310102b Current Voltage and Resistance Mar 2025 (TF) (2)

Instrument Technician Overview

• Course Code: 310102b

• Main Topics:

  • Current

  • Voltage

  • Resistance

  • Ohm's Law

Objectives

• The student will be able to:

  • Describe an electrical current.

  • Describe voltage.

  • Describe resistance and state and apply Ohm’s Law.

Three Basic Properties of an Electrical Circuit

• Current:

  • Definition: Flow of electrons through a complete conductive path.

• Voltage:

  • Definition: Potential energy difference (force, pressure, EMF, V) between two points.

• Resistance:

  • Definition: Opposition to electron flow.

Law of Electrical Charges

• States that similar charges repel each other and dissimilar charges attract each other.

• Known as Coulomb’s Law.

Quantity of Charge

• An electron:

  • Extremely small particle with a negative charge.

• Coulomb:

  • Unit of charge representing a large number of electrons, specifically:

    • 1 Coulomb ≈ 6.24 x 10^18 electrons.

Standard Concepts about Charge

• Symbol: Q

• Unit of Measurement: Coulomb (C)

Standard Concepts about Current

• Definition: The quantity of charge flowing past a point within a specific amount of time.

• Symbol for Current: I

• Unit of Measurement: Ampere (A)

Current Measurement

• An Ampere is defined as:

  • The flow of one coulomb of charge past a point in a conductor in one second.

• Equation:

  • I = Q/t

  • This formula is not found in the formula sheet.

Current Flow Descriptions

• Conventional Flow (Hole Flow):

  • Describes the flow of charge from positive to negative source.

• Electron Flow:

  • Movement of charge from negative to positive source, reflecting actual electron movement.

Measuring Current

• Procedure:

  • Red lead (positive) and black lead (negative).

  • Selector switch must be set to dcA scale.

  • Connect leads in series with the load (LIGHT).

  • Circuit must be de-energized during connection, then energized for measurement.

Voltage or Potential Difference

• Voltage Description:

  • An imbalance of electrons or potential energy difference between two points.

  • Creation of voltage requires energy; moving electrons from valence shells creates charge difference.

• Charge Collection:

  • Positively charged ions at one terminal and negatively charged at the other terminal.

Standard Rules about Voltage

1. Exists between two points.

2. Measured across a device.

3. A measurement of charge difference between two points.

4. An electromotive force (emf).

5. Source Voltage Symbol: E

6. Load Voltage Symbol: V (also the IR drop).

7. Unit of Measurement: Volt (V).

Methods of Producing an EMF

• Magnetic: Movement of a conductor in a magnetic field.

• Chemical: Reaction between two or more substances.

• Friction: Rubbing of two materials (e.g., Van de Graf Generator).

• Heat: Application of heat to dissimilar metals.

• Pressure: Application of pressure to certain crystals.

• Light: Application of light to elements.

Measuring Voltage

• Procedure:

  • Red lead (positive) and black lead (negative).

  • Selector switch must be set to dcV scale.

  • Connection in parallel with the load (LIGHT).

  • Circuit must be energized for measurement.

Resistance

• Definition:

  • Opposition to current flow due to material composition; current flow generates heat.

  • All materials have some resistance; low resistance (conductors) vs. high resistance (insulators).

Standard Concepts about Resistance

• Symbol: R

• Unit of Measurement: Ohm, represented by the Greek letter Ω (Omega).

• Example: If a device has a resistance of 40 ohms, represent as R = 40 Ω.

Measuring Resistance

• Procedure:

  • Red lead (positive) and black lead (negative).

  • Selector switch must be set to Ω scale.

  • Connection in parallel with the load (LIGHT).

  • Circuit must be de-energized throughout measurement.

Electrical Components of a Circuit

1. A voltage source (potential difference, emf).

2. A conductive path.

3. Resistance (the load).

Ohm’s Law

• Formula: I = E/R

• Relationship between:

  • Current (I)

  • Voltage (E)

  • Resistance (R)

Ohm's Law Insights

• An increase in voltage (emf) increases the force pushing more current (current increases).

• An increase in resistance implies more opposition against current (current decreases).

Ohm’s Law Summary

• Current in a circuit is directly proportional to the voltage applied and inversely proportional to resistance:

  • Formula: I = E/R, E = I x R.