Current Electricity | Jamb(UTME)

Electromagnetic Force (EMF)

1. EMF is the energy supplied by a source (e.g., battery or generator) per unit charge.
2. It is measured in volts (V).
3. EMF represents the maximum potential difference a cell can provide.
4. It is generated through chemical reactions in a battery or electromagnetic induction in a generator.
5. EMF is not affected by the circuit connected to the source.
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Potential Difference (P.D.)

6. Potential difference is the energy difference per unit charge between two points in a circuit.
7. It is also measured in volts (V).
8. P.D. is the "push" that drives current through a circuit.
9. Unlike EMF, P.D. depends on the resistance in the circuit and the current flowing.
10. P.D. drops across resistors and other components in a circuit.
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Current

11. Current is the flow of electric charge through a conductor.
12. It is measured in amperes (A).
13. The formula for current is $I = \frac{Q}{t}$, where $Q$ is charge, and $t$ is time.
14. Current flows from higher to lower potential in a circuit.
15. There are two types of current: direct current (DC) and alternating current (AC).
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Internal Resistance of a Cell and Lost Volt

16. Internal resistance is the resistance within a battery or cell that opposes the flow of current.
17. It causes a voltage drop within the cell when current flows.
18. The "lost volt" is the voltage drop across the internal resistance.
19. The formula for terminal voltage is $V = E - Ir$, where $E$ is EMF, $I$ is current, and $r$ is internal resistance.
20. Internal resistance increases with age or use of the battery.
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Ohm’s Law

21. Ohm's Law states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance.
22. The formula is $V = IR$, where $V$ is voltage, $I$ is current, and $R$ is resistance.
23. Ohm’s Law applies only to ohmic materials, which have a constant resistance.
24. Non-ohmic materials, like diodes, do not follow Ohm’s Law.
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Resistivity and Conductivity

25. Resistivity is a material's property that opposes the flow of current.
26. It is measured in ohm-meters $(\Omega \cdot m)$.
27. The formula is $R = \rho \frac{L}{A}$, where $R$ is resistance, $\rho$ is resistivity, $L$ is length, and $A$ is cross-sectional area.
28. Conductivity is the reciprocal of resistivity and measures how easily a material allows current.
29. Good conductors, like copper, have low resistivity and high conductivity.
30. Insulators, like rubber, have high resistivity and low conductivity.
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Measurement of Resistance

31. Resistance can be measured using an ohmmeter or a multimeter.
32. Wheatstone bridges are used for precise resistance measurements.
33. The formula for resistance in a Wheatstone bridge is based on the ratio of known and unknown resistances.
34. A rheostat is a variable resistor used to control resistance in a circuit.
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Meter Bridge

35. The meter bridge is a practical setup for measuring unknown resistance.
36. It uses the principle of the Wheatstone bridge.
37. A uniform wire of one-meter length acts as the bridge.
38. The unknown resistance is balanced against a known resistance using a sliding contact.
39. The formula is $\frac{R_1}{R_2} = \frac{l_1}{l_2}$, where $l_1$ and $l_2$ are lengths on the bridge wire.
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Resistance in Series

40. In a series circuit, resistances are connected end-to-end.
41. The total resistance is the sum of individual resistances: $R_{total} = R_1 + R_2 + R_3 + \cdots$.
42. The same current flows through all resistors in series.
43. The voltage across the circuit is divided among the resistors.
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Resistance in Parallel

44. In a parallel circuit, resistances are connected side-by-side.
45. The total resistance is less than the smallest individual resistance.
46. The formula is $\frac{1}{R_{total}} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} + \cdots$.
47. The voltage across each resistor is the same in parallel.
48. Parallel circuits allow independent paths for current to flow.
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Combination of Series and Parallel Resistances

49. Complex circuits often involve a combination of series and parallel resistances.
50. The total resistance is calculated by reducing series and parallel parts step by step.
51. Combination circuits optimize both resistance and current flow in electrical systems.
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Potentiometer Method for Measuring EMF

52. A potentiometer is an instrument used to measure EMF accurately.
53. It consists of a long wire with a uniform cross-section and a sliding contact.
54. The voltage across the wire is proportional to its length.
55. The unknown EMF is compared to a known EMF by balancing the circuit.
56. This method does not draw current from the cell, giving precise results.
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Potentiometer Method for Measuring Current

57. To measure current, a known resistance is connected in series with the circuit.
58. The voltage drop across the resistance is measured using the potentiometer.
59. The current is calculated using $I = \frac{V}{R}$, where $V$ is the measured voltage.
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Potentiometer Method for Measuring Internal Resistance

60. Internal resistance is measured by comparing the EMF of a cell to its terminal voltage when a known external resistance is connected.

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