Electricity (2)

Question 1

Capacitors

Answer 1

Capacitors:

• Store energy in circuits by storing electric charge, creating electric potential energy.
• Consist of two conductive plates separated by a dielectric, preventing charge flow.

===

Parallel Plate Capacitor:

• Q: Charge stored on the plates.
• V: Potential difference across the plates.
• One plate holds +Q, the other –Q, with a potential difference V between them.

Question 2

Capacitance

Answer 2

Capacitance:

• Defined as the charge stored per unit potential difference.
• Units: Farads (F), often in smaller units like μF, nF, or pF.

===

Capacitance Equation:

• C = Q / V where:
  
  • C = capacitance.
  • Q = charge stored.
  • V = potential difference.

===

Key Points:

• Charge stored refers to the magnitude of charge on each plate or surface of a spherical conductor.
• Higher capacitance means the capacitor can store more charge for the same potential difference.

Question 3

Use of capacitors

Answer 3

• Energy storage: Capacitors store electric potential energy for various applications.
• Camera flashes: Provide a bright flash of light during discharge.
• Smoothing currents: Stabilize current in electronic circuits.
• Backup power: Supply power during unexpected outages for memory devices like calculators.
• Timing circuits: Used in electronic timers for precise operations.

Question 4

Charging capacitor

Answer 4

1. Initial Setup:
   
   • A circuit with a battery (e.m.f. ε), resistor (R), capacitor (C), and switch connected in series.
2. Switch Closed:
   
   • Electrons flow from the negative terminal of the battery, through the resistor, to the negative plate of the capacitor.
3. Plate Charging:
   
   • The positive terminal pulls electrons from one plate, leaving it positively charged.
   • The negative terminal pushes electrons onto the other plate, making it negatively charged.
4. Insulator Role:
   
   • The insulator between the plates prevents charge flow, forcing charge to accumulate.
5. Electrostatic Repulsion:
   
   • As negative charge builds, it repels incoming electrons, slowing the flow of charge.
6. Current and Voltage:
   
   • Current decreases exponentially over time.
   • Potential difference (V) across the plates increases as charge accumulates, eventually matching the supply voltage.
7. Fully Charged:
   
   • The capacitor stops charging when it reaches maximum charge, determined by its capacitance (C) and the supply voltage.

Question 5

Discharging Capacitors

Answer 5

1. Initial Setup:
   
   • A circuit with a resistor (R), switch, and capacitor (C) in series. No power supply is present.
2. Switch Closed:
   
   • The potential difference (V) across the capacitor causes a current (I) to flow through the circuit.
3. Current Flow:
   
   • Electrons flow from the negative plate of the capacitor, through the resistor, to the positive plate, reducing charge on both plates.
4. Exponential Decay:
   
   • Current, potential difference, and charge decrease exponentially over time.
   • The rate of decrease is proportional to the amount remaining.
5. Discharge Completion:
   
   • The capacitor is fully discharged when potential difference (V) and current (I) fall to zero.
6. Energy Dissipation:
   
   • The electrical energy stored in the capacitor is transferred to thermal energy in the resistor.
7. Graphs:
   
   • Current, potential difference, and charge follow an identical exponential decay pattern over time.

Question 6

Energy Stored by a Capacitor

Answer 6

Charge and Potential Difference:

• Charge (Q) is directly proportional to the potential difference (V), forming a straight-line graph.

===

Energy Stored:

• The electrical energy stored in the capacitor is represented by the area under the potential-charge graph, forming a triangle.

Question 7

Capacitors in Series

Answer 7

1. Charge and Potential Difference:
   
   • The potential difference (p.d.) is shared between capacitors, but each stores the same charge (Q).
   • A negative charge on the left plate of C₁ induces an equal positive charge on its right plate.
   • This causes a negative charge on the left plate of C₂, equal to the positive charge on its right plate.
2. Total Potential Difference:
   
   • V_total = V₁ + V₂
   • Substituting V = Q/C
     V_total = (Q/C₁) + (Q/C₂)
3. Total Capacitance:
   
   • Since current (and charge Q) is the same in series, Q cancels out:
     1/C_total = (1/C₁) + (1/C₂)

Question 8

Capacitors in Parallel

Answer 8

1. Potential Difference:
   
   • All capacitors have the same potential difference (p.d.) across them.
2. Charge Distribution:
   
   • The current splits across each junction, so the charge stored on each capacitor is different.
   • The total charge (Q) is the sum of the charges on each capacitor:
     Q = Q₁ + Q₂.
3. Charge on Each Capacitor:
   Q₁ = C₁V and Q₂ = C₂ V, where V is the common p.d.
   
   • Therefore, Q_total = (C₁ + C₂) V
4. Total Capacitance:
   
   • C_total = C₁ + C₂ + C₃ + ….

Question 9

Time Constant

Answer 9

Time to Half (t_1/2):

• The time it takes for the charge, current, or voltage of a discharging capacitor to decrease to half its initial value.
• Equation: t_1/2 = ln(2) τ

===

Time Constant (τ):

• Measures how long it takes for the charge, current, or voltage of a discharging capacitor to decrease to 37% of its original value, or for a charging capacitor to rise to 63% of its maximum value.
• Equation: τ = RC where:
  
  • R = resistance (Ω).
  • C = capacitance (F).

Question 10

How to verify if potential difference (V) or charge (Q) on a capacitor decreases exponentially?

Answer 10

Constant ratio method:

• Plot a V-T graph and check if the time constant is constant,
• when t = τ the potential difference on the capacitor will have decreased to approximately 37% of its original value

===

Logarithmic graph method

• Plot a graph of ln(V )against time (t) and check if a straight-line graph is obtained.

Question 11

Charging and Discharging graphs

Answer 11

Charging Graphs:

1. p.d. and Charge vs. Time:
   
   • Both graphs have identical shapes, starting at 0 and increasing to a maximum value.
2. Current vs. Time:
   
   • An exponential decay curve, starting at 0 and decreasing exponentially.

===

Discharge Graphs:

1. Current, p.d., and Charge vs. Time:
   
   • All graphs show exponential decay curves with decreasing gradients.
   • Initial values (I₀, V₀, Q₀) start on the y-axis and decrease exponentially.
2. Rate of Discharge:
   
   • High resistance: Slower discharge, current decreases, capacitor discharges slowly.
   • Low resistance: Faster discharge, current increases, capacitor discharges quickly.

Question 12

Electric field

Answer 12

A region where a unit charge experiences an electrostatic force.

Question 13

Electric Field Lines In a uniform electric field

Answer 13

Uniform Electric Field:

• Field lines are equally spaced at all points.
• Electric field strength is constant at all points.
• The force on a test charge has the same magnitude and direction everywhere.

===

Electric Field Between Parallel Plates:

• When a potential difference is applied, the plates become charged.
• The electric field between the plates is uniform.
• The field beyond the edges is non-uniform.
• Field lines are directed from the positive to the negative plate.
• A uniform electric field has equally spaced field lines.

Question 14

Electric Field Lines In a radial electric field:

Answer 14

Radial Electric Field:

• Field lines are equally spaced near the charge but spread out with distance.
• Electric field strength and force on a test charge decrease with distance from the charge.

===

Around a Point Charge:

• The field is radial, with lines:
  
  • Radially inwards for negative charges.
  • Radially outwards for positive charges.

===

Field Strength:

• The field is stronger where lines are closer together and weaker where lines are further apart.

Question 15

Electric Field Strength

Answer 15

• Electrostatic force per unit positive charge
• acting on a charge at a specific point
• or on a stationary point charge

===

• Describes how strong or weak an electric field is

Question 16

Coulomb’s Law

Answer 16

Electric Fields and Coulomb’s Law:

• All charged particles produce an electric field, exerting a force on other charges within range.

===

Coulomb’s Law:

• The force between two charges is:
  
  • Proportional to the product of their charges (Qq).
  • Inversely proportional to the square of their separation (r²).

===

Charge Interactions:

• Like charges (Qq > 0) Repulsion.
• Opposite charges (Qq < 0) Attraction.

Question 17

Electric Field strength of a Point Charge

Answer 17

Radial Field

• Charged sphere acts as a point charge
• Follows an inverse square law (1/r²)

===

Direction

• Towards a negative charge
• Away from a positive charge.

Question 18

Electric Field Strength in a Uniform Field

Answer 18

Electric Field Strength in a Uniform Field:

• Equation: E = V/d
  where:
  
  • E = electric field strength.
  • V = potential difference.
  • d = distance between plates.

===

Key Points:

• Greater voltage (V): Results in a stronger field (E).
• Greater separation (d): Results in a weaker field (E).
• Does not apply to a radial field around a point charge.
• Field direction: From the positive to the negative plate.

===

Derivation:

1. Work done on a charge ( Q ):
   W = ΔV × Q
2. Work done as force × distance:
   W = F × d
3. Equating the two:
   F × d = ΔV × Q.
4. Rearranging:
   F/Q = ΔV/d.
5. Since E = F/Q
   E = ΔV/d.

Question 19

Relative permittivity

Answer 19

Permittivity:

• Measures how easy it is to generate an electric field in a material.

===

Relative permittivity (εr) (dielectric constant)

• ratio of permittivity of a material to permittivity of free space (ε0):
• εr = ε/ε0.
• Dimensionless because it’s a ratio of two quantities with the same unit.

Question 20

Effect of Dielectric on Capacitance

Answer 20

Dielectric in a Capacitor:

1. Polar Molecules:
   
   • Align with the applied electric field, creating an opposing electric field.
   • This reduces the overall electric field, lowering the potential difference between the plates.
2. Permittivity:
   
   • Reflects how well polar molecules align with the field; higher alignment = higher permittivity.
3. Parallel Plate Capacitor:
   
   • Plates of area A, separated by distance d, with a dielectric of permittivity ε between them.
   • The reduction in potential difference increases the capacitance of the plates.

Question 21

Motion of Charged Particles in an Electric Field

Answer 21

Charged Particles in an Electric Field:

1. Force and Motion:
   
   • Charged particles experience a force, causing them to move.
   • In a uniform electric field, particles move parallel to the field lines (direction depends on charge).
2. Perpendicular Motion:
   
   • A particle moving perpendicular to the field follows a parabolic trajectory due to the constant force.
3. Deflection:
   
   • Positive charges: Deflect towards the negative plate.
   • Negative charges: Deflect towards the positive plate.
   • Deflection depends on mass, charge, and speed:
     
     • Heavier particles deflect less.
     • Larger charges and slower particles deflect more.

===

Formulas:

• Force: F = EQ
• Work Done: W = Fd
• Kinetic Energy: The force increases the particle’s kinetic energy, causing constant acceleration (Newton’s second law).
• Perpendicular Velocity: If the particle’s velocity has a component perpendicular to the field, it remains unchanged (Newton’s first law).

Question 22

Electric Potential

Answer 22

Electric Potential:

• Defined as the work done per unit positive charge to bring a test charge from infinity to a defined point.
• A scalar quantity (no direction) but can be positive, negative, or zero:
  
  • Positive around an isolated positive charge.
  • Negative around an isolated negative charge.
  • Zero at infinity.

===

Total Electric Potential:

• The total potential at a point from multiple charges is the sum of the potentials from each individual charge.

Question 23

The graph of potential V against distance r for a negative or positive charge

Answer 23

Electric Potential for a Positive Charge:

• As distance (r) decreases, electric potential (V) increases.
• More work is required to overcome the repulsive force as the test charge moves closer.
• V starts positive and increases as r decreases, approaching zero as r approaches infinity.

===

Electric Potential for a Negative Charge:

• As distance (r) decreases, electric potential (V) decreases (becomes more negative).
• Less work is required due to the attractive force, which pulls the test charge closer.
• V starts negative and decreases (in magnitude) as r decreases, approaching zero as r increases towards infinity.

Question 24

Electric Potential Energy

Answer 24

Work in an Uniform Electric Field:

• Work is done when a charge moves through an electric field.
  
  • Positive charge: Work is done when it moves against the field.
  • Negative charge: Work is done when it moves with the field.

Key Points

• The work done equals the change in electric potential energy.
• When the electric potential is zero, the electric potential energy is also zero.
• The work done depends on the distance the charge moves in the field.
• q = charge being moved; Q = charge producing the potential. Do not confuse the two in calculations.

===

Work in a Radial Field:

• Work is required to:
  
  • Move a positive charge closer to another positive charge (overcoming repulsion).
  • Move a positive charge away from a negative charge (overcoming attraction).

Key points

• Potential energy increases when moving a charge towards a repelling charge and decreases when moving away from an attracting charge.

Question 25

Force-Distance Graph for a Point Charge

Answer 25

**Force-Distance Graph**: - **Force (F)** values are all **positive**. - As **r** increases, **F** follows a **1/r² relation** (inverse square law). - The **area under the graph** represents **work done (ΔW)**. - The graph shows a **steep decline** as **r** increases. === **Estimating Area**: - Use methods like **counting squares** or **summing areas of trapeziums**.

Question 26

Electric field between two point charges

Answer 26

**Opposite Charges**: - **Field lines** are directed from the **positive** charge to the **negative** charge. - As the charges get closer, the **attractive force** becomes **stronger**. === **Same Type Charges**: - **Field lines** are directed **away** from two positive charges or **towards** two negative charges. - As the charges get closer, the **repulsive force** becomes **stronger**. - A **neutral point** exists at the midpoint where the resultant electric force is **zero**.

Question 27

Capacitance of an Isolated Sphere

Answer 27

**Capacitance of a Charged Sphere**: - Defined as the **charge per unit potential** at the surface: **C = Q / V** === **Key Equations**: 1. **Potential of an Isolated Point Charge**: **V = Q / (4πε₀R)** 2. **Capacitance of an Isolated Sphere**: **C = 4πε₀R** === **Variables**: - **Q** = charge on the sphere (considered as a point charge at its center). - **R** = radius of the sphere. - **ε₀** = permittivity of free space.

Question 28

Electric Fields vs Gravitational Fields

Answer 28

**Similarities Between Gravitational and Electrostatic Forces**: 1. Both follow the **inverse square law**. 2. **Field lines** around a **point mass** and a **negative point charge** are identical. 3. **Field lines** in **uniform gravitational** and **electric fields** are identical. 4. **Field strengths** in a **radial field** have a **1/r** relationship. 5. **Potential** in both fields has a **1/r** relationship. 6. **Equipotential surfaces** are: - **Spherical** around a **point mass** or **charge**. - **Parallel** in uniform fields. 7. **Work done** is the product of: - **Mass** and change in **gravitational potential**. - **Charge** and change in **electric potential**. === **Differences**: 1. **Gravitational force** acts on **mass**; **electrostatic force** acts on **charge**. 2. **Gravitational force** is always **attractive**; **electrostatic force** can be **attractive** or **repulsive**. 3. **Gravitational potential** is always **negative**; **electric potential** can be **negative** or **positive**.

Question 29

Magnetic Fields

Answer 29

**Magnetic Field**: - A **field of force** created by **moving electric charges** or **permanent magnets**, also called a **B-field**. === **Sources of Magnetic Fields**: - **Permanent magnets** produce magnetic fields. - **Current-carrying wires** create magnetic fields due to the movement of **electrons** (stationary charges do not produce magnetic fields). === **Observing Magnetic Fields**: - Although **invisible**, their effects can be observed through: - The **force** acting on magnetic materials (e.g., **iron**). - The movement of a needle in a **plotting compass**.

Question 30

Field Lines in a Current-Carrying Wire

Answer 30

**Magnetic Field Around a Current-Carrying Wire**: - **Field lines** are **circular rings** centered on the wire. - The field is **strongest near the wire** and weakens with distance. - **Reversing the current** reverses the direction of the field lines. === **Maxwell’s Right-Hand Screw Rule**: - Point your **thumb** in the direction of the **conventional current** (positive to negative). - The **curled fingers** indicate the direction of the magnetic field around the wire.

Question 31

Magnetic Field Lines in Solenoids and Coils

Answer 31

**Field Lines in a Solenoid**: - **Electromagnets** use solenoids (coils of wire) to **concentrate magnetic fields**. - One end becomes the **north pole**, and the other becomes the **south pole**. - **Magnetic field lines** resemble those of a **bar magnet**: - **Emerge** from the **north pole**. - **Return** to the **south pole**. === **Field Lines in a Flat Circular Coil**: - Behaves like a single loop of a solenoid. - **Field lines**: - **Emerge** from one side (north pole). - **Return** to the other side (south pole). - **Multiple coils** in a solenoid combine to create a **stronger, uniform field**. === **Right-hand thumb rule**: - Thumb shows the magnetic field direction. - Fingers show the current direction.

Question 32

Factors Affecting the Magnetic Field Strength

Answer 32

1. **Add a Ferrous Core**: - Use a core made from a **ferrous (iron-rich) material** (e.g., an iron rod). - When current flows, the core becomes **magnetised**, increasing the field strength by **several hundred times**. 2. **Add More Turns to the Coil**: - Concentrates the **magnetic field lines**, increasing the field strength.

Question 33

Fleming's Left-Hand Rule

Answer 33

**Fleming’s Left-Hand Rule**: - Determines the direction of **magnetic force** on a moving charged particle in a magnetic field: - **First Finger**: Direction of the **magnetic field**. - **Second Finger**: Direction of **conventional current** (velocity of a moving positive charge). - **Thumb**: Direction of the **magnetic force**. === **Magnetic Field Direction in 3D**: - **Dots** (tip of an arrow): Magnetic field coming **out of the page**. - **Crosses** (back of an arrow): Magnetic field going **into the page**.

Question 34

Magnetic Flux Density

Answer 34

- Defined as the **force acting per unit current per unit length** on a current-carrying conductor placed **perpendicular** to the magnetic field. - **Units**: Tesla (T), where 1 T = **1 N m⁻¹** force on a conductor carrying **1 A** current normal to the field. - Also referred to as **magnetic field strength**.

Question 35

Force on a Current-Carrying Conductor

Answer 35

**Magnetic Force on a Current-Carrying Conductor**: - A **current-carrying conductor** produces its own magnetic field. - An **external magnetic field** exerts a **magnetic force** on the conductor. - The **maximum force** occurs when the current is **perpendicular** to the magnetic flux lines. === **Magnitude of Magnetic Force (F)**: - Proportional to: - **Current (I)**. - **Magnetic flux density (B)**. - **Length of conductor in the field (L)**. - **Sine of the angle (θ)** between the conductor and the magnetic flux lines. - **No force** is experienced if the current is **parallel** to the magnetic field.

Question 36

Force on a Moving Charge

Answer 36

**Magnetic Force on a Moving Charged Particle**: - **Equation**: F = BQv, where: - F = magnetic force (N). - B = magnetic flux density (T). - Q = charge of the particle (C). - v = speed of the particle (m/s). - This is the **maximum force** when F, B, and v are **mutually perpendicular**. - If the particle travels **parallel** to the magnetic field, it **does not experience a magnetic force**. === **Force at an Angle θ**: - When the particle moves at an **angle θ** to the magnetic field lines: **F = BQv sin θ**

Question 37

Motion of a Charged Particle in a Magnetic Field

Answer 37

**Circular Motion of a Charged Particle in a Magnetic Field**: - A **charged particle** in a uniform **magnetic field** perpendicular to its motion travels in a **circular path** because: - The **magnetic force (F)** is always **perpendicular** to its velocity (**v**), causing circular motion. - The **magnetic force** always points towards the **center** of the circular path. === **Centripetal Force**: - Provides the force required for circular motion: **mv² / r = BQv** - Rearranging for the radius **r**: **r = mv / BQ** === **Key Points**: - **Faster particles (v)**: Move in **larger circles** **r ∝ v** - **Greater mass (m)**: Move in **larger circles** **r ∝ m** - **Greater charge (q)**: Move in **smaller circles** **r ∝ 1 / q** - **Stronger magnetic field (B)**: Move in **smaller circles** **r ∝ 1 / B** --- ### **Centripetal Acceleration**: - Calculated using **Newton’s second law**: F = ma

Question 38

Charged Particles in a Velocity Selector

Answer 38

**Velocity Selector**: - Filters **charged particles** by using **perpendicular electric and magnetic fields** to allow only particles with a **specific velocity** to pass through. - Used in devices like **mass spectrometers** to create a beam of particles moving at the **same speed**. === **Setup**: - **Two oppositely charged plates** create an **electric field (E)**. - A **magnetic field (B)** is applied perpendicular to the electric field. === **Force Balance**: - **Electric force**: F_E = EQ (independent of velocity). - **Magnetic force**: F_B = BQv (depends on velocity). - For a particle to pass through **undeflected**, the forces must balance: **F_E = F_B**. - The selected velocity is: **v = E / B**. === **Deflection**: - Particles with velocities **different from v** are **deflected** and removed from the beam.

Question 39

Magnetic Flux

Answer 39

**Magnetic Flux (Φ)**: - Defined as the product of **magnetic flux density (B)** and the **cross-sectional area (A)** perpendicular to the magnetic field: Φ = B A - **Units**: Webers (Wb). === **Key Points**: - **Maximum flux**: Occurs when the magnetic field lines are **perpendicular** to the area. - **Minimum flux**: Occurs when the magnetic field lines are **parallel** to the area. - Represents the amount of **magnetic field** passing through a given area.

Question 40

Magnetic Flux Linkage

Answer 40

**Magnetic Flux Linkage**: - Commonly used for solenoids with **N turns** of wire. - Defined as the product of **magnetic flux (Φ)** and the **number of turns (N)** in the coil: ΦN = Φ × N = B × A × N** where: - B = magnetic flux density. - A = cross-sectional area of the coil. - N = number of turns in the coil. - **Units**: Weber turns (Wb turns). === **General Equation**: -**ΦN = B × A × N × cos(θ)** where θ is the angle between the magnetic field lines and the **normal** to the coil.

Question 41

Faraday's Law

Answer 41

- The magnitude of the **induced e.m.f.** (electromotive force) is directly **proportional to the rate of change of magnetic flux linkage**. - The equation form of Faraday's Law is: **ε = Δ(Nɸ) / Δt** Where: - **ε** = induced e.m.f (V) - **Δ(Nɸ)** = change in flux linkage (Wb turns) - **Δt** = time interval (s)

Question 42

Lenz's Law

Answer 42

**Lenz’s Law**: - The **induced e.m.f.** produces effects that **oppose the change causing it**. === **Example**: - If a **north pole approaches the coil**, the **induced e.m.f.** creates an **opposing north pole** to repel the incoming magnet. === **Right-Hand Grip Rule**: - Curl fingers around the coil in the direction of the **current**. - The thumb points along the direction of the **magnetic flux** (north to south).

Question 43

Induced E.m.f.

Answer 43

**Faraday’s Law with Lenz’s Law**: The equation that combines **Faraday's Law** and **Lenz's Law** is written as: **ε = - Δ(Nɸ) / Δt** - The **negative sign** represents **Lenz’s Law**.

Question 44

EMF Inducted in a Rotating Coil

Answer 44

**Induced E.M.F. in a Rotating Coil**: - When a **coil** rotates in a **uniform magnetic field**, the **magnetic flux** through the coil changes, causing the **induced e.m.f.** to change. - The **e.m.f.** is: - **Maximum** when the coil cuts through the most **magnetic field lines** (normal to the coil is **perpendicular** to the field). - **Zero** when the coil is aligned with the field (normal to the coil is **parallel** to the field). === **E.M.F. Formula**: - **ε = BANω sin(θ)** (not in formula sheet) - where **θ = ωt** - The **e.m.f. varies sinusoidally** and is **90° out of phase** with the flux linkage.

Question 45

Alternators

Answer 45

**Alternator**: - A device that converts **mechanical energy** into **electrical energy** using a rotating coil in a magnetic field. === **Components**: 1. **Coil**: A **rectangular coil** rotates within a **uniform magnetic field**, generating electricity. 2. **Slip Rings**: Metal rings that rotate with the coil, maintaining **continuous electrical contact**. 3. **Brushes**: Metal brushes press against the slip rings to transfer current to the **external circuit**. === **Operation**: - As the coil rotates, it cuts through **magnetic field lines**, inducing a **potential difference (voltage)** across the coil. - The induced current changes direction as the coil spins, creating an **alternating current (AC)**. - The **meter pointer** deflects first in one direction, then the opposite, as the current reverses. - The induced voltage and current **alternate direction** continuously, producing a steady **AC waveform**.

Question 46

Dynamos

Answer 46

**Dynamo**: - A **direct-current (D.C.) generator** that uses a **split-ring commutator** instead of slip rings. - Consists of a **rotating coil** in a **magnetic field**. === **Operation**: - The **split-ring commutator** ensures the current stays in **one direction**. - The **induced potential difference** varies only in the **positive region** of the graph, never reversing direction. - The **current** is always **positive** (or negative), never alternating. === **Key Process**: - As the coil rotates, it **cuts through magnetic field lines**, inducing a **potential difference** between the coil’s ends. - The **split-ring commutator** changes the coil's connection to the **brushes** every half turn, keeping the current in the **same direction**. - This change occurs when the coil is **perpendicular** to the magnetic field lines.

Question 47

Transformer Basics

Answer 47

**Transformer**: - Changes **high alternating voltage at low current** to **low alternating voltage at high current**, and vice versa. - Increases **transmission efficiency** by reducing **heat energy loss** in power lines. === **Power Loss**: - Given by **P = I²R**, so reducing **current** reduces **power loss** during transmission. === **Applications**: - Used in the **National Grid** for efficient power transmission. - **Step-up transformers**: Increase voltage and decrease current for **long-distance transmission**. - **Step-down transformers**: Reduce voltage and increase current for **local use** near homes and businesses.

Question 48

Transformer Components and Functioning

Answer 48

**Step-Up Transformer**: - The **secondary coil** has more turns than the **primary coil**, increasing voltage. === **Operation**: - The **primary coil** is powered by an **alternating current (AC)**, creating a **changing magnetic field** in the **iron core**. - The **changing magnetic field** induces an **e.m.f.** in the **secondary coil**. - The **secondary voltage** depends on the number of turns: - More turns = **step-up** (voltage increases). - Fewer turns = **step-down** (voltage decreases). === **Components**: - **Primary coil**, **secondary coil**, and **soft iron core**. - The **soft iron core** focuses and directs the magnetic field between the coils, and is used because it can be easily **magnetised and demagnetised**.

Question 49

Eddy Currents

Answer 49

**Transformer Efficiency**: - Transformers are **not 100% efficient**, and **power loss** occurs due to various factors. === **Eddy Currents**: - **Looping currents** in the core caused by the **changing magnetic flux**. - Effects: - **Generate heat**, leading to **energy loss**. - Create a **magnetic field** that opposes the inducing field, reducing **field strength**. === **Reducing Eddy Currents**: - The core is **laminated**, with layers separated by thin insulating material to prevent current flow.