Revision Notes: Physics: OCR A: Unit 2: Module 1: Charge and Current

(a) explain that electric current is a net flow of charged particles
Current is defined as the rate of flow of charge

(b) explain that electric current in a metal is due to the movement of electrons, whereas in an electrolyte the current is due to the movement of ions

For a current to flow, there must be charge carriers. In metals, these are electrons but in liquids and gases, they are ions

Ionic crystals are insulators but when molten or aqueous, the liquid conducts: positive and negative ions are the charge carriers.

Gases are insulators, but if you apply a high enough voltage, electrons get ripped out of atoms giving you ions along a path so a current can flow.

(c) explain what is meant by conventional current and electron flow

Electrons are the charge carriers in a circuit, and they are negatively charged particles.

This means that they are repelled by the negative terminal in a battery and attracted by the positive terminal, so electrons flow from negative to positive.

However conventional current, which was decided when electricity was first discovered, flows from positive to negative.

(d) select and use the equation DQ = IDt

As said, electric current is a transfer of charge from one place to another. The size of the current is the rate of charge transferred per second.

average current (A) = amount of charge transferred (C) / time taken (s) ...... !=Q/t ...... Q=It

(e) define the coulomb

The coulomb is the unit used to measure electrical charge.

1 Coulomb is defined as the amount of charge that passes in 1 second when the current is 1 ampere.

It isn't an SI base unit. This is because it is easier to define the unit of electrical current, which is linked to electric charge, and then derive the coulomb from that.

(f) describe how an ammeter may be used to measure the current in a circuit

Ammeters must be connected in series.

This is so that their resistance is 0 ohms so it takes no energy from the circuit (V=IR so V=0)

(g) recall and use the elementary charge e = 1.6 ´ 10-19 C

e = 1.6× 10-19 C

Electrons have a charge of -e and protons have a charge of +e

(h) describe Kirchhoff’s first law and appreciate that this is a consequence of conservation of charge

Kirchhoff first law states that the sum of the current entering a junction is equal to the sum of the current leaving a junction.

This is because charge is conserved so whatever current flows into a junction must come out of it.

(i) state what is meant by the term mean drift velocity of charge carriers

In a wire, the electrons all move in different directions however overall they drift in one direction. Drift velocity is the average velocity.

(j) select and use the equation I = Anev

I is the current, n is the number of charge carriers per m3, A is the cross sectional area is m2, v is drift velocity and q is the charge each charge carrier has, so in a metal it is e

(k) describe the difference between conductors, semiconductors and insulators in terms of the number density n

In a metal, the charge carriers are delocalised electrons and there are lots of them so the current is usually high, even if there's a low drift velocity

Semiconductors have fewer charge carriers, however if energy is supplied, n can increase so they make good sensors for detecting a change in their environment

A perfect insulator has no charge carriers so n = 0 and no current can flow, but insulators have a very small n

PRACTICE QUESTIONS

1. A battery delivers 4500C of electric charge to a circuit in 10 minutes. Calculate the average current

[2 marks]

2. Copper has 1.0× 1029 free electrons per m3. Calculate the drift velocity of the elctrons in a copper wire of cross-sectional area 5× 10-6 m2 when it is carrying a current of 13 A.

[3 marks]

3. What is Kirchhoff's first law?

[2 marks]

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AS level Physics, OCR A, Unit 2: Electrons, Waves and Photons, Module 2.1.1. Charge and Current.