B.5 Current and circuits

Elementary charge

The magnitude of charge of a proton/electron.

1e = 1.6 × 10^-19 C

Number of electrons = Total charge / elementary charge

Potential difference

Potential difference (ΔV) is the work done per unit charge in moving a positive charge between two points.

ΔV = W/q

Electromotive force (ε - emf)

Emf is the work done by a cell per unit charge

ΔV = W/q

Electron volt

The energy gained when an electron is accelerated by a potential difference of 1 volt. 1ev = 1.6 × 10^-19 J

Convectional current

Convectional current (I) is defined as the amount of positive charge flowing past a point per second. Flowing from high potential (+) to low potential (-).

I = Δq / Δt

Electron current

From low potential (-) to high potential (+).

Resistance

The ratio of the potential difference across a component to the current through it. R = V/I

Ohm’s Law

The potential difference across a metallic conductor is proportional to the current flowing through it, provided the temperature does not change.

• Materials that obey Ohm’s law (Ohmic conductors) will have a constant resistance at a constant temperature.

Explain the Non Ohmic conductors graph - Filament Lamp

• As pd across the filament lamp is increased, the electrons collide more frequently with the vibrating fixed ions, transferring energy.

• The ions vibrate with greater amplitude and the temperature of the metal filament increases.

• This makes it more difficult for electrons to flow (resistance increases).

Power

The rate of energy transfer (amount of work done per second)

P = W / t

Explain the Non Ohmic conductors graph - Diode

• Diodes only allow current to flow in one direction.

• For a negative V values there is no current

• There is no significant current in the forward direction until a certain pd (threshold voltage) is exceeded. High resistance in the backward direction and high then low resistance in the forward direction.

Explain the Non Ohmic conductors graph - LDR

• Resistance is inversely proportional to light intensity.

• Bright - low resistance

• Dark - high resistance

Explain the Non Ohmic conductors graph - Thermistor

• Resistance is inversely proportional to temperature

• Hot - low resistance

• Cold - high resistance

What physical properties of a wire change its resistance?

• Length (L)

• Cross sectional area (A) - πr²

• Resistivity (ρ) - a property of material equal to the resistance of a sample of the material with a length of 1m and a cross sectional are of 1m².

Resistivity formula

ρ = AR / L

Potential dividers

• A circuit made of two (or more) series resistors that allow us to put any voltage we want that is less than the battery voltage.

• Input voltage is the emf of the battery.

• Output voltage is the voltage drop across a component.

What is a potentiometer?

A is a single wire with sliding contact that can be used to build potential divider circuit.

Battery vs Cell

• A battery consists of two or more cells that are joined together

• The cell consists of conductive components and a store of chemical energy.

Primary vs Secondary vs Solar

• A primary cell cannot be recharged, the chemicals in the cell get used in a non reversible reaction.

• A secondary cell can be recharged by applying an external reversed voltage, reversing the chemical reaction.

• In solar cells the electrons in semiconductors can absorb energy from sunlight, energy is converted from solar to electrical energy.

• All cells produce a direct current.

Internal resistance and cell emf formula

The conductive components of a cell have resistance - internal resistance of a cell / battery.

ε = V + v, V - terminal pd, v - lost volts

ε = IR + Ir, IR - external resistance, Ir - internal resistance.

Power formula

P = IV = I²R = V²/R

P_T = I²R + I²r, I²R - power transferred to external resistance I²r - power overcoming internal resistance.