Electricity and Magnetism Overview

Overview of Electricity and Magnetism

• The unit is divided into electricity and magnetism.
• The syllabus changed in 2023, removing and adding certain topics.
• Digital Electronics, including digital/analog circuits and logic gates, has been removed from the O Level physics syllabus.
• The concept of digital and analog as Information Systems has been moved to the waves unit, concerning communication.
• A new unit of energy, the kilowatt hour, has been added to measure electrical energy consumption.
• Extended content is beneficial for core students to enhance understanding.
• Today's study session covers static electricity, current electricity, magnetism basics, electromagnets/coils, and electromagnetic applications (motor, generator, transformer).

Static Electricity

• Static electricity involves non-moving charges.
• Charges move to create positively or negatively charged objects.
• Charge is a property of matter that experiences force near other charges or within an electric field.
• Atoms contain protons (positive), electrons (negative), and neutrons (neutral).
• Neutral objects have equal amounts of positive and negative charges.
• Charge is measured in Coulombs.
• Like charges repel each other, while opposite charges attract. Neutral objects are attracted to both positive and negative objects.

Electric Fields

• An electric field is a region where charges experience a force.
• Electric field lines indicate the force direction on a positive particle, pointing away from positive charges and toward negative charges.
• Field strength is indicated by the spacing of field lines; closer lines mean a stronger field.
• When positive and negative charged objects are placed together, their electric fields combine.
• A uniform field has consistent spacing between field lines, resulting in constant field strength. This is achieved with two flat plates, one positive and one negative.

Charging Objects

• Insulators can be charged by rubbing them together, causing electron transfer through friction
• The object that loses electrons becomes positively charged, while the object that gains electrons becomes negatively charged.
• Conductors can be charged by induction by bringing a charged rod near a metal ball.
• The charge distribution in the ball changes, separating positive and negative charges
• Grounding the ball allows electrons to be repelled out, then removing the ground leaves the metal sphere charged.
• Using a negative rod results in a positively charged metal ball and vice versa.

Current Electricity: Circuits and Components

• A circuit delivers energy from a source (battery) to a device (light bulb).
• Closing the switch completes the circuit, allowing positive charges to flow from the positive side of the battery.
• Charges carry energy and are pushed through the circuit by the battery.
• Resistance in the light bulb slows down charges, releasing electrical energy as light and heat.
• Circuit symbols include batteries (long line is positive), switches (always drawn open), and light bulbs.

Voltage

• Voltage (V) measures the energy per charge, with units in volts.
• A 10-volt battery delivers 10 joules of energy per Coulomb.
• Potential difference is voltage across a component (e.g., light bulb), with units in Volts.
• EMF (electromotive force) is voltage of a battery/power supply, and is the energy gained by a charge.
• EMF drives the charge through each component, while potential difference is energy lost through a conductor.

Current

• Current (I) measures the rate of charge flow, or how many charges go through a circuit per second.
• Measured in Amperes, so a current of 2 amperes means 2 charges go through the circuit every second.
• Conventional current flows from positive to negative, while electrons move from negative to positive.

Resistance

• Resistance (R), measured in Ohms, opposes the flow of charges.
• Atoms inside a conductor vibrate, colliding with electrons and causing them to lose energy.
• Good conductors have low resistance.
• Higher resistance results in lower current, and lower resistance yields higher current.

Measuring Voltage and Current

• Voltmeters measure voltage and are connected in parallel to a component.
• Ammeters measure current and are placed in series in a circuit.

Ohm's Law

• Ohm's Law is expressed by the equation V = IR, where V is voltage, I is current, and R is resistance.
• Rearranging this formula into R = V/I or I = V/R is crucial for solving electricity problems.
• For Ohm's Law to be accurate, the resistance and the temperature of the resistor must remain constant.
• If temperature increases, atoms vibrate faster, hindering electron flow and increasing resistance.

Current and Resistance

• With a voltage of 10 volts and resistance, the current (speed of charge) is constant throughout the circuit (e.g., 0.02) from beginning to end.
• The battery determines the current (I = V/R) based on voltage (V) and resistance (R) and releases it at a fixed speed from the start.
• Increasing resistance (adding a resistor or using a higher resistance lamp) decreases current throughout the wire, not just in the lamp.
• Charges don't speed up, slow down at the resistor, then speed up again; they move at a constant, slower current from the start due to the resistance.
• All charges in the wire move together like a train; if one slows down due to resistance, they all slow down.
• Resistance doesn't initially affect voltage; the choice of resistance (lamp) and voltage (battery) are independent, but both impact the current.
• Wires have resistance, dependent on both length and cross-sectional area, even copper wires.

Wire Resistance Factors

• Resistance is directly proportional to wire length: doubling the length doubles resistance.
• Resistance is inversely proportional to wire's cross-sectional area: doubling the area halves the resistance.
• Material also affects resistance, but is outside the scope of the topic

Power, Energy and the Kilowatt Hour

• Power is energy over time, but in electricity, it's calculated as voltage (V) times current (I): P = VI.
• Energy is Power times time: E = Pt = VIt.
• Electric fan example: at 230 volts and 0.4 amperes, power = 92 watts.
• Energy consumed by the fan in one minute: 92 watts * 60 seconds = 5520 joules.
• Two alternative formulas for power: P = I²R and P = V²/R (derived from P=VI and V=IR).
• Use P = VI and V = IR, if familiar, otherwise memorize the power formulas.
• Always convert time to seconds when calculating energy in joules, which is an SI unit (kg, meters, seconds).
• Kilowatt hour is a unit of energy used in billing, equivalent to 1000 watts of power consumed in one hour.
• One kilowatt hour equals 3,600,000 joules.
• Calculating electrical bill: multiply daily kilowatt-hour consumption by the number of days in the month, then by the cost per kilowatt hour.

Introduction to Resistors

• Three types of resistors exist: fixed, variable, and thermistors.
• Fixed resistors have a fixed resistance.
• Variable resistors' resistance changes based on the length of the resistor wire in the circuit.
• Thermistors' resistance changes based on temperature.

Thermistors

• The hotter the thermistor, the lower the resistance, and vice versa, opposite to fixed resistors.
• Thermistors are made of non-metallic conductors; when heated, they gain more free-moving electrons and conduct electricity better.
• Thermistor's resistance and temperature share an inversely proportional relationship.

Light Dependent Resistors (LDRs)

• LDRs are light-dependent resistors: resistance decreases when light increases and vice versa.
• Important to realize the LDR doesn't emit light it receives light.
• Light Dependent Resistors don't emit light.

Diodes

• The main function of a diode is to convert an AC voltage to a DC voltage
• Allows current to flow only in one direction; acts as a one-way gate.
• A diode allows current flow in the direction of the arrow, but blocks it in the opposite direction.
• When a diode allows current to flow, it's forward biased; when it blocks current, it's reverse biased.
• A direct current (DC) from a battery flows in one direction with a constant value.
• An alternating current (AC) from an electrical socket alternates direction.

AC to DC Conversion using Diodes

• A single diode allows only part of the AC current/voltage to pass.
• Four diodes connected can completely convert AC voltage to DC.
• In a bridge rectifier setup of 4 diodes, the output across the resistor is always DC.

IV Graphs

• Current increases constantly as voltage increases for fixed resistors.
• As voltage increases across a light bulb, the current increases at first, but then increases less and less.
• As a light bulb gets brighter (due to increased voltage), its temperature/resistance increases, causing less current increase.

Series Circuits

• Total resistance increases when resistors are connected in series: Rtotal = R1 + R2 + R3.
• If a 5-volt supply is connected to two 10-ohm resistors in series, the current is 0.25 ampere (I = V/R = 5/20).
• Current is the same throughout a series circuit.
• The voltage of is divided across resistors is split according to the ratio of the resistors
• In a series circuit with two 10-ohm resistors, the 5-volt supply is split equally and the voltage will be 2.5 volts

Unequal Resistors in Series

• If the two series resistors have unequal values (e.g., 5 ohms and 15 ohms), the total resistance remains 20 ohms.
• The current remains the same (0.25 ampere) if the voltage is the same and the total resistance the same.
• With a 0.25 ampere current through a 5-ohm resistor, the voltage across it is 1.25 volts (V = IR = 0.25 * 5).
• The voltage across the 15-ohm resistor will be 3.75 volts, which is found by subtracting 1.25 (5 ohm) from 5 (Vtotal).
• In a series connection, the larger resistor gets a larger portion of the voltage, and the smaller resistor gets a smaller portion.

Parallel Circuits

• Total resistance decreases in parallel.
• For resistors in parallel, one uses the formula: 1/Rtotal = 1/R1 + 1/R2, or the product over the sum Rtotal = (R1 * R2) / (R1 + R2)

Parallel Resistors

• The combined resistance of resistors in parallel decreases.
• When two 10 ohm resistors are connected in parallel, the combined resistance is 5 ohms.
• Total resistance calculation: 100/20 = 5 ohms

Calculating Total Current

• Total current is calculated using I = V/R.
• For a 5-volt supply and 5 ohms resistance, the total current is 1 ampere (5V / 5 ohms = 1A).
• At a junction, current splits.
• With equal resistors in parallel, current splits equally.
• For example, 1 ampere splits into 0.5 ampere through each 10 ohm resistor.

Voltage in Parallel Circuits

• In parallel circuits, each component receives 100% of the supply voltage.
• If the supply voltage is 5 volts, each resistor in parallel gets 5 volts.

Individual Current Calculation

• Individual current (I1) is calculated by V/R1.
• With 5 volts and a 10 ohm resistor, I1 is 0.5 ampere (5V / 10 ohms = 0.5A).
• If resistors are not equal, the current does not split equally with current calculated using I = V/R for each resistor.

Battery Behavior

• The battery behaves as if it is facing a 5 ohm resistance.
• The battery doubles the current due to the split.
• Increased current in a circuit indicates decreased resistance, assuming constant voltage.

Current-Voltage (IV) Graph

• In an IV graph, current will plateau but not decrease with increasing voltage under normal circumstances.
• Decreasing current with increasing voltage is not typical IV behavior.

Advantages of Parallel Connections

• Components can be switched on and off separately.
• If one component breaks, the others are unaffected.
• All components receive the full voltage.
• Household electricity is wired in parallel to ensure appliances receive full voltage (220-240 volts).

Light Bulbs and Voltage

• Light bulbs connected in parallel receive the full voltage of the power supply.
• Series connections lead to decreased voltage per bulb as more bulbs are added, eventually preventing them from lighting up.
• Parallel connections ensure each bulb receives the same, full voltage regardless of how many are added.

Potential Dividers

• Potential dividers consist of two resistors in series used to split the voltage.
• In a 10-volt supply with two equal resistors in series, each resistor receives 5 volts.
• Adding a light bulb in parallel to only one resistor in a potential divider means the light bulb receives the same voltage as the resistor it's parallel to.
• Potential dividers allow for voltage control across a component, unlike variable resistors which control current.
• A thermistor or LDR can be used as one of the resistors to make the light bulb sensitive to heat or light.

Variable Potential Dividers

• Variable potential dividers have both ends connected to the battery, supplying the entire wire with the full voltage.
• The position of the sliding contact determines the voltage across a component.
• A sliding contact in the middle results in half the voltage, while at the beginning yields zero volts, and at the end, the full voltage.
• Variable potential dividers control voltage across a component, whereas variable resistors control current in a circuit.

Voltage Distribution

• The voltage across a light bulb connected in parallel to a resistor in a potential divider is equal to the voltage across that resistor.
• Instead of physically replacing fixed resistors, a sliding contact can be used to easily control the voltage.
• Dimmer switches use variable potential dividers to control voltage, not current.

Electrical Safety at Home

• Mains electricity uses live and neutral wires in sockets.
• The live wire carries 240 volts AC, while the neutral wire is at zero volts.
• Current flows in through the live wire and out through the neutral wire in a closed circuit.

Fuses

• Fuses protect circuits from current overloads.
• A fuse contains a thin wire with a specific ampere rating.
• If the current exceeds the fuse's rating, the wire melts, stopping the current.
• Replacing a blown fuse is easy, protecting devices from getting burnt out.

Circuit Breakers

• Circuit breakers protect homes from current overloads using an electromagnet.
• When the current is too high, the electromagnet attracts and opens a switch, stopping the current flow.
• Unlike fuses, circuit breakers can be reset by flipping the switch back up.

Earth Wire

• The earth wire connects the metal case of a device to the ground.
• If a live wire touches the case, the current flows through the earth wire, cutting off the electricity and preventing electric shock.
• Earth wires redirect current away from metal cases.

Magnets and Magnetism

• Magnets have North and South Poles that attract or repel each other.
• Opposite poles attract (North and South), while like poles repel (South and South, North and North).
• Magnets attract certain metals like iron and steel through magnetization.

Magnetization

• Magnetization is when Iron and Steel become temporarily or permanantly magnetized
• Iron becomes a soft, temporary magnet when magnetized; steel becomes a hard, permanent magnet.

Magnetic Fields

• A magnetic field is the region around a magnet where other magnets experience a force.
• Magnetic field lines are drawn from the North Pole to the South Pole.
• Closer field lines indicate a stronger field, while lines farther apart indicate a weaker field.
• A uniform field is created when field lines are parallel, such as between a North and South Pole.

Visualizing Magnetic Fields

• Iron filings can be used to visualize a magnetic field, showing its shape and strength.
• A compass can be used to determine the direction of a magnetic field by marking points along the compass needle's direction.

Magnetic Fields and Electric Current

• Electric current produces a magnetic field around a wire.
• The magnetic field around a wire is circular.
• The direction of the magnetic field is determined by the right-hand grip rule, where the thumb indicates the direction of the current and the fingers indicate the direction of the field.

Electromagnets

• An electromagnet is formed when a wire is coiled, creating a magnetic field similar to that of a bar magnet.
• The strength of an electromagnet can be controlled by adjusting the current or the number of coils.
• Inserting an iron core inside the coil increases the strength of the electromagnet.

Direction of Electromagnet Field

• For electromagnets, the right-hand grip rule can determine the poles, with the fingers indicating the current and the thumb pointing to the North Pole.

Electromagnet Applications: Relays

• Relays use a low voltage circuit or current to turn on a high voltage circuit.
• The low voltage circuit passes through a coil, which magnetizes and attracts an iron switch.
• The iron switch closes, allowing the high voltage circuit to turn on.

Electromagnet Applications: Speakers

• Speakers use electromagnets to produce sound.
• A coil around a magnet is connected to a cone.
• When current passes through the coil, it becomes magnetized, repelling or attracting the magnet, and moving the cone back and forth which creates sound.
• The alternating current from a music signal causes the coil to vibrate the cone.

Magnetizing Steel

• Steel can be magnetized by stroking it with a magnet in one direction.
• Other method for magnetizing steel involves hammering the magnet while aligned with the Earth's magnetic field.
• The best method to magnetize steel involves placing a steel bar inside a coil connected to a DC supply.

Demagnetizing Steel

• Steel can be demagnetized by heating it to high temperatures.
• Hammering it while holding the magnet east to west as well as placing it inside a coil connected to an AC supply can demagnetize steel.

Force on a Current-Carrying Conductor

• Placing a current-carrying wire between the poles of a magnet causes the wire to move due to the interaction of the magnetic fields.
• The force exerted on the wire depends on the current, number of loops, and strength of the magnet

Fleming's Left-Hand Rule

• Fleming's left-hand rule is used to determine the direction of the force.
• The index finger points in direction of magnetic field
• The middle finger points in direction of the current
• The thumb then indicates the direction of the force.
• Reversing the current reverses the direction of the force.

DC Motor

• A DC motor consists of a coil placed in a magnetic field, a battery, a split ring, and carbon brushes.
• Current flowing through the coil experiences a force, causing it to rotate.
• Increasing the number of turns, increasing the current, or using stronger magnets increases rotation speed.
• Carbon brushes conduct electricity from the battery to the coil without breaking the circuit.
• Split ring reverses the current every half cycle to keep the coil rotating.

Electromagnetic Induction (Generator Effect)

• Moving a copper wire up and down through a magnetic field generates a current.
• The changing magnetic field forces charges inside the wire to move generating electric current.
• Electromagnetic induction generates a voltage when a wire cuts through the magnet field

Increasing Voltage Generation

• Voltage generation can be increased by moving the wire or by increasing the number of Loops / Coils.
• Using a stronger magnet can increase the voltage generation.
• Key is that the cutting must happen, so it is important to cut in the coil.

Lenz's Law

• Lenz's Law is the generation of current to oppose change causing it.
• Implies that If trying to move the magnet towards the coil, current generated so that side becomes North.
• Magnet away from coil will change so the side is south.

Angle of Cutting Field

• The best effect occurs at perpendicular angles.
• When wire is moving perpendicular to the field generates maximum voltage.
• When wire is parallel to the field generates no voltage.
• Moving the coil around in a circle makes an alternating current.

Generator

• The generator is just like the motor, except there is no battery.
• In these devices instead humans or external forces rotate/spin the device
• When this happens, if the coil rotates, the magnetic field is cut which generates electricity.

Transformer

• Transforms electricity to voltage, everywhere.
• Transforms consist of three parts: Primary coil , Secondary coil and Iron Core.
• Put AC voltage through primary coil , current goes thru coil and produces magnetic field then the field goes through the iron core and then cuts through the secondary coil.
• Electromagnet induction generates electricity or voltage.
• Can control with numbers of terms of the coil.
• This transformer is called a Step Up transformer, since it increases the voltage.
• A step down transformer decreases the voltage
• VP over VS = NP / NS