Electrical Quantities

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Notes

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)** (Extended tier).

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 (Extended Tier)

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 (Extended tier).
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} (Extended tier).
**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} .

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

A simple series circuit with cell, switch, bulb, and ammeter.

Particle arrangement in solids, liquids, and gases (for context of conductivity).

Practice questions

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1.Which of the following is a correct definition of electric current?
Easy
- AThe rate of flow of electrons
- BThe energy transferred per unit charge
- CThe opposition to the flow of charge
- DThe force on a charged particle
2.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
3.Which of the following is a good conductor of electricity?
Easy
- ARubber
- BPlastic
- CCopper
- DGlass
4.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
5.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?
Medium
- A0.05 V
- B5 V
- C20 V
- D10.5 V
6.A student connects a voltmeter to measure the potential difference across a lamp. How should the voltmeter be connected?
Medium
- AIn series with the lamp
- BIn parallel with the lamp
- CIn series with the power supply
- DIn parallel with the power supply
7.Which graph shows the current-voltage characteristic of a filament lamp?
Medium
- AA straight line through the origin
- BA curve that becomes less steep as voltage increases
- CA curve that becomes steeper as voltage increases
- DA horizontal line
8.A charge of 270 kC passes through a 12 V battery. How much electrical energy is transferred?
Hard
- A22.5 J
- B3.24 × 10⁶ J
- C3.24 × 10³ J
- D2.25 × 10⁴ J

Slides

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 they are electrically neutral.

Which particles are negatively charged?

Protons
Neutrons
Electrons
Nucleus

Attraction and Repulsion

Like charges repel each other.
Opposite charges attract each other.
For example, two positive charges repel, while a positive and a negative charge attract.

Which of these are true?

A positive charge and a negative charge attract.
Two negative charges attract each other.
Two positive charges repel each other.

Charging by Friction

Rubbing two insulating materials together can transfer electrons from one to the other.
The material that gains electrons becomes negatively charged.
The material that loses electrons becomes positively charged.
For example, rubbing a polythene rod with a cloth makes the rod negative (gains electrons) and the cloth positive (loses electrons).

When a polythene rod is rubbed with a cloth, ____ move from the cloth to the rod, giving the rod a ____ charge.

Conductors and Insulators

Conductors allow electric charge to flow through them easily (e.g., metals like copper).
Insulators do not allow charge to flow easily (e.g., plastic, rubber, glass).
Metals conduct because they have delocalised electrons that are free to move.

Which of the following is a good conductor of electricity?

Rubber
Plastic
Copper
Wood

Electric Current

Electric current is the rate of flow of electric charge.
Current is measured in amperes (A) using an ammeter connected in series.
In metal wires, current is a flow of negatively charged electrons.
Conventional current flows from positive to negative terminal, opposite to electron flow.

How is an ammeter connected in a circuit?

In parallel
In series
Across the battery
In any way

Potential Difference and EMF

Potential difference (p.d.) is the work done per unit charge passing through a component.
Electromotive force (e.m.f.) is the work done per unit charge by a power source.
Both are measured in volts (V).
A voltmeter is connected in parallel to measure p.d.

A voltmeter must be connected in ____ to measure the potential difference across a component.

Ohm's Law and Resistance

Resistance is the opposition to current, measured in ohms (Ω).
Ohm's law: R = V / I, where V is p.d. and I is current.
For a fixed resistor at constant temperature, current is directly proportional to p.d. (ohmic conductor).
For a filament lamp, resistance increases with temperature (non-ohmic).

A resistor has a p.d. of 6 V and a current of 0.5 A. What is its resistance?

3 Ω
12 Ω
0.083 Ω
30 Ω

Resistance of a Wire

Resistance of a wire depends on length, cross-sectional area, and material.
Resistance is directly proportional to length: longer wire = higher resistance.
Resistance is inversely proportional to cross-sectional area: thicker wire = lower resistance.

If you double the length of a wire, what happens to its resistance?

It halves
It stays the same
It doubles
It quadruples

Electrical Energy and Power

Electrical energy transferred E = V × Q, where V is p.d. and Q is charge.
Power P = V × I, measured in watts (W).
Energy can also be calculated as E = P × t.

The power of a device is 60 W and it operates at 12 V. The current is ____ A.

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