Electricity — Physics Unit 8 practice.
This unit covers electric charge, circuits, Ohm's law and series and parallel — essential concepts for Physics. Use our interactive study games to test your understanding, or review questions in traditional format below.
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This unit covers electric charge, circuits, Ohm's law and series and parallel — essential concepts for Physics. Use our interactive study games to test your understanding, or review questions in traditional format below.
Key Concepts Breakdown
1 Electric Charge
Electric charge comes in two types: positive (protons) and negative (electrons). Like charges repel and opposite charges attract. Charge is conserved — it cannot be created or destroyed, only transferred.
Key Points
- Unit of charge is the Coulomb (C); elementary charge e = 1.6 × 10⁻¹⁹ C
- Objects become charged by friction, conduction, or induction
- Conductors allow charge to flow freely; insulators do not
- Coulomb's Law: F = kq₁q₂/r² — force doubles if charge doubles, quadruples if distance halves
Two charges, q₁ = +2 μC and q₂ = −3 μC, are placed 0.1 m apart. Find the electrostatic force between them.
Use F = kq₁q₂/r² with k = 9 × 10⁹ N·m²/C². Substituting: F = (9 × 10⁹)(2 × 10⁻⁶)(3 × 10⁻⁶) / (0.1)² = 5.4 N. Because the charges are opposite, the force is attractive.
2 Electric Circuits
A circuit is a closed loop through which electric current flows. Current (I) is the rate of charge flow, measured in Amperes. Voltage (V) is the energy per unit charge that drives current through the circuit.
Key Points
- Current I = Q/t, where Q is charge (C) and t is time (s)
- Voltage is measured in Volts (V); resistance in Ohms (Ω)
- A complete, unbroken path is required for current to flow
- Conventional current flows from positive to negative terminal outside the battery
A charge of 30 C flows through a wire in 6 seconds. What is the current?
Apply I = Q/t: I = 30 C ÷ 6 s = 5 A. This means 5 Coulombs of charge pass any point in the wire every second. No additional steps are needed — this is a direct substitution problem.
3 Ohm's Law
Ohm's Law states that voltage equals current times resistance: V = IR. This relationship holds for ohmic materials (constant resistance). Exams test your ability to rearrange and apply this equation in circuit problems.
Key Points
- V = IR; rearranges to I = V/R and R = V/I
- Resistance depends on material, length, and cross-sectional area — not on V or I for ohmic conductors
- Power formulas: P = IV = I²R = V²/R
- A resistor with higher resistance allows less current at the same voltage
A 12 V battery is connected to a resistor. If the current measured is 0.4 A, what is the resistance? What power is dissipated?
For resistance: R = V/I = 12 V ÷ 0.4 A = 30 Ω. For power: P = IV = 0.4 A × 12 V = 4.8 W. Alternatively, P = I²R = (0.4)² × 30 = 4.8 W — both methods must give the same answer.
4 Series And Parallel Circuits
In a series circuit, components share the same current but voltage is split across them. In a parallel circuit, components share the same voltage but current is split. Total resistance differs between the two configurations.
Key Points
- Series: R_total = R₁ + R₂ + … ; same current through all resistors
- Parallel: 1/R_total = 1/R₁ + 1/R₂ + … ; total resistance is always less than the smallest resistor
- In series, removing one component breaks the entire circuit
- In parallel, each branch operates independently; household wiring is parallel
Two resistors, 4 Ω and 6 Ω, are connected in parallel across a 12 V source. Find the total resistance and total current drawn from the source.
Find R_total: 1/R = 1/4 + 1/6 = 3/12 + 2/12 = 5/12, so R_total = 12/5 = 2.4 Ω. Then find total current: I = V/R = 12 V ÷ 2.4 Ω = 5 A. Notice the total resistance (2.4 Ω) is less than either individual resistor, which is always true for parallel circuits.
Questions, answered.
What is Electricity?
Electricity is Unit 8 of Physics, covering electric charge, circuits, Ohm's law and series and parallel.
How to study for Physics Unit 8?
Start with the Quick Summary above, review the Key Concepts, then test yourself with our interactive study games. Aim for 80%+ accuracy before moving on.
How many questions are in this unit?
This unit has 30+ review questions across 5 different game modes.