AQA GCSE Physics

Revision Notes

2.1.2 Charge & Current

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 is always called the work done
  • This is calculated using the equation:

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

  • 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 voltage for

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

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.

Step 1: Write down the known quantities

    • Charge, Q = 5 C
    • Potential difference, V = 20 V

Step 2: Write down the relevant equation

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

electrons-and-current, IGCSE & GCSE Physics revision notes

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 equation, downloadable AS & A Level Physics revision notes

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

Current charge time triangle, IGCSE & GCSE Physics revision notes

Worked Example

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

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

Step 4: Substitute in the values

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, 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)

  • 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

  • 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

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

The current is the same at each point in a closed-loop

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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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