Current, Potential Difference & Resistance (Edexcel GCSE Physics): Revision notes

Section 1

What is inside an atom, and what is electric current?

An atom has a small, dense nucleus (containing protons and neutrons) surrounded by electrons. Protons are positively charged, neutrons are uncharged, and electrons are negatively charged.

Electric current is the rate of flow of electric charge.
In metals, current is a flow of electrons through the material.
Current equation: charge = current × time, or Q = I × t (charge in coulombs, C; current in amperes, A; time in seconds, s).
A closed circuit including a source of potential difference (such as a cell) will have a current flowing in it.
Current is conserved at a junction — the total current flowing into a junction equals the total current flowing out.

Key termselectric currentcharge

Example

A current of 2 A flows for 10 s. Charge = I × t = 2 × 10 = 20 C.

Section 2

What is potential difference?

Potential difference (voltage) is the energy transferred per unit charge passed between two points in a circuit.

Since energy is measured in joules and charge in coulombs, one volt is equal to one joule per coulomb.
Energy transferred equation: E = Q × V (energy transferred in joules; charge in coulombs; potential difference in volts).
A voltmeter is connected in parallel with a component to measure the potential difference across it.
An ammeter is connected in series with a component to measure the current through it.

Key termspotential differencevolt

Section 3

How are current, potential difference and resistance related?

Resistance opposes the flow of current in a circuit.

Equation: V = I × R (potential difference in volts = current in amperes × resistance in ohms).
A variable resistor can be used to change the resistance in a circuit and therefore control the current — increasing resistance decreases current for a given potential difference, and vice versa.
Series and parallel circuits behave differently: a circuit diagram can show cells, batteries, switches, resistors, variable resistors, lamps, motors, diodes, thermistors, LDRs and LEDs using standard symbols.

Key termsresistancevariable resistor

Exam tip

Always state the equation with correct subject before substituting numbers — examiners award a mark for correct rearrangement.

Section 4

How does resistance differ in series and parallel circuits?

	Series circuit	Parallel circuit
Current	Same at every point in the circuit	Splits between branches; total current = sum of branch currents
Potential difference	Shared between components (adds up to the source p.d.)	Same across each parallel branch
Effect of adding resistors	Total resistance increases (resistors add in series)	Total resistance decreases (extra paths for current to flow)

Adding resistors in series increases total resistance because the current has to pass through each resistor in turn — each one adds opposition.
Adding resistors in parallel decreases total resistance because the current has more paths to flow through, making it easier overall for charge to pass.
Currents, potential differences and resistances in series circuits can be calculated using V = I × R applied to each component, and adding potential differences/resistances around the loop.

Common mistake

A common error is assuming adding any resistor always increases total circuit resistance — this is only true in series; in parallel it decreases.

Must Know

Current is the rate of flow of charge; in metals, it is a flow of electrons. Q = I × t.
Potential difference is energy transferred per unit charge; 1 volt = 1 joule per coulomb. E = Q × V.
Voltmeter connects in parallel; ammeter connects in series.
V = I × R relates potential difference, current and resistance.
Current is conserved at a junction (total in = total out).
Adding resistors in series increases total resistance; adding resistors in parallel decreases total resistance.
A variable resistor changes resistance to control current in a circuit.

That's the notes covered.

Carry on to the next subtopic.