JAMB Physics · Section A
Study notes for Current Electricity — part of the JAMB UTME Physics syllabus. 8 learning objectives with explanations and exam tips.
When you connect cells in parallel, you join all their positive terminals together and all their negative terminals together. Think of it like connecting multiple power banks to charge your phone simultaneously—the voltage stays the same as a single cell, but you get more current. Each cell in parallel provides the same voltage (usually 1.5V for a dry cell), so the total voltage remains 1.5V whether you use one cell or ten cells connected this way.
The real advantage is the current. If one cell produces 2A, two identical cells in parallel produce 4A total. This is useful in torches where you need longer-lasting power without brightness loss. Nigerian torch manufacturers often use this principle to make torches run longer.
Think of a battery like a water pump pushing water through pipes in your home. The electromotive force (EMF) is the total energy the battery supplies to push electrons around the circuit. It's the "push power" of the battery itself, measured in volts.
Potential difference (P.D.) is the energy lost by electrons as they move through a component like a bulb or resistor. It's the voltage drop across that specific device. Current is simply the flow rate of electrons through the circuit, measured in amperes (A).
Using Nigeria's PHCN electricity: EMF is like the total voltage leaving the power station, while P.D. is what you actually measure at your wall socket after losses in transmission lines. Current is how much electrical charge flows through your kettle when you switch it on.
The key difference: EMF is the source's total energy, P.D. is energy used by components, and current is the rate of electron flow.
Every battery or cell has two important properties. The electromotive force (EMF) is the total energy each unit of charge gains as it moves through the cell. Think of it like how much "push" the battery gives to electrons. However, the cell itself has internal resistance—just like how a water pipe has friction that slows water flow. This internal resistance reduces the voltage you actually measure when the cell is supplying current.
Consider a torch battery in Nigeria: its EMF might be 1.5V, but when you connect it to a bulb, the voltage measured is less because some energy is lost inside the battery itself. The relationship is V = E - Ir, where V is terminal voltage, E is EMF, I is current, and r is internal resistance.
Ohm's law states that the current flowing through a conductor is directly proportional to the voltage applied across it and inversely proportional to its resistance. Simply put, V = IR, where V is voltage in volts, I is current in amperes, and R is resistance in ohms. Think of it like water flowing through a pipe: more pressure (voltage) means more water flows (current), but a narrower pipe (higher resistance) restricts the flow.
Consider a Nigerian fan rated at 220V. If the fan's heating coil has a resistance of 44 ohms, the current flowing through it would be I = V/R = 220/44 = 5 amperes. Understanding this relationship helps you solve circuit problems and predict how electrical devices behave.
The metre bridge is a simple wire used to find unknown resistances accurately. Think of it like a balanced scale—when everything is equal on both sides, you've found your answer. The bridge has a one-metre wire stretched across it, and you place your known resistance on one side and unknown resistance on the other side. When you slide a contact point along the wire and find the balance point where no current flows through the galvanometer, you can calculate the unknown resistance using the formula: R₁/R₂ = L₁/L₂. For example, if you're testing the resistance of copper wire from a local electrical shop in Lagos against a standard resistor, the metre bridge gives you precise measurements without damage. The ratio of lengths on the wire equals the ratio of resistances—it's that straightforward.
When resistors connect in a circuit, you need to find the total resistance they create together. For series connection, resistors line up one after another, so you simply add them: R_total = R₁ + R₂ + R₃. For parallel connection, resistors sit side by side offering multiple paths for current, so the calculation differs: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃.
Think of it like traffic flow on Lagos roads. Series is one road—all vehicles pass through every checkpoint. Parallel is multiple roads—vehicles split up and bypass congestion. The parallel route always gives lower total resistance because current finds easier paths.
Most circuits combine both types. Calculate series sections first, then treat them as single resistors in parallel arrangements. This systematic approach prevents confusion when solving complex problems.
Series arrangement means connecting components one after another in a single loop, like beads on a string. The same current flows through each component. Think of Christmas lights strung together—when one bulb fails, the whole line goes dark. Parallel arrangement means connecting components side by side with separate paths. Current divides among the branches. Nigerian homes use parallel wiring; each socket gets full voltage, and switching off one light doesn't affect others.
In series circuits, total resistance increases as you add more resistors. In parallel circuits, total resistance decreases. For batteries in series, voltages add up. For parallel batteries of equal voltage, voltage stays the same but current capacity increases—useful for powering devices longer.
Understanding these arrangements helps explain why household wiring uses parallel connections for safety and convenience.
Resistivity is a material property that tells you how much a substance opposes the flow of electric current. Think of it like asking: "How stubborn is this material about letting electricity pass through?" Different materials have different resistivity values. Copper, for example, has very low resistivity, which is why it's used for electrical wires in Nigerian homes. Nichrome wire, used in electric heaters, has higher resistivity, so it gets hot when current flows through it.
The relationship between resistivity (ρ), resistance (R), length (L), and cross-sectional area (A) is given by: R = ρL/A. This means a longer wire has more resistance, while a thicker wire has less resistance. Understanding resistivity helps you predict how different materials will behave in circuits.