Definitions (AQA A-Level Physics): Revision Notes

Definitions

1. Ammeter

• Definition: A device used to measure the current flowing in a circuit.
• Explanation: Ammeters are connected in series with the circuit loop to measure the flow of charge. An ideal ammeter has zero resistance to avoid affecting the circuit.

2. Current ($I$)

• Definition: The rate of flow of charge through a conductor, measured in amperes ($A$).
• Formula: $I = \frac{Q}{t}$, where $Q$ is charge and $t$ is time.

3. Electromotive Force ($emf$)

• Definition: The energy supplied per unit charge by a source, such as a battery, measured in volts ($V$).
• Explanation: This is not a force but rather the total energy per charge provided by the source to drive current around a complete circuit.

4. Internal Resistance

• Definition: The resistance within a power source that causes energy loss within the source itself.
• Explanation: Due to internal resistance, not all the emf from a source is available to the external circuit. This is why batteries warm up during use.

5. Light Dependent Resistor ($LDR$)

• Definition: A semiconductor component whose resistance decreases as light intensity increases.

6. Ohmic Conductor

• Definition: A conductor for which current is directly proportional to potential difference at constant temperature.

7. Ohm's Law

• Definition: The current through an ohmic conductor is directly proportional to the potential difference across it, provided temperature remains constant.
• Formula: $V = IR$
• Explanation: Doubling the voltage across a resistor will double the current if the resistor's temperature is unchanged.

8. Parallel Circuits

• Definition: Circuits where components are connected across multiple paths, allowing current to split and travel through separate branches.
• Explanation: In parallel circuits, each branch receives the same voltage, and the total current is the sum of currents through each branch.

9. Potential Divider

• Definition: A configuration of resistors in series that splits the potential difference in proportion to their resistances.
• Application: Potential dividers are used to adjust voltage levels, for example, to provide a specific input voltage to a sensor.

10. Resistance ($R$)

• Definition: A measure of how difficult it is for current to flow through a material, measured in ohms $(Ω)$.
• Formula: $R = \frac{V}{I}$
• Explanation: Higher resistance means less current for a given voltage.

11. Resistivity ($ρ$)

• Definition: A property of a material that affects its resistance, defined as resistance across a specific length and area.
• Formula: $R = \rho \frac{L}{A}$, where $L$ is length, and $A$ is cross-sectional area.

12. Resistors in Parallel

• Definition: A configuration where resistors are connected so each has the same potential difference, but the current splits across them.
• Formula: $\frac{1}{R_{\text{total}}} = \frac{1}{R_1} + \frac{1}{R_2} + \dots$
• Explanation: Adding resistors in parallel decreases the overall resistance, allowing more current to flow.

13. Resistors in Series

• Definition: A configuration where resistors are connected one after another, sharing the same current.
• Formula: $R_{\text{total}} = R_1 + R_2 + \dots$
• Explanation: The total resistance increases as more resistors are added in series, reducing the current for a given voltage.

14. Series Circuits

• Definition: Circuits in which components are connected end-to-end, so there is only one path for current.
• Explanation: All components share the same current, and the total voltage is divided among the components based on their resistances.

15. Superconductor

• Definition: A material that has zero resistivity below a critical temperature, allowing current to flow without energy loss.
• Application: Superconductors are used in MRI machines and high-speed maglev trains due to their ability to conduct large currents without resistance.

16. Terminal Potential Difference

• Definition: The voltage across the terminals of a power source, equal to the source's emf minus any drop due to internal resistance.
• Explanation: As current flows, internal resistance reduces the available voltage to the external circuit.

17. Thermistor

• Definition: A temperature-sensitive resistor whose resistance decreases as temperature increases (NTC type).

18. Voltmeter

• Definition: A device that measures potential difference across components in a circuit.
• Explanation: Voltmeters are connected in parallel to avoid altering the current in the component being measured. An ideal voltmeter has infinite resistance to prevent current flow through it.