Science 9 · Electrical principles
Current, voltage and the energy they carry
Every device you plug in does the same three things: takes energy from a source, turns some of it into something useful, and wastes the rest as heat. This unit gives a number to each part of that, and one rule, Ohm's law, that ties them together.
- 1. The words, first
- 2. Series and parallel circuits
- 3. Ohm's law
- 4. Power, energy and efficiency
- 5. Making and storing electricity
- 6. What costs marks
The words, first
The idea: Current is the flow, voltage is the push, resistance is what gets in the way. Power and energy are different, and the bill charges for energy.
| Word | What it means |
|---|---|
| Static / current electricity | Static: charge built up on an object, usually by rubbing, that stays put until it jumps as a spark. Current: charge flowing steadily around a circuit. |
| Conductor / insulator | A conductor lets charge flow easily (copper, most metals); an insulator does not (rubber, plastic, glass). |
| Circuit / load | A circuit is a complete, closed path for current. A load is a device in it that turns electrical energy into another form: a bulb, a motor, a heater. |
| Current (I) | How fast charge flows, in amperes (A). |
| Voltage (V) | The energy the source gives each bit of charge — the push — in volts (V). |
| Resistance (R) | How strongly something opposes the current, in ohms (Ω). |
| Series / parallel | Series: one path, the same current through every load. Parallel: two or more branches, each getting the full source voltage. |
| Ammeter / voltmeter | An ammeter measures current and goes in series, in the path. A voltmeter measures voltage and goes in parallel, across a load. |
| Power (P) | How fast energy is used, in watts (W). 1 W is 1 joule per second; 1 kW is 1000 W. |
| Energy (E) | How much is used, in joules (J) or kilowatt-hours (kWh). 1 kWh = 3 600 000 J. |
| Efficiency | Useful energy out ÷ total energy in × 100%. |
| Motor / generator | A motor turns electrical energy into mechanical energy (the energy of motion and position). A generator does the reverse. |
| Renewable / non-renewable | Renewable sources are replaced naturally as fast as they are used: wind, sunlight, flowing water. Non-renewable ones are not: coal, natural gas, uranium. |
Series and parallel circuits
The idea: In series the current is the same everywhere and the voltage is shared. In parallel every branch gets the full voltage and the current splits.
Static first. Rub a balloon on your hair and electrons move from hair to balloon, then stay put; a doorknob shock and lightning are static jumping. Current electricity keeps moving, and needs a complete circuit to do it.
Worked: series. A 9.0 V battery runs two bulbs in series. The ammeter reads 0.20 A, and would read 0.20 A anywhere in the loop, because there is only one path. The voltmeter across bulb 1 reads 4.0 V, so bulb 2 has the rest: 9.0 − 4.0 = 5.0 V.
Worked: parallel. A 6.0 V battery runs two bulbs on separate branches, and each has the full 6.0 V across it. The branch currents join on the way back: 0.30 + 0.50 = 0.80 A through the main ammeter.
| Series | Parallel | |
|---|---|---|
| Current | The same everywhere | Branch currents add to the total |
| Voltage | Shared; the bulbs' voltages add up to the battery's | Every branch gets all of it |
| One bulb burns out | They all go out | The others stay on |
Why houses are wired in parallel. Every outlet gets the full 120 V, and switching off a lamp does not switch off the fridge.
Ohm's law
The idea: Voltage, current and resistance are tied by one equation. Know any two and you can find the third.
V = I × R · I = V ÷ R · R = V ÷ I
Worked. A toaster on 120 V draws 10.0 A: R = 120 ÷ 10.0 = 12.0 Ω. A 30 Ω resistor on a 6.0 V battery: I = 6.0 ÷ 30 = 0.20 A.
On the test: a data table. A student measures the current through one resistor at four voltages:
| Voltage (V) | Current (A) | V ÷ I (Ω) |
|---|---|---|
| 1.5 | 0.05 | 30 |
| 3.0 | 0.10 | 30 |
| 4.5 | 0.15 | 30 |
| 6.0 | 0.20 | 30 |
Doubling the voltage doubles the current, and V ÷ I is the same every time, so the resistance is 30 Ω. A light bulb is less tidy: its filament heats up as the current rises, and hot metal resists more.
What raises resistance: a longer wire, a thinner wire, a poorer conductor, and, for metals, a higher temperature. That is why a heavy-duty extension cord has thicker wires.
Power, energy and efficiency
The idea: Power is how fast energy is used; energy is how much. Multiply power by time to get energy, and compare useful energy out with energy in to get efficiency.
P = I × V · E = P × t · efficiency = useful energy out ÷ total energy in × 100%
Worked: power. A kettle on 120 V draws 12.5 A: P = 12.5 × 120 = 1500 W, or 1.5 kW.
Worked: energy, two ways. It runs for 4.0 minutes. In joules, time must be in seconds: E = 1500 W × 240 s = 360 000 J. In kilowatt-hours, power in kW and time in hours: E = 1.5 kW × (4 ÷ 60) h = 0.10 kWh. The same energy, since 360 000 ÷ 3 600 000 = 0.10.
Worked: efficiency. The water gains 306 000 J of that 360 000 J: 306 000 ÷ 360 000 × 100% = 85%. The other 15% warmed the kettle and the air — not destroyed, just not useful.
The EnerGuide label on a Canadian appliance gives its yearly energy use in kWh. Fridge A uses 450 kWh a year, fridge B 350 kWh. At 15¢ per kWh, B saves 100 × $0.15 = $15 a year, or $225 over 15 years — which is how the fridge with the higher price tag can be cheaper to own.
The toolkit Volts, amps, watts and the power bill follows this chain all the way to dollars. Science 10 picks up energy and efficiency again and adds work, kinetic energy and heat.
Making and storing electricity
The idea: A cell turns chemical energy into electrical energy; a generator turns motion into it. Every source works one of those ways, and each has costs.
Cells and batteries. A cell has two different metals, the electrodes, in a chemical that conducts, the electrolyte; a battery is several cells joined. A wet cell has a liquid electrolyte (most car batteries), a dry cell a paste (an AA battery), and a rechargeable cell, like a phone's, has its reaction run backwards by the charger.
Motors and generators. Current in a coil of wire makes it an electromagnet. In a motor, that magnet pushes against a permanent magnet and the coil turns. Spin the coil yourself and a current appears: the motor has become a generator. Most grid electricity comes from spinning generators; solar panels are the main exception.
| Source | Renewable? | The trade-off |
|---|---|---|
| Natural gas | No | Available on demand; releases carbon dioxide |
| Coal | No | The most carbon dioxide; Alberta's last coal-fired plant stopped burning coal in 2024 |
| Wind | Yes | No fuel or emissions; output rises and falls with the wind |
| Hydro | Yes | Reliable; dams flood valleys |
| Solar | Yes | No emissions; nothing at night |
In a gas plant the energy goes chemical → thermal → kinetic (a spinning turbine) → electrical, and every arrow loses some as heat.
On the test: societal decisions. A question may give cost, emissions and reliability for several sources and ask which is best for a stated goal. Read the goal: the cheapest, the cleanest and the most reliable are often three different answers.
What costs marks
The idea: Six, and most are about units or where a meter goes.
- An ammeter in parallel or a voltmeter in series.
- Saying the first bulb uses up the current. Current is the same all round a series loop; the bulbs use energy.
- Saying parallel branches share the voltage. Each gets all of it; the current splits.
- Mixing units in E = P × t. Watts × seconds gives joules; kilowatts × hours gives kWh. Minutes are neither.
- Confusing power and energy. A 1500 W kettle uses 1500 joules each second; how much it uses depends on how long it runs.
- Giving the wasted percentage as the efficiency.