Electricity & Magnetism
Electric Circuits
1st Year · 2nd Year · 3rd Year (Junior Cert)
- ✓By the end of this lesson students will be able to define voltage, current, and resistance.
- ✓By the end of this lesson students will be able to state and apply Ohm's Law to solve problems.
- ✓By the end of this lesson students will be able to distinguish between series and parallel circuits.
- ✓By the end of this lesson students will be able to calculate total resistance, current, and voltage in simple series and parallel circuits.
- ✓By the end of this lesson students will be able to draw and interpret circuit diagrams using standard symbols.
Key concepts
Electric current is the rate of flow of electric charge. It is carried by moving electrons in a conductor. The unit of current is the Ampere (A). Current is measured using an ammeter, which must be connected in series with the component through which the current is being measured.
Voltage, also known as potential difference, is the 'push' or 'energy' provided per unit charge to move electrons around a circuit. It represents the difference in electrical potential energy between two points in a circuit. The unit of voltage is the Volt (V). Voltage is measured using a voltmeter, which must be connected in parallel across the component whose voltage is being measured.
Resistance is the opposition to the flow of electric current in a material. Materials with high resistance are poor conductors, while materials with low resistance are good conductors. The unit of resistance is the Ohm (Ω).
Ohm's Law states that for a metallic conductor at a constant temperature, the current (I) flowing through it is directly proportional to the voltage (V) across it. The constant of proportionality is the resistance (R) of the conductor.
In a series circuit, components are connected end-to-end, forming a single path for the electric current. This means the current is the same through all components in the circuit. The total voltage supplied by the power source is divided among the components. The total resistance of a series circuit is the sum of the individual resistances.
In a parallel circuit, components are connected across the same two points, providing multiple paths or branches for the electric current. This means the voltage across each component in parallel is the same. The total current from the power source is divided among the branches. The reciprocal of the total resistance of a parallel circuit is the sum of the reciprocals of the individual resistances.
Key facts to remember
- 1Electric current is the flow of electric charge, measured in Amperes (A).
- 2Voltage is the 'push' or energy per unit charge, measured in Volts (V).
- 3Resistance is the opposition to current flow, measured in Ohms (Ω).
- 4Ohm's Law states that V = IR.
- 5In a series circuit, current is the same everywhere, and total resistance is the sum of individual resistances (R_total = R1 + R2 + ...).
- 6In a parallel circuit, voltage is the same across all branches, and the reciprocal of total resistance is the sum of reciprocals (1/R_total = 1/R1 + 1/R2 + ...).
- 7Ammeters are connected in series to measure current.
- 8Voltmeters are connected in parallel to measure voltage.
Worked examples
Example 1
A 12 V battery is connected to a resistor of 4 Ω. Calculate the current flowing through the resistor.
Answer
3 A
Remember to always include the correct unit with your final answer.
Example 2
Two resistors, R1 = 3 Ω and R2 = 5 Ω, are connected in series to a 16 V power supply. (a) Calculate the total resistance of the circuit. (b) Calculate the total current flowing in the circuit. (c) Calculate the voltage across R1.
Answer
(a) 8 Ω (b) 2 A (c) 6 V
In a series circuit, the sum of voltages across individual components equals the total supply voltage (6V + 10V = 16V).
Example 3
Two resistors, R1 = 6 Ω and R2 = 3 Ω, are connected in parallel to a 6 V power supply. (a) Calculate the total resistance of the circuit. (b) Calculate the total current flowing from the power supply.
Answer
(a) 2 Ω (b) 3 A
For two resistors in parallel, a quicker formula is R_total = (R1 * R2) / (R1 + R2). Try it!
Common mistakes
- ✗Confusing the rules for current and voltage in series versus parallel circuits.
- ✗Incorrectly calculating total resistance for parallel circuits (e.g., forgetting to invert the final fraction).
- ✗Not including the correct units (A, V, Ω) with numerical answers.
- ✗Mixing up the variables in Ohm's Law (e.g., calculating I = VR instead of I = V/R).
- ✗Connecting ammeters in parallel or voltmeters in series.
Exam tips
- ★Always draw a clear circuit diagram if one is not provided, labelling all known values.
- ★Write down all given values and the quantity you need to find before starting calculations.
- ★Show all steps in your calculations clearly, as partial marks may be awarded for correct methods.
- ★Double-check your answers and ensure units are included and correct.
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