Electric charge and current

Charge interactions, current, e.m.f., potential difference, resistance and wire dimensions.

SEC G3 Combined Science Physics component 2027
2 lessons

Before you begin

Every electric circuit works by moving charge. The lessons start with positive and negative charge, then define current as the rate of flow of charge and use Q = It. They finish with the e.m.f. of a source, the potential difference across a component, and resistance, R = V/I, including how a wire’s length and thickness change it.

Be comfortable with: SI prefixes, because currents are often given in mA, and energy stores and transfers, because p.d. is energy transferred per unit charge.

Learning goals

State that charges are positive or negative and are measured in coulombs
State that like charges repel and unlike charges attract
State current as rate of charge flow measured in amperes
Distinguish conventional current from electron flow
Apply charge equals current multiplied by time
State that source e.m.f. is measured in volts
Define component potential difference as work done per unit charge
State resistance as potential difference divided by current
Apply resistance equals potential difference divided by current
Apply wire-resistance proportionalities for length and cross-sectional area

Syllabus statements covered

state that there are positive and negative charges and that charge is measured in coulombs
state that unlike charges attract and like charges repel
state that current is the rate of flow of charge and that it is measured in amperes
distinguish between conventional current and electron flow
recall and apply the relationship charge = current × time to new situations or to solve related problems
state that the electromotive force (e.m.f.) of an electrical source of energy is measured in volts
state that the potential difference (p.d.) across a component in a circuit is the work done per unit charge in driving charges through the component and that it is measured in volts
state that resistance = p.d. / current
apply the relationship R = V / I to new situations or to solve related problems
recall and apply the relationship of the proportionality between resistance and the length and cross-sectional area of a wire to new situations or to solve related problems

Lessons

Work through them in order.

Practise and check

All practice for this topic

Topic reference

Quick reference

Quantity	Relationship	Unit
charge	charge = current × time, Q = It	coulomb, C
current	rate of flow of charge	ampere, A
e.m.f.	energy given to each unit of charge by a source	volt, V
potential difference	work done per unit charge in a component	volt, V
resistance	R = V/I	ohm, Ω
resistance of a wire	R ∝ length; R ∝ 1/(cross-sectional area)	for the same material and temperature

Core knowledge to remember

There are positive and negative charges, and charge is measured in coulombs. Unlike charges attract and like charges repel.
Current is the rate of flow of charge, measured in amperes.
Conventional current flows from the positive terminal to the negative; electrons in a metal flow the opposite way.
The e.m.f. of an electrical source of energy is measured in volts.
The potential difference across a component is the work done per unit charge in driving charges through it, measured in volts.
Resistance = p.d. / current.

Common mistakes

Using time in minutes instead of seconds in Q = It.
Saying conventional current is the direction electrons flow.
Calling p.d. “the current through” a component. P.d. is across a component; current flows through it.
Saying a thicker wire has more resistance. A larger cross-sectional area gives a smaller resistance.