The charge passing a point when a current of one ampere flows for one second.

Define potential difference.

The energy transferred per unit charge as it moves between two points: V = W / Q.

What is one volt in terms of joules and coulombs?

One joule per coulomb (J C⁻¹).

The ratio of the potential difference across a component to the current through it: R = V / I.

The current in a conductor is directly proportional to the potential difference across it, provided the temperature is constant.

What does a graph of I against V look like for an ohmic conductor?

A straight line through the origin.

Why does a filament lamp not obey Ohm's law?

As the current rises, the filament heats up and its resistance increases, so I is not proportional to V.

Why does the resistance of a metal increase as its temperature rises?

The lattice ions vibrate more, so the electrons collide with them more often.

How do you find the number of electrons passing a point in time t?

Charge Q = It, divided by the charge of one electron, 1.60 × 10⁻¹⁹ C.

How should an ammeter and a voltmeter be connected to a component?

Ammeter in series with it; voltmeter in parallel across it.

Current, potential difference and resistanceEdexcel A-Level Physics: Flashcards

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Define electric current.

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Define electric current.: The rate of flow of charge: I = Q / t.
What is one coulomb?: The charge passing a point when a current of one ampere flows for one second.
Define potential difference.: The energy transferred per unit charge as it moves between two points: V = W / Q.
What is one volt in terms of joules and coulombs?: One joule per coulomb (J C⁻¹).
Define resistance.: The ratio of the potential difference across a component to the current through it: R = V / I.
What is one ohm?: One volt per ampere.
State Ohm's law.: The current in a conductor is directly proportional to the potential difference across it, provided the temperature is constant.
What does a graph of I against V look like for an ohmic conductor?: A straight line through the origin.
Why does a filament lamp not obey Ohm's law?: As the current rises, the filament heats up and its resistance increases, so I is not proportional to V.
Why does the resistance of a metal increase as its temperature rises?: The lattice ions vibrate more, so the electrons collide with them more often.
How do you find the number of electrons passing a point in time t?: Charge Q = It, divided by the charge of one electron, 1.60 × 10⁻¹⁹ C.
How should an ammeter and a voltmeter be connected to a component?: Ammeter in series with it; voltmeter in parallel across it.
What is the energy transferred when charge Q passes through a p.d. V?: W = QV.

Exam questions on Current, potential difference and resistance

A phone charger delivers a constant current of 2.0 A at a potential difference of 5.0 V. A phone is charged for 45 minutes.
Calculate the number of electrons that pass through the charger cable each second. The charge of an electron is 1.60 × 10⁻¹⁹ C.2 marks
A student investigates a filament lamp. When the potential difference across the lamp is 6.0 V the current is 1.5 A, and when it is 12 V the current is 2.0 A.
Use the data to explain why the filament lamp does not obey Ohm's law.2 marks
The heating element of an electric kettle is connected to a 230 V supply. When the water is boiling the element carries a steady current of 8.7 A for 3.0 minutes.
Calculate the charge that passes through the element and the energy it transfers in this time.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).