Chapter 6: Conduct electrical experiment

Electrical experiments are fundamental in Science Laboratory Technology, enabling the understanding and application of electrical principles in various scientific analyses and instrumentations. Mastery of electrical quantities and circuit configurations is essential for laboratory technologists who routinely work with measurement devices, sensors, and experimental setups that involve electricity. In Kenya’s laboratories, such as those in universities or county health facilities, precise knowledge of electrical concepts ensures accurate data collection and safe handling of equipment. This chapter focuses on the core electrical quantities and the behavior of circuits configured in series and parallel arrangements, forming the foundation for conducting electrical experiments.

6.1 Electrical quantities

Electrical quantities are the measurable properties that describe what happens in an electric circuit: how much charge flows, how fast it flows, how much it is opposed, and how much energy each unit of charge carries. A laboratory technician sets up power supplies, connects meters and reads values from a bench circuit, so every reading needs its correct quantity and unit. The five quantities in this topic are current, resistance, voltage, electromotive force (emf) and potential difference (pd). Electric charge and electrical power are needed to define them, so they are explained first.

Electric charge

Electric charge is the property of matter that produces electric force. It is measured in coulombs (C). Charge exists in whole multiples of the charge on one electron, which is about 1.6 × 10⁻¹⁹ C, so a coulomb is a very large number of electrons. Charge is conserved: it is never created or destroyed, only moved from one body to another. Charge is measured indirectly. A steady current I flowing for a time t carries a charge Q = I × t. Capacitors store charge, and experiments with them rely on the same relationship.

Electrical power and energy

Electrical power is the rate at which a device converts electrical energy into another form, such as heat, light or movement. It is measured in watts (W). The formulas are P = V × I, P = I² × R and P = V² ÷ R, where V is the pd across the device, I the current through it and R its resistance. Electrical energy is E = P × t, in joules when t is in seconds. Power ratings guide the choice of components: a resistor rated at a lower power than the circuit delivers will overheat.

Worked example: a heating coil on a 12 V supply draws 2.0 A.

• Formula: P = V × I
• Substitution: P = 12 V × 2.0 A
• Result: P = 24 W
• Energy in 10 minutes: E = P × t = 24 W × 600 s = 14 400 J = 14.4 kJ

6.1.1 Current

Electric current is the rate of flow of electric charge past a point in a circuit. It is measured in amperes (A), where one ampere is one coulomb of charge passing a point each second. In a metal wire the charge carriers are free electrons, which drift slowly through the metal when a source pushes them. The formula is I = Q ÷ t. Milliamperes (mA) are one thousandth of an ampere, so 25 mA = 0.025 A.

Characteristics of electric current

• Direction: by convention current is drawn flowing from the positive terminal of a source round the circuit to the negative terminal. The electrons actually move the opposite way. Meters are connected with the positive terminal on the side nearer the source's positive terminal.
• Magnitude: the size of the current decides how much energy is delivered each second. Too large a current damages sensitive components and may overheat wires.
• Direct and alternating current: direct current (DC) flows in one direction, as from a battery or a bench DC supply. Alternating current (AC) reverses direction regularly, as from the mains.
• Effects: a current heats the conductor it flows through, produces a magnetic field around it, and causes chemical changes in an electrolyte.
• Measurement: an ammeter measures current. It is connected in series, so the same current flows through it, and it has a very low resistance so that it disturbs the circuit as little as possible. How to read and look after meters is taught in Electrical measuring instruments.

Worked example: a current of 1.5 A flows for 2 minutes.

• Formula: Q = I × t
• Substitution: Q = 1.5 A × 120 s
• Result: Q = 180 C
• Number of electrons: 180 C ÷ (1.6 × 10⁻¹⁹ C) = 1.125 × 10²¹, or about 1.1 × 10²¹ electrons.

6.1.2 Resistance

Resistance is the opposition a component or material offers to the flow of electric current. It is measured in ohms (Ω). For a component, resistance is the potential difference across it divided by the current through it: R = V ÷ I. A conductor such as copper has a low resistance, an insulator such as rubber or dry glass has a very high resistance, and a semiconductor lies between them. The energy lost to resistance appears as heat, which is how a heating element works.

Resistors

• Fixed resistors: have a set value and are used to limit the current in a circuit or to divide a voltage.
• Variable resistors: the resistance can be changed by moving a slider or turning a knob. A rheostat controls the current in a circuit and a potentiometer varies the potential difference tapped from a supply.
• Power rating: a resistor must be rated for at least the power it will dissipate, or it overheats.
• Tolerance: the printed value is only correct within a stated percentage, such as 5 %.
• Colour code: the coloured bands on a small resistor give the value. The digit colours are black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8 and white 9. The first two bands give two digits and the third band gives the number of zeros to add. A gold fourth band means a tolerance of 5 % and silver means 10 %. For example, brown, black, red, gold means 1, 0, then 2 zeros, which is 1000 Ω with a tolerance of 5 %.

Finding resistance

• Calculation: resistance can be found by measuring the pd across the component and the current through it, then dividing: R = V ÷ I.
• Open and short circuits: an open circuit has an extremely high resistance and a short circuit has a resistance near zero ohms.

Worked example: a pd of 6.0 V is measured across a resistor carrying a current of 0.020 A.

• Formula: R = V ÷ I
• Substitution: R = 6.0 V ÷ 0.020 A
• Result: R = 300 Ω

6.1.3 Voltage

Voltage is the electrical push that moves charge round a circuit. It is the energy transferred to or by each coulomb of charge, measured in volts (V), where one volt is one joule per coulomb: V = W ÷ Q. The word voltage is the everyday name for the two closely related quantities defined in the next sections: the electromotive force of a source and the potential difference across a component. A greater voltage drives a larger current through the same resistance. Millivolts (mV) are one thousandth of a volt, so 250 mV = 0.250 V.

Importance of voltage in experiments

• Driving force: no current flows in a closed circuit without a voltage, and too low a voltage means the circuit does not work.
• Measurement: a voltmeter measures voltage. It is connected in parallel with the component or source, and it has a very high resistance so that it takes almost no current from the circuit.
• Typical values: a dry cell gives about 1.5 V, a car or small laboratory battery about 12 V, and bench supplies are adjustable over a range. The mains supply is far more dangerous than any of these.
• Safety: high voltages can drive a dangerous current through the body, so work on live circuits only at low voltage, with dry hands, insulated leads and the supply off while connecting.
• Power calculation: voltage together with current gives the power of a device.

Worked example: 12 J of energy is transferred when 4.0 C of charge passes between two points.

• Formula: V = W ÷ Q
• Substitution: V = 12 J ÷ 4.0 C
• Result: V = 3.0 V

6.1.4 Electromotive force (emf)

Electromotive force (emf) is the energy a source supplies to each coulomb of charge that passes through it, to drive the charge round the complete circuit. It is measured in volts and given the symbol ε: ε = W ÷ Q, where W is the energy converted from chemical, mechanical, thermal or light energy into electrical energy. A cell, battery, generator and solar cell are all sources of emf. The emf is a property of the source itself and is not the same as the voltage at its terminals when current flows.

Internal resistance and terminal pd

Every real source has a small internal resistance r, because current must also flow through the material of the source. When current I flows, a pd of I × r is used up inside the source, so the pd at the terminals is smaller than the emf: V = ε − I × r, or ε = I × (R + r) in a circuit with external resistance R. The pd used up inside the source is sometimes called the lost volts. When no current flows, nothing is lost, and the reading of a high-resistance voltmeter across the terminals is the emf.

Measuring emf

• Open circuit: connect a high-resistance voltmeter directly across the cell with no other load, so that practically no current flows and nothing is lost to the internal resistance.
• Reading: record the steady reading in volts as the emf.
• Loaded reading: connect a resistor and read the pd across it again. The reading is lower, and the difference shows the internal resistance effect.
• Calculating r: from the two readings, r = (ε − V) ÷ I.
• Fresh and old cells: an old cell often shows an emf close to that of a new one but a much larger internal resistance, so its terminal pd falls sharply on load.

Worked example: a cell has an emf of 1.50 V. When it is connected to a 4.0 Ω resistor, the current is 0.35 A. Find the terminal pd and the internal resistance.

• Terminal pd: V = I × R = 0.35 A × 4.0 Ω = 1.40 V
• Lost volts: ε − V = 1.50 V − 1.40 V = 0.10 V
• Internal resistance: r = 0.10 V ÷ 0.35 A = 0.2857 Ω
• Check: ε = I × (R + r) = 0.35 × (4.0 + 0.2857) = 1.50 V
• Result: the terminal pd is 1.40 V and the internal resistance is about 0.29 Ω.

6.1.5 Potential Difference (pd)

The potential difference (pd) between two points in a circuit is the energy transferred from each coulomb of charge as it moves from one point to the other, for example through a resistor or a lamp. It is measured in volts and is calculated as V = W ÷ Q, so one volt is the pd across which one joule of energy is transferred for each coulomb of charge. Where emf describes what a source supplies, the pd describes what a component uses: the electrical energy is converted to heat, light or movement in the component.

Emf compared with pd

In a circuit, the sum of the pds across the components equals the emf of the source, less the lost volts across its internal resistance.

Measuring pd

• Voltmeter in parallel: place the voltmeter in parallel across the two points whose pd is wanted, with its positive terminal towards the more positive point.
• Calculation: for a resistor, pd = I × R.
• Between any two points: a pd can be measured between any two points of a circuit, and the pds along a path add up.
• Zero pd: points joined by a wire of negligible resistance have almost no pd between them.

Worked example: a current of 0.50 A flows through an 8.0 Ω resistor for 60 s. Find the pd and the energy transferred.

• pd: V = I × R = 0.50 A × 8.0 Ω = 4.0 V
• Charge passed: Q = I × t = 0.50 A × 60 s = 30 C
• Energy: W = Q × V = 30 C × 4.0 V = 120 J
• Check using power: P = V × I = 4.0 V × 0.50 A = 2.0 W, and 2.0 W × 60 s = 120 J
• Result: the pd is 4.0 V and 120 J of electrical energy is converted to heat in the resistor.

Chapter Summary

This chapter explored fundamental electrical quantities including current, resistance, voltage, electromotive force, and potential difference, each essential for understanding electrical phenomena. It examined electrical circuits, distinguishing between series and parallel configurations and their effects on circuit behavior. Various electrical measuring instruments were introduced to facilitate accurate experimentation and data collection. Ohm's law was presented as a key principle linking voltage, current, and resistance in conductors. The chapter detailed factors influencing resistance such as the length and cross-sectional area of a conductor, temperature changes, and the intrinsic resistivity of materials. Finally, resistor networks were analyzed, focusing on how resistors combine in parallel and series arrangements, which affects overall resistance and circuit performance. This comprehensive overview equips students with the foundational knowledge to conduct electrical experiments effectively.