OCR AS Physics

Revision Notes

4.1.1 Electric Current & Charge

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Electric Current

Electric Current

  • Electric current is defined as the rate of flow of positive charge carriers
    • It is measured in units of amperes (A) or amps
    • The symbol for current is I

  • The charge, current and time are related by the equation:

Current Equation
  • Where:
    • I = current (A)
    • ΔQ = change in charge (Q)
    • Δt = time interval (s)

  • When two oppositely charged conductors are connected together (by a length of wire), charge will flow between the two conductors, causing a current

Flow of charge, downloadable AS & A Level Physics revision notes

Charge can flow between two conductors. The direction of conventional current in a metal is from positive to negative

  • There are several examples of electric currents, including in household wiring and electrical appliances
  • Current is measured using an ammeter
  • Ammeters should always be connected in series with the part of the circuit you wish to measure the current through

Ammeter in series, downloadable AS & A Level Physics revision notes

An ammeter can be used to measure the current around a circuit and always connected in series

Worked example

When will 8 mA of current pass through an electrical circuit?

A.     When 1 J of energy is used by 1 C of charge

B.     When a charge of 4 C passes in 500 s

C.     When a charge of 8 C passes in 100 s

D.     When a charge of 1 C passes in 8 s

ANSWER: B

Step 1: Write out the equation relating current, charge and time, rearranging for charge Q

Q = It

Step 2: Rule out any obviously incorrect options

    • Option A does not contain charge or time, so can be ruled out

Step 3: Try the rest of the options to determine the correct answer

    • Consider option B:

I = 4 / 500 = 8 × 10–3 = 8 mA

    • Consider option C:

I = 8 / 100 = 80 × 10–3 = 80 mA

    • Consider option D:

I = 1 / 8 = 125 × 10–3 = 125 mA

    • Therefore, the correct answer is B

Exam Tip

Although electric charge can be positive or negative, since the conventional direction of current is the flow of positive charge the current should always be a positive value for your exam answers.

Electric Charge

  • Charge is a property certain particles have. It can either be:
    • A positive charge (+) (eg. proton)
    • A negative charge (–) (eg. electron)
    • A neutral (no) charge (eg. neutron)

  • An atom is neutral. This is because it has an equal number of protons (positive charge) and electrons (negative) charge
    • However, just the nucleus which is made up of protons and neutrons is positively charged

  • In physics, the charge is represented by the symbol Q or q

The Coulomb

  • The unit of charge is the Coulomb (C)
    • This is defined as the quantity of charge that passes a fixed point per second when a current of 1 A is flowing

  • The coulomb (C), in SI base units, is equal to the quantity of electricity conveyed in one second by a current of one ampere i.e. 1 C = 1 A s

 

The Coulomb, downloadable AS & A Level Physics revision notes

Definition of the Coulomb

Quantisation of Charge

  • The charge on charge carriers is quantised
    • This means the charge comes in definite, finite quantities

  • In this way, the quantity of charge can be quantised depending on how many protons or electrons are present
    • Positive and negative charge has a definite minimum magnitude and comes in multiples of that magnitude

  • This magnitude is the elementary charge, e = 1.60 × 10-19 C
    • The magnitude of the charge just refers to its value, rather than whether it is positive or negative

  • The net charge on a particle can be quantised, meaning it is always a multiple of the charge of an electron by convention
    • The charge of an electron, e is -1.60 × 10-19 C
    • The charge of a proton, +e is +1.60 × 10-19 C

Worked example

Determine the charge of an ion with charge 3e. State an appropriate unit for your answer.

Step 1: Calculate the charge

1e = –1.60 × 10–19

3e = 3 × (–1.60 × 10–19) = –4.8 × 10–19

Step 2: Include the unit for charge

The units of charge is coulombs (C)

Therefore 3e = –4.8 × 10–19 C

Exam Tip

Although the charge of the electron is given on your data sheet, you will be expected to remember that the charge of the proton has the same magnitude

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