Electricity and Magnetism 1

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Quiz 2:
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Fundamentals of Magnets

Magnets are objects that have two poles - a north (N) pole and a south (S) pole. Every magnet always has these two poles, and cannot have only one pole (Ruoho & Arkkio, 2008). Properties of Magnets:

• Like poles repel each other
• Opposite poles attract each other
• The strength of the magnetic force decreases as the distance increases

Magnetic Field

The magnetic field is the region around a magnet where the magnetic force can still be felt. The strength of the magnetic field is shown by the magnetic field lines:

• The closer the field lines, the stronger the magnetic field
• The direction of the magnetic field is always from the north pole to the south pole

Types of Magnetic Materials

Magnetic Materials:

• Examples are iron, steel, nickel, and cobalt
• Can be attracted by magnets
• Can be magnetized

Non-magnetic Materials:

• Examples are aluminum and wood
• Cannot be attracted by magnets
• Cannot be magnetized

Permanent Magnets vs. Temporary Magnets

Permanent Magnets:

• Made of steel
• Retain their magnetism for a long time
• Difficult to magnetize but also difficult to demagnetize

Temporary Magnets:

• Made of soft iron
• Easily lose their magnetism
• Easy to magnetize but also easy to demagnetize

Magnetic Induction

When a magnetic material (such as an iron nail) is brought near or in contact with a magnet, the material also becomes a magnet. This phenomenon is called magnetic induction. The magnetic induction property in iron is temporary, while in steel it is permanent (Ruoho & Arkkio, 2008).

Magnetization and Demagnetization

Ways to Magnetize:

• Placing a ferromagnetic material in a solenoid (coil of wire) carrying direct current (DC)
• Striking/tapping the magnetic material while in a magnetic field
• Stroking the magnetic material repeatedly with a permanent magnet in one direction
• The stronger the current in the solenoid, the stronger the magnetic field

Ways to Demagnetize:

• Placing the magnet in a solenoid carrying alternating current (AC) that is gradually reduced
• Dropping or heating the magnet
• Striking/tapping the magnet without a magnetic field around it

Electromagnets

Definition: Electromagnets are magnets made by winding wire around a soft iron core and passing electric current through the wire. Their magnetism is temporary - they can be turned on and off. Ways to Strengthen Electromagnets:

• Increase the electric current flowing
• Increase the number of wire turns
• Bring the two poles closer together (like in a C-shaped electromagnet)

Applications of Magnets

Permanent Magnets:

• Compass
• Computer hard drives
• Electric motors
• Electric generators
• Microphones and speakers
• Credit/debit cards
• Do not require electric current to maintain their magnetism

Electromagnets:

• Scrap metal lifting cranes
• Electric bells
• Magnetic locks
• Relays
• Electric motors and generators
• Can adjust the strength of the magnetic field
• Can be turned on/off as needed

Earth's Magnetic Field

• The Earth has a natural magnetic field
• The direction of the Earth's magnetic field does not exactly point to the geographic north-south
• The difference between the magnetic north and geographic north is called declination

Static Electricity

Static electricity occurs when electrons (negatively charged particles) transfer from one object to another through friction. A simple example is when we:

• Comb our hair
• Walk on synthetic carpets
• Remove nylon clothing

Two Types of Electric Charge

Positive and Negative Charge

• Like charges repel each other (positive with positive, negative with negative)
• Opposite charges attract each other (positive with negative)

How Static Electricity Forms

The process of static electricity formation can be explained with a simple example:

1. When we rub a plastic ruler with a cloth
2. Electrons from one object transfer to the other
3. The object that loses electrons becomes positively charged
4. The object that gains electrons becomes negatively charged

Examples in Nature

Lightning is the most spectacular example of static electricity in nature. When a negatively charged cloud meets a positively charged ground, a lightning bolt occurs.

Conductors and Insulators

• Conductors: Materials that can allow electricity to flow (like metals), electric charge can move freely
• Insulators: Materials that cannot allow electricity to flow (like plastic), electric charge will remain stationary

Atomic Structure

Atoms consist of:

• A nucleus containing protons (positive charge)
• Electrons (negative charge) orbiting the nucleus
• In a neutral state, the number of protons and electrons are equal

The simplest example is a hydrogen atom, which has 1 proton in the nucleus and 1 electron orbiting it.

Electric Charge Unit

Electric charge is measured in the unit of Coulomb (C). One electron has a charge of 1.6 × 10−19 C.

Conductors vs. Insulators

Main Difference:

• Insulators: All electrons are tightly bound to their atoms. Examples:
  • Plastic (polythene)
  • Perspex
  • Nylon
  • Dry air
• Conductors: Some electrons can move freely between atoms. Examples:
  • All metals
  • Carbon
  • Water

Semi-Conductors:

• Wood
• Paper
• Cotton
• Human body
• Soil

Electric Field

The electric field is the region around an electric charge that can exert a force on other electric charges. Some properties of the electric field:

• Works without direct contact
• Can be uniform or non-uniform
• Has direction and magnitude (vector quantity)

Dangers of Static Electricity

Lightning Rods:

• Made of thick copper
• Installed on top of tall buildings
• Connects the tip to a metal plate in the ground
• Provides a safe path for lightning to reach the ground

Other Dangers:

• Can trigger explosions near flammable materials
• Can damage sensitive electronic equipment
• Hazardous during refueling

Benefits of Static Electricity

1. Inkjet Printers:

• Ink droplets are given an electric charge
• Pass through positively and negatively charged plates
• Controlled by a computer to produce precise prints

2. Other Applications:

• Cleaning ash from power plant chimneys
• Spray painting
• Crop dusting
• Photocopiers

To avoid dangers, electronic equipment and vehicles during refueling must always be grounded (connected to the earth).

Electric Current

Electric current is the flow of electric charge. Imagine it like water flowing through a pipe, but what's flowing is electric charge through a wire.

Measuring Electric Current

• The unit of electric current is Ampere (A)
• 1 milliampere (mA) = 1/1000 ampere
• Basic formula: Current = Charge ÷ Time
• Or in symbols: I = Q/t
  • I = current (in amperes)
  • Q = charge (in coulombs)
  • t = time (in seconds)

Three Effects of Electric Current

1. Heat and Light

• Example: A light bulb glows because the filament wire is heated by the electric current

2. Magnetism

• Electric current produces a magnetic field around the wire

3. Chemical

• Electric current can cause chemical reactions
• Example: Bubbles of gas formed when current flows through an acid solution

Simple Calculation Examples

Example 1:

• If a current of 2 amperes flows for 20 seconds, the total charge that flows is:
  • Q = I × t
  • Q = 2 × 20 = 40 coulombs

Example 2:

• If a charge of 3 coulombs flows for 7 seconds, the current is:
  • I = Q ÷ t
  • I = 3 ÷ 7 = 0.43 amperes

Important Points to Remember

• Electric current requires a complete circuit to flow
• In metals, it is the free electrons that move
• The actual direction of electron flow is opposite to the conventional current direction

Conventional Current vs. Electron Current

Conventional Current:

• Flows from positive to negative terminal
• This was the agreed convention before electrons were discovered
• Used in electrical circuit diagrams

Electron Current:

• Actually flows from negative to positive terminal
• Opposite to conventional current direction
• This is what actually happens in the wire

Current Measuring Instrument (Ammeter)

How to Use an Ammeter:

• Must be connected in series in the circuit
• Positive terminal of ammeter connected to positive side of power source
• Choose the appropriate scale range for the current to be measured

Types of Ammeters:

1. Analog:
   • Uses a needle pointer
   • Can only measure DC current
   • Has multiple measurement scales
2. Digital (Multimeter):
   • Displays digital readout
   • Can measure AC and DC current
   • Easier to read
   • Can also measure voltage and resistance

Simple Problem Examples

Example 1:

• What is the current if a charge of 10 coulombs flows in 2 seconds?
  • I = Q/t = 10/2 = 5 amperes

Example 2:

• How long will it take for a charge of 5 coulombs to flow if the current is 2 amperes?
  • t = Q/I = 5/2 = 2.5 seconds

Important Tips

• Always start with a large scale when measuring an unknown current
• If the needle/reading is too small, then switch to a smaller scale
• Ensure the ammeter connections are correct to avoid damaging the instrument

Electromotive Force (EMF) and Potential Difference

Electromotive Force (EMF)

• EMF is the work done by a power source to move charge around a complete circuit
• Measured in volts (V)
• Example: A car battery has an EMF of 12 volts, household electricity is 220-240 volts

Potential Difference

• Potential difference is the work done to move a unit of charge through a component
• Also measured in volts (V)
• Often referred to as "voltage"

Electromotive Force and Potential Difference

Electromotive Force (EMF)

Electromotive force (EMF) is the work done by a source of electrical energy to move a unit of electric charge around a complete circuit. It is measured in volts (V).

For example, a car battery has an EMF of 12 volts, and a household electrical system has an EMF of 220-240 volts.

Potential Difference (Voltage)

Potential difference, also known as voltage, is the work done to move a unit of electric charge through a component. It is also measured in volts (V).

Understanding EMF and Potential Difference

Imagine a battery as a water pump that pushes electrons around a circuit. Each electron carries a "bundle of energy" with it. The higher the voltage, the larger the "bundle of energy" carried by each electron.

Important Formulas

1. EMF and Potential Difference:

   V = W/Q
   W = Q × V
   Where:

   • V = voltage (volts)
   • W = energy/work (joules)
   • Q = charge (coulombs)

2. For Constant Current:

   W = I × t × V
   Where:

   • I = current (amperes)
   • t = time (seconds)

Simple Examples

1. If a lamp requires 6 joules of energy to move 2 coulombs of charge:

   Potential difference = 6/2 = 3 volts

2. If a lamp with a 5-volt voltage is supplied with a 2-ampere current for 5 seconds:

   Charge = 2 × 5 = 10 coulombs
   Energy = 10 × 5 = 50 joules

Voltage Effect

A 12V lamp and a 230V lamp can have the same current, but the 230V lamp will be brighter because it transfers more energy per unit of charge.

Voltmeters

A voltmeter is a device used to measure the potential difference (voltage) across a component. It is connected in parallel with the component being measured, and the positive terminal of the voltmeter must be connected to the side where the current is entering.

Types of Voltmeters

1. Analog Voltmeter:
   • Uses a needle pointer
   • Usually has two voltage scales (e.g., 0-5V and 0-10V)
   • Can only measure DC voltage
2. Digital Voltmeter (Multimeter):
   • Displays digital readings
   • More accurate
   • Can measure both AC and DC voltage
   • Has a very high input resistance (10 MΩ)

Reading Analog Voltmeter Scales

For a 0-5V scale:

• Each small division = 0.1 volts
• Can be read with an accuracy of 0.05 volts

For a 0-10V scale:

• Each small division = 0.2 volts
• Lower accuracy than the 0-5V scale

Voltmeter Usage Tips

1. Choose the appropriate voltage scale for the measurement
2. Start with a larger scale if the voltage is unknown
3. Avoid parallax error by reading the meter straight on
4. Use a smaller scale for more accurate measurements

Resistance

Electrical Resistance

Electrical resistance is the measure of the difficulty for electric current to flow through a material. The higher the resistance, the more difficult it is for current to flow. Resistance is measured in ohms (Ω).

Factors Affecting Resistance

1. Material Type:
   • Good conductors have low resistance
   • Poor conductors have high resistance
2. Cable Size:
   • Thick cables have low resistance, suitable for high currents
   • Long cables have high resistance
   • Short cables have low resistance

Basic Resistance Formulas

Using Ohm's law:

• R = V/I (Resistance = Voltage ÷ Current)
• V = I × R (Voltage = Current × Resistance)
• I = V/R (Current = Voltage ÷ Resistance)

Simple Calculation Examples

1. Example 1:
   • Voltage = 4.5 volts
   • Current = 1.5 amperes
   • Resistance = 4.5 ÷ 1.5 = 3 ohms
2. Example 2:
   • Current = 0.5 amperes
   • Resistance = 5 ohms
   • Voltage = 0.5 × 5 = 2.5 volts

Applications in Life

• Electrical Cables: Use low resistance to minimize energy loss
• Heating Elements: Use high resistance to generate heat
• Car Starter: Use thick cables for high current
• Electric Oven: Requires thick cables due to high current

Resistors

Resistors are electrical components designed to have a specific resistance value. They can be made of wire or carbon and have resistance values ranging from a few ohms to millions of ohms.

Variable Resistors

Variable resistors, also called potentiometers, are used to adjust the resistance in a circuit. They can be used in two ways:

1. As a rheostat: to change the current in a circuit
2. As a voltage divider: to change the voltage supplied to a device

Metal Wire Resistance

The resistance of a metal wire is affected by:

1. Length of the wire: Longer wires have higher resistance
2. Cross-sectional area: Larger area has lower resistance
3. Material of the wire: Different metals have different resistances

Important Formulas

• Resistance ∝ length of wire
• Resistance ∝ 1 / (cross-sectional area)

Understanding these concepts allows us to calculate changes in resistance when the dimensions of a wire are modified, which is useful in designing electrical circuits.

Electrical Working

Electrical Power

Power is the rate of energy transfer. It is measured in watts (W).

Formulas:

• P = V × I
• P = I² × R
• P = V²/R

Power Units

• 1 watt = 1 joule per second
• 1 kilowatt (kW) = 1,000 watts
• 1 megawatt (MW) = 1,000,000 watts

Important Formulas

1. Power:
   • P = V × I
   • P = I² × R
   • P = V²/R
2. Energy:
   • E = P × t
   • E = V × I × t

Calculation Examples

1. Example 1: 240V Lamp
   • Current = 0.25 amperes
   • Power = 240 × 0.25 = 60 watts
   • The lamp uses 60 joules of energy per second
2. Example 2: 12Ω Lamp
   • Current = 0.5 amperes
   • Voltage = 12 × 0.5 = 6 volts
   • Power = 6 × 0.5 = 3 watts
   • Energy in 6 seconds = 3 × 6 = 18 joules

Key Points to Remember

• Electrical energy is converted into other forms (heat, light)
• If current doubles, the heat energy increases 4 times
• Power determines the rate of energy usage
• Total energy depends on power and duration of use

Electrical Energy Measurement

Joule Meter: Measures electrical energy directly in joules

Electricity Meter: Measures in kilowatt-hours (kWh), where 1 kWh = 3,600,000 joules

Common Household Appliance Power Ratings

• DVD Player: 20 watts
• Laptop: 50 watts
• Lamps: 60-100 watts
• TV: 100 watts
• Refrigerator: 150 watts
• Iron: 1000 watts (1 kW)
• Space Heater: 1-3 kW
• Kettle: 2 kW
• Water Heater: 3 kW
• Electric Stove: 6.4 kW

Calculating Electricity Costs

Example:

• 3000-watt (3 kW) heater used for 3 hours
• Energy = 3 kW × 3 hours = 9 kWh
• If 1 kWh costs 10 cents
• Total cost = 9 × 10 cents = 90 cents

Important Considerations

• Electric stoves (6.4 kW) require special thick cables due to the high current
• Formula: Current = Power ÷ Voltage
• Example: 6400W ÷ 230V = 28 amperes, too high for regular cables

Understanding these concepts is important for:

• Calculating monthly electricity bills
• Selecting energy-efficient appliances
• Avoiding overloading in electrical installations