Electric charge is a fundamental property of matter. It is measured in coulombs (C). One coulomb is a very large amount of charge - approximately 6.24 × 1018 electrons.
Electric current is the rate of flow of electric charge. In metallic conductors, current is the flow of electrons. Current is measured in amperes (A), where 1 ampere = 1 coulomb per second.
Voltage (or potential difference) is the energy transferred per unit charge. It is measured in volts (V).
A voltage of 12V means that 12 joules of energy are transferred for every coulomb of charge that flows.
Resistance is a measure of how difficult it is for current to flow through a component. It is measured in ohms (Ω). The resistance of a conductor depends on its length, cross-sectional area, temperature, and the material it is made from.
Ohm's Law states that the current through a conductor is directly proportional to the voltage across it, provided the temperature remains constant.
The graph should be a straight line through the origin, showing that current is directly proportional to voltage. The gradient of the line equals 1/R.
Steeper line = Lower resistance
The graph shows I = V/R. As you change resistance, you can see how the line changes slope.
Power is the rate of energy transfer. In electrical circuits, power is the rate at which electrical energy is transferred. It is measured in watts (W).
One watt is equal to one joule per second (1 W = 1 J/s).
The total energy transferred depends on the power and the time for which the current flows.
Alternatively, using the relationship between energy, charge, and voltage:
A kettle has a power rating of 2000 W and is used for 3 minutes. Calculate the energy transferred.
Solution:
Time = 3 minutes = 3 × 60 = 180 seconds
E = P × t = 2000 × 180 = 360,000 J = 360 kJ
In a series circuit, components are connected end-to-end, forming a single path for current to flow.
Rtotal: 80Ω
Current (same everywhere): 0.15 A
V1 across R1: 4.5 V
V2 across R2: 7.5 V
In a parallel circuit, components are connected across each other, forming multiple paths for current to flow.
Important Note:
In parallel circuits, the total resistance is ALWAYS LESS than the smallest individual resistance. More paths for current = easier for current to flow = lower total resistance.
Req: 20Ω
I1 through R1: 0.20 A
I2 through R2: 0.40 A
Itotal: 0.60 A
Notice: More current flows through the lower resistance path!
Different components have different current-voltage relationships. Understanding these characteristics is crucial for circuit design.
Ohmic Resistor: Current is directly proportional to voltage. The graph is a straight line through the origin. Resistance remains constant.
An LDR is a component whose resistance decreases as light intensity increases. They are used in automatic lighting systems and light sensors.
Resistance: 5000Ω
An NTC thermistor is a component whose resistance decreases as temperature increases. They are used in temperature sensors and control systems.
Resistance: 5000Ω
LEDs are semiconductor devices that emit light when current flows through them. They have a threshold voltage (typically 0.6-0.7V) below which no current flows. LEDs are highly efficient and are used in displays, indicators, and lighting.
Important:
LEDs must be connected with the correct polarity (anode to positive, cathode to negative) and should always be used with a series resistor to limit current and prevent damage.
Test your understanding of electricity concepts with this interactive quiz!
A current of 2 A flows for 10 seconds. What is the total charge transferred?
A circuit has a voltage of 10 V and a current of 2 A. What is the resistance?
What is the power dissipated in a circuit with 5 A current and 12 V voltage?
In a series circuit with three bulbs, if one bulb breaks, what happens to the others?
When an LDR is placed in the dark, what happens to its resistance?
What is the unit of electrical energy?
In a parallel circuit, the voltage across each branch is:
A 2000 W kettle is used for 3 minutes. How much energy is transferred?
A diode allows current to flow in how many directions?
When an NTC thermistor is heated, its resistance: