AQA Physics A-level: Electricity

• This document contains study notes for AQA Physics A-level, Section 5: Electricity

Basics of Electricity

• Electric current (I) refers to the charge per time, defining the rate of charge flow and is measured in Amperes
• I = ΔQ/Δt, where ΔQ is the amount of charge in Coulombs that passes in a time Δt in seconds
• Potential difference (V) represents energy transferred per unit charge between two points in a circuit.
• V = W/Q, where W represents energy transferred and is measured in Volts
• Resistance (R) indicates how difficult it is for charge carriers to pass through a component.
• R = V/I and is measured in Ohms

Current-Voltage Characteristics

• Ohm's law says current is directly proportional to potential difference in an ohmic conductor if physical conditions such as temperature are constant.
• Current-voltage graphs show the properties of components, where current or voltage is on the y-axis

Ohmic Conductor

• An Ohmic conductor follows Ohm's law, and its current-voltage graph is a straight line through the origin if physical conditions are constant.

Semiconductor Diode

• The current-voltage graph must consider forward and reverse bias.
• Forward bias in a diode allows current to flow easily past the threshold voltage, the minimum voltage for current flow.
• Reverse bias causes extremely high resistance and allows almost no current flow.

Filament Lamp

• As current increases, a filament lamp's wire heats up.
• Increased heat raises resistance with current
• At low currents, the wire does not heat significantly, so Ohm's law applies.
• As current increases in either direction, the graph begins to curve as resistance increases.

Ammeters and Voltmeters

• Unless stated, assume ammeters have zero resistance, meaning they don't affect current measurement.
• Voltmeters have infinite resistance, meaning no current flows through them.
• Voltmeters give an exact potential difference measurement.

Resistivity

• Resistivity (ρ) measures how easily a material conducts electricity
• It's calculated as the product of resistance and cross-sectional area, divided by length
• Resistivity gives the resistance value for a material of 1m length and 1 m² cross-sectional area
• The formula to calculate is ρ = RA / L where R is the resistance in Ohms, A is the cross sectional area in m^2 and L is the length in meters.
• Temperature affects resistivity
• When the temperature rises in a metal conductor, resistance increases.
• Increased kinetic energy of metal atoms causes more collisions with charge carriers, slowing current.

Thermistors

• The opposite effect of a metal conductor happens in thermistors
• When temperature rises, a thermistor's resistance decreases.
• Increasing temperature causes electrons to be emitted from atoms, increasing charge carriers, and allowing current to increase, thus decreasing resistance.
• Thermistors act as temperature sensors where an event can be triggered if temperature changes

Superconductors

• A superconductor, below its critical temperature, has zero resistivity.
• Critical temperature depends on the material; most superconductors have extremely low critical temperatures close to 0 K (-273°C).
• Applications for superconductors include power cables that would reduce energy loss through heating and strong magnetic fields needing no power source, like in maglev trains or medical applications.

Circuits

• There are two rules for adding resistance in circuits, based on series or parallel configurations.
• In a series circuit, R_T = R_1 + R_2 + R_3 ...
• R_T represents the total resistance
• R_n represents the resistance of resistor n
• In parallel circuits, 1/R_T = 1/R_1 + 1/R_2 + 1/R_3 ...

Power

• Power (P) is the energy transferred over time, or the rate of energy transfer.
• P = E/t, where E is energy transferred and t is time.
• Another formula for power is P = VI, which can be combined with V = IR to form two variations: P = VI = I^2/R = V^2/R
• The product of power and time is the energy transferred therefore E = VIt.

Series Circuits

• Current is the same everywhere in a series circuit.
• The battery p.d is shared across all elements
• The total sum of the voltages across all elements is equal to the supply p.d.

Parallel Circuits

• The sum of the currents in each parallel set of branches is equal to the total current.
• The potential difference across each branch is the same.

Battery Cells

• Battery cells can be combined in series or parallel.
• When combined in series, total voltage is the sum of individual cell voltages: VT=V1+V2+ V3 + ...
• When identical cells are joined in parallel, the total voltage is equal to the voltage of one cell: Vr = V1=V2 = V 3 = ...

Kirchhoff's Laws

• In DC circuits, charge and energy are always conserved.
• Kirchhoff's first law states the total current into a junction equals the current flowing out of that junction.
• Kirchhoff's second law states the sum of all voltages in a series circuit equals the battery voltage.

Potential Dividers

• Potential dividers are circuits with several resistors in series across a voltage source.
• Potential dividers produce a fraction of the source potential difference.
• Variable resistors support adjustable potential differences, and varying resistance allows variances in the potential difference output.

Sensors

• Using light dependent resistors and thermistors form light or temperature sensors
• Ex: If light intensity falls, resistance across R₁ (light dependent resistor) will increase so circuit current decreases and resistance across R₂ decreases, so the p.d decreases

Electromotive Force

• Batteries have internal resistance (r) caused by electrons colliding with battery atoms
• Energy is lost before electrons leave the battery
• Electromotive force (emf / ε) represents energy transferred per coulomb of charge, ε = E / Q
• Internal (r) and load (R) resistance sum to total resistance (RT) in the circuit: RT = R + r

Formula

• Emf is the product of total resistance and circuit current: ε = IR + Ir = I(R + r)
• The p.d across resistor R is known as the terminal p.d (V).
• The p.d across resistor r is known as lost volts (v) and equals energy wasted by the cell per charge unit
• V = IR and v = Ir
• Therefore, emf is the sum of terminal p.d and lost volts: ε = V + v
• Battery emf is measured by measuring voltage across a cell in an open circuit.