Basics of Electricity (AQA A Level Physics): Revision Note

Electric current & potential difference

• A simple electrical circuit requires:

  • a conductor through which current can flow

  • a source of potential difference

Electric current

• Electric current is defined as

  The rate of flow of electric charge

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

  • 1 amp is equivalent to a charge of 1 coulomb flowing in 1 second, or 1 A = 1 C s⁻¹

• Current can be described using the equation:

$I=\frac{∆Q}{∆t}$

• Where:

  • $I$ = current (A)

  • $∆Q$ = amount of charge flowing (C)

  • $∆t$ = time interval (s)

• Current flows when a circuit is formed

  • This is when a conductor, such as a wire, connects two oppositely charged terminals of a source, such as a cell

• In electric circuits, the current is a flow of electrons

• Conventional current is defined as the flow of positive charge

  • This is from the positive terminal of a cell to the negative terminal

  • This is the opposite of the direction of electron flow

    • Electrons are negatively charged so they flow from the negative terminal of a cell to the positive terminal

• Current can be measured using an ammeter

• Ammeters must be connected in series with the component being measured

Potential difference

• Potential difference is defined as

  The electrical work done per unit charge flowing between two points

• Potential difference is measured in units of volts (V)

  • 1 volt is equivalent to the transfer of 1 joule of electrical energy by 1 coulomb of charge, or 1 V = 1 J C⁻¹

• Potential difference can be described using the equation:

$V=\frac{W}{Q}$

• Where:

  • $V$ = potential difference (V)

  • $W$ = electrical work done by the charge (J)

  • $Q$ = charge flow (C)

• A simple cell creates a potential difference through the separation of charge

  • One end (terminal) of the cell has an excess of positive charge and the other an excess of negative charge

  • Negatively charged electrons are repelled by the negative terminal and attracted to the positive terminal

  • Therefore, when a wire is connected between the two terminals, the potential difference causes the flow of electrons (current)

• As electrons flow through a cell, they gain energy

  • For example, in a 12 V cell, every coulomb of charge passing through gains 12 J of energy

• As electrons flow through a circuit, they lose energy

  • For example, after leaving the 12 V cell, each coulomb of charge will transfer 12 J of energy to the wires and components in the circuit

• Potential difference can be measured using a voltmeter

• Voltmeters must be set up in parallel with the component being measured

Resistance

• Resistance is defined as

  The opposition of a component to the flow of electric current through it

• The resistance $R$ of a conductor is defined as the ratio of the potential difference $V$ across to the current $I$ in it

$R=\frac{V}{I}$

• Where:

  • $R$ = resistance of a conductor (Ω)

  • $V$ = potential difference across the conductor (V)

  • $I$ = current in the conductor (A)

• Resistance is measured in units of ohms (Ω)

  • A resistance of 1 Ω is equivalent to a potential difference across a component of 1 V which produces a current of 1 A through it

• The resistance of a component controls the size of the current in a circuit

• For a given potential difference across a component:

  • The higher the resistance, the lower the current that can flow

  • The lower the resistance, the higher the current that can flow

• All electrical components, including wires, possess some value of resistance

• Wires are often made from copper because of its low electrical resistance

  • This is why it is known as a good conductor