Home / iGCSE / Coordinated Sciences / CIE iGCSE Co-ordinated Sciences-P4.2.3 Voltage (electromotive force and potential difference)- Study Notes

CIE iGCSE Co-ordinated Sciences-P4.2.3 Voltage (electromotive force and potential difference)- Study Notes- New Syllabus

CIE iGCSE Co-ordinated Sciences-P4.2.3 Voltage (electromotive force and potential difference) – Study Notes

CIE iGCSE Co-ordinated Sciences-P4.2.3 Voltage (electromotive force and potential difference) – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.

Key Concepts:

Core

  • Describe the voltage of the source as the cause of current in the circuit
  • Know that the voltage of the source is shared between the components in a series circuit
  • Describe the use of voltmeters (analogue and digital) with different ranges

Supplement

  • Define electromotive force (e.m.f.) as the electrical work done by a source in moving a unit charge around a complete circuit
  • Know that e.m.f. is measured in volts (V)
  • Define potential difference (p.d.) as the work done by a unit charge passing between two points in a circuit
  • Know that the p.d. between two points is measured in volts (V)

CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics

Voltage and Current in a Circuit

 Voltage as the Cause of Current:

The voltage of a source (also called potential difference, p.d.) is the driving force that pushes charge around a circuit.

  • Voltage provides energy to charges → this energy is transferred to components in the circuit (lamps, resistors, motors).
  • Higher voltage means stronger “push” on charges, so a larger current flows (for the same resistance).

Relationship:

\(\mathrm{V = \dfrac{W}{Q}}\), where

  • \(\mathrm{V}\) = voltage (volts, V) 
  • \(\mathrm{W}\) = energy transferred (joules, J)
  • \(\mathrm{Q}\) = charge (coulombs, C)

Voltage in Series Circuits:

In a series circuit, the source voltage is shared between components.

  • The sum of the voltages across each component = total voltage of the source.
  • If two identical resistors are connected in series across a 12 V battery, each resistor has 6 V across it.
  • The greater the resistance of a component, the larger the share of voltage it receives.

Everyday Understanding:

  • A 9 V battery pushes charges around a torch circuit → the voltage provides energy for the bulb to light.
  • In fairy lights (series bulbs), the supply voltage is divided among the bulbs so that each gets a smaller share, preventing overheating.

Example :

Two resistors of equal resistance are connected in series across a 12 V battery. What is the voltage across each resistor?

▶️ Answer/Explanation

Step 1: Total voltage of the source = 12 V.

Step 2: In series, voltage is shared according to resistance. Since resistances are equal, the voltage divides equally.

Step 3: Each resistor gets 6 V.

Final Answer: Voltage across each resistor = 6 V.

Use of Voltmeters

A voltmeter is an instrument used to measure the voltage (potential difference) between two points in a circuit.

General Features:

  • A voltmeter is always connected in parallel with the component across which the voltage is measured, because voltage is the difference in potential between two points.
  • It has a very high resistance so that it does not allow significant current to pass through it → preventing disturbance of the circuit.
  • The reading is in volts (V).

 Analogue Voltmeters:

  • Use a pointer (needle) moving across a scale to show the voltage.
  • Different ranges are available, e.g. 0–5 V, 0–10 V, 0–50 V.
  • The correct range must be chosen before connecting: – Too low → risk of damaging the meter. – Too high → less sensitive reading.
  • Analogue meters are useful for observing gradual changes in voltage.

Digital Voltmeters:

  • Show voltage as a numerical value on a screen.
  • Often more accurate and easier to read than analogue meters.
  • Many digital voltmeters automatically select the correct range, or allow manual selection.
  • Can measure very small voltages (mV) as well as large voltages (hundreds of volts).

 Choosing the Range:

  • Always start with the highest range to avoid damaging the voltmeter.
  • Then reduce the range to get the most accurate measurement.

 Applications:

  • Checking the voltage of a household battery (e.g. 1.5 V, 9 V).
  • Measuring mains supply voltage (about 230 V in many countries).
  • Monitoring voltage levels in electrical circuits or devices.

Example :

A student wants to measure the voltage across a lamp connected to a 12 V supply. Which voltmeter range should be used?

▶️ Answer/Explanation

Step 1: The expected voltage is about 12 V.

Step 2: Always select a range higher than the expected value to prevent damage.

Step 3: A 0–20 V range is suitable (0–10 V would be too low).

Final Answer: The student should use the 0–20 V range.

Electromotive Force (e.m.f.)

The electromotive force (e.m.f.) of a source is the electrical work done by the source in moving a unit charge around a complete circuit.

Explanation:

When a battery or power supply drives charge through a circuit, it transfers energy to the charges.

  • The e.m.f. represents the energy supplied per coulomb of charge.

Formula:

$\mathrm{e.m.f. = \dfrac{W}{Q}} $

where: 

  • \(\mathrm{W}\) = work done / energy supplied (joules, J) 
  • \(\mathrm{Q}\) = charge moved (coulombs, C)
  • This definition is very similar to voltage, but specifically refers to the energy provided by the source.

 Unit of e.m.f.:

  • e.m.f. is measured in volts (V).
  • 1 volt = 1 joule of energy supplied per coulomb of charge.

Everyday Understanding:

  • A 1.5 V cell supplies 1.5 joules of energy for every coulomb of charge it moves around the circuit.
  • A 12 V car battery supplies 12 joules per coulomb of charge.

Example :

A battery does 24 J of work in moving 4 C of charge around a circuit. What is the e.m.f. of the battery?

▶️ Answer/Explanation

Step 1: Use formula \(\mathrm{e.m.f. = \dfrac{W}{Q}}\).

Step 2: Substitute: \(\mathrm{e.m.f. = \dfrac{24}{4} = 6 \, V}\).

Final Answer: The battery has an e.m.f. of 6 V.

Potential Difference (p.d.)

The potential difference (p.d.) between two points in a circuit is the work done by a unit charge in passing between those two points.

Explanation:

When charge flows through a component (e.g. lamp, resistor), it loses energy as it transfers it to the component.

  • The p.d. between two points measures the amount of energy transferred per coulomb of charge.

Formula:

$ \mathrm{V = \dfrac{W}{Q}}$

  • where: 
  • \(\mathrm{V}\) = potential difference (volts, V) 
  • \(\mathrm{W}\) = work done / energy transferred (joules, J) 
  • \(\mathrm{Q}\) = charge (coulombs, C)

 Unit of Potential Difference:

  • P.d. is measured in volts (V).
  • 1 volt = 1 joule of energy transferred per coulomb of charge.

Difference Between e.m.f. and p.d.:

  • e.m.f.: Energy supplied per coulomb by the source.
  • p.d.: Energy transferred per coulomb to a component in the circuit.

Everyday Understanding:

  • A lamp connected across 6 V has 6 joules of energy transferred to it by each coulomb of charge.
  • A phone charger supplying 5 V means that each coulomb of charge transfers 5 J of energy to the phone’s circuit.

Example :

12 J of energy is transferred when 3 C of charge passes through a resistor. Calculate the potential difference across the resistor.

▶️ Answer/Explanation

Step 1: Use formula \(\mathrm{V = \dfrac{W}{Q}}\).

Step 2: Substitute: \(\mathrm{V = \dfrac{12}{3} = 4 \, V}\).

Final Answer: The potential difference across the resistor is 4 V.

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