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Electricity (Chapters 16-22) Done by: Nathaniel Seah (23) 4Eng by Mind Map: Electricity (Chapters 16-22)
Done by: Nathaniel Seah (23)
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Electricity (Chapters 16-22) Done by: Nathaniel Seah (23) 4Eng


Magnetic Field around a straight line wire

Magnetic field is stronger in the region around the wire

Magnetic Field is weaker when further away from the wire

When the magnitude of the current flowing through the wire is increased, more magnetic field lines will be formed indicating a stronger field

When current is reversed, the direction of the magnetic field will also increase

Right hand grip rule

Thumb = direction of current

Fingers curl = direction of the magnetic field

Magnetic current of a flat coil

Magnetic field due to current in a flat coil is stronger inside the coil. Because fields from each part of the wire are in the same direction and are confined to a small area

Magnetic field of a solenoid

Magnetic field can be increased by, Increasing the magnitude of the current, Increasing the number of turns around the solenoid, Inserting a soft iron core into the solenoid

Fleming's left hand grip rule

Can be applied to all moving charges

Forces between two parallel, current-carrying conductors

Conductors with currents flowing in the same direction attract

Conductors with currents flowing in the opposite directions repel

D.C motor

Turning effects can be increased by...

Static Electricity


Induction, Production of electric charge on the surface of a conductor due to the influence of an electric field, Can be done over and over again, Aim is to get either a positive or negative charge, Earthling process, Earthling causes electrons to flow from body to earth or earth to body, Charging by induction

Rubbing, Transferring charges from one object to another


Negative charge, Amber rubbed with fur, Rubber rubbed with fur, Polythene rubbed with wool

Positive charge, Glass rubbed with silk, Perspex rubbed with wool

Charge does not flow in electrical insulations

Electrical conductore allow charge to flow throught them.Metals are good electrical conductors



Laser printer

Drawing Field Lines

Line begins from positive charge and ends on negative charge

Lines drawn leaving a positive charge or ending on a negative charge are proportional to the magnitude of the charge


van de Graff generator


Not necessarily due to friction but by contact

Measured in Coulomb (C)

Region in which and electric charge experiences a charge, Experience weaker charges when placed further apart

Law of electrostatics, Like charges repel, Unlike charges attract

Positive,negative and neutral charges, Positive Charge, Electric field lines point outwards, Positively charged if there are more positive charges than negative charges on the object, Negative Charge, Electric field lines point inwards, Negatively charged if there are more negative charges than positive charges on the object, The object is neutral if there are equal numbers of positive and negative charges on the object, Strength, Nearer, Stronger, Furthur, Weaker

Current of Electricity


Rate of flow of Charge

Q= It Charge = Current × time

SI unit is coulomb per second (C/s) or ampere (A)

Conventional Current, Direction a positive charge will flow, Protons move from + to -

Motion of Electrons, Direction a negative charge will flow, Electrons move from - to +

Measured using an ammeter, Must be connected in series to the circuit, Measure the same current throughout as the same current flows through the simple circuit

Electromotive Force (e.m.f)

The work done by the electrical source in driving a unit charge around a complete circuit

Electromotive force= work done/charge

SI unit is joule per coulomb (J/C) or volt (V)

Series, Connection gives an increased e.m.f because charges gain potential energy from all the cells they pass through., eg. 1.5+1.5+1.5=4.5

Parallel, Charges flowing around the circuit splits into 3 equal portions, Gain in potential energy for any charge in each portion is only from one of the three cells, Advantages, The cells last longer before going flat, the cells are able to supply a larger current

Potential difference

Work done to drive a unit charge through the component

Potential difference=Work done/charge

SI unit is joule per coulomb (J/C) or volt (V)

Measured by a voltmeter, Connected in parallel to the component, Total energy gained =total energy lost.

Sum of all the e.m.f.s of the cells must be equal to the sum of potential differences across all the components in the circuit


The ratio of the potential difference across the component to the current flowing through it

R = V/I

SI unit is Ω

Factors affecting resistance, Length, A long wire has more resistance than a short wire, Area, A thin wire has more resistance than a thick wire, Temperature, Conductors, The hotter the wire the higher the resistance, Semi-conductors, The higher the temperature the lower the resistance, R=pl/A, p= resistivity, l= length, A= area

Ohm's Law, Current through a metal conductor is directly proportional to the potential difference across the ends of the conductor provided that the physical condition and temperature remain constany, V/I= constant


Formulas, I =I1=I2=I3, Current at every point in the circuit is the same, V= V1+V2+V3, The component with the largest resistance has the highest potential difference across it, R= R1 + R2 + R3

Advantages, Higher voltage

Disadvantages, The current will cease to flow if there is a break in the circuit, Smaller battery life


Formulas, I= I1 + I2 + I3, V= V1=V2=V3, (1/R) = (1/R1) + (1/R2) + (1/R3), Component with the smallest resistance has the highest current flowing through it

Advantages, Any breaks in the circuit does not affect the current flow in the other branches of the circuit, Longer battery life

Disadvantages, Lower voltage


Variable Resistor, V1=R1/(R1+R2)X V, Potential divider circuit, Cannot reduce current and potential difference to zero, Potentiometer, Can reduce current and potential difference to zero

Transducers, Output transducer, Converts electrical energy into other forms of energy, Input transducer, Changes non-electrical energy into electrical energy, Thermistors, Resistance and voltage decreases when temperature rises, Light-dependent resistors, Resistance and voltage decreases when light increases

Cathode-ray Oscilloscope

Uses, Measure a.c and d.c voltages, Displaying wave forms, Measures time and frequency, speed of sound

axis, y-axis, voltage, x-axis, time


Law of Magnetism

Like Poles Repel, Unlike Poles Attract


Found in magnetic materials, Metals

Not found in non-magnetic materials, Plastics, Fabrics, Wood

Induced magnetism

eg. a single bar magnet can attract a iron nail hence becoming a magnet. In turn attracting another nail. When the bar magnet is removed,Both the nails fall off due to the lost of their induced magnetism.

Magnetic Materials

May Undergo Magnetisaton, Stroking, Take one permenant magnet and stroke a magnetic material lenghtwise. Stroke from end to end over 20 continuous strokes, End of the magnetised bar where the strokes end has the opposite polarity of the end of which the stroking magnets is in contact with, Using Direct Current, A steel bar is inserted into a solenoid and a direct current is made to flow through the solenoid. A strong magnetic current is produced by the solenoid leading to the steel bar becoming magnetised., When the current is switched off, the steel bar remains magnetised

May Undergo Demagnetisation, Heating, magnet must lie in an east-west direction, Hammering, magnet must lie in an east-west direction and heated till red-hot then cooled

Magnets, Permanent Magnet, Steel, Hard magnetic material., Hard to magnetise but also hard to lose its magnetism, Uses, Loudspeakers, Compasses, Magnetic door catches, Refrigerator door seals, Temporary Magnet, Iron, Soft magnetic material, Easy to magnetise but loses its magnetism easily, Electromagnets, How to increase the magnetic field strength, Increasing the current that flows through the coil, Increasing the number of coils around the soft iron, Addition of a core made out of soft magnetic materials, Made from coils of insulated copper wire coiled around a soft magnetic material (iron) with current flowing through, Magnetic only when a current is flowing through the wires., Uses, Sorting out scrap metals, Audio and Video tapes, Credit, debit and ATM cards, Televisions


Compass, Discovered by the ancient Chinese in 2000BC, A magnet, 2 Poles, North pole, South pole, Similar to a bar magnet

Bar magnet(1)

with..., Bar magnet (2), North pole of 1 attracts south pole of 2, North pole of 1 repels north pole of 2, soft iron rod (magnetic material) (2), North pole of 2 can be attracted by either poles of 1, a non-magnetic material (wood) (2), 1 and 2 remain stationary

Strength, Peak strength is not at the poles but is around 0.5cm away from the poles.

Field Lines

Drawing magnetic field lines, Things required, Pencil, Paper, Compass, Bar magnet, how to..., Put the bar magnet on the sheet of paper, and trace out its shape, Make a dot at the edge of a pole, Put the end of the compass on top of the dot and make a dot at where the needle points., Repeat Steps 1-3 till the dots reach either the end of the paper or the other pole, Draw a line to connect the dots. Yay you've got your first field line :), When finished, pick another spot near the magnet and repeat steps 1-5 for more field lines

Properties, Lines start from the north pole and end at either the south pole or simply away from the north pole, The lines cannot overlap or cross each other, The lines are closer to each other in a stronger magnetic field

Practical electricity

Heating Elements

Nichrome, High resistivity, High melting points

Calculations and Formulas

Calculation of Electrical energy, Enrgy = Charge x potential difference E = QV, E = VIt, E= I²RT, E = (V²/R)t, Measuring Electricity consumption, Energy released = Power x time

Calculation of electrical power, Power = Energy released/Time taken P = E/t, P = VI = I²R = V²/R, SI unit is the watt 1 watt (W) = 1 joule per second (J/s), 1 milliwatt (mW) = 1/1000 W, 1 kilowatt (kW) = 1000W, 1 megawatt (MW) = 1 000 000 W


Damaged insulation, Exposed live wires may cause an electric shock if someone touches it, May also cause a short circuit, Which may eventually lead to a fire

Overheating of cables, Thin wires = higher resistance = more heat, Use of multi-way adaptors, If used at the same time, total current drawn through the cables from the main supply may be too large that the cable becomes overloaded and overheated. This may result in a fire, Weight of the many plugs on an adaptor may pull it partly out of the socket, revealing live prongs which may result in a bad connection or an electric shock if someone touches it., Damp conditions, Dry skin = Higher resistance approx.100 000 ohms, Wet skin = low resistance approx a few hundred ohms


Fuses, Placed on the live wire, Melts and breaks when the current exceeds a certain value causing an open circuit, Protects the appliance, Protects us, Must take a current which is slightly larger than the maximum current that is allowed to pass through, Fuses will not melt prematurely

Switches, Placed on the live wire, Used as a safety device to cut off the flow of the current when someone accidentally touches a live wire

Earth wire, Joined to the metal case of an appliance. If the live wire accidentally touches the case, the current will flow to the earth and blow the fuse in the live wire. The case is prevented from being live and the person avoids getting an electric shock

Double insulation, Casing is made of plasic, Electrical parts can never come into contact with the outer casing of the device.

Three pin plug

Live wire = brown

Neutral wire = blue

Earth wire = yellow and green stripes, Longest pin., Open safety shutters that cover the slots for the earth and neutral pins, Ensures that the appliance is properly earthed before a current flows through