GCSEScienceAQACombined Science: TrilogyPhysicsRevision NotesElectricityCurrent, Potential Difference & ResistanceCharge & Current

Charge & Current (AQA GCSE Combined Science: Trilogy): Revision Note

Exam code: 8464

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Written by: Ashika

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AQA Combined Science: Trilogy: PhysicsAQA Physics

Source of Potential Difference

  • For electrical charge to flow through a closed circuit, it must include a source of potential difference (p.d)

  • Sources of potential difference include:

    • A cell

    • Batteries (multiple cells)

    • Electrical generator

  • 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 symbol for potential difference is V

  • The potential difference across a component in a circuit is defined as

    The energy transferred per unit charge flowing from one point to another

  • The energy transferred can also be called the work done

  • This is calculated using the equation:

potential difference (V) = fraction numerator work space done space open parentheses straight J close parentheses over denominator charge space open parentheses straight C close parentheses end fraction

V = W over Q

  • Potential difference is measured in volts (V). This is the same as a Joule per coulomb (J C-1)

    • If a bulb has a voltage of 3 V, every coulomb of charge passing through the bulb will lose 3 J of energy

  • Potential difference (or voltage) is measured using a voltmeter

  • A voltmeter is always set up in parallel to the component you are measuring the potential difference across

Circuit diagram showing a voltmeter connected in parallel across a component to measure the potential difference across it
Potential difference can be measured by connecting a voltmeter in parallel between two points in a circuit

Worked Example

Calculate the work done in moving a charge of 5 C through a potential difference of 20 V.

Answer:

Step 1: Write down the known quantities

  • Charge, Q = 5 C

  • Potential difference, V = 20 V

Step 2: Write down the relevant equation

V = W over Q

Step 3: Rearrange for the work done, W

W = VQ

Step 4: Substitute in the values

W = 20 × 5 = 100 J

Electric Current

  • Electric current is the flow of electrical charge

    • It is measured in units of amperes (A) or amps

    • The symbol for current is I

  • The size of the electric current is the rate of flow of electrical charge

    • In other words, how much charge passes through a point each second

  • In metals, such as a copper wire, the electrical charge that flows is electrons

    • Therefore, the current in a circuit is a flow of electrons

Diagram of a metal wire showing delocalised electrons flowing from the negative terminal to the positive terminal of a cell, with an arrow indicating the direction of electron flow
In metals, the current is caused by a flow of electrons

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

  • Charge flow, current and time are related by the equation:

charge (C) = current (A) x time (s)

Q = It

  • This equation can be rearranged with the help of the following formula triangle:

Formula triangle for charge, current and time: charge (Q) at the top, current (I) and time (t) at the bottom, used to rearrange Q = It

Worked Example

Calculate the current through a lamp when a charge of 4 C passes through it in 500 s.

Answer:

Step 1: Write down the known quantities

  • Charge, Q = 4 C

  • Time, t = 500 s

Step 2: Write down the relevant equation

Q = It

Step 3: Rearrange for current, I

I = Q over t

Step 4: Substitute in the values

I = 4 over 500 = 8 x 10-3 A

Examiner Tips and Tricks

Students often confuse charge and current. When we say charge in terms of electrical circuits, we are really talking about a quantity of electrons. A unit of charge, one coulomb (1 C) is 6.24 × 1018 electrons. When delocalised electrons flow around a circuit, the amount of charge passing a fixed point (like an ammeter) per second is the current. So an ammeter is basically counting the number of electrons or the amount of charge flowing through it each second. One amp (1 A) of current is one coulomb of charge per second (1 C / s).

Current in a Loop

  • Electrons are negatively charged

    • Therefore, 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 opposite to the direction of electron flow, this is because conventional current was being put to use before the discovery of the electron

Circuit diagram showing that electrons flow from the negative terminal to the positive terminal of a cell, while conventional current flows in the opposite direction — from positive to negative
By definition, conventional current always goes from positive to negative (even through electrons go the other way)

  • 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

Circuit diagram showing an ammeter connected in series in a circuit to measure the current flowing through a component
An ammeter can be used to measure the current around a circuit and always connected in series

  • In a circuit that is a closed-loop, such as a series circuit, the current is the same value at any point

    • This is because the number of electrons per second that passes through one part of the circuit is the same number that passes through any other part

  • This means that all components in a closed-loop have the same current

Circuit diagram of a series circuit showing ammeters at different points, all reading the same current value, illustrating that current is the same at every point in a closed loop
The current is the same at each point in a closed-loop

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Ashika

Author: Ashika

Expertise: Physics Content Creator

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.