked to calculate the current, potential difference or resistance in d.c. series circuits, you don't have to do anything different from what you have already learned!

Answer 33

A conductor is a material that allows charge (usually electrons) to flow through it easily Examples of conductors are: Silver Copper Aluminium Steel Conductors tend to be metals

Answer 34

On the atomic scale, conductors are made up of positively charged metal ions with their outermost electrons delocalised This means the electrons are free to move Metals conduct electricity very well because: Current is the rate of flow of electrons So, the more easily electrons are able to flow, the better the conductor

Answer 35

Aim of the experiment The aim of this experiment is to investigate how insulating materials can be charged by friction Variables Independent variable = Rods of different material Dependent variable = Charge on the rod Control variables: Time spent rubbing the rod Using the same type of cloth Using the same length of rod

Answer 36

An insulator is a material that has no free charges, hence does not allow the flow of charge through them very easily Examples of insulators are: Rubber Plastic Glass Wood Some non-metals, such as wood, allow some charge to pass through them Although they are not very good at conducting, they do conduct a little in the form of static electricity For example, two insulators can build up charge on their surfaces and if they touch this would allow that charge to be conducted away

Answer 37

1. Take a polythene rod, hold it at its centre and rub both ends with a cloth 2. Suspend the rod, without touching the ends, from a stand using a cradle and nylon string 3. Take an acrylic rod and rub it with another cloth 4. Without touching the ends of the acrylic rod bring each end of the acrylic rod up to, but without touching, each end of the polythene rod (if the ends do touch, the rods will discharge and the forces will no longer be present) 5. Record any observations of the polythene rod's motion 6. Repeat, changing out the acrylic rod for rods of different materials

Answer 38

When two insulating materials are rubbed together, electrons will transfer from one insulator onto the other insulator A polythene rod is given a negative charge by rubbing it with the cloth This is because electrons are transferred to the polythene from the cloth Electrons are negatively charged, hence the polythene rod becomes negatively charged Conversely, an acetate rod is given a positive when rubbed with a cloth This is because electrons are transferred away from the acetate to the cloth Electrons are negatively charged, so when the acetate loses negative charge, it becomes positively charged If the material is repelled by (rotates away from) the polythene rod, then the materials have the same charge If the material is attracted to (moves towards) the polythene rod, then they have opposite charges In the example from the diagram above, the acetate rod would be attracted to the polythene rod, as they have opposite charges

Answer 39

Random errors Ensure the experiment is done in a space with no draft or breeze and the table is free of vibrations (e.g. from electrical equipment in the room), as this could affect the motion of the polythene rod If the deflection of the rod is very small, the direction could be misinterpreted – rub the other material for a longer period to transfer more charge and produce a more visible deflection This experiment can be carried out in several different ways To improve the outcome of the experiment, consider investigating a variable with a numerical result For example: the independent variable could stay the same (using rods of different material) the dependent variable could change to be the number of paper circles picked up by each rod Collecting numerical data allows: more analysis to be carried out e.g. creating a graph or a chart better conclusions to be drawn e.g. the rod made of ___ picked up more circles of paper than the other rods, therefore it became the most charged

Answer 40

he charge of a particle can be: positive negative neutral (no charge) Electrons are negatively charged particles, whilst protons are positive and neutrons are neutral This is why in a neutral atom, the number of electrons is equal to the number of protons This is so the equal (but opposite) charges cancel out to make the overall charge of the atom zero Therefore, an object becomes negatively charged when it gains electrons and positively charged when it loses electrons When two charged particles or objects are close together, they also exert a force on each other This force could be: Attractive (the objects get closer together) Repulsive (the objects move further apart) Opposite charges attract, like charges repel

Answer 41

Whether two objects attract or repel depends on their charge If the charges are the opposite, they will attract If the charges are the same, they will repel – charges which are the same (e.g. both positive) are often referred to as like charges The force is stronger if the objects are closer together Exam Tip Remember the saying “opposites attract” when answering questions about forces between charged particles.

Answer 42

Static electricity refers to the accumulation of charge on an object, which then attracts other objects or can even produce sparks When certain insulating materials are rubbed against each other they become electrically charged This is called charging by friction The charges remain on the insulators and cannot immediately flow away One becomes positive and the other negative An example of this is a plastic or polythene rod being charged by rubbing it with a cloth Both the rod and cloth are insulating materials This occurs because negatively charged electrons are transferred from one material to the other The material, in this case, the rod, gains electrons Since electrons are negatively charged, the rod becomes negatively charged As a result, the cloth has lost electrons and therefore is left with an equal positive charge Exam Tip At this level, if asked to explain how things gain or lose charge, you must discuss electrons and explain whether something has gained or lost them. Remember when charging by friction, it is only the electrons that can move, not any 'positive' charge, therefore if an object gains a negative charge, something else must have gained a positive charge by losing negative electrons.

Answer 43

All objects are initially electrically neutral, meaning the negative and positive charges are evenly distributed However, when the electrons are transferred through friction, one object becomes negatively charged and the other positively charged The object the electrons are transferred to becomes negatively charged The object the electrons transfer from becomes positively charged This difference in charges leads to a force of attraction between itself and other objects which are also electrically neutral This is done by attracting the opposite charge to the surface of the objects they are attracted to In the example below, when the cloth and acetate rod are rubbed together, the electrons are transferred from the rod to the cloth This results in an attractive force between the two objects once separated

Answer 44

Electrostatic charges are used in everyday situations such as photocopiers and inkjet printers

Answer 45

Photocopiers use static electricity to copy paper documents, most commonly in black and white 1. An image of the document is projected onto a positively charged copying plate 2. The plate loses its charge in the light areas and keeps the positive charge in the dark areas (i.e the text) 3. A negatively charged black toner powder (the ink) is applied to the plate and sticks to the part where there is a positive charge 4. The toner is then transferred onto a new blank sheet of white paper 5. The paper is heated to make sure the powder sticks (hence why photocopied paper feels warm) The photocopy of the document is now made Inkjet printers work in a similar way, but instead of the black toner powder, a small jet of coloured ink is negatively charged and attracted to the correct place on the page

Answer 46

Insecticides are chemicals used to kill pests in order to protect crops In order to spray crops effectively whilst using a minimal amount of chemicals, the sprayer has to deliver the chemicals as a fine mist and cover a large area To achieve this, the insecticide is given an electrostatic charge (e.g. positive) as it leaves the sprayer The droplets of insecticide then repel each other since they are the same charge This ensures that the spray remains fine and covers a large area They are also attracted to the negative charges on Earth, so they will fall quickly and are less likely to be blown away A similar technique is used in the spray painting of cars

Answer 47

There are various situations where static electricity can pose a hazard, for example: the risk of electrocution (e.g from lightning or a loose connection in an electrical appliance) the risk of a fire or explosion due to a spark close to a flammable gas or liquid Static electricity can cause sparking This is where a large amount of charge builds up, producing a large potential difference across a gap If the potential difference is large enough, current can travel through the air between objects – this is a spark There are dangers of sparking in everyday situations such as fuelling vehicles such as cars and planes Earthing is used to prevent the dangerous build-up of charge This is done by connecting the vehicles to the Earth with a conductor

Answer 48

A build-up of static charge is a potential danger when refuelling aeroplanes Fuel runs through pipes at a fast rate This fuel is very flammable The friction between the fuel (a liquid insulator) and the pipe causes the fuel to gain charge If this charge were to cause a spark, the fuel could ignite and cause an explosion This is prevented by the fuel tank being connected to the Earth with a copper wire called the bonding line during the refuelling The conductor earths the plane by carrying the charge through to the Earth which removes the risk of any sparks It is easier for charge to flow down the bonding line than to spark, so sparks are very unlikely to occur Exam Tip You could be asked to explain other dangers and uses in your exams They may ask you to explain the movement of charge in terms of electrons If asked to explain a danger: State what the danger is (electrocution? fire?) Explain how the charge can be removed to get rid of the risk i.e earthing (think about which way the electrons have to move) If asked to explain a use, think carefully about the forces exerted due to static electricity and what they will do

Answer 49

An inkjet printer uses electrostatic charge to direct the tiny ink droplets to the correct place on the page. Coloured ink is passed through a very small hole called a nozzle which separates the ink into many tiny droplets. The tiny droplets are given an electrostatic charge. The direction in which the charged ink droplets move can be controlled by electrically charged metal plates. A voltage on the plates means that the charged ink droplets will be attracted to one plate and repelled by the other. This is very similar to a cathode ray oscilloscope where an electron beam is directed to a particular place on a screen. In the picture below, the ink droplets have a positive charge. The ink droplets are attracted to the negative plate and repelled by the positive plate. By controlling the voltage on the plates a particular ink drop can be precisely positioned on the paper. There are many nozzles, and the final picture is made up from a very large number of coloured ink drops, each in exactly the right place for the image.

Electricity Flashcards

(74 cards)