Electricity

The One Thing

Electric current is the flow of charge through a closed conducting circuit, driven by potential difference and opposed by resistance. The relationships among current, potential difference, resistance, power, heating, and energy are expressed through the stated equations.

Reactions, Processes and Experiments

Key Terms

• Electric charge: A property of matter responsible for electrical effects. Its SI unit is the coulomb (C).
• Electric current: The rate of flow of electric charge through a conductor, given by I = Q/t and measured in ampere (A).
• Potential difference: The work done to move a unit charge between two points, given by V = W/Q and measured in volt (V).
• Circuit: A closed conducting path through which electric current can flow.
• Ohm’s law: At constant temperature, the current through a conductor is directly proportional to the potential difference across it, expressed as V = IR.
• Resistance: The opposition offered by a conductor to the flow of current, given by R = V/I and measured in ohm (Ω).
• Resistivity: A property of a material indicating how strongly it resists current. It is represented by ρ and measured in ohm metre (Ω m).
• Series combination: A connection in which resistors are placed one after another. The same current flows through each resistor, with equivalent resistance R = R1 + R2 + R3.
• Parallel combination: A connection in which resistors are connected across the same two points. The potential difference is the same across each resistor, with 1/R = 1/R1 + 1/R2 + 1/R3.
• Electric power: The rate at which electrical energy is consumed or electrical work is done, given by P = VI, P = I²R, or P = V²/R, and measured in watt (W).
• Electrical energy: The total electrical work done or energy consumed, given by E = Pt and measured in joule (J).
• Commercial unit of energy: The kilowatt-hour (kWh), commonly called one unit of electricity. One kWh equals 3.6 × 10⁶ J.
• Heating effect of current: The conversion of electrical energy into heat when current passes through resistance, described by Joule’s law, H = I²Rt.

Easily Confused

• Conventional current and electron flow: Conventional current is taken from the positive terminal to the negative terminal through the external circuit, whereas electrons flow in the opposite direction.
• Series and parallel combinations: In series, current is the same through all resistors and total resistance is greater than any individual resistance; in parallel, potential difference is the same across each branch and equivalent resistance is less than the smallest branch resistance.
• Resistance and resistivity: Resistance depends on a conductor’s material, length, cross-sectional area, and temperature; resistivity depends on the nature of the material and temperature, but not on the conductor’s dimensions.
• Electric power and electrical energy: Power is the rate of electrical energy consumption, whereas energy is the total electrical work done over time.
• Ohm’s law and resistance: Ohm’s law applies to an ohmic conductor at constant temperature, while resistance is the opposition to current and is given by R = V/I.
• Heating effect and electric power: The heating effect describes electrical energy changing into heat and is given by H = I²Rt; electric power describes the rate of electrical energy consumption and is given by P = VI, P = I²R, or P = V²/R.

What Gets Asked

• Define electric current and use I = Q/t; marks are lost by confusing charge Q with current I, or by omitting the time t.
• Calculate potential difference using V = W/Q; marks are lost by confusing work done W with electrical energy or by using incorrect units.
• Apply Ohm’s law using V = IR; the relationship is valid only when temperature and other physical conditions remain constant.
• Determine resistance in series or parallel circuits; marks are lost by using the series equation for a parallel circuit or reversing the stated current and potential-difference rules.
• Calculate power and energy using P = VI = I²R = V²/R and E = Pt = VIt = I²Rt; marks are lost by treating power as total energy.
• Explain the heating effect of current or the operation of a fuse using H = I²Rt; marks are lost by stating that a fuse merely reduces current rather than melting and breaking the circuit when current becomes excessive.