Current, Potential Difference & Resistance Notes

Edexcel GCSE Physics: Revision notes

Key facts

Electric current is the rate of flow of charge. In metals it is a flow of electrons. $Q = I \times t$ .
Current is conserved at a junction: current in = current out.
Potential difference is energy transferred per unit charge: $E = Q \times V$ . 1 volt = 1 joule per coulomb.
$V = I \times R$ . Ammeters go in series; voltmeters go in parallel.
Resistors in series increase the total resistance; resistors in parallel decrease it.

Charge $Q = I \times t$
Energy $E = Q \times V$
Potential difference $V = I \times R$

Charge and current

Current is the rate of flow of charge, and in a metal it is a flow of electrons.

An atom has a small, dense nucleus of protons (positive) and neutrons (uncharged), surrounded by electrons (negative).

Electric current is the rate of flow of charge, and a closed circuit with a cell has a current in it. Charge $Q$ is in coulombs (C), current $I$ in amperes (A) and time $t$ in seconds (s). Current is conserved at a junction.

c = 2current I (A)

Charge flowing against time at a steady current: the gradient is the current (Q = I × t). Drag the slider to change the current.

Charge (C) $Q = I \times t$

Worked example

A current of 2 A flows for 10 s. Calculate the charge that flows.

1
Write the equation: $Q = I \times t$ .
2
Substitute: $Q = 2 \times 10$ .

3 A flows into a junction that splits into two branches. One branch carries 1 A. What does the other carry?

Potential difference

Potential difference is the energy transferred per unit of charge passing between two points: one volt is one joule per coulomb.

Potential difference (voltage) is the energy transferred per unit charge passed between two points. Energy $E$ is in joules (J), charge $Q$ in coulombs (C) and potential difference $V$ in volts (V).

A voltmeter is connected in parallel with a component; an ammeter is connected in series.

Energy (J) $E = Q \times V$

Ammeter in series, voltmeter across the resistor

Worked example

20 C of charge passes through a lamp with a potential difference of 6 V across it. How much energy is transferred?

1
Write the equation: $E = Q \times V$ .
2
Substitute: $E = 20 \times 6$ .

How should a voltmeter be connected to measure the potential difference across a lamp?

Resistance

Resistance opposes current: for the same potential difference, more resistance means less current.

Resistance opposes the flow of current. It is in ohms ( $\Omega$ ). A variable resistor changes the resistance, and so controls the current: increasing the resistance reduces the current for a given potential difference.

Fixed resistor: current is directly proportional to potential difference, so the resistance is constant

Potential difference (V) $V = I \times R$

Worked example

A resistor has 12 V across it and 3 A through it. Calculate its resistance.

1
Rearrange: $R = V \div I$ .
2
Substitute: $R = 12 \div 3$ .

The resistance in a circuit is increased but the potential difference stays the same. What happens to the current?

Series and parallel circuits

Adding resistors in series raises the total resistance; adding them in parallel lowers it.

In series, current passes through each resistor in turn, so each adds opposition. In parallel, current has more paths, so the total resistance falls. Use $V = I \times R$ on each component, and add p.d.s and resistances around a series loop.

Series Parallel Current Same at every point Splits; total = sum of branches Potential difference Shared; adds up to the source p.d. Same across each branch Adding resistors Total resistance increases Total resistance decreases

	Series	Parallel
Current	Same at every point	Splits; total = sum of branches
Potential difference	Shared; adds up to the source p.d.	Same across each branch
Adding resistors	Total resistance increases	Total resistance decreases

Series

Current:: Same at every point
Potential difference:: Shared; adds up to the source p.d.
Adding resistors:: Total resistance increases

Parallel

Current:: Splits; total = sum of branches
Potential difference:: Same across each branch
Adding resistors:: Total resistance decreases

Parallel circuit: each lamp has its own branch

A second identical resistor is added in parallel with the first. What happens to the total resistance?

Try an exam question

A 6 V cell is connected across a 12 Ω resistor. Calculate the current in the resistor, and the charge that flows in 30 s.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.