Electricity

All of the topics I think

Potential Difference

Energy transferred per unit charge

Unit: V

V = E/Q

Voltmeter

Connected in parallel

Current

Rate of flow of charge

Unit: A

I = Q/t

Ammeter

Connected in series

Resistance

A measure of how much a component resists the flow of current

Unit: Ω

Ohm’s Law

V = IR → R = V/

Ohmic Resistor

Obeys Ohm’s Law:

• V-I graph is a straight line

Filament Lamp

Resistance is not constant, therefore V-I graph curves

• Delocalised electrons collide with ionic lattice, causing them to vibrate more and increase temperature

• Larger current results in increased resistance - NON-OHMIC

Diode

Lets current flow in one direction

• Resistance in forward direction is LOW

• Resistance in reverse direction is HIGH

Resistivity, ρ

How resistive a material is to the flow of charge.

Unit: Ωm

Measuring resistivity

• Measure diameter of wire in multiple places using micrometer and find mean.

• Use this to find the cross-sectional area

• Change L of wire measuring with meter rule by moving croc clip

• Plot graph of R against L

  • ρ = RA / L

Series Circuit

• Total p.d is shared between all components

• Current is the same for all components

• Total Resistance is sum of all resistances

Parallel Circuit

• P.d across each branch = e.m.f of battery

• Current is split between branches

• Adding more resistors in parallel reduces total R

Kirchoff’s 1st Law

Charge (and therefore current since Q = It) is conserved at any junction (I_in = I_out)

Kirchoff’s 2nd Law

Sum of EMF’s (pd rises) must equal the sum of pd drops in a closed loop

Thermistor

If temperature increases, resistance decreases: opposite to a metal wire essentially

Thermistor circuit

If temperature increases, R of thermistor decreases, so thermistor gets a smaller share of V_total

Bottom resistor gets a bigger share of V_total which could be used to turn off heating

LDR

When light intensity increases, resistance decreases

LDR circuit

If light intensity decreases, R of LDR decreases, so LDR gets a larger share of V_total

Bottom resistor gets a bigger share of V_total which could be used to turn on a street lamp

Power

P = IV

P = I²R

P = V²/R

Direct Current

Result of a direct p.d (p.d that acts in one direction) from a battery

Alternating Current

Result of alternating p.d (both reverse direction at regular intervals)

Electromotive Force

Energy supplied to each unit charge

Internal resistance

Some p.d is lost in the circuit due to internal resistance, r, of the cell.

Terminal Potential difference

Potential difference across the terminals of a power source

Electromotive Force, ε

Energy supplied to each unit charge

Internal Resistance

Resistance in the source. This means that the circuit will not receive the full EMF of the cell

Electromotive force and internal resistance

ε = I(R+r)

ε = IR + Ir

ε = V + Ir

• V = Terminal p.d

• I = Current

• r = internal resistance

• R = Total Resistance of circuit

Number Density, n

Free charge carriers that can flow per m³ of a material.

Higher number density = better conductivity

Drift Velocity

Distance travelled by an electron per unit time in a wire

Mean drift velocity and current

I = Anev → v = I / Ane

• I = current (A)

• A = Cross-sectional Area (m²)

• n = number density (m^-3)

• e = elementary charge (1.6×10^-19)

• v = mean drift velocity (ms^-1)