Electricity — GCSE Physics Revision
Everything you need to revise electricity for GCSE Physics: clear notes, the key facts and terms to learn, the mistakes that cost students marks, and practice questions with answers.
Revision notes
Electrical Charge and Current
Electrical current is the rate of flow of electrical charge. In metals, this charge is carried by electrons. We measure current in Amperes (A) using an ammeter, which must be connected in series within the circuit. The direction of conventional current is from the positive terminal to the negative terminal, even though electrons flow in the opposite direction. Charge (Q) is measured in Coulombs (C) and current (I) is related to time (t) by the equation Q = I × t. This fundamental relationship is crucial for understanding all electrical circuits.
Potential Difference and Resistance
Potential difference, or voltage, is the energy transferred per unit charge. It is measured in Volts (V) using a voltmeter connected in parallel across the component. Resistance (R) is a measure of how much a component opposes the flow of current, measured in Ohms (Ω). Ohm's Law states that for an ohmic conductor, current is directly proportional to potential difference, provided temperature is constant: V = I × R. This law helps us understand how components affect current flow and energy transfer within a circuit.
Series and Parallel Circuits
In a series circuit, components are connected end-to-end, forming a single path for current. The current is the same everywhere, and the total potential difference is shared across the components. The total resistance is the sum of individual resistances. In a parallel circuit, components are connected across the same two points, providing multiple paths for current. The potential difference is the same across each branch, and the total current is the sum of currents in each branch. Total resistance in parallel is less than the smallest individual resistance.
Electrical Power and Energy Transfer
Electrical power is the rate at which energy is transferred by an electrical component. It is measured in Watts (W). Power (P) can be calculated using various formulae: P = V × I, P = I² × R, or P = V²/R. The amount of energy transferred (E) over a period of time (t) is given by E = P × t. This energy is measured in Joules (J). Understanding power allows us to calculate how much energy electrical appliances consume and convert into other forms, such as heat or light.
Key facts
- Current is the flow of charge (electrons in metals).|Voltage is energy per unit charge.|Resistance opposes current flow.|Ohm's Law: V = I × R.|Series circuits have one path for current.|Parallel circuits have multiple paths for current.
Key terms
- Current::The rate of flow of electrical charge, measured in Amperes (A).|Potential Difference::The energy transferred per unit charge between two points in a circuit, measured in Volts (V).|Resistance::A measure of how much a component opposes the flow of electrical current, measured in Ohms (Ω).|Ohm's Law::For an ohmic conductor, current is directly proportional to potential difference, provided temperature is constant.|Electrical Power::The rate at which energy is transferred by an electrical component, measured in Watts (W).|Charge::A fundamental property of matter that causes it to experience a force when placed in an electromagnetic field, measured in Coulombs (C).
Common mistakes
- Confusing the direction of electron flow with conventional current.|Connecting voltmeters in series or ammeters in parallel.|Incorrectly calculating total resistance for parallel circuits (e.g., just adding them).|Forgetting that resistance can change with temperature for non-ohmic components.
Exam tips
- Always state units in your answers for numerical questions.|Draw clear circuit diagrams for complex problems if not provided.|Rearrange formulae before substituting values to minimise errors.|Remember the relationships for current, voltage, and resistance in both series and parallel circuits.
Quick quiz
1. Which of the following is the correct unit for electrical current?
- Volts
- Ohms
- Amperes
- Watts
Show answer
Amperes — Electrical current is measured in Amperes (A). Volts measure potential difference, Ohms measure resistance, and Watts measure power.
2. In a series circuit, what happens to the total resistance if more resistors are added?
- It decreases
- It increases
- It stays the same
- It becomes zero
Show answer
It increases — In a series circuit, the total resistance is the sum of the individual resistances (R_total = R1 + R2 + ...), so adding more resistors increases the total resistance.
3. Which equation correctly links charge (Q), current (I), and time (t)?
- I = Q × t
- t = Q × I
- Q = I × t
- Q = I / t
Show answer
Q = I × t — The definition of current is the rate of flow of charge, so I = Q/t, which can be rearranged to Q = I × t.
4. A 12V battery is connected to a 4Ω resistor. What is the current flowing through the resistor?
- 3A
- 48A
- 0.33A
- 12A
Show answer
3A — Using Ohm's Law, V = I × R, we can rearrange to I = V / R. So, I = 12V / 4Ω = 3A.
5. What happens to the potential difference across components connected in parallel?
- It is shared between them
- It is the same across each component
- It is inversely proportional to their resistance
- It is always zero
Show answer
It is the same across each component — In a parallel circuit, all components connected in parallel have the same potential difference across them, equal to the supply voltage.
Exam-style questions
A student sets up a circuit with a 6V power supply and two 3Ω resistors connected in series. Calculate the total resistance of the circuit. [2 marks]
Show mark scheme
- R_total = R1 + R2
- R_total = 3Ω + 3Ω = 6Ω
State two differences between a series circuit and a parallel circuit. [2 marks]
Show mark scheme
- In series, current is the same everywhere; in parallel, current splits / is shared between branches.
- In series, potential difference is shared; in parallel, potential difference is the same across each branch.
A 1500W electric kettle is used for 5 minutes. Calculate the energy transferred by the kettle. [3 marks]
Show mark scheme
- Convert time to seconds: 5 minutes = 300 seconds
- Use formula E = P × t
- E = 1500W × 300s = 450000 J (or 450 kJ)
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