3.2 Current Electricity

• Measurement of current, potential difference and resistance
• Relationship between current, voltage and resistance; Ohm's law
• Electromotive force and internal resistance; E = I(R + r)
• Resistivity and factors affecting resistance
• Resistor colour codes; ohmic and non-ohmic conductors
• Resistors and resistor networks; laws of current and voltage in a circuit
• Electrical power and the heating effect of an electric current

• explain the terminologies used in current electricity
• verify the relationships V = IR and E = I(R + r)
• determine the resistance and resistivity of various conductors using different methods
• determine the effective resistance of various resistor networks
• determine the relationship of potential difference and current to power in the heating effect of an electric current
• appreciate the applications of current electricity in day-to-day life

“How does temperature affect the resistance of conductors and semiconductors?”

“Why are resistors used in electrical circuits?”

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1. Current, voltage and resistance
   40 min · 2 practice tasks · quiz
   The three quantities every circuit is described by, and what a cell really supplies.
2. Ohm's law, resistivity and resistor codes
   40 min · 2 practice tasks · quiz
   The straight-line law and its exceptions, what actually decides a wire's resistance, and how to read a resistor.
3. Resistor networks, power and heating
   40 min · 2 practice tasks · quiz
   Combining resistors, the rules that govern any circuit, and where the energy goes.
4. Circuit symbols and drawing circuits
   40 min · 1 practice tasks · quiz
   The standard symbols for a cell, switch, lamp, resistor and meters, and the rules for drawing a circuit as a map of connections.
5. Measuring current with an ammeter
   40 min · 1 practice tasks · quiz
   How to connect an ammeter, choose a range, and convert a reading into charge using I = Q/t.
6. Measuring potential difference
   40 min · 1 practice tasks · quiz
   How a voltmeter is connected across a component, what the reading means in joules per coulomb, and how terminal p.d. differs from e.m.f.
7. Verifying Ohm's law
   40 min · 1 practice tasks · quiz
   A full ammeter-voltmeter practical: vary the current, plot V against I, and read resistance from the gradient.
8. Factors affecting the resistance of a wire
   40 min · 1 practice tasks · quiz
   How length, cross-sectional area, material and temperature change resistance, and why metals and semiconductors disagree about heat.
9. Measuring the resistivity of a wire
   40 min · 1 practice tasks · quiz
   Use R = ρL/A on several lengths of a uniform wire, plot R against L, and find resistivity from the gradient and the cross-sectional area.
10. Determining e.m.f. and internal resistance
    40 min · 1 practice tasks · quiz
    Plot terminal p.d. against current for a cell, read E from the intercept and r from the slope, and explain lost volts.
11. Laws of current and voltage in a circuit
    40 min · 1 practice tasks · quiz
    Charge is conserved at a junction and energy is conserved around a loop: the two laws that govern every network.
12. Series circuits
    40 min · 1 practice tasks · quiz
    One path, one current: voltages add, resistances add, and a single break kills the whole loop.
13. Parallel circuits
    40 min · 1 practice tasks · quiz
    Several paths, one voltage: currents add, reciprocals of resistance add, and one branch can fail without killing the others.
14. Mixed networks and voltage dividers
    40 min · 1 practice tasks · quiz
    Reduce a mixed circuit from the inside outwards, then use two series resistors as a voltage divider - the reason many circuits contain resistors at all.
15. Electrical energy and the kilowatt-hour
    40 min · 1 practice tasks · quiz
    Energy W = VIt = Pt, the joule versus the kilowatt-hour, and the cost of a Kenya Power token.
16. Heating effect of an electric current
    40 min · 1 practice tasks · quiz
    P = I²R as wanted heat in nichrome elements and as wasted heat in cables, and a check that P = VI for a lamp.
17. Fuses, ratings and choosing cable
    40 min · 1 practice tasks · quiz
    A fuse is a deliberate weak link on the live wire; choose its rating just above the working current, and match the cable thickness to that current.
18. Kenya domestic wiring and electrical safety
    40 min · 1 practice tasks · quiz
    Live, neutral and earth in a Kenyan house, colour codes old and new, the three-pin plug, and the habits that keep people alive.