StemUp: OCR A A level Physics 4.1: Charge and current

What is the definition of electric current? (1)

Current is defined as the rate of flow of charge.

What is the equation for electric current? (1)

Current is given by: I = ∆Q / ∆t

What is the S.I. unit for current? (1)

The unit for current is the Ampere (A).

What is meant by charge? (1)

Charge is a physical quantity of an object and it can be either positive or negative.

What is charge measured in? (2)

- Charge is measured in coulombs (C) where 1 coulomb is the flow of charge in one second when the current is 1 Ampere.

- The S.I. units of charge are Is (Ampere-seconds).

What is the elementary charge (e)? (1)

This is the smallest possible charge with a value of 1.6 x 10^-19C and it can also be negative.

What is the charge of an electron? (1)

This is equal to the negative of the elementary charge (-e) so it has a value of -1.6 x 10^-19C​

What is the charge of a proton? (1)

This is equal to the elementary charge (e) so it has a value of 1.6 x 10^-19C

How is charge quantised? (1)

The charge on any object is equal to a multiple of the elementary charge (e).

How can electric current flow through metals? (2)

- A metal has a lattice of positive ions surrounded by free electrons.

- If one side of the metal is positive and one is negative there will be a flow of electrons creating a movement of charge and therefore generating a current.

How can electric current flow through electrolytes? (2)

- Electrolytes are commonly ionic solutions containing positive and negative ions.

- When an anode and cathode are placed in the solution there will be a movement of ions creating a flow of charge and therefore generating a current.

What is meant by conventional current? (1)

This is the rate of flow of charge from the positive to negative terminal of the electric circuit.

Which direction do electrons flow in metals? (2)

- In metals the electrons flow from negative to positive as electrons are attracted to the positive terminal.

- This is in the opposite direction to the conventional current.

What is Kirchhoff's first law? (2)

- This states that for any point in an electrical circuit, the sum of the currents into that point is equal to the sum of the currents coming out of that point.

- It is a consequence of the conservation of charge.

What is the conservation of charge? (1)

This states that charge cannot be created or destroyed and must be conserved.

What is meant by the mean drift velocity? (1)

This is defined as the average velocity of the electrons as they travel through a material.

What are the factors affecting mean drift velocity? (3)

- Current (I)

- Number density (n)

- Cross-sectional area (A)

How does current affect mean drift velocity? (1)

A larger current means there is a greater rate of flow of charge and there will be less collisions between electrons and metal ions causing a higher velocity.

How does cross-sectional area affect mean drift velocity? (1)

If the cross-sectional area of the material is smaller, there will be more frequent collisions between metal ions and electrons. This will slow electrons down and the mean drift velocity will be smaller.

How does the number density (n) affect mean drift velocity? (1)

This represents the number of free electrons per unit volume. If there is a higher value of n, there are more electrons flowing and a higher mean drift velocity.

How does the value of number density change for different materials? (3)

- Conductors (such as metals) have a very high number density.

- Insulators (such as plastics) have a very small number density.

- Semi-conductors (such as silicon) have a value in between conductors and insulators.

What is the equation of current in terms of number density (n)? (1)

This is given by I = Anev​

Where I is the current (A), A is the cross-sectional area (m²). n is the number density (m⁻³), e is the elementary charge (1.6 x 10^-19) and v is the average drift velocity (m s⁻¹).

How is the equation of current in terms of number density (n) derived? (3)

- Current is given by I = ∆Q / ∆t

-​ Q can be rewritten to be the product of the volume of the object multiplied by n and the elementary charge (e) so Q = neV and I = neV \ ∆t

- As V = AL where A is the cross-sectional area and L is the length and L/t is velocity the current can be rewritten to be I = Anev​