CURRENT AND VOLTAGE

Key Vocabulary

  • Current: The rate of flow of electric charge in a circuit.

  • Electricity: The flow of electric charge that transfers energy.

  • Resistance: The opposition to the flow of electric current.

  • Voltage (Potential Difference): The energy given to each charge to move it through a component.

Learning Outcomes

Objectives

  • Describe resistance and how to calculate resistance of a component.

  • Establish the relationship between resistance and current.

  • Utilize the formula relating voltage, current, and resistance.

  • Understand the relationship between the resistance of a wire and its length and thickness.

Main Concepts

Concept of Resistance

  • The size of the current flowing in a circuit depends on:

    • Voltage of the cell or power pack

    • Ease for current to flow through components

  • High Resistance correlates with Low Current.

  • Low Resistance correlates with High Current.

  • Resistance indicates how easy or difficult it is for current to flow through a component. Resistance can be controlled using resistors in a circuit.

Factors Affecting Resistance

  1. Length of the wire:

    • Longer wires have higher resistance than shorter wires.

  2. Thickness of the wire:

    • Thinner wires have higher resistance compared to thicker wires.

  3. Material of the wire:

    • Some metals are better conductors than others; namely, copper, silver, gold, and aluminum.

    • Insulating materials have very high resistances, inhibiting the flow of current.

Ohm's Law

  • Definition: Resistance is the opposition to current.

  • For a given potential difference, as resistance increases, current decreases.

  • Equation:

    • V = I \times R
      Where:

    • R = resistance (ohms, Ω)

    • V = potential difference (volts, V)

    • I = current (amperes, A)

  • Interpretation: Current is directly proportional to potential difference, provided the temperature remains constant.

Calculating Resistance

  • Relation: The resistance of a component can be defined as the ratio of voltage across it to the current flowing through it:

    • R = \frac{V}{I}

  • Example Calculation:

    • For a voltage of 10V and current of 0.5A:

    • R = \frac{10V}{0.5A} = 20Ω

Example Problems

Practice Problem (TIMSS)

  1. A washing machine heater has a resistance of 22Ω and operates at 230V.

    • (i) Calculate the current using
      V = I \times R \rightarrow I = \frac{V}{R} \rightarrow I = \frac{230}{22} = 10.45 A

    • (ii) The equation relating voltage, current, and resistance is:
      V = I \times R

Calculating Required Voltage

  1. Determine the necessary voltage to generate 20A in a circuit with 5.5Ω resistance:

    • V = I \times R = 20 A \times 5.5 Ω = 110 V

  2. A light bulb with a resistance of 8Ω and maximum current of 10A can tolerate up to:

    • V = I \times R = 10 A \times 8 Ω = 80 V

Graphical Representation and Observation

  • Current-Voltage Graph: The relationship between current and voltage is linear for ohmic conductors, indicating that the current is directly proportional to voltage.

    • Example: Given a current of 2A for a resistance of 5Ω, the required voltage is

    • V = I \times R = 2A \times 5Ω = 10 V

Practical Applications and Experimental Tasks

Experimental Setup

  1. Set up a circuit with a battery, resistor, bulb, and switch.

  2. Vary wire lengths and observe the current changes. Record the observations.

  3. Alter battery voltage and analyze the impact on current and bulb brightness.

  4. Use adjustable resistors to see their effect on circuit behavior.

Reflection Questions for Discussion

  1. How does wire length influence current?

  2. What happens to the current when resistance increases?

  3. How do changes in battery voltage impact circuit behavior?

Comparative Analysis of Resistances

  • Compare different resistors by their size and measurements, observing how resistance varies with different configurations in series and parallel.

  • Use experimental data to determine resistance values, ensuring a fair comparison between like materials and designs.

Self Assessment and Peer Review

  • Provide answers and explanations to assessment queries regarding units of measurement and calculations involving resistance and voltage, assessing understanding through discussion and peer feedback.