AQA A Level Physics

Revision Notes

5.1.1 Basics of Electricity

Electric Current & Potential Difference

Electric Current

  • Electric current is defined as the flow of positive charge carriers
    • It is measured in units of amperes (A) or amps
  • The charge, current and time are related by the equation:

Charge equation, downloadable AS & A Level Physics revision notes

  • Charge is sometimes written as ΔQ which means ‘change in charge’
    • Similarly, time is written as Δt means ‘change in time’
  • 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

  • In electrical wires, the current is a flow of electrons
  • Electrons are negatively charged; they flow away from the negative terminal of a cell towards the positive terminal
  • Conventional current is defined as the flow of positive charge from the positive terminal of a cell to the negative terminal
    • This is the opposite to the direction of electron flow, as the conventional current was described before electric current was really understood

Electric current flow, downloadable AS & A Level Physics revision notes

By definition, conventional current always goes from positive to negative (even through electrons go the other way)

  • 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

Potential Difference

  • A cell makes one end of the circuit positive and the other negative
  • This sets up a potential difference across the circuit
    • This is sometimes known as the voltage
  • The potential difference is defined as the work done per unit charge and is measured in units of volts (V)

Potential difference equation, downloadable AS & A Level Physics revision notes

  • Potential difference (or voltage) is measured using a voltmeter
  • A voltmeter is always set up in parallel to the component you are measuring the voltage for

Voltmeter in a circuit, downloadable AS & A Level Physics revision notes

Potential difference can be measured by connecting a voltmeter in series between two points in a circuit

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

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.

Think of potential difference as being the energy per coulomb of charge transferred between two points in a circuit

Resistance

  • Resistance is defined as the opposition to current
    • For a given potential difference: The higher the resistance the lower the current
  • Wires are often made from copper because copper has a low electrical resistance. This is also known as a good conductor
  • The resistance R of a conductor is defined as the ratio of the potential difference V across to the current I in it

Resistance equation, downloadable AS & A Level Physics revision notes

Resistance of a component is the ratio of the potential difference and current

 

  • Resistance is measured in Ohms (Ω)
  • An Ohm is defined as one volt per ampere
  • The resistance controls the size of the current in a circuit
    • A higher resistance means a smaller current
    • A lower resistance means a larger current
  • All electrical components, including wires, have some value of resistance

Exam Tip

Although all electrical components have resistance, the resistance of wires is taken to be 0 in exam questions.

Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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