Charge, current, voltage and resistance

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Lesson notes

Big idea: charge, current, voltage and resistance

Key concept: Relationships. Current is charge flow per unit time. Potential difference is energy transferred per unit charge; resistance equals potential difference divided by current.
Related concepts: Models and evidence. Use a scientific explanation to make predictions, then test it against observations.
Global context: Scientific and technical innovation. Electrical relationships help choose components for low-voltage devices.

Electric Charge

There are two types of electric charge: positive and negative.
Inside an atom, electrons are negatively charged, protons are positively charged, and neutrons are neutral.
Atoms have equal numbers of protons and electrons, so overall they are neutral.
Like charges repel, opposite charges attract.
Electric charge is measured in coulombs (C).

Structure of an atom showing protons, neutrons and electron shells.

Charging by Friction

Rubbing two insulating materials together transfers electrons from one to the other.
The material that gains electrons becomes negatively charged; the material that loses electrons becomes positively charged.
Example: rubbing a polythene rod with a cloth transfers electrons from the cloth to the rod → rod becomes negative, cloth becomes positive.
Example: rubbing an acetate rod with a cloth transfers electrons from the rod to the cloth → rod becomes positive, cloth becomes negative.
Only electrons move during charging by friction; protons do not move.

Apparatus for demonstrating charging by friction using a polythene rod and another insulating material.

Electric Fields

An electric field is a region where a charge experiences a force.
Field lines are directed from positive to negative charge.
Around a point charge, field lines are radial: outward for positive, inward for negative.
Between two parallel plates, the field is uniform with parallel, equally spaced lines.
Field lines are always perpendicular to the surface of a conductor.

Demonstration of a Van de Graaff generator with streamers and field lines around a charged sphere.

Conductors and Insulators

Conductors (e.g., metals) allow charge to flow easily because they have delocalised electrons.
Insulators (e.g., plastic, rubber, glass) have no free charges and do not allow charge to flow easily.
A gold-leaf electroscope can test conductivity: a good conductor makes the leaf fall quickly; an insulator makes it fall slowly or not at all.

Current

Electric current is the rate of flow of charge.
Current is measured in amperes (A) using an ammeter connected in series.
Conventional current flows from positive to negative terminal (opposite to electron flow).
Equation: I = \frac{Q}{t} , where Q is charge in coulombs, t is time in seconds.
In metals, current is a flow of negatively charged electrons.

Direct and Alternating Current

Direct current (d.c.) flows in one direction only; produced by cells and batteries.
Alternating current (a.c.) changes direction periodically; produced by mains electricity (UK: 230 V, 50 Hz).
Graph of d.c. is a horizontal line; graph of a.c. is a sine wave.

Electromotive Force and Potential Difference

Electromotive force (e.m.f.) is the energy supplied per unit charge by a source: E = \frac{W}{Q} .
Potential difference (p.d.) is the energy transferred per unit charge across a component: V = \frac{W}{Q} .
Both are measured in volts (V).
Voltmeter is connected in parallel to measure p.d.

Resistance and Ohm's Law

Resistance is the opposition to current; measured in ohms (Ω).
Ohm's law: R = \frac{V}{I} .
For a fixed resistor at constant temperature, current is directly proportional to voltage (ohmic conductor).
For a filament lamp, resistance increases with temperature (non-ohmic).
A diode allows current in one direction only (forward bias).

Resistance of a Wire

Resistance is directly proportional to length: longer wire → higher resistance.
Resistance is inversely proportional to cross-sectional area: thicker wire → lower resistance.
Increasing temperature increases resistance due to more frequent collisions with vibrating metal ions.

Electrical Energy and Power

Electrical power P = VI , measured in watts (W).
Electrical energy E = VIt , measured in joules (J).
Alternative: P = I²R and P = \frac{V²}{R} .

Think like a scientist

Measure current through a resistor at several low voltages in a teacher-approved circuit.
Comparison: the potential difference across the resistor. Outcome: current in amperes.
Control: keep the resistor's temperature approximately constant. Explain why this makes the comparison fairer.
Evidence: Use a consistent method, repeated observations where appropriate and a table with labelled quantities and units. Keep unexpected results and investigate their cause.
Safety: Practical activities need teacher supervision and an appropriate risk assessment. Use the provided data or simulation where the investigation specifies it.
Inquiry task: State a testable question, predict the outcome using the science, then explain how your observations would support or challenge the prediction.

Evaluate the science

Electrical relationships help choose components for low-voltage devices.
A resistor that heats up may change resistance, so constant resistance must be tested rather than assumed.
Evaluation task: Link your conclusion to evidence, identify a limitation and suggest a specific improvement. Distinguish a measured result from an explanation of its cause.

Slides

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Practice questions

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1.A polythene rod is rubbed with a cloth and becomes negatively charged. This happens because
Easy
- Aelectrons move from the cloth to the rod
- Belectrons move from the rod to the cloth
- Cprotons move from the cloth to the rod
- Dprotons move from the rod to the cloth
2.Which of the following is a good conductor of electricity?
Easy
- ARubber
- BPlastic
- CCopper
- DGlass
3.A lamp is rated at 12 V, 50 W. What is the current through the lamp when operating normally?
Medium
- A0.24 A
- B4.17 A
- C600 A
- D38 A
4.The resistance of a wire is 10 Ω. What is the potential difference across the wire when a current of 0.5 A flows through it?
Easy
- A0.05 V
- B5 V
- C20 V
- D10.5 V
5.A student connects a voltmeter to measure the potential difference across a lamp. How should the voltmeter be connected?
Easy
- AIn series with the lamp
- BIn parallel with the lamp
- CIn series with the power supply
- DIn parallel with the power supply
6.A charge of 270 kC passes through a 12 V battery. How much electrical energy is transferred?
Medium
- A22.5 J
- B3.24 × 10⁶ J
- C3.24 × 10³ J
- D2.25 × 10⁴ J
7.Two identical resistors are connected in parallel. The total resistance is 2 Ω. What is the resistance of each resistor?
Medium
- A1 Ω
- B2 Ω
- C4 Ω
- D0.5 Ω
8.What is the unit of electromotive force (e.m.f.)?
Easy
- AAmpere
- BVolt
- COhm
- DCoulomb

Revision notes

## Big idea: charge, current, voltage and resistance - **Key concept: Relationships.** Current is charge flow per unit time. Potential difference is energy transferred per unit charge; resistance equals potential difference divided by current. - **Related concepts: Models and evidence.** Use a scientific explanation to make predictions, then test it against observations. - **Global context: Scientific and technical innovation.** Electrical relationships help choose components for low-voltage devices. ## Electric Charge - There are two types of electric charge: **positive** and **negative**. - Inside an atom, **electrons** are negatively charged, **protons** are positively charged, and **neutrons** are neutral. - Atoms have equal numbers of protons and electrons, so overall they are **neutral**. - **Like charges repel**, **opposite charges attract**. - Electric charge is measured in **coulombs (C)**. ## Charging by Friction - Rubbing two insulating materials together transfers **electrons** from one to the other. - The material that **gains electrons** becomes **negatively charged**; the material that **loses electrons** becomes **positively charged**. - Example: rubbing a polythene rod with a cloth transfers electrons from the cloth to the rod → rod becomes **negative**, cloth becomes **positive**. - Example: rubbing an acetate rod with a cloth transfers electrons from the rod to the cloth → rod becomes **positive**, cloth becomes **negative**. - Only **electrons** move during charging by friction; protons do not move. ## Electric Fields - An **electric field** is a region where a charge experiences a force. - Field lines are directed from **positive** to **negative** charge. - Around a point charge, field lines are **radial**: outward for positive, inward for negative. - Between two parallel plates, the field is **uniform** with parallel, equally spaced lines. - Field lines are always **perpendicular** to the surface of a conductor. ## Conductors and Insulators - **Conductors** (e.g., metals) allow charge to flow easily because they have **delocalised electrons**. - **Insulators** (e.g., plastic, rubber, glass) have no free charges and do not allow charge to flow easily. - A gold-leaf electroscope can test conductivity: a good conductor makes the leaf **fall quickly**; an insulator makes it fall slowly or not at all. ## Current - **Electric current** is the **rate of flow of charge**. - Current is measured in **amperes (A)** using an **ammeter** connected in **series**. - **Conventional current** flows from **positive** to **negative** terminal (opposite to electron flow). - Equation: \( I = \frac{Q}{t} \), where \( Q \) is charge in coulombs, \( t \) is time in seconds. - In metals, current is a flow of **negatively charged electrons**. ## Direct and Alternating Current - **Direct current (d.c.)** flows in **one direction** only; produced by cells and batteries. - **Alternating current (a.c.)** **changes direction** periodically; produced by mains electricity (UK: 230 V, 50 Hz). - Graph of d.c. is a horizontal line; graph of a.c. is a sine wave. ## Electromotive Force and Potential Difference - **Electromotive force (e.m.f.)** is the energy supplied per unit charge by a source: \( E = \frac{W}{Q} \). - **Potential difference (p.d.)** is the energy transferred per unit charge across a component: \( V = \frac{W}{Q} \). - Both are measured in **volts (V)**. - **Voltmeter** is connected in **parallel** to measure p.d. ## Resistance and Ohm's Law - **Resistance** is the opposition to current; measured in **ohms (Ω)**. - Ohm's law: \( R = \frac{V}{I} \). - For a fixed resistor at constant temperature, current is **directly proportional** to voltage (ohmic conductor). - For a filament lamp, resistance **increases** with temperature (non-ohmic). - A **diode** allows current in **one direction only** (forward bias). ## Resistance of a Wire - Resistance is **directly proportional** to length: longer wire → higher resistance. - Resistance is **inversely proportional** to cross-sectional area: thicker wire → lower resistance. - Increasing temperature increases resistance due to more frequent collisions with vibrating metal ions. ## Electrical Energy and Power - **Electrical power** \( P = VI \), measured in **watts (W)**. - **Electrical energy** \( E = VIt \), measured in **joules (J)**. - Alternative: \( P = I^2R \) and \( P = \frac{V^2}{R} \). ## Think like a scientist - Measure current through a resistor at several low voltages in a teacher-approved circuit. - **Comparison:** the potential difference across the resistor. **Outcome:** current in amperes. - **Control:** keep the resistor's temperature approximately constant. Explain why this makes the comparison fairer. - **Evidence:** Use a consistent method, repeated observations where appropriate and a table with labelled quantities and units. Keep unexpected results and investigate their cause. - **Safety:** Practical activities need teacher supervision and an appropriate risk assessment. Use the provided data or simulation where the investigation specifies it. - **Inquiry task:** State a testable question, predict the outcome using the science, then explain how your observations would support or challenge the prediction. ## Evaluate the science - Electrical relationships help choose components for low-voltage devices. - A resistor that heats up may change resistance, so constant resistance must be tested rather than assumed. - **Evaluation task:** Link your conclusion to evidence, identify a limitation and suggest a specific improvement. Distinguish a measured result from an explanation of its cause.

Electric Fields: An **electric field** is a region where a charge experiences a force.
Conductors and Insulators: **Conductors** (e.g., metals) allow charge to flow easily because they have **delocalised electrons**.
Electric Current: Define electric current as rate of flow of charge and measure it with an ammeter.
Electric Charge Interactions: Understand positive and negative charges, attraction and repulsion, and charging by friction.
Designing scientific inquiries: Plan a fair comparison: Measure current through a resistor at several low voltages in a teacher-approved circuit.
Evaluating evidence and impacts: Electrical relationships help choose components for low-voltage devices. A resistor that heats up may change resistance, so constant resistance must be tested rather than assumed.
Electric Charge: There are two types of electric charge: **positive** and **negative**.
Charging by Friction: Rubbing two insulating materials together transfers **electrons** from one to the other.
Current: **Electric current** is the **rate of flow of charge**.
Direct and Alternating Current: **Direct current (d.c.)** flows in **one direction** only; produced by cells and batteries.
Electromotive Force and Potential Difference: **Electromotive force (e.m.f.)** is the energy supplied per unit charge by a source: \( E = \frac{W}{Q} \).
Resistance and Ohm's Law: **Resistance** is the opposition to current; measured in **ohms (Ω)**.
Resistance of a Wire: Resistance is **directly proportional** to length: longer wire → higher resistance.
Electrical Energy and Power: **Electrical power** \( P = VI \), measured in **watts (W)**.
positive: **Conventional current** flows from **positive** to **negative** terminal (opposite to electron flow).
negative: **Conventional current** flows from **positive** to **negative** terminal (opposite to electron flow).

Flashcards

A polythene rod is rubbed with a cloth and becomes negatively charged. This happens because

electrons move from the cloth to the rod When a polythene rod is rubbed with a cloth, electrons are transferred from the cloth to the rod, giving the rod a negative charge. Protons do not move in charging by friction.

Which of the following is a good conductor of electricity?

Copper Copper is a metal and a good conductor of electricity. Rubber, plastic, and glass are insulators.

A lamp is rated at 12 V, 50 W. What is the current through the lamp when operating normally?

4.17 A Power = voltage × current, so current = power / voltage = 50 W / 12 V = 4.17 A. Option A results from multiplying instead of dividing, C from 12×50, D from 50/1.3.

The resistance of a wire is 10 Ω. What is the potential difference across the wire when a current of 0.5 A flows through it?

5 V Using Ohm's law V = IR = 0.5 A × 10 Ω = 5 V. Option A is I/R, C is R/I, D is sum.

A student connects a voltmeter to measure the potential difference across a lamp. How should the voltmeter be connected?

In parallel with the lamp A voltmeter must be connected in parallel with the component across which the potential difference is to be measured. Series connection is for ammeters.

A charge of 270 kC passes through a 12 V battery. How much electrical energy is transferred?

3.24 × 10⁶ J Energy = charge × voltage = 270,000 C × 12 V = 3,240,000 J = 3.24 × 10⁶ J. Option A is charge/voltage, C is misplacing decimal, D is 270×12×10^?

Two identical resistors are connected in parallel. The total resistance is 2 Ω. What is the resistance of each resistor?

4 Ω For two identical resistors in parallel, total resistance R_total = R/2, so R = 2 × R_total = 4 Ω. Option A is half of total, B is same as total, D is quarter.

What is the unit of electromotive force (e.m.f.)?

Volt E.m.f. is measured in volts (V). Ampere is for current, ohm for resistance, coulomb for charge.

With the switch closed, 0.8 A flows through the first lamp. What current flows through the second lamp?

0.8 A There is one path for current, so both lamps carry 0.8 A.

A wire of length L and cross-sectional area A has resistance R. What is the resistance of a wire of the same material with length 2L and cross-sectional area 3A?

(2/3)R Resistance is proportional to length and inversely proportional to area. New resistance = (2L/ L) × (A/3A) × R = 2/3 R. Option B reverses ratio, C multiplies both, D inverts.

A student charges a polythene rod by rubbing it with a cloth. The rod becomes negatively charged. Which statement explains how the rod becomes negatively charged?

The rod gains electrons from the cloth. When rubbed, electrons transfer from the cloth to the rod, giving the rod a negative charge. Protons are not free to move.

The current in a lamp is 0.20 A and the potential difference across it is 6.0 V. What is the resistance of the lamp?

30 Ω Using Ohm's law, R = V/I = 6.0 V / 0.20 A = 30 Ω.

What is the unit of electric charge?

Coulomb Electric charge is measured in coulombs (C). Ampere is for current, volt for potential difference, ohm for resistance.

A positively charged rod is brought near a suspended, charged balloon. The balloon is repelled. What is the charge on the balloon?

Positive Like charges repel. Since the rod is positive, the balloon must also be positive to be repelled.

What is the correct definition of electric current?

The rate of flow of charge Current is defined as the rate of flow of electric charge, measured in amperes.

Which component is used to measure potential difference?

Voltmeter A voltmeter measures potential difference (voltage) and is connected in parallel.

How would you design the inquiry for charge, current, voltage and resistance?

Measure current through a resistor at several low voltages in a teacher-approved circuit. Compare the potential difference across the resistor; record current in amperes; keep the resistor's temperature approximately constant.

What limitation matters when applying charge, current, voltage and resistance?

A resistor that heats up may change resistance, so constant resistance must be tested rather than assumed.

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