What is the difference between current and potential difference?

Current is the rate of flow of electric charge, measured in amperes (A). It tells you how much charge is flowing per second. Potential difference (voltage) is the energy transferred per unit charge, measured in volts (V). It tells you how much energy each coulomb of charge gains or loses as it moves through a component. Think of current as the flow of water, and voltage as the pressure pushing the water.

How do I calculate total resistance in a series circuit?

In a series circuit, the total resistance is simply the sum of all individual resistances. For example, if you have resistors of 2 Ω, 3 Ω, and 5 Ω in series, the total resistance is 2 + 3 + 5 = 10 Ω. This is because the current has to pass through each resistor in turn, so the overall opposition to current flow adds up.

Why does a filament lamp's resistance increase as current increases?

As current flows through the filament, it heats up due to the resistance. The increased temperature causes the metal atoms in the filament to vibrate more vigorously, which makes it harder for electrons to pass through. This increases the resistance. That's why the I-V graph for a filament lamp is a curve: at higher voltages (and currents), the resistance is higher, so the current does not increase as much.

How does a diode behave in a circuit?

A diode allows current to flow in only one direction, called the forward bias direction. When connected the correct way, it has a very low resistance and current flows easily. If connected in reverse (reverse bias), it has a very high resistance and almost no current flows. This makes diodes useful for converting AC to DC in rectifier circuits.

What is the formula for Ohm's Law and how do I use it?

Ohm's Law is V = I × R, where V is potential difference in volts, I is current in amperes, and R is resistance in ohms. You can rearrange it to find current: I = V/R, or resistance: R = V/I. For example, if a 12 V battery is connected to a 4 Ω resistor, the current is I = 12/4 = 3 A. Always ensure units are consistent.

Current, potential difference and resistance

Q: What is an ohmic conductor?

An ohmic conductor is a component that obeys Ohm's Law, meaning its resistance stays constant as long as the temperature is constant. The current through it is directly proportional to the potential difference across it. Examples include fixed resistors and metal wires at constant temperature. The I-V graph for an ohmic conductor is a straight line through the origin.

WJEC

GCSE

This topic explores the fundamental concepts of electric charge, current, potential difference, and resistance within electrical circuits. It examines the relationships between these quantities, including the use of Ohm's Law and power equations, while investigating the behavior of various circuit components such as lamps, diodes, thermistors, and LDRs.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

Topic Overview

This topic explores the fundamental relationship between current, potential difference (voltage), and resistance in electrical circuits. Current is the flow of electric charge (measured in amperes, A), potential difference is the energy transferred per unit charge (measured in volts, V), and resistance is the opposition to current flow (measured in ohms, Ω). Ohm's Law states that for a metallic conductor at constant temperature, the current is directly proportional to the potential difference, giving the equation V = IR. Understanding these concepts is essential for analysing series and parallel circuits, where current and voltage behave differently.

In the WJEC GCSE Physics specification, this topic builds on basic circuit knowledge and is crucial for later topics such as electrical power, energy transfer, and the National Grid. You will learn to calculate resistance using the formula R = V/I, interpret current-voltage (I-V) characteristic graphs for resistors, filament lamps, and diodes, and understand how components like variable resistors and thermistors affect circuits. Mastery of this topic is vital for practical investigations, including determining resistance in a circuit and exploring the factors affecting resistance, such as length, cross-sectional area, and temperature of a wire.

This knowledge is not just academic; it explains everyday phenomena like why bulbs glow brighter in parallel circuits or why a wire heats up when current flows. It also forms the basis for understanding electrical safety, such as why fuses and circuit breakers are used. By the end of this topic, you should be able to predict and explain circuit behaviour, perform calculations confidently, and interpret graphical data accurately.

Key Concepts

Core ideas you must understand for this topic

→Ohm's Law: For a conductor at constant temperature, current is directly proportional to potential difference. The equation V = IR links voltage (V), current (I), and resistance (R).
→Series circuits: Current is the same at all points; potential difference is shared across components; total resistance is the sum of individual resistances (R_total = R1 + R2 + ...).
→Parallel circuits: Potential difference is the same across each branch; current is split between branches; total resistance is less than the smallest individual resistance (1/R_total = 1/R1 + 1/R2 + ...).
→I-V characteristics: A resistor (ohmic conductor) gives a straight line through the origin; a filament lamp shows a curve due to increasing resistance as temperature rises; a diode allows current only in one direction (forward bias).
→Factors affecting resistance: Length (longer wire = higher resistance), cross-sectional area (thicker wire = lower resistance), material (conductors like copper have low resistance), and temperature (for metals, resistance increases with temperature).

What You Need to Demonstrate

Key skills and knowledge for this topic

Current is the rate of flow of charge (Q = It)
A source of potential difference and a closed circuit are required for charge to flow
Current has the same value at any point in a single closed loop
Relationship between potential difference, current, and resistance (V = IR)
Power transfer equations (P = IV, P = I²R, P = E/t)
Energy transferred equations (E = QV, E = Pt)
Variation of resistance in components like lamps, diodes, thermistors, and LDRs

Marking Points

Key points examiners look for in your answers

Current is the rate of flow of charge (Q = It)
A source of potential difference and a closed circuit are required for charge to flow
Current has the same value at any point in a single closed loop
Relationship between potential difference, current, and resistance (V = IR)
Power transfer equations (P = IV, P = I²R, P = E/t)
Energy transferred equations (E = QV, E = Pt)
Variation of resistance in components like lamps, diodes, thermistors, and LDRs

Examiner Tips

Expert advice for maximising your marks

💡Always check the units in calculations and convert to SI units if necessary
💡When describing I-V characteristics, clearly distinguish between linear and non-linear components
💡Use the correct circuit symbols for diodes, thermistors, and LDRs in diagrams
💡Remember that power equations can be combined; choose the one that fits the known variables
💡Always show your working in calculations. Write the formula (e.g., V = IR), substitute values with units, and then give the answer with correct units. This ensures you get method marks even if your final answer is wrong.
💡When drawing or interpreting I-V graphs, label axes correctly (current on y-axis, voltage on x-axis) and describe the shape. For a filament lamp, mention that as current increases, temperature increases, causing resistance to increase, hence the curve.
💡In practical questions, remember that a variable resistor is used to change the current or potential difference in a circuit. For example, to investigate the I-V characteristic of a component, you adjust the variable resistor to vary the voltage across the component.

Common Mistakes

Pitfalls to avoid in your exam answers

Confusing the units for charge (Coulombs), current (Amperes), and potential difference (Volts)
Assuming resistance remains constant for all components, particularly lamps
Incorrectly rearranging the V = IR or P = IV equations
Failing to recognize that current is constant in a single closed loop
Misconception: Current is 'used up' as it flows around a circuit. Correction: Current is the flow of charge; it is not consumed. In a series circuit, the same current flows through all components. Energy is transferred by the components, not the current itself.
Misconception: A higher voltage always means a higher current. Correction: According to Ohm's Law, current depends on both voltage and resistance. If resistance also increases, current may stay the same or even decrease. For example, a filament lamp's resistance increases with temperature, so the I-V graph is curved.
Misconception: In a parallel circuit, the total resistance is always less than the smallest resistor because 'current takes the path of least resistance'. Correction: While it's true that total resistance is less, the reason is that adding more paths for current reduces overall opposition. The formula 1/R_total = 1/R1 + 1/R2 shows this mathematically.

Frequently Asked Questions

Common questions students ask about this topic

Before You Start

Prior knowledge that will help with this topic

•Basic circuit symbols and components (cell, battery, bulb, switch, resistor, ammeter, voltmeter).
•Understanding of electric charge and the concept of current as flow of charge.
•Simple series and parallel circuit diagrams (recognising how components are connected).

Likely Command Words

How questions on this topic are typically asked

Calculate

Describe

Explain

Recall

Use

Ready to test yourself?

Practice questions tailored to this topic

Current, potential difference and resistance

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

Before You Start

Likely Command Words

Ready to test yourself?

Related Topics in WJEC GCSE Physics

Absorption and emission of ionising radiations and of electrons and nuclear particles

Atomic structure

Black body radiation (qualitative only)

Colour and frequency; differential effects in transmission, absorption and diffuse reflection

Topic Synopsis

Key Concepts & Core Principles

Exam Tips & Revision Strategies

Common Misconceptions & Mistakes to Avoid

Examiner Marking Points

Current, potential difference and resistance

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

What is the difference between current and potential difference?

How do I calculate total resistance in a series circuit?

Why does a filament lamp's resistance increase as current increases?

What is an ohmic conductor?

How does a diode behave in a circuit?

What is the formula for Ohm's Law and how do I use it?

Before You Start

Likely Command Words

Ready to test yourself?

Related Topics in WJEC GCSE Physics

Absorption and emission of ionising radiations and of electrons and nuclear particles

Atomic structure

Black body radiation (qualitative only)

Colour and frequency; differential effects in transmission, absorption and diffuse reflection