Learn ยท Physics ยท Class 10

Current Electricity

What is actually moving in a wire, and why one broken bulb can kill a whole string of lights.

8 parts
Scroll to begin โ†“
๐Ÿ“˜ DefinitionCurrent, voltage, resistancePart 1 of 8
SeriesL1L2one path ยท same currentvoltage sharedParallelbranches ยท current splitseach gets full voltageRemove one lamp: series goes dark, parallel keeps working
Series shares voltage and stops together; parallel shares current and keeps working.
๐Ÿงฎ EquationsOhm's law and powerPart 3 of 8
โœ๏ธ Worked exampleSolving a circuitPart 4 of 8
๐ŸŒ In the real worldSafety and the homePart 5 of 8
โš ๏ธ Common mistakesWhere marks get lostPart 6 of 8
โœ… SummaryWhat to rememberPart 7 of 8
โœ๏ธ Worked exampleWatch one worked throughPart 8 of 8
1๐Ÿ“˜ DefinitionPart 1 / 8

Current, voltage, resistance

Three quantities describe every circuit, and confusing them is the main source of trouble.

  • Current (I): the rate of flow of charge, in amperes. Measured with an ammeter in series.
  • Voltage (V): energy given to each unit of charge, in volts. Also called potential difference. Measured with a voltmeter in parallel.
  • Resistance (R): opposition to current, in ohms.

A water analogy helps if you do not lean on it too hard: current is the flow rate, voltage is the pressure pushing it, resistance is the narrowness of the pipe.

I = Q / t   (charge per second)

Note that current in a metal is a flow of electrons, which are negative, so they actually move opposite to the conventional current direction drawn on diagrams.

2๐Ÿงฉ The partsPart 2 / 8

Series and parallel

Series: one path only.

  • Current is the same everywhere.
  • Voltage is shared between components.
  • Resistances add: R = Rโ‚ + Rโ‚‚.
  • One breaks, everything stops.

Parallel, branches.

  • Voltage is the same across each branch.
  • Current splits between branches.
  • Total resistance is less than the smallest branch: 1/R = 1/Rโ‚ + 1/Rโ‚‚.
  • One breaks, the rest keep working.

House wiring is parallel for exactly that reason, and so every appliance gets the full mains voltage.

3๐Ÿงฎ EquationsPart 3 / 8

Ohm's law and power

V = I R

Ohm's law holds for a metal conductor at constant temperature. A component obeying it is "ohmic" and gives a straight-line I-V graph through the origin. A filament lamp does not, as it heats, resistance rises and the graph curves.

Electrical power and energy:

P = V I  =  IยฒR  =  Vยฒ/R
E = P t

The IยฒR form explains why power lines run at very high voltage: for the same power, higher V means lower I, and since heat loss goes with Iยฒ, halving the current cuts losses to a quarter.

4โœ๏ธ Worked examplePart 4 / 8

Solving a circuit

Question. Two resistors, 4 ฮฉ and 6 ฮฉ, are connected in series to a 12 V battery. Find the total resistance, the current, and the voltage across each.

Step 1: total resistance. Series, so add:

R = 4 + 6 = 10 ฮฉ

Step 2: current. Same everywhere in series:

I = V/R = 12 รท 10 = 1.2 A

Step 3: voltage across each.

Vโ‚ = 1.2 ร— 4 = 4.8 V    Vโ‚‚ = 1.2 ร— 6 = 7.2 V

Step 4: check. 4.8 + 7.2 = 12 V, matching the supply. The voltages must add to the source in series, so this confirms the working.

Notice the larger resistor takes the larger share of the voltage, that is the principle behind a potential divider.

5๐ŸŒ In the real worldPart 5 / 8

Safety and the home

Fuses and circuit breakers protect against excessive current. A fuse is a thin wire that melts above its rating, breaking the circuit. It must be fitted in the live wire, so the appliance is isolated from the supply when it blows.

Earthing gives fault current a safe path. If a live wire touches a metal casing, current flows to earth rather than through anyone touching it, and the resulting surge blows the fuse.

Choosing a fuse: a 2 kW heater on 230 V draws I = P/V = 8.7 A, so a 13 A fuse is right and a 3 A fuse would blow immediately.

Why bills are in kWh. Energy, not power, is what you pay for. A 100 W bulb left on for 10 hours uses 1 kWh, one unit.

6โš ๏ธ Common mistakesPart 6 / 8

Where marks get lost

  • "Current is used up as it goes round." It is not. In series it is identical everywhere, energy is transferred, charge is not consumed.
  • Ammeter in parallel, voltmeter in series. Backwards, ammeter series, voltmeter parallel.
  • Adding parallel resistances directly. Use 1/R = 1/Rโ‚ + 1/Rโ‚‚, and remember the total is smaller than any branch.
  • Applying Ohm's law to a filament lamp. Its resistance changes with temperature.
  • Fitting the fuse in the neutral wire. It must be in the live wire.
7โœ… SummaryPart 7 / 8

What to remember

  • V = IR ยท P = VI = IยฒR = Vยฒ/R ยท E = Pt
  • Series: same current, shared voltage, resistances add.
  • Parallel: same voltage, split current, total R is smallest.
  • Ammeter in series, voltmeter in parallel.
  • Fuse in the live wire; earth protects metal casings.
8โœ๏ธ Worked examplePart 8 / 8

Watch one worked through

Here is one done slowly, with the reason for every move. Then one for you.

Watch one first

Two resistors of 4 ฮฉ and 6 ฮฉ are connected in series to a 12 V supply. Find the current and the voltage across each.

  1. In series, resistances add.
    R = 4 + 6 = 10 ฮฉ
    There is only one path, so the current meets both in turn. Series adds; parallel does not, and mixing those up is the usual error.
  2. Apply Ohm's law to the whole circuit.
    I = V รท R = 12 รท 10 = 1.2 A
    In series the current is the same everywhere, so this one value applies to both resistors.
  3. Find each voltage with the same law.
    Vโ‚ = 1.2 ร— 4 = 4.8 V Vโ‚‚ = 1.2 ร— 6 = 7.2 V
    The larger resistor takes the larger share of the voltage. That is what a potential divider is.
  4. Check the voltages add to the supply.
    4.8 + 7.2 = 12 V โœ“
    In a series loop the voltages must add to the supply. A free check on every question of this kind.

Answer: 1.2 A, with 4.8 V across the 4 ฮฉ and 7.2 V across the 6 ฮฉ

Now you try

Two 5 ฮฉ resistors in series on a 20 V supply. What is the current in amps?

Show the answer

2

Practise Current Electricity

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๐Ÿง  5 quizzes on Current Electricity (2 easy, 2 medium, 1 hard)

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๐Ÿ–จ๏ธ Printable worksheet

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