What is the unit of charge, and how is it defined?

The coulomb; one coulomb is the charge passing a point each second when the current is 1 A.

Define potential difference.

The work done (energy transferred) per unit charge between two points: V = W / Q.

One joule of energy transferred per coulomb of charge.

The pd across a component divided by the current through it: R = V / I.

What does a battery's pd of 9.0 V mean?

9.0 J of energy is transferred to each coulomb of charge that passes through the battery.

What is the common unit conversion error with current in this topic?

Using mA as A; divide by 1000 to convert mA to A.

Why is a bird on a bare cable not harmed by the large current in the cable?

The pd between its feet is tiny, so the current through its body is tiny.

Current, potential difference and resistanceAQA 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 the unit of charge, and how is it defined?: The coulomb; one coulomb is the charge passing a point each second when the current is 1 A.
Define potential difference.: The work done (energy transferred) per unit charge between two points: V = W / Q.
Define the volt.: One joule of energy transferred per coulomb of charge.
Define resistance.: The pd across a component divided by the current through it: R = V / I.
Define the ohm.: One volt per ampere.
A current of 3.0 A flows for 20 s. How much charge passes?: Q = I t = 3.0 × 20 = 60 C.
How much energy is transferred when 5.0 C passes through a pd of 12 V?: W = Q V = 5.0 × 12 = 60 J.
What current flows through a 40 Ω resistor with 6.0 V across it?: I = V / R = 6.0 / 40 = 0.15 A.
What does a battery's pd of 9.0 V mean?: 9.0 J of energy is transferred to each coulomb of charge that passes through the battery.
What is the common unit conversion error with current in this topic?: Using mA as A; divide by 1000 to convert mA to A.
Why is a bird on a bare cable not harmed by the large current in the cable?: The pd between its feet is tiny, so the current through its body is tiny.

Exam questions on Current, potential difference and resistance

A mobile phone is charged for 40 minutes with a constant current of 1.5 A. The pd across the phone's battery is 3.7 V while it charges.
State what is meant by a pd of 3.7 V across the battery, and calculate the energy transferred to the battery while 3.6 × 10³ C of charge flows through it.2 marks
A student connects a resistor to a power supply. When the pd across the resistor is 6.0 V, the current through it is 24 mA. The resistor is an ohmic conductor and its temperature stays constant.
The pd of 6.0 V is applied for 5.0 minutes. Calculate the charge that passes through the resistor and the energy transferred to the resistor in this time.2 marks
A car starter motor is supplied by a 12 V battery. During one start the motor draws a constant current of 150 A for 3.0 s. The battery can supply a total charge of 1.6 × 10⁵ C before it is flat.
Calculate the charge that flows through the starter motor during one start, the energy transferred to the motor, and the resistance of the motor circuit.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).