Voltage, Current and Resistance

Voltage

Voltage is the electric potential difference between two points in an electrical circuit.

It provides the electrical push that causes free electrons to move through a conductor.

The greater the potential difference between two points, the greater the voltage.

Voltage is measured using an instrument called a voltmeter.

Symbol

V

SI Unit

Volt (V)

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Current

Electric current is the rate of flow of electric charge through a conductor.

In metallic conductors, electric current is associated with the movement of free electrons.

The greater the amount of charge passing through a conductor per unit time, the greater the current.

Electric current is measured in amperes.

Symbol

I

SI Unit

Ampere (A)

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Formula of Electric Current

Electric current can be calculated using:

I = Q / t

Where:

• I = Electric Current
• Q = Electric Charge
• t = Time

Since charge is measured in coulombs and time in seconds:

Ampere = Coulomb / Second

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1 Ampere Current

A current of 1 ampere flows through a conductor when 1 coulomb of electric charge passes through it in 1 second.

Therefore:

1 A = 1 C / 1 s

Current can be compared with the flow of water through a pipe.

A larger quantity of charge passing per second means a larger electric current.

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Resistance

Resistance is the opposition offered by a material to the flow of electric current.

A material with high resistance makes it more difficult for current to flow, while a material with low resistance allows current to flow more easily.

Symbol

R

SI Unit

Ohm (Ω)

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Formula of Resistance

Resistance can be calculated using:

R = V / I

Where:

• R = Resistance
• V = Potential Difference or Voltage
• I = Current

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One Ohm Resistance

If:

V = 1 Volt
I = 1 Ampere

then:

R = V / I

Therefore:

R = 1 / 1

Hence:

1 Ohm = 1 Volt / 1 Ampere

A conductor has a resistance of 1 ohm when a potential difference of 1 volt produces a current of 1 ampere through it.

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Relationship Between Voltage, Current and Resistance

The basic relationship is:

V = I × R

From this relation:

I = V / R

and:

R = V / I

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Movement of Electrons in a Conductor

Conductors contain free electrons that can move through the material.

Without an applied electric field, these electrons move randomly.

When a conductor is connected to a source such as a battery, an electric field is established inside the conductor.

This electric field causes the free electrons to develop an overall movement in a particular direction.

This directed movement contributes to electric current.

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Drift Velocity

Drift velocity is the average velocity acquired by free charge carriers in a conductor due to an applied electric field.

Electrons in a conductor continuously undergo random motion and collide with atoms or ions of the material.

When an electric field is applied:

1. Electrons are accelerated by the electric field.
2. They collide with particles inside the conductor.
3. Energy may be transferred during these collisions.
4. The electrons develop a small overall drift.

This overall motion is called drift velocity.

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Heating Due to Electron Collisions

As electrons move through a conductor, they collide with atoms and ions.

During these collisions, some electrical energy is converted into heat energy.

This is why a conductor may become warm when current flows through it.

This effect is important in devices such as:

• Electric Heaters
• Toasters
• Heating Coils

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Sources of Electricity

Electricity can be generated using different sources.

The major sources discussed in this chapter are:

1. Hydropower
2. Nuclear Energy
3. Wind Energy
4. Thermal Energy
5. Solar Energy

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1. Hydropower

Hydropower or hydroelectric power uses the energy of moving or falling water to generate electricity.

Many hydroelectric power plants use dams to store water.

The basic process is:

Stored Water
     ↓
Potential Energy
     ↓
Moving Water
     ↓
Kinetic Energy
     ↓
Turbine
     ↓
Generator
     ↓
Electricity

Water stored at a height possesses potential energy.

When released:

1. Water flows downward.
2. Potential energy changes into kinetic energy.
3. Moving water rotates a turbine.
4. The turbine drives a generator.
5. The generator produces electrical energy.

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2. Nuclear Fission and Fusion

Nuclear Fission

Nuclear fission is a process in which a heavy atomic nucleus splits into smaller nuclei and releases energy.

Nuclear power plants use controlled fission reactions to produce heat.

That heat can be used to produce steam and ultimately generate electricity.

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Nuclear Fusion

Nuclear fusion is a process in which lighter atomic nuclei combine and release energy.

The source discusses fusion power as the idea of using energy released from fusion reactions for electricity generation.

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3. Wind Energy

Wind energy uses the kinetic energy of moving air to generate electricity.

A wind turbine converts wind energy into electrical energy.

The process can be represented as:

Wind
  ↓
Turbine Blades
  ↓
Rotating Shaft
  ↓
Generator
  ↓
Electricity

When wind strikes the turbine blades:

1. The blades rotate.
2. The connected shaft rotates.
3. The generator is driven.
4. Electrical energy is produced.

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4. Thermal Power

In a thermal power plant, heat is used to generate electricity.

Steam is commonly used to rotate a turbine.

The general process is:

Heat
 ↓
Water
 ↓
Steam
 ↓
Turbine
 ↓
Generator
 ↓
Electricity

The rotating turbine drives an electrical generator.

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5. Solar Energy

Solar electricity is produced by converting sunlight into electrical energy.

Solar panels contain photovoltaic (PV) cells.

When sunlight falls on these cells, electrical energy is produced.

The basic process is:

Sunlight
   ↓
PV Cells
   ↓
DC Electricity
   ↓
Inverter
   ↓
AC Electricity

Solar panels initially generate Direct Current (DC) electricity.

An inverter can convert DC into Alternating Current (AC) for use in homes and businesses.

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Conventional Direction of Electric Current

The conventional direction of current is considered to be from the positive terminal to the negative terminal of a power source.

Therefore:

Conventional Current:
Positive (+) → Negative (-)

In metallic conductors, electrons themselves move in the opposite direction:

Electron Flow:
Negative (-) → Positive (+)

The conventional direction of current continues to be used in electrical engineering and circuit analysis.

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Electrical Resistance and Its Unit

Electrical resistance measures how strongly a material opposes electric current.

The greater the resistance, the smaller the current for a given applied voltage.

The unit of electrical resistance is:

Ohm

Symbol:

Ω

The relationship between resistance, voltage, and current is:

R = V / I

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Uses and Applications of Resistance in a Circuit

Resistors are important components in electrical and electronic circuits.

They are used for:

1. Current Control
2. LED Protection
3. Transistor Circuits
4. Heating
5. Timing Circuits
6. Voltage Division

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1. Use of Resistor to Control Current

A resistor is used to limit or regulate electric current in a circuit.

It can prevent excessive current from flowing through sensitive components.

Resistors can also be used to produce specific voltage drops.

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2. Resistor in LED Circuits

An LED can be damaged if too much current passes through it.

A resistor connected in series with an LED limits the current.

Example Arrangement

Power Supply → Resistor → LED

The resistor helps keep the current within a safe operating range.

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3. Resistor in Transistor Circuits

Resistors are used in transistor circuits to establish suitable operating conditions.

They may help control:

• Current
• Voltage
• Biasing
• Operating Point

This helps the transistor operate correctly.

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4. Resistor for Heating

When current passes through a resistor, electrical energy can be converted into heat.

This is called the heating effect of electric current or Joule heating.

This principle is used in devices such as:

• Electric Heaters
• Toasters
• Heating Elements

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5. Resistor in Timing Circuits

Resistors can be used together with capacitors in timing circuits.

In an RC circuit, resistance affects how quickly a capacitor charges or discharges.

This principle is used in circuits that require:

• Time Delays
• Oscillations
• Timing Control

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6. Resistor for Dividing Voltage

Two or more resistors can be connected to form a voltage divider.

A voltage divider produces a fraction of the input voltage.

It may be used for:

• Reference Voltages
• Measurement Circuits
• Biasing
• Reducing Voltage Levels

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Classification of Materials Based on Electrical Resistance

Materials can be classified according to their resistance and ability to conduct electricity.

They are:

1. Conductors
2. Semiconductors
3. Insulators

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1. Conductors

Conductors are materials that allow electric current to flow easily.

They contain a large number of free charge carriers.

Conductors have low electrical resistance.

Examples

• Copper
• Aluminium
• Silver
• Gold

Conductors are commonly used in:

• Electrical Wiring
• Electronic Circuits
• Power Transmission

Copper is widely used for electrical wiring because it is a good conductor.

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2. Semiconductors

Semiconductors are materials whose electrical conductivity lies between that of conductors and insulators.

Their conductivity can be changed by:

• Adding impurities
• Changing temperature
• Applying light or other external conditions

This process of adding controlled impurities is called doping.

Common semiconductor materials include:

• Silicon
• Germanium
• Gallium Arsenide

Semiconductors are used in:

• Diodes
• Transistors
• Integrated Circuits
• Computer Chips
• Solar Cells

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Types of Doped Semiconductor

The source refers to two basic types:

N-Type Semiconductor

An n-type semiconductor has additional free electrons available as charge carriers.

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P-Type Semiconductor

A p-type semiconductor contains holes that behave as positive charge carriers.

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3. Insulators

Insulators are materials that strongly resist the flow of electric current.

They contain very few freely moving charge carriers.

Therefore, insulators have high electrical resistance.

Examples

• Rubber
• Glass
• Plastic
• Ceramic

Insulators are used for:

• Electrical Safety
• Wire Coating
• Separating Conductors
• Preventing Short Circuits

Rubber and plastic are commonly used to cover electrical wires.

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Difference Between Conductor, Semiconductor and Insulator

Conductor	Semiconductor	Insulator
Allows current to flow easily.	Conductivity lies between conductor and insulator.	Strongly opposes current flow.
Has very low resistance.	Has intermediate resistance.	Has very high resistance.
Has many free charge carriers.	Charge carriers can be controlled.	Has very few free charge carriers.
Example: Copper.	Example: Silicon.	Example: Rubber.
Used for wiring.	Used in electronic devices.	Used for electrical insulation.

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Factors Affecting Resistance

The resistance of a conductor depends on several factors.

The major factors are:

1. Length of the Conductor
2. Cross-Sectional Area
3. Temperature
4. Nature of the Material

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1. Length of the Conductor

Resistance is directly proportional to the length of a conductor.

R ∝ L

Where:

• R = Resistance
• L = Length

This means:

Longer conductor → Greater resistance

A longer conductor causes charge carriers to experience more collisions while moving through the material.

Therefore, resistance increases as length increases.

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2. Cross-Sectional Area of the Conductor

Resistance is inversely proportional to the cross-sectional area of a conductor.

R ∝ 1 / A

Where:

• R = Resistance
• A = Cross-Sectional Area

Therefore:

Larger area → Lower resistance

and:

Smaller area → Higher resistance

A thicker wire provides more space for charge carriers to move.

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3. Temperature of the Conductor

For most metallic conductors, resistance increases when temperature increases.

Temperature ↑ → Resistance ↑

As temperature rises:

1. Atoms vibrate more strongly.
2. Electrons experience more collisions.
3. Current flow becomes more difficult.
4. Resistance increases.

The source also notes that some materials, particularly semiconductors, can behave differently.

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4. Nature of the Conducting Material

Different materials have different electrical resistance properties.

This property is described by resistivity.

Symbol:

ρ

Materials with low resistivity allow current to flow more easily.

Examples include:

• Copper
• Silver

Materials with higher resistivity oppose electric current more strongly.

Examples mentioned in the source include:

• Nichrome
• Constantan

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Relationship Between Resistance, Length, Area and Resistivity

The relationships in the chapter can be summarized as:

R ∝ L

and:

R ∝ 1 / A

Combining them:

R ∝ L / A

Including resistivity:

R = ρL / A

Where:

• R = Resistance
• ρ = Resistivity
• L = Length of Conductor
• A = Cross-Sectional Area

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Quick Revision

Voltage

Voltage is electric potential difference.

Unit

Volt (V)

Instrument

Voltmeter

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Current

Current is the rate of flow of electric charge.

Formula

I = Q / t

Unit

Ampere (A)

---

1 Ampere

1 A = 1 C / 1 s

---

Resistance

Resistance is opposition to the flow of electric current.

Formula

R = V / I

Unit

Ohm (Ω)

---

Drift Velocity

Drift velocity is the average directed velocity of charge carriers caused by an applied electric field.

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Sources of Electricity

• Hydropower
• Nuclear Energy
• Wind Energy
• Thermal Energy
• Solar Energy

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Conventional Current

Positive → Negative

Electron flow in a metallic conductor is opposite:

Negative → Positive

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Uses of Resistors

• Limiting Current
• Protecting LEDs
• Transistor Circuits
• Heating
• Timing Circuits
• Voltage Division

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Materials Based on Resistance

• Conductor
• Semiconductor
• Insulator

---

Factors Affecting Resistance

• Length
• Cross-Sectional Area
• Temperature
• Nature of Material

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Important Formulae

Current

I = Q / t

Resistance

R = V / I

Voltage

V = I × R

Current from Voltage and Resistance

I = V / R

Resistance and Length

R ∝ L

Resistance and Area

R ∝ 1 / A

Resistance of a Conductor

R = ρL / A

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Important Exam Points

• Voltage is the potential difference between two points.
• Voltage is measured using a voltmeter.
• The SI unit of voltage is volt.
• Electric current is the rate of flow of electric charge.
• Current is measured in amperes.
• I = Q/t is the formula for current.
• One ampere means one coulomb of charge passes in one second.
• Resistance is opposition to electric current.
• Resistance is measured in ohms.
• R = V/I gives resistance.
• Drift velocity refers to the average directed motion of charge carriers under an electric field.
• Hydropower uses moving water to generate electricity.
• Wind turbines convert wind energy into electrical energy.
• Thermal power plants use heat and steam.
• Solar panels use photovoltaic cells.
• Conventional current flows from positive to negative.
• Electron flow in metallic conductors is from negative to positive.
• Resistors are used to limit current.
• Resistors protect LEDs from excessive current.
• Resistors can be used in timing and voltage-divider circuits.
• Conductors have low resistance.
• Semiconductors have conductivity between conductors and insulators.
• Insulators have high resistance.
• Resistance increases with conductor length.
• Resistance decreases when cross-sectional area increases.
• Resistance of most metallic conductors increases with temperature.
• Resistance depends on the nature and resistivity of the material.