Current Electricity - AQA A-Level Physics

Current

The rate of flow of charge

Potential difference

The energy transferred per unit of charge between two points

Resistance

The opposition to current

Charge carriers

The charged particles that move through a substance as a current

What are the charge carriers in a metal?

The conduction electrons which collide with each other and the fixed, positive ions.

Conventional current

The flow of a positive charge from an anode (+ve) to a cathode (-ve)

What direction is electron flow?

From negative to positive charge.

2 conditions for the flow of current

• There are free-moving charge carriers

• There is a source of electrical potential energy

What is the emf of a source?

(electromotive force) The electrical energy transferred to each unit charge passing through the source

What is the internal resistance of a source?

The loss of potential difference per unit current passing through the source

Ohm’s law

The pd across a metallic conductor is proportional to the current provided conditions do not change.

Resistivity

How strongly a material resists current

How does increasing temperature in a metal wire increase resistance?

• Ions will have more KE and vibrate more

• The electrons will collide with the ions at a higher rate, increasing resistance

Describe the component this IV graph represents

A fixed, metal, ohmic resistor

Describe the component this IV graph represents

A filament wire (the greater the current, the greater the resistance)

Describe the component this IV graph represents

A diode (current flows about 0.7V and there is a forward bias)

Why does the temperature in a filament lamp increase with potential difference? (3)

• As potential increases, current will also increase

• Electrons will collide more frequently with metal ions

• This transfer KE to the metal ions, meaning they will vibrate more frequently

Current in series

The same across all components

Current in parallel

Separated between branches

total potential difference in series

The sum of potential difference across individual components

potential difference in parallel

The same across components in parallel

Kirchhoff’’s laws (2)

• At any junction in a circuit, the sum of currents arriving at the junction = the sum of currents leaving

• In any loop around a circuit, the sum of emfs = the sum of pds

Super conductor

A wire/device that has zero resistivity at and below a critical temperature.

How come the current passing through a superconductor has no heating effect?

Because there is no pd across it

Positive temperature coefficient

Resistance increases with increase in temperature

Negative temperature coefficient

Resistance decreases with increase in temperature