Charge & Current

What is electric current?

The rate of flow of charge

What is the equation for electric current?

I = dQ/dt

What is the SI base unit of current?

Ampere A

How is current measured in a circuit?

Using an ammeter

How is an ammeter connected?

In series

What is electric charge?

A physical quantity that can be positive or negative

What is the unit of charge?

Coulomb C

How is one coulomb defined?

The charge flowing when a current of 1 A flows for 1 s

What are the SI base units of charge?

A s

How do like charges interact?

They repel

How do opposite charges interact?

They attract

What is meant by charge being quantised?

Charge exists in discrete amounts

What is the charge of a proton?

+1 elementary charge

What is the charge of an electron?

-1 elementary charge

What is the value of the elementary charge e?

1.6 × 10⁻¹⁹ C

What causes the net charge of a particle?

Gain or loss of electrons

Why is an atom electrically neutral?

Equal numbers of protons and electrons

What produces a negative ion?

Gaining electrons

What produces a positive ion?

Losing electrons

What is the equation for net charge?

Q = ne

What does n represent in Q = ne?

The number of electrons gained or lost

What are charge carriers?

Particles that carry electric charge

What carries current in metals?

Electrons

Describe the structure of a metal.

Lattice of positive ions with free electrons

Why can electrons move in metals?

They are delocalised and free to move

What happens when a potential difference is applied to a metal?

Electrons drift towards the positive terminal

What carries current in electrolytes?

Positive and negative ions

What is an electrolyte?

A conducting liquid containing ions

Give an example of an electrolyte.

Salt dissolved in water

What ions are formed when NaCl dissolves in water?

Na⁺ and Cl⁻

What is the anode?

The positive electrode

What is the cathode?

The negative electrode

Which ions move to the cathode?

Cations

Which ions move to the anode?

Anions

What is conventional current?

The flow of charge from positive to negative

Why was conventional current defined this way?

It was defined before electrons were discovered

In which direction do electrons flow in metals?

From negative to positive

How does electron flow compare to conventional current?

It is in the opposite direction

State Kirchhoff’s first law.

The sum of currents into a junction equals the sum out

What principle explains Kirchhoff’s first law?

Conservation of charge

Why must charge be conserved?

It cannot be created or destroyed

What is mean drift velocity?

The average velocity of electrons through a wire

Why do electrons move randomly in a metal?

Frequent collisions with positive ions

What causes electrons to have a drift velocity?

An applied electric field

What is number density n?

Number of free electrons per unit volume

What are typical number densities for conductors?

Around 10²⁸ m⁻³

How do insulators compare in number density?

Very low number density

How do semiconductors compare in number density?

Between conductors and insulators

What happens to drift velocity if number density decreases?

It increases for the same current

How can current be defined using charge flow?

I = dQ/dt

What is the equation for total charge in a wire?

Q = neV

What does V represent in Q = neV?

Volume of the wire

How is the volume of a wire calculated?

Cross-sectional area × length

What is the relationship between length time and drift velocity?

v = length / time

How is current related to drift velocity?

I = Anev

What does A represent in I = Anev?

Cross-sectional area

What does e represent in I = Anev?

Elementary charge

What can I = Anev be rearranged to find?

Mean drift velocity