Electrical Energy in the Home

Section 1: Society has become increasingly dependent on electricity over the last 200 years.

Discuss how the main sources of domestic energy have changed over time.o Wood (to produce fire)

Chemical energy in wood converted into heat energy. First energy source used by humans. Wood allowed food to be cooked and helped to keep fierce animals at bay. Provided warmth.

o Domesticated animals Source of mechanical energy. Used to pull carts and turn grindstones. Improvement of agricultural practices. Transportation increased.

o Wind and water Source of mechanical energy. Wind for boats and mills. Water wheels used to move grindstones. More efficient production of food.

o Coal Main fuel in 19th century due to dwindling supplies of wood and superior

energy content of coal. Development of steam engine. High energy content allowed production of steel. Atmospheric pollution of cities. Driving force of modern industrial age.

o Coal gas Produced by burning coal in absence of air. Source of heat and light in houses in cities. No longer requires wood to keep

warm. Assess some of the impacts of changes in, and increased access to, sources of energy for a

community.o Advantages

Increased production of food and construction methods leading to increased population.

Improved transport and trade. Increased convenience and improved quality of life. Increase in efficient for human labour and production methods. Greater employment opportunities for people with expertise in electrical

energy generation and management.o Disadvantages

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Overcrowding in cities leading to poverty. Overcrowding also leads to disease. Pollution and climate change from coal-fired power stations.

Discuss some of the ways in which electricity can be provided in remote locations.o Electricity is produced at a power station at a voltage of around 10000 volts and

10000 amps.o This is then sent to transformers which change the voltage and current to suitable

values for each section of the distribution system.o Step-up transformers increase voltage but decrease current to minimise energy loss

in transmission through heat. From 10000 volts to 500000 volts.o Step-down Transformers near homes gradually decrease the voltage from 330000

volts to 240 volts.

o As the number of power stations increased, they become linked together in a power grid.

o Power stations connected this way can exchange energy so a station with low demand can assist stations with high demand.

o Grids can also connect remote energy sources such as solar panels or wind farms.o Some remote areas are not connected to the main grid.

o Remote areas not connected to the main grid frequently use small generators in which a coil is rotated by an internal combustion engine using diesel or petrol.

o Solar cells and small wind generators are also used. Identify data sources, gather, process and analyse secondary information about the differing

views of Volta and Galvani about animal and chemical electricity and discuss whether their different views contributed to increased understanding of electricity.

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Power station.

10,000V

Step-up transformer

500, 000V

Transmission lines

500, 000V

Step-down transformers

33, 000V

Suburban substation step-down transformer

11, 000V

Street transformers

240V or 415V

Industry

415V

Homes

240V

o Luigi Galvani Galvanic found that a dissected frog’s leg would contract if a scalpel touched

the frog’s nerves when the electrostatic machine was working or when there was a lightning strike in vicinity.

Later he found that he could make a frog’s leg contract with just 2 different metals attached to the dissected frog – one to a leg muscle and the other to the spinal cord. The leg muscle would contact if the 2 metals touched.

Believed that his observations showed that electricity was being generated in frog tissue. Called animal electricity.

o Alessandro Volta Carried out experiments to verify Galvani’s work. Believed that muscle movement was due to the contact between two

different metals. The contact of two different metals produced electric charge.

He believed that when 2 different metals are brought into contact, one becomes positively charged and the other becomes negatively charged.

Created the Voltaic pile using zinc and brass discs to demonstrate this.o Volta’s construction of the Voltaic pile became the forerunner of modern batteries

and so increased our understanding of producing electricity through chemical reactions.

o Galvani’s observation of frog leg contractions suggested that there were electrical nerve signals from the brain to the nerves in muscles and increased our understanding of the human body and bioelectricity.

Section 2: One of the main advantages of electricity is that is can be moved with comparative ease from one place to another through electrical circuits

Describe the behaviour of electrostatic charges and the properties of the fields associated with them.

o Electric charge – a property of electrons and protons by which they exert electric forces on one another.

o There are 2 types of charges: positive and negative.o Like charges repel; unlike charges attract.o Protons (+), Electrons (-) and Neutrons (no charge).o A body which has equal number of protons and electrons will be neutral.o If a body has gained electrons it will acquire a negative charge due to excess

electrons.o If a body has lost electrons it will acquire a positive charge due to a deficiency of

electrons.o A charge on a body due to excess or deficiency of electrons is called an electrostatic

charge. Define the unit of electric charge as the coulomb.

o 1 C = charge of 6.25 x 1018 electrons. OR

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o 1 e- = - 1.60 x 10-19 C.o 1 p = +1.60 x 10-19.o Q or q used to represent electric charge.

Define the electric field as a field of force with a field strength equal to the force per unit charge at the point: E=f/q

o An electric field is a region of influence in which a charged object will experience a non-contact electrostatic force.

o An object can be charged by: Friction (rubbing). Conduction (by bringing in contact a charged object) Induction (by moving a charged object close to an uncharged object, this will

induce charges).o The Strength of an electric field is given by:

E = f / q Where E is the electric field strength in N C-1 (newtons per coulomb). F is the force acting on the charge. Q is the charge.

o In other words, the electric field strength is the force which a coulomb of charge experiences when placed in an electrical field.

o The direction of the electric field a t a point is defined as the direction of the force that acts on a positive electric charge placed at the point.

o i.e.

o Treat the electric field as a gravitational field.o The electric field strength is the force acting on the charge in the field.o The spacing of the lines indicates the magnitude of the electric field. The closer

together the lines, the stronger the field. Define the electric current as the rate at which charge flows (coulombs/ second or amperes)

under the influence of an electric field.o The electric current is the net movement of electric charge.o SI unit: A or ampere.o Moving charges are referred to as charge carriers.o This is given by:

I = Q/t Where Q is charge, t is time in seconds.

o Other definition.

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F

F

1 ampere = 6.25 x 1018 electrons passing through a cross-section of a conductor in 1 second.

o In a metal, there are free electrons which are detached from the atoms and are able to move around freely.

o These free electrons are the charge carriers.o If there is an electric field in a metal, the free electrons, being negatively charged,

will experience a force in the opposite direction to the field.o When the electric field is superimposed on the random movement of free electrons,

there will be a net movement in the direction opposite of the electric field.o This net movement is known as electron drift and constitutes an electric current.o When a negative charge moves in the opposite direction to an electric field, it lsoes

electric potential energy and gains kinetic energy.o As the free electrons gain kinetic energy and collide with the positive ions in the

electron drift, the kinetic energy is transferred to the positive ions causing them to vibrate with greater energy.

o Thus when an electric current flows through a metal, the electric potential energy is transformed into heat energy.

o Current is measured using an ammeter. Denoted by an encircled A.o The ammeter must have very low and optimally no resistance.

This is because it is connected in series with the circuit. Its purpose is to sample the current and so as little interruption with the

circuit is desired. Identify that current can be either direct with the net flow of charge carriers moving in one

direction or alternating with the charge carriers moving backwards and forwards.o DC (Direct current)

The current flows continuously from the positive to the negative terminal. (Conventional current)

The polarity of the terminals remains the same. E.g. batteries, power packs in school.

o AC (Alternating current) The polarity of the terminals continuously alternate. This results in an alternating voltage. The charge carriers move backwards and forwards periodically.

o Conventional current consist of positive charges and flows from positive to negative.o Real current is the flow of electrons and goes from negative to positive terminal.o The negative current moving in one direction is equivalent to the same amount of

positive current.o When doing any work involving current direction always follow conventional

current. Describe electric potential difference (voltage) between two points as the change in

potential energy per unit charge moving from one point to the other (joules/coulomb or volts).

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o The potential difference across a resistor is the number of joules of electric potential energy dissipated by each coulomb of charge that passes through the resistor.

o In other words, the change in electric potential energy per coulomb of charge that moves between two points.

o This is given by: V = W / Q

Where W is work done. Energy (joules). Q is the amount of charge. V is voltage.

o Let +q be a charge that moves from A to B under the influence of the electric field of strength E. (Blue is +q)

Plate A

E +

d -

Plate B

o The force acting on q, F = E x qo The work done by the field is therefore:

W = F x d = E x q x d.

o As +q moves from A to B, its electrical potential energy changes to kinetic energy through work done by the electric field.

o Therefore: V = E x Q x D Where E is electric field strength. Q is charge. D is distance between the 2 plates.

o Voltage is measured using a voltmeter. This is connected in parallel to the circuit. The voltmeter should have a large amount of resistance. This is so that it would be able to use up as much of the electrical potential

energy as possible to determine the voltage across a resistor. Discuss how potential difference changes at different points around a DC circuit.

o In a circuit, the electric power supply provides the source of electric potential energy.

o The strength is described by its voltage between the two terminals.o As the electric current passes through various resistors of the circuit, electrical

potential energy is converted into other forms which include heat and light.o This is achieved through work done by appliances.

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+ + + + + + + +

- - - - - - - - -

o In a circuit, the sum of the voltages across the resistors must equal to the voltage setting of the terminal.

o If there are no resistors, then the energy will not be used up and will continue to circulate faster and faster in the circuit leading to a short-circuit as energy builds up.

Identify the difference between conductors and insulatorso A conductor is a material that let electric charges (current) flow easily with minimal

energy loss.o This is because electrons are free to move. They are typically metals such as copper,

silver and gold.o Insulators are materials that do not let electric current flow at all.o The electrons are held tightly by the atomic nuclei or by covalent bonds. Commonly

non-metals such as glass and plastics. Define resistance as the ratio of voltage to current for a particular conductor: R = V/I.

o Resistance is the opposition to the flow of electrical charge.o Resistance is defined by: R = V/I

Where R is resistance in ohms (Ω). V is voltage in volts (V). I is current in amps (A).

o This is also known as Ohm’s law The potential drop across a resistor is proportional to the current passing

through the resistor. Ohm’s Law only applies to resistors with constant resistance.

o Ohmic resistors. Resistors that obey Ohm’s Law are called ohmic resistors. For an ohmic resistor, its V vs. I graph will be a straight line.

o Non-ohmic resistance. A resistor that does not obey Ohm’s Law is known as a non-ohmic resistor. One of the reasons this occurs is because as more current passes through

the resistor its temperature increases. An increase in temperature means that the atoms vibrate more energetically

and so the electrons bump into the atoms more often resulting in high resistance.

For a non-ohmic resistor, its V vs I graph is curved. These resistors do not obey Ohm’s Law.

o A good conductor is one in which there is negligible conversion of electric potential energy into heat energy.

o Therefore a good conductor has 0 resistance as V=0 and I=0. Describe qualitatively how each of the following affects the movement of electricity through

a conductor: length, cross sectional area, temperature and material.

Length

o The longer the wire, the higher its resistance.o This is because as the wire gets longer, there is a greater chance of a collision

between the free electron with an ion in the lattice.

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o If two conductors differ in length only, then: R1 / R2 = l1 / l2.

o If the length of a conductor is doubled and all other factors are kept the same, then its resistance will double.

Cross-sectional area

o The smaller the area of cross-section of a wire, the greater its resistance.o This is because a small cross-sectional area means that there is a greater chance for

an electron to collide with an ion in the lattice.o If 2 conductors differ in length only, then:

R1 / R2 = A2 / A1.o If the area of cross-section is doubled and all factors are constant, then resistance is

halved.

Temperature

o When the temperature of a conductor is increased, the resistance will increase.o This is because as temperature increases, the ions in the lattice will vibrate with

greater amplitude. This increases the chances for collision between an electron and ion.

Material

o When a free electron travels through a conductor, the chances it will collide with an ion in the lattice is dependent on the material the conductor is made of.

o The factors contributing to the resistance of a material can be combined to form the equation:

R = (P x L x T) / A Where P is material type (given as a constant). L is the length of the conductor. T is temperature. A is cross-sectional area.

Present diagrammatic information to describe the electric field strength and direction: between charged parallel plates, about and between a positive and negative charge.

Electric field surrounding a positive point

o If a small positive charge +q is placed near the electric field surrounding +Q, it will move away from +Q.

o If a small negative charge –q is placed near the electric field surrounding +Q it will move towards +Q.

o This can be used to determine the direction of the electric field lines.o Notice that as the distance from +Q increases the electric field strength decreases.

Preliminary Physics: Electrical Energy in the Home Page 8

Blue is +q

E +Q Red is -q

Electric field surrounding a negative point

o When a positive charge +q is placed close to –Q it will move towards –Q.o When a negative charge –q is place close to –Q it will move away from –Q.o This can also be used to determine the direction of the electric field lines.

Blue is +q

-Q -Q Red is –q

Solve problems and analyse information applying: R = V/I.o Resistance can be added in a circuit. o Series

In a series circuit, the resistance can simply be added up. i.e. Rtotal = R1 + R2 + R3 + ... + Rn.

o Parallel In a parallel circuit the resistance cannot be added up i.e. 1 / Rtotal = 1/R1 + 1/R2 + 1/R3 ... 1/Rn. Or Rtotal = (R1 R2 R3 ... Rn) / (R1 + R2 + R3 + ... Rn )

o When performing calculations, apply Ohm’s Law only to a certain region or aspect of the circuit each time.

i.e. Only apply Ohm’s Law to resistor 1 when concerning resistor 1. Use only R1, V1 and I1.

Only apply Ohm’s Law to the whole circuit when concerning the whole circuit. Use only Rtotal, Vtotal and I total.

o In a series circuit, the voltage is the only value that changes in the circuit. Current remains constant.

Preliminary Physics: Electrical Energy in the Home Page 9

+Q

o In a parallel circuit, the current is split at each branch but the voltage at the beginning of each branch is constant.

o Thus in a parallel circuit, the more branches there is, the greater the current is. However, the voltage is the same across each branch and so it is more efficient than a series circuit.

Plan, choose equipment for and perform a first-hand investigation to gather data and use the available evidence to show the variations in potential difference between different points around a DC circuit.

o Equipment: Power pack, rheostat, light globe, alligator clips, voltmeter, ammeter.o In a circuit the sum of the voltages across the circuit should be 0.o Connect the ammeter in series with the rheostat, power pack and light globe.o Connect the voltmeter in parallel to the globe.o Take reading on the ammeter.o Set the voltage setting on the power pack to 12V.o Take voltmeter reading at the light globe.o Change voltmeter position to the ends of the power pack.o Take another reading on the voltmeter.o Change the voltmeter’s position to the rheostat. o Take another reading on the voltmeter.o Change the voltage setting to 10V and adjust the rheostat so that the amps is the

same as the previous circuit.o Repeat with different locations for voltmeter.o The sum of the volt readings around the circuit should add to give the voltmeter

reading at the power pack. Gather and process secondary information to identify materials that are commonly used as

conductors to provide household electricity.o Materials commonly used to provide householdo Aluminium and iron (steel) are often used for transmission lines as copper is too

expensive and not mechanically strong enough.o Copper is often used in household circuits due to its high conductivity and its

cheapness.

Section 3: Series and parallel circuits serve different purposes in households

Identify the difference between series and parallel circuits.o Series

A series circuit is where there is only one current pathway. The current is constant throughout the circuit. If one appliance does not work then the whole circuit breaks down.

o Parallel A circuit where there are multiple current pathways. The current splits at each branch of the circuit.

Preliminary Physics: Electrical Energy in the Home Page 10

If one appliance does not work then the others connected in parallel will continue to function.

Compare parallel and series circuits in terms of voltage across components and current through them.

o Series The voltage drop across each resistor adds up to give the voltage provided

by the power pack. The resistance of a series circuit can be added simply by adding the

resistance of each resistor. i.e. Rtotal = R1 + R2 + ... + Rn. The current throughout the circuit is constant. i.e. Itotal = I1 = I2 =I3 = ... = In. The greater the resistance of a resistor, the greater the power dissipated in

it. The greater the resistance of a resistor, the greater the voltage drop across

the resistor.o Parallel

The voltage drop across each branch is the same as the voltage provided by the power source.

However, the current from the power source is split at each branch of the circuit.

Thus, the more branches you have, the greater the current flow must be so sufficient current can split at each branch to power the resistors.

This is the cause of overloading in a circuit as connecting too many resistors greatly increases the current which leads to vast temperature increases in the wires. This makes the wire hot and increases its resistance until the current can no longer support the resistors.

Sum of current in circuit branches equals the current provided by the power source.

i.e. Itotal = I1 + I2 + I3 +... +In

The sum of resistance in a parallel circuit is given by: Rtotal = (R1 * R2 * R3 ... * Rn ) / (R1 + R2 + ... Rn ). 1 / Rtotal = 1/R1 + 1/R2 + 1/R3 + ... 1/Rn.

The greater the resistance of a resistor, the smaller the current passing through it.

The greater the resistance of a resistor, the smaller the power dissipated in it.

o In a combined circuit add the resistance in the minor circuit first. i.e. if there was a parallel circuit connected in series with the rest of the circuit. First add the resistance in the parallel then it becomes one resistor in the series circuit.

o If there was a series circuit in a parallel circuit, then add the resistance first in the series circuit. Treat the series circuit as one resistor and continue as normal.

Preliminary Physics: Electrical Energy in the Home Page 11

o When dealing with each current. i.e. current of entire circuit or current over a certain resistor. Only use the other factors associated with that particular component only.

o i.e. When dealing with entire circuit use Itotal, Rtotal and Vtotal.

o When dealing with one particular resistor use I1, R1 and V1. Identify the uses of ammeters and voltmeters.

o Ammeter The ammeter is used to measure the current size of a circuit. It is connected in series with the other resistors as it needs to sample the

current in the circuit. Thus it must have minimal resistance as minimal interference is desired in

the circuit.o Voltmeter

The voltmeter is used to measure the potential difference across each resistor.

The potential difference between two points across a power source is called a potential rise.

The potential difference between two points across a resistor is called a potential drop.

It is connected in parallel to each component of the resistor. It also has extremely high resistance. This is because it must use all the energy per coulomb of charge across the

resistor to see how much energy is used or gained across the component. Explain why ammeters and voltmeters are connected differently in a circuit.

o See previous dot point. Explain why there are different circuits for lighting, heating and other appliances in a house.

o Each type of circuit has different voltage and current requirements. Thus different circuits are required to support different types of appliances. This allows for specification in design of circuit capacity and installation of appropriate fuses.

o If one circuit fails, then the rest will not be affected as other circuits are separate. i.e. if the lighting circuit fails then the hot water system will not be affected. This makes it easier to maintain and isolate problems.

o Each circuit has a design capacity. Separation of circuits helps divide the current as opposed to having only one

circuit. Too many appliances connected in parallel will raise the current to unsustainable levels.

o The separation of circuits allows for the turning off of certain circuits such as those supporting the air conditioner while leaving vital circuits on such as those supporting the alarm system.

o Each individual circuit has a safety feature known as a fuse or circuit breaker. A fuse is made of a metal wire with a low melting point. When more appliances are added to the circuit, the current increases which

also leads to a rise in temperature.

Preliminary Physics: Electrical Energy in the Home Page 12

When the current is too strong, the heat will melt a component in the fuse/circuit breaker thereby activating the fuse.

Plan, choose equipment or resources for and perform a first-hand investigation to gather data and use available evidence to compare measurements of current and voltage in series and parallel circuits in computer simulations or hands-on equipment.

o Series Set up 2 or more resistors in series. Use an ammeter to measure the current between the negative terminal and

one resistor, between the positive terminal and the other resistor, and between both resistors.

Use voltmeter to measure the voltage across each resistor, across both resistors and across the power pack.

Current same throughout circuit. Voltage across each resistor is proportional to resistance. Sum of voltages across all resistors is equal to the potential rise across the

power pack.o Parallel

Set up 2 or more resistors in parallel. Use ammeter to measure the current before the 1st branch, along the 1st

branch and along the 2nd branch. Also measure current after the branches have re-joined.

Use voltameter to measure voltage across each resistor and power pack. Current across each resistor is inversely proportional to its resistance. Sum of currents across both resistors equals the current before and after the

branching. Voltage drop across each resistor (or branch) is equal.

Plan, choose equipment or resources and perform a first-hand investigation to construct simple model household circuits using electrical components.

o Household circuits are roughly divided into the following components: Ring main circuit (in the wall surrounding house). Lighting. Kitchen. Hot water / heater.

Section 4: The amount of power is related to the rate at which energy is transformed.

Explain that power is the rate at which energy is transformed from one form to another.o Power is the rate at which work is done per unit time.

P = W / t Where P is power (Watts). W is work (joules). T is time (seconds).

o Work is the transformation of different forms of energy.o Therefore, power is the rate at which energy is transformed.

Preliminary Physics: Electrical Energy in the Home Page 13

o How power is defined in electricity. The current is I amps. Therefore, I coulombs of charge passes through the resistor in one second. Each coulomb of charge dissipates V joules of electric potential energy. Therefore in one second, VI joules of electric energy are dissipated.

o There are 3 different formulas for power relating to voltage, resistance and current.o P = V x I.

Where P is power (Watts). V is voltage. I is current. This is only used when current does not change across the resistor. i.e. in series only.

o P = I2 x R Where R is resistance. I is current. Only used when current does not change. i.e. in series only.

o P = V2 / R. Where R is resistance. V is voltage. To be used in parallel circuits when other 2 formulas cannot be used.

o More useful formulas relate to work: W = V x I x t.

Where W is work (Joules). V is voltage. I is current. T is time (seconds).

W = I2 x R x t. Where W is work. I is current R is resistance. T is time.

o Finding power helps to calculate the work done by a load. This is useful in brightness questions where the question asks which of the

light globes in the circuit are brightest. To find the brightest globe, find the power of each globe and whichever one

has the highest is the brightest. Identify the relationship between power, potential difference and current.

o See previous dot point. Identify that the total amount of energy used depends on the length of time the current is

flowing and can be calculated using: Energy = VIt.o Power is the amount of energy transformed per unit time.o Thus the total amount of energy used depends on the length of time the current has

flowed.

Preliminary Physics: Electrical Energy in the Home Page 14

o W = Pt. Where W is work. P is power and t is time (seconds).

o This in turn gives rise to Energy = V x I x t. Where V is voltage. I is current T is time (seconds).

o A Watt is the same as 1 Js-1.

o If the currents through 2 resistors are equal, the resistor with the greater resistance will have the greatest power dissipation. (Series circuit).

o If the voltage drops across 2 resistors are equal, the resistor with greater resistance will have the smaller power dissipation. (Parallel circuit).

Explain why the kilowatt-hour is used to measure electrical energy consumption rather than the joule.

o The kilowatt-hour is used because the joule is too small to be used for practical purposes.

o When expressing the amount of energy used in everyday life in joules, the figure becomes too large and unmanageable for calculations.

o 1 kw-hour of energy is the amount of energy used by an appliance of 1000 watts in power over 1 hour.

o Energy in kilowatt-hours = Power in kilowatts x time in hours. Perform a first-hand investigation, gather information and use available evidence to

demonstrate the relationship between current, voltage and power for a model 6V to 12V electric heating coil.

Solve problems and analyse information using: P =VI and Energy = VIt.

Section 5: Electric currents also produce magnetic fields and these fields are used in different devices in the home.

Describe the behaviour of the magnetic poles of bar magnets when they are brought close together.

o A magnet is always made of two poles: North and South.o There is no such thing as a magnet with only one pole. i.e. if a magnet was cut in

half, then one end will automatically become the conjugate pole of the existing end.o Unlike poles attract and like poles repel.o There is a magnetic field around each pole which allows poles to exert non-contact

forces on each other.o The forces between 2 magnets are the interaction between 2 magnetic fields.o Magnetic field – A region of influence in which another magnet/magnetic material

will experience a non- contact force.o B is the label magnetic field.o Magnetic field lines never intersect.

Preliminary Physics: Electrical Energy in the Home Page 15

Define the direction of the magnetic field at a point as the direction of force on a very small north magnetic pole when placed at that point.

o The direction of a magnetic field can found be placing a small magnet in the magnetic field.

o The direction in which the north pole on the magnet points is the direction of the magnetic field.

B

o When a compass is placed on Earth, the suspended needle is allowed to be influenced by the Earth’s magnetic field.

o The direction in which the compass points is the geographic North pole which is really the magnetic south as the north end of the compass is attracted to it.

Describe the magnetic field around pairs of magnetic poles.o Field lines never intersect.o All field lines form self-enclosed loops.o Inside the magnet, the field lines flow from South to North.o Outside the magnet, the field lines flow from North to South.

Preliminary Physics: Electrical Energy in the Home Page 16

S N

Describe the production of a magnetic field by an electric current in a straight current-carrying conductor and describe how the right hand grip rule can determine the direction of current and field lines.

o An electric current flowing through a wire is always accompanied by circular magnetic fields.

o The shape of this magnetic field depends in a way on the shape of the conducting wires.

o Straight wire. The magnetic field lines around a straight wire are concentric circles. This can be determined by the right hand grip rule. When the thumb points in the direction of the conventional current passing

through the wire, curling the other 4 fingers will give the direction of the concentric magnetic fields.

Preliminary Physics: Electrical Energy in the Home Page 17

o Solenoid. The magnetic field lines in a solenoid are similar to the ones found in

magnets. The direction of the magnetic field can also be determined by the right hand

grip rule. Determine the direction in which current is flowing in the curled wires

around the solenoid. Curl the 4 fingers in the same direction as the conventional current flow. The thumb will point in the direction of the magnetic field. (It will also point

at the north pole of the solenoid.). Like a magnet, the magnetic field lines outside the solenoid will curve in a

loop back to the other end.

o Another way to determine which end is north and south is to look at the end of a solenoid.

o If the conventional current is rotating anti-clockwise then the end is a north pole.o If the conventional current is rotating clockwise then the end is a south pole.

Preliminary Physics: Electrical Energy in the Home Page 18

NS

Compare the nature and generation of magnetic fields by solenoids and a bar magnet.o Both a bar magnet and the magnetic fields of solenoid form self – enclosed loops

from N to S on the outside and from S to N inside.o A permanent magnet is created by placing a ‘hard iron’ core inside a solenoid for a

sufficient amount of time. These magnets can be demagnetised by hitting or heating. This is because through hitting and heating, the magnetic domains of the

magnets are randomised once more resulting in no overall magnetism. Hard iron is used to refer to any alloy of iron that magnetises slowly but

retains its magnetism for longer periods of time after it is removed from the magnetic field.

o Soft iron is a type of iron that becomes magnetised quickly but loses its magnetism quickly once it is removed from the magnetic field.

o A solenoid with a soft iron core can be turned into an electromagnet by passing an electric current through the coils of the solenoid.

However, when the current ceases to pass through the solenoid, the soft iron core quickly becomes demagnetised.

o Materials commonly used in magnets: Fe, Ni and Co.

o Domain theory A ferromagnetic material is made up of many magnetic domains. In each domain, nearly all the atomic magnets are lined up in the same

direction. When a ferromagnetic material is placed in a magnetic field, the directions

of magnetism of domains line up in the direction of the magnetic field. When all domains are lined up, the material is fully magnetised. When demagnetised, the domains are all magnetised in random directions

resulting in no overall magnetism. Plan, choose equipment or resources for, and perform a first – hand investigation to build an

electromagnet. Perform a first – hand investigation to observe magnetic fields by mapping lines of force:

around a bar magnet, surrounding a straight DC current-carrying conductor, a solenoid, present information using x and to show the direction of a current and direction of a magnetic field.

o Solenoid and straight DC current. Equipment : Uninsulated wire, cardboard piece, solenoid, power pack, wires,

alligator clips, retort stand, G-clamp, retort ring, compass. Use a hole-puncher to punch a hole in the cardboard piece. String the piece of uninsulated wire through the cardboard. Balance the cardboard on the retort ring. Connect wires to the two ends of the wire. Wind one end of the wire onto the G – clamp at the top of the retort stand

so the wire is taught. Apply voltage from power pack and place the compass on the cardboard. Record the direction in which the compass is pointing.

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Repeat by placing compass in various positions. Switch the positive and negative end of the wires to reverse current

direction. Place compass on cardboard again in various positions. The magnetic field direction should be reversed when current is reversed.

Set up solenoid and power pack. Turn on power pack and place compass close to solenoid. Record direction and place compass in various places. Swap the positive and negative terminals to reverse current. The magnetic field direction should be reversed when current is reversed.

o An x surrounded by a circle and is used to indicate current direction. An x surrounded by a circle indicates that current is going away from you.

i.e. into the page. indicates that current is coming towards you. i.e. out of the page.

o An x and a · are used to indicate magnetic field direction when 2 D field lines cannot describe the 3 D magnetic field.

An x means that the magnetic field is going away from you. i.e. into the page.

A · indicates that the magnetic field is going towards you. i.e. out of the page.

B (Out of page) Conventional current

B (Into page

Identify data sources, gather, process and analyse information to explain one application of magnetic fields in household appliances.

o Loudspeakers

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Conventional current coming out of the page. The concentric rings are magnetic field lines.

Conventional current going into the page. The concentric rings are magnetic field lines.

Circular permanent magnet surrounding a freely moving coil. Coil attached to cone-shaped diaphragm. When a current passes through the coil, magnetic forces will act on the coil

causing it to move in or out depending on current direction. If an alternating current passes through the coil, it will move alternately in

and out as current changes direction. Coil vibrates at same frequency as alternating current. Alternating current derived from sound waves converted by a microphone. Vibrations in coil causes the diaphragm to vibrate and generate sound waves

that reproduce the sound waves from the original source.

Section 6: Safety devices are important in household circuits Discuss the dangers of an electric shock from both a 240 volt AC mains supply and various

DC voltages, from appliances, on the muscles of the body.o An electric shock is caused when electric current passes through the body and

causes a violent disturbance of the nervous system.o DC current

Produced when the positive and negative terminals do not change while the current flows.

E.g. batteries. A continuous flow of current flows through the circuit at a constant rate. DC shocks are not fatal, they do however cause burns and other physical

injuries. DC shocks do not lead to fibrillation.

o AC current Produced when the positive and negative terminals alternate. The current changes direction according to the frequency of the alternating

power supply. AC shocks are more fatal as its alternating frequencies are close to the

frequency of heart beats, thus increasing the risk of fibrillation of heart muscles.

Fibrillation is a condition in which the heart stops beating regularly and oscillates rapidly.

o The time the current passes through the body affects severity of shock. The longer the exposure, the more serious the shock.

Sometimes, when a current passes through a muscle, it contracts and the person can no longer pull away.

When this happens someone else needs to hit the person with a non-conducting material to break his or her hold from the wires.

o Resistance of the body affects severity of shock. Current depends on the voltage across the body and the resistance of the

body. This is determined by the relationship I = V /R. Thus the larger the resistance, the smaller the current.

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Wet skin has much lower resistance than dry skin.

Effects of alternating currents with a frequency of 50Hz from hand to hand. (240V)

Current (mA) Physiological effects0 – 1 Not perceptible1 – 15 Pain at points of entry and exit of current.

Involuntary contraction of muscles in path of current.

15 – 100 Inability to release grip. Muscular contractions of chest and diaphragm. Contractions may prevent breathing and if prolonged leads to death.

100 – 200 Fibrillation leading to death.Over 200 Reversible stopping of heart. Loss of

consciousness.

Describe the functions of circuit breakers, fuses, earthing, double insulation and other safety devices in the home.

o A short circuit occurs when the active wires comes in contact with the neutral wire or with a conductor connected to the Earth.

This creates a circuit with very little resistance leading to an extremely high current.

Can cause fire.o Overloading

Current is proportional to temperature. Each piece of wiring is designed to carry a certain max current without

overheating. Wires that have to carry a high current are made thicker to reduce

resistance and decrease heat generated. If a wire is overloaded, it can become sufficiently hot to cause fire. Most likely occurs when too many appliances are connected in parallel to

the household circuit raising the current through the wire.o Circuit breakers

Same function as fuses. Uses an electromagnet to mechanically break the circuit when current

exceeds max value. Can be reset after being tripped.

o Fuses Used to prevent overloading of household circuits. Fuse made of resistance wire with low melting point. Fuse is in series with rest of circuit. The greater the current, the higher the temperature. Fuse designed to melt when current through circuit exceeds limit and this

breaks the circuit. Fuse wire is placed inside an insulated, high melting point casing to prevent

fuse from becoming fire hazard.

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Made with different ratings to melt at different currents. Must be replaced every time a fuse ‘blows’.

o Double insulation Prevent live electric current from escaping wires and causing shocks. Normally insulated with plastics (PVC). Must be flexible as wires are flexible. Some appliances are double insulated in case the inner insulation fails.

o Earth wire There are usually 3 slots on power outlets: active, neutral and earth. The earth wire is connected to the fuse box and to the earth.. The earth wire provides protection in case the active wire comes in contact

with the casing of the appliance or someone touches the active. A large current will flow to the earth through the earth wire as soon as the

active wire touches the appliance. This is because the current will flow through the shortest path

towards the Earth. This is through the earth wire which goes through the fuse box.

The large current will go to the fuse box and cause the fuse to blow or circuit breaker to trip.

Double insulated appliances do not need earth wires. o Residual current devices.

Detects any leakage of current to the Earth. Designed to switch current off before it reaches harmful levels.

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