Current, Potential Difference and ResistanceAQA GCSE Physics: Revision notes

Electric current is the rate of flow of electric charge through a circuit. It is measured in amperes (A).

Current is defined by the equation:

Q = It

Where:

• Q = charge in coulombs (C)
• I = current in amperes (A)
• t = time in seconds (s)

This equation tells us that the total charge flowing depends on how much current flows and for how long it flows.

Rearranging the equation:

• To find current: I = Q/t
• To find time: t = Q/I

In a circuit, current is the flow of electrons through a conductor. The larger the current, the more charge passes through a point per second.

Potential difference (p.d.) is the energy transferred per unit charge in a circuit. It is measured in volts (V).

Potential difference is defined by the equation:

V = W/Q

Where:

• V = potential difference in volts (V)
• W = energy transferred in joules (J)
• Q = charge in coulombs (C)

This equation shows that potential difference tells us how much energy is given to (or taken from) each coulomb of charge.

Rearranging the equation:

• To find energy: W = VQ
• To find charge: Q = W/V

Key point: A potential difference of 1 volt means 1 joule of energy is transferred for every coulomb of charge that flows.

Potential difference across a component is often called the voltage across that component.

Resistance is a measure of how much a material opposes the flow of electric current through it. It is measured in ohms (Ω).

The relationship between potential difference, current, and resistance is given by Ohm's Law:

V = IR

Where:

• V = potential difference in volts (V)
• I = current in amperes (A)
• R = resistance in ohms (Ω)

Rearranging the equation:

• To find current: I = V/R
• To find resistance: R = V/I

Key points:

• For a given potential difference, higher resistance means lower current
• For a given current, higher resistance means higher potential difference
• A resistance of 1 ohm means 1 volt produces a current of 1 ampere

Ohmic conductors (like resistors at constant temperature) follow Ohm's Law—the potential difference is directly proportional to the current.

At constant temperature, the resistance of a metal conductor remains constant (for an ohmic conductor).

As temperature increases, the resistance of a metal increases.

Explanation in terms of ion vibration (Higher Tier):

• At higher temperatures, the metal ions vibrate more vigorously about their fixed lattice positions
• Free electrons in the metal move through the lattice between these ions
• As ion vibration increases, electrons collide more frequently with the vibrating ions
• Each collision reduces the electron's momentum, slowing down the flow of charge
• More collisions = more opposition to current flow = higher resistance

In summary:

• High temperature → vigorous ion vibration → more frequent electron-ion collisions → higher resistance
• Low temperature → less ion vibration → fewer electron-ion collisions → lower resistance

This is why metal resistors are often temperature-dependent, and why calculations assume constant temperature unless otherwise stated.

Strategy for multi-step problems:

1. Identify what you know – list all given values with their units (V, I, R, Q, t, W)
2. Identify what you need to find – the unknown value required by the question
3. Choose the correct equation – use one of these:
   • Q = It (for charge and current relationships)
   • V = W/Q (for potential difference and energy)
   • V = IR (for resistance problems)
4. Rearrange if necessary – solve for the unknown variable
5. Substitute values – include units at each step
6. Calculate and state the answer – with correct units

Common equation combinations:

Always show working – examiners award marks for method, not just the final answer.