Aqa A-level Physics Electricity

Ammeter

A device that measures the current in the loop of the circuit that it is connected in series with. An ideal ammeter is modelled to have zero resistance.

Current

The rate of flow of charge. It is measured in amperes (A) and indicates how much electric charge passes a point in the circuit per unit time.

Electromotive Force

The amount of energy transferred by a source to each unit of charge that passes through it.

Internal Resistance

The resistance to the flow of charge within a source. Internal resistance results in energy being dissipated within the source.

Light Dependant Resistor

A light sensitive semiconductor whose resistance increases when light intensity decreases.

Ohmic Conductor

A conductor for which the current flow is directly proportional to the potential difference across it, when under constant physical conditions.

Ohm’s Law

The current and potential difference through an ohmic conductor held under constant physical conditions are directly proportional, with the constant of proportionality being resistance.

Parallel circuits

Components are said to be connected in parallel when connected across each other (separate loops). In a parallel circuit, the voltage across each component is the same, while the total current is the sum of the currents through each component.

Potential Divider

A method of splitting a potential difference, by connecting two resistors in series. The total potential difference is split in the ratio of their resistances. This allows for varying output voltages from a single source.

Resistance

A measure of how difficult it is for current to flow through a material. It is defined as the ratio of voltage to current, measured in ohms.

Resistivity

A quantity that is proportional to an object’s resistance and cross sectional area, and inversely proportional to the object’s length. It is a material property, typically measured in ohm-meters.

Resistors In Parallel

The potential difference across resistors connected in parallel is identical for each resistor. The current is split between the resistors. The total resistance is equal to the inverse sum of the inverses of the resistances of the resistors.

Resistors In Series

The current through resistors connected in series is identical for each resistor. The potential difference is split in the ratio of their resistances. The total resistance is equal to the sum of the resistances of the resistors.

Series Circuits

Components are said to be connected in series when they are connected end to end (in one loop).

Superconductor

A material which has zero resistivity when the temperature is decreased to, or below, the materials critical temperature. Superconductors can be used to produce strong magnetic fields and reduce energy loss when transmitting electric power.

Terminal Potential Difference

The potential difference across the terminals of a power source. It is equal to the source’s emf minus any voltage drop over the source’s internal resistance.

Thermistor

A temperature sensitive semiconductor whose resistance increases when temperature decreases. It is commonly used in temperature sensing and control applications.

Voltmeter

A device used to measure the potential difference across components. An ideal voltmeter is modelled to have infinite resistance.

Current is defined as:

The change in charge divided by the change in time. I = ΔQ/Δt, where Q is charge.

Potential difference is defined as:

The work done divided by the charge. V = W/Q where W is work done and Q is charge.

Resistance is defined as:

Potential difference divided by current. R = V/I, where V is potential difference and I is current.

Resistivity is defined as:

The resistance of a material per unit length and cross-sectional area. ρ = R*A/L, where ( R ) is resistance, ( A ) is cross-sectional area, and ( L ) is length.

Power is defined as:

Work done divided by time. P = W/t, where P is power, W is work done, and t is time. It can also be represented by P = I*V = I²R = V²/R, where I is current, R is resistance and V is potential difference.

EMF is defined as:

Current times resistance add current times internal resistance. ε = I*R + I*r or ε = V +I*r where ε is electromotive force, I is current, V is potential difference, R is external resistance, and r is internal resistance.