Current and potential differenceIB MYP Physics: Revision notes
Section 1
Electric current
An electric current is the rate of flow of charge. In a metal wire the charge is carried by free electrons.
Q = I × t
- Q is charge, in coulombs (C)
- I is current, in amperes (A)
- t is time, in seconds (s)
Worked example: a current of 2.0 A flows for 3 minutes. Convert the time first: 3 × 60 = 180 s. Then Q = 2.0 × 180 = 360 C.
One ampere means one coulomb of charge passes a point every second.
Always change minutes or hours into seconds before using Q = I × t.
Section 2
Conventional current and electron flow
Before electrons were discovered, scientists decided that current flows from the positive terminal of a cell to the negative terminal. This is called conventional current.
We now know that the charge carriers in metals are electrons, which are negative. They are repelled by the negative terminal and attracted to the positive one, so electron flow goes from negative to positive: the opposite direction to conventional current.
In circuit diagrams, arrows for current always show the conventional direction.
Section 3
Potential difference
Potential difference (p.d.), also called voltage, is the energy transferred per unit charge as charge moves between two points.
V = E ÷ Q, or E = Q × V
- V is potential difference in volts (V)
- E is energy in joules (J)
- Q is charge in coulombs (C)
One volt means one joule of energy is transferred for each coulomb of charge. A 6.0 V battery gives 6.0 J to every coulomb that passes through it.
Worked example: 60 C of charge pass through a lamp with 6.0 V across it. E = 60 × 6.0 = 360 J.
Section 4
Cells and batteries
A cell is a single source of potential difference that uses a chemical reaction to push charge round a circuit. A battery is two or more cells connected together.
The cell does work on the charge, transferring chemical energy to the charge. This energy is then transferred to components such as lamps, where it is transferred as light and heat.
When cells are connected in series (end to end, positive to negative), their potential differences add up.
Section 5
Measuring current and potential difference
An ammeter measures current. It is connected in series, in the same loop as the component, so all the current passes through it.
A voltmeter measures potential difference. It is connected in parallel, across the component, so it forms a separate branch.
Mistakes here are common: a voltmeter in series would reduce the current almost to zero, and an ammeter placed in parallel would cause a short circuit.
Remember: Ammeter = A for Along the wire (series); Voltmeter = across the component (parallel).
Section 6
Circuit symbols and diagrams
Circuit diagrams use standard symbols so that anyone can read them. You should know the symbols for a cell, battery, switch (open and closed), lamp, resistor, ammeter (A in a circle), voltmeter (V in a circle), fuse and variable resistor.
A diagram is drawn with straight wires and right-angle corners. Components are joined in a closed loop, because a current only flows if the circuit is complete and the switch is closed.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Current and potential difference
- A camping torch has a small lamp that is powered by a 6.0 V battery. A family on a night walk in the Hajar Mountains switches the torch on and leaves it running for a short time.Calculate the energy transferred to the lamp when 60 C of charge passes through it from the 6.0 V battery.2 marks
- A student builds a simple circuit from a cell, a switch and a lamp, joined together with copper wires. She wants to find out how charge moves in the wires and to measure the current through the lamp and the potential difference across it.State the direction of conventional current in the circuit and explain why this is opposite to the direction in which the electrons move.2 marks
- A student investigates how the current through a small filament lamp depends on the potential difference across it. She connects the lamp to a variable power supply, with an ammeter and a voltmeter, and measures the current for four different values of potential difference.State a testable hypothesis for this investigation, give a scientific reason for it, and identify the dependent variable.3 marks
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).