B.5
Current and Circuits
EMF and potential difference, charge carriers, current, resistance, Ohm's law, IV characteristics, series and parallel circuits, power, resistivity, thermistors, LDRs, internal resistance and potential dividers.
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Key Concepts, Current and Circuits
EMF and Potential Difference
Electromotive force (EMF) is the energy transferred to charges by a source such as a battery or solar cell: it is defined as the energy given to each unit of charge, with units of volts. Potential difference (PD) is the energy given up by charges as they pass through a component: it is also defined as the energy transferred per unit charge, V = E/Q. The key distinction is that EMF is about gaining energy (at the source) and PD is about losing energy (to a component). Both are measured in volts and both use the same formula. A solar cell converts light energy into electrical energy, and a chemical cell converts chemical energy into electrical energy.
Charge, Current and Charge Carriers
Electric current is the rate of flow of charge: I = ΔQ/Δt, measured in amperes (A), where 1 A = 1 C/s. In a metal, the charge carriers are free electrons, which move randomly at high speed but drift slowly in one direction when a voltage is applied. In electrolytes (solutions), the charge carriers are positive and negative ions. Charge is quantized: all charges are integer multiples of the elementary charge e = 1.6 × 10⁻¹⁹ C. Conductors have many free electrons; insulators have very few; semiconductors lie in between and conduct when an impurity is added or in response to heat or light.
Resistance, Ohm's Law and IV Characteristics
Resistance is defined as R = V/I and has units of ohms (Ω). Ohm's law states that current is proportional to potential difference provided temperature is constant: V = IR. A component that obeys Ohm's law is called an ohmic conductor, and its IV graph is a straight line through the origin. A metal wire at constant temperature is an example. A filament lamp is non-ohmic: as current increases, the filament heats up, ions vibrate more and resist electron flow more, so resistance increases. Its IV graph is an S-curve. A diode allows current in only one direction: in reverse it has infinite resistance; in forward it has very high resistance until the threshold voltage, after which resistance drops to nearly zero.
Resistivity and Heating Effects
When electrons flow through a wire they collide with vibrating metal ions, transferring energy to them and causing the wire to heat up. This is the origin of electrical resistance. Resistivity ρ is a property of the material and links to resistance by ρ = RA/L, where R is resistance, A is cross-sectional area and L is length. A longer wire has more resistance; a wider wire has less resistance. Everyday devices that rely on the heating effect of current include toasters, electric fires, hair dryers and filament bulbs: all use a high-resistance wire that heats up when current flows through it.
Thermistors, LDRs and Potentiometers
A thermistor (NTC type) has resistance that decreases as temperature increases: as temperature rises, more electrons are released and can carry charge, reducing resistance. Uses include thermostats, car temperature gauges and fire alarms. A light-dependent resistor (LDR) has resistance that decreases as light intensity increases: uses include digital cameras, security lights and street lamps. A potentiometer is a variable resistor with a sliding contact; it can divide a voltage continuously from zero up to the supply voltage and is used in potential divider circuits to produce a variable output voltage. The output voltage from a potential divider depends on the ratio of the resistances in the two parts of the divider.
Series and Parallel Circuits
In a series circuit, current is the same at every point, total resistance is the sum of individual resistances (RT = R1 + R2), and the total PD is shared between components in proportion to their resistance. In a parallel circuit, the total current splits between branches (IT = I1 + I2), all branches have the same PD as the supply, and adding more branches reduces the total resistance of the circuit. Kirchhoff's rules (conservation of charge and energy) underpin both. Ammeters are connected in series and must have zero resistance so they do not affect the circuit. Voltmeters are connected in parallel and must have infinite resistance so they do not draw current.
Power and Energy in Circuits
Power is the rate of energy transfer. In electrical circuits: P = IV (power from current and voltage), P = I²R (power in a resistor from current and resistance), and P = V²/R. Energy is related to power by E = Pt, and can also be found using E = QV (energy from charge and voltage) or E = IVt. These equations apply to any component in a circuit. The power dissipated in a resistor becomes thermal energy (heat), which is why wires warm up when current flows through them.
Internal Resistance and EMF
Real cells and power supplies contain internal resistance r, which means the voltage available to the external circuit is less than the EMF. When current flows, some voltage is lost across the internal resistor: this is called lost volts = Ir. The terminal potential difference (the voltage across the external circuit) is V = ε - Ir, which rearranges to the internal resistance equation ε = I(R + r). On a V-I graph for a cell, the y-intercept gives the EMF and the gradient gives the internal resistance. The higher the current drawn, the greater the lost volts and the lower the terminal PD. In a solar cell module, connecting cells in series increases the total EMF; connecting branches in parallel decreases the internal resistance and provides redundancy if one cell fails.
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Frequently Asked Questions, IB Physics Current and Circuits
What is Current and Circuits in IB Physics? ↓
EMF and potential difference, charge carriers, current, resistance, Ohm's law, IV characteristics, series and parallel circuits, power, resistivity, thermistors, LDRs, internal resistance and potential dividers. This topic is part of Theme B (The Particulate Nature of Matter) in the current IB Physics syllabus.
Is Current and Circuits SL or HL in IB Physics? ↓
Current and Circuits is covered by both SL and HL students in the current IB Physics syllabus. HL students study additional depth and extension content beyond the SL core.
What equations do I need for IB Physics Current and Circuits? ↓
The key equations for Current and Circuits are covered in the concept tutorial above. For a structured set of notes with all equations, conditions and worked examples, the GradePod Exam Pack includes a revision note template for every topic.
What are common exam mistakes in IB Physics Current and Circuits? ↓
Common mistakes are covered in detail in the exam technique video above. The GradePod Exam Pack also includes exam-style questions with mark schemes so you can see exactly how marks are awarded and where students typically drop them.
How do I revise Current and Circuits for the IB Physics exam? ↓
Follow the GradePod three-step method. First, watch the concept tutorial and tick off each learning objective on the checklist above as you go. Second, watch the exam technique video to see how IB-style questions are answered under exam conditions. Third, use the Exam Pack to practise independently with knowledge questions, exam questions and mark schemes. That's it. It works. I promise.