Understanding current, voltage, resistance, and how circuits behave.
Charge is a property of matter. Electrons carry negative charge. When these charges flow through a conductor, they create an electric current.
Real-world Anchor: Static shock from a doorknob is a sudden movement of stored charge trying to balance out.
Current ($I$) is the rate of flow of charge. It tells you how much charge passes a specific point every second.
$$ I = \frac{Q}{t} $$
What it calculates: Current in Amperes (A).
When it applies: In any complete circuit where charge is moving.
Common misuse: Thinking current is "used up" by components. Current is always conserved in a single loop.
Q1: If 12 Coulombs of charge flow past a point in 3 seconds, calculate the current.
$$ I = 12 / 3 = 4 \, A $$
Q2: A current of 0.5 A flows for 20 seconds. How much charge has moved?
$$ Q = I \times t = 0.5 \times 20 = 10 \, C $$
Voltage is the energy transferred per unit of charge. It represents the "push" provided by a power source to move the charges.
$$ V = \frac{E}{Q} $$
What it calculates: Potential Difference in Volts (V).
When it applies: To find the energy provided by a source or used by a component.
Common misuse: Confusing voltage with current. Voltage is what causes current to flow.
Link to Topic 2: This energy ($E$) is measured in Joules, the same energy unit used for KE and GPE.
Q1: A 6V battery transfers 30J of energy to a circuit. How much charge flowed?
$$ Q = E / V = 30 / 6 = 5 \, C $$
Q2: Why is it incorrect to say current "is" voltage?
Current is the flow of charge; Voltage is the energy per charge that pushes that flow.
Resistance is the opposition to current. It is caused by electrons colliding with the positive ions in the metal lattice of a wire.
Real-world Anchor: A narrow hallway slows down a crowd of people; a thin wire resists current more than a thick one.
$$ V = I \times R $$
What it calculates: Resistance, Voltage, or Current.
When it applies: For "Ohmic" conductors at a constant temperature.
Common misuse: Forgetting that resistance changes if a component (like a bulb) gets hot.
Ohmic Conductor
Directly proportional (straight line).
Filament Lamp
Resistance increases as it heats up (curved).
Diode
Current flows in one direction only.
Diodes have very high resistance in the reverse direction, preventing current flow.
Q1: Calculate the resistance of a component if 12V causes a current of 3A.
$$ R = V / I = 12 / 3 = 4 \, \Omega $$
Q2: What happens to the resistance of a filament lamp as the current increases?
Resistance increases because the lamp gets hotter, causing more ion collisions.
Series Circuit
Parallel Circuit
| Feature | Series Circuit | Parallel Circuit |
|---|---|---|
| Current ($I$) | Same everywhere | Splits at junctions |
| Voltage ($V$) | Shared across components | Same across all branches |
| Total Resistance | Increases ($R_1 + R_2$) | Decreases (more paths) |
*These rules apply when the circuit is operating steadily.
Q1: In a series circuit with a 2Ω and 5Ω resistor, what is the total resistance?
$$ 2 + 5 = 7 \, \Omega $$
Q2: Why does total resistance decrease when you add a resistor in parallel?
Because you are providing extra paths for the current to flow, similar to adding lanes to a motorway.
$$ P = I \times V $$
Note: A device with high voltage but very small current can still have low power; current matters equally.
Future Link: This links to Topic 1 (Forces) and Topic 2 (Energy) as power is the rate of energy transfer.
Q1: Calculate the power of a 230V device using a current of 0.2A.
$$ P = 0.2 \times 230 = 46 \, W $$
Q2: Which wire in a plug is designed to carry the alternating potential to the device?
The Live wire (Brown).
1. Don't say current is "lost" or "used up"; it is the same everywhere in a series loop.
2. Don't forget that resistance only stays constant if temperature is constant.
3. Don't confuse Power ($W$) with Energy ($J$).