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4.2.8Electricity and Magnetism

Potential Difference — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 26 of 65 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

4.2.8 Potential Difference — IGCSE PHYSICS notes: Define potential difference as energy transferred per unit charge between two points, use V = W / Q and measure p.d.

  • Potential difference (p.d.)
  • Also called voltage
  • What the symbols mean
  • Through a cell
  • Through components
  • High resistance

Part 1 of 7Definition of p.d.

Definition of p.d.
Potential difference (p.d.)
  • Potential difference (p.d.) the work done by a unit charge passing through a component, or the energy transferred per unit charge between two points.
Unit
  • Unit the volt (V), where 1 V = 1 J/C.
  • A p.d. of 1 V means 1 J of energy is transferred for each coulomb that passes.
Also called voltage
  • Also called voltage the term potential difference is better because it describes a difference between two points.

Part 2 of 7The equation V = W / Q (Extended)

V
W / Q
  • V is p.d.
  • (V), W is energy transferred (J) and Q is charge (C).
  • Rearranged, W = V Q.
    What the symbols mean: E or W = energy transferred (J) · Q = charge (C) · V = potential difference (V) · I = current (A) · t = time (s)
  • If a current I flows for a time t then Q = I t, so W = V I t.
Units
  • Units convert kJ to J and minutes to seconds before calculating.

Part 3 of 7Energy changes as charge flows

1Through a cell
  • Through a cell charge gains energy, so each coulomb gains 12 J in a 12 V cell.
2Through components
  • Through components charge loses energy, giving it to the wires and components as heat, light or kinetic energy.
3Lamp
  • Lamp a p.d. of 6.0 V means each coulomb gives up 6.0 J as heat and light.
4Resistor
  • Resistor all the energy becomes heat.

Part 4 of 7Measuring p.d.

Voltmeter
  • Voltmeter connected in parallel across the component, because p.d. is the difference between two points.
High resistance
  • High resistance a voltmeter has a very high resistance so only a tiny current flows through it.
Analogue voltmeter
  • Analogue voltmeter read from directly in front to avoid parallax error.
Digital voltmeter
  • Digital voltmeter gives a direct reading and may flicker between values.
Zero error
  • Zero error check the reading is zero first or subtract the zero value.
1
Build the circuit first and check it works.
2
Connect the voltmeter leads one on each side of the component.
3
Connect the + terminal of the voltmeter nearer the + side of the supply.
4
Read the p.d. in volts.

Part 5 of 7What a p.d. does

1Drives current
  • Drives current a p.d. across a component makes charge flow, and a larger p.d. gives a larger current.
2Zero p.d.
  • Zero p.d. between two points at the same potential no current flows.
3Voltmeter across a closed switch
  • Voltmeter across a closed switch it reads zero because a closed switch has almost no resistance.
4Voltmeter across an open switch
  • Voltmeter across an open switch it reads nearly the full supply p.d., so the gap showing the supply voltage is the break in the circuit.

Part 6 of 7P.d. in simple circuits

Series
  • Series the p.d.s across the components add up to the supply p.d.
Parallel
  • Parallel each branch has the same p.d. as the supply.
Energy split
  • Energy split with a 12 V supply and 8 V across the first resistor, 4 V is across the second,
    • so each coulomb gives 8 J to the first and 4 J to the second.

Part 7 of 7More from the specification

More from the specification
Triboelectric Charging by Friction
  • Rubbing an insulating polythene rod with a cloth transfers negative electrons from one material to the other, leaving the rod negatively charged and the cloth positively charged.
Gold Leaf Electroscope Function
  • When a charged object is brought near or touches the metal plate of an electroscope, like electrical charges repel down into the stem and movable gold leaf,
  • causing the leaf to diverge in the electric field.
Electric Potential Difference Definition
  • Potential difference (voltage) V across a component is defined as the electrical energy transferred per unit charge, V = W / Q (measured in volts,
  • where 1 V = 1 J/C).
Electrical Power Dissipation
  • The rate of electrical energy delivered to a component by power supplies is given by P = I V = I2 R, describing how potential difference and current dictate power dissipation.
Triboelectric Charging by Friction
  • Rubbing an insulating polythene rod with a cloth transfers negative electrons from one material to the other, leaving the rod negatively charged and the cloth positively charged.
Gold Leaf Electroscope Function
  • When a charged object is brought near or touches the metal plate of an electroscope, like electrical charges repel down into the stem and movable gold leaf,
  • causing the leaf to diverge in the electric field.
Electric Potential Difference Definition
  • Potential difference (voltage) V across a component is defined as the electrical energy transferred per unit charge, V = W / Q (measured in volts,
  • where 1 V = 1 J/C).
Electrical Power Dissipation
  • The rate of electrical energy delivered to a component by power supplies is given by P = I V = I2 R, describing how potential difference and current dictate power dissipation.
Electromotive force
  • Energy transferred from other forms to electrical energy per unit charge.
Gold-leaf electroscope
  • A device in which a charged gold leaf falls when photoelectrons discharge the plate.
Mains supply
  • Domestic a.c. supply, rated 230 V rms at 50 Hz in the UK.
Electrical power
  • Rate of energy transfer, P = I V.
    What the symbols mean: P = power (W) · I = current (A) · V = potential difference (V)
Electron
  • Negatively charged particle in an atom.
Electroscope
  • Instrument with a metal cap, stem and gold leaf used to detect charge.
Conductor
  • Material that allows current to flow.
Insulator
  • Material that does not allow current to flow.
Electric field
  • Region of space in which a charge experiences a force.
Alternating current
  • Current that continually reverses direction.
Field lines
  • Lines showing the direction and strength of a magnetic field.
Electric charge
  • Property of matter that causes electric forces; positive or negative, measured in coulombs (C).

Key terms

TermMeaning
Potential differenceWork done by a unit charge passing through a component.
VoltOne joule per coulomb.
VoltmeterInstrument connected in parallel that measures p.d.
Energy transferredW = V Q, in joules.
Zero errorA non-zero reading when the meter should read zero.

Avoid these mistakes

1
Connecting a voltmeter in series — it must be in parallel across the component.
2
Saying p.d. flows through a component — current flows, p.d. is across it.
3
Mixing up e.m.f. and p.d. — e.m.f. is energy supplied per coulomb, p.d. is energy transferred per coulomb.
4
Using time in minutes in W = VIt — it must be in seconds.

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