Electric Circuits: Current, Voltage & Resistance Lesson Slides

Slide-by-slide content

1. 1. Electric Circuits

   Physics · Current, voltage, resistance, and series vs parallel

   Speaker notes

   Show a torch or a desk lamp and ask: what has to happen inside for the bulb to light? Collect ideas. Many students will say electricity 'flows into the bulb and gets used up'. Tell them the lesson will test that idea.

2. 2. By the end of this lesson you can

   • Explain why a circuit must be a complete loop
   • Describe current, potential difference and resistance
   • Calculate with V = IR
   • Compare series and parallel circuits
   • Explain how homes are wired
   Speaker notes

   Point out that 'voltage' and 'potential difference' mean the same thing in this lesson; some courses prefer one term, some the other. Ask which objective students think will be hardest.

3. 3. A circuit is a complete loop

   • Charge flows only around a complete, unbroken loop
   • The cell or battery pushes the charge round
   • Opening a switch breaks the loop and the current stops
   • Conventional current flows from + to − outside the cell
   • Electrons actually drift the other way, from − to +
   Speaker notes

   Draw a cell, a switch and a lamp in a loop using standard circuit symbols. Ask what happens if one wire is disconnected anywhere in the loop: everything stops. The direction convention was chosen before electrons were discovered, which is why the two directions differ.

4. 4. Current and potential difference

   Current (I)

   • The rate of flow of charge
   • Unit: ampere (A); 1 A = 1 coulomb per second
   • Measured with an ammeter
   • Ammeter goes in series, in the loop

   Potential difference (V)

   • Energy transferred per unit of charge
   • Also called voltage; unit: volt (V)
   • Measured with a voltmeter
   • Voltmeter goes in parallel, across a component
   Speaker notes

   A useful picture: current is how much charge passes a point each second; potential difference is how much energy each unit of charge gives up in a component. Ask students where the ammeter and voltmeter must go in a circuit and why: the ammeter must have all the current pass through it, the voltmeter compares two points.

5. 5. Resistance and Ohm's law

   • Resistance opposes the current in a component
   • Unit: ohm (Ω); 1 Ω = 1 volt per ampere
   • V = I × R
   • For a fixed resistance, more voltage means more current
   • Copper wires have low resistance; lamp filaments have high resistance
   Speaker notes

   Show the rearrangements I = V ÷ R and R = V ÷ I. Components that keep the same resistance as the current changes are called ohmic; a filament lamp is not, because its resistance rises as it heats up. Older students can sketch a current-voltage graph for each.

6. 6. Worked example: a torch lamp

   • A lamp is connected to a 3.0 V battery
   • The ammeter reads 0.25 A
   • R = V ÷ I
   • R = 3.0 ÷ 0.25
   • R = 12 Ω
   Speaker notes

   Write each line on the board and ask students to check the units: volts divided by amperes gives ohms. Follow-up: if the same 12 Ω lamp were connected to a 6.0 V battery and its resistance did not change, the current would be 0.5 A. In reality the filament gets hotter, so the current would be a little less.

7. 7. Series circuits: one path

   • All components sit in one loop
   • The current is the same at every point
   • Supply voltage is shared between components
   • Total resistance = R₁ + R₂ + …
   • If one lamp breaks, the whole circuit stops
   Speaker notes

   Add a second lamp in series and the lamps get dimmer, because the total resistance goes up and the current goes down. Show ammeter readings before and after each lamp: they are the same. Current is not used up; energy is transferred to the lamps.

8. 8. Parallel circuits: several paths

   • Each component has its own branch to the supply
   • Each branch gets the full supply voltage
   • Branch currents add up to the total current
   • Adding a branch lowers the total resistance
   • Each branch can be switched on or off separately
   Speaker notes

   Add a second lamp in parallel and both stay as bright as a single lamp, because each gets the full supply voltage. The battery now supplies more current in total, so it runs down faster. For ages 14–15, introduce 1/R = 1/R₁ + 1/R₂ as an extension.

9. 9. Series vs parallel at a glance

   Series

   • One path for the current
   • Same current everywhere
   • Voltage shared between components
   • More lamps: each lamp dimmer
   • One break stops everything

   Parallel

   • Two or more paths
   • Current splits, then rejoins
   • Same voltage across each branch
   • More lamps: each stays bright
   • One break stops only that branch
   Speaker notes

   Ask students to copy this table and add a sketch of each circuit. Then ask a hinge question: a string of decorative lights goes out completely when one bulb fails. Is it wired in series or parallel? Series.

10. 10. Worked example: two 6 Ω resistors, 6 V battery

    • Series: total R = 6 + 6 = 12 Ω
    • Series: I = 6 ÷ 12 = 0.5 A; each resistor gets 3 V
    • Parallel: each resistor gets the full 6 V
    • Parallel: each branch I = 6 ÷ 6 = 1 A
    • Parallel: total current = 1 + 1 = 2 A
    Speaker notes

    Same parts, very different results. In parallel the total resistance is 6 V ÷ 2 A = 3 Ω, less than either resistor on its own. Check with older students: 1/R = 1/6 + 1/6 = 1/3, so R = 3 Ω. Ask why the battery would run down four times faster in the parallel circuit (four times the current).

11. 11. Why homes are wired in parallel

    • Every appliance gets the full mains voltage
    • Each light or socket can be switched on its own
    • One appliance failing does not switch off the rest
    • Mains electricity can kill: never experiment with it
    • Fuses and circuit breakers cut off dangerous currents
    Speaker notes

    Mains voltage differs between countries, for example about 230 V in many countries and about 120 V in others, so avoid quoting one figure as universal. Make the safety message explicit: classroom experiments use low-voltage cells or power packs only.

12. 12. Series and parallel on one page

    Series: one path, current the same everywhere, voltage shared. Parallel: several paths, voltage the same across each branch, currents add up.

    Speaker notes

    Use the poster as a recap or print it for the classroom wall. Ask students to cover one column and recall it from memory, then swap with a partner.

13. 13. Quick check

    Two identical lamps are in series with a 6 V battery. What is the potential difference across each lamp?

    • 6 V
    • 12 V
    • 3 V
    • 0 V
    Speaker notes

    Answer: C, 3 V. In series the supply voltage is shared, and identical lamps share it equally. Students who chose A are thinking of a parallel circuit; ask them to explain what would change if the lamps were in parallel.

14. 14. Key takeaways

    • Current flows only in a complete circuit and is not used up
    • Current in amperes, potential difference in volts, resistance in ohms
    • V = IR links all three
    • Series: same current, shared voltage
    • Parallel: same voltage, currents add up
    Speaker notes

    Return to the opening question about the torch and ask students to answer it again with the words current, potential difference and energy. Then move to the five-question quiz. Homework idea: find three things at home that are switched independently and explain how that shows parallel wiring.

Key terms

Electric current

The rate of flow of electric charge, measured in amperes (A) with an ammeter.

Potential difference (voltage)

The energy transferred per unit of charge between two points, measured in volts (V) with a voltmeter.

Resistance

How much a component opposes the current, measured in ohms (Ω); R = V ÷ I.

Ohm's law

For a component at constant temperature, the current is proportional to the potential difference: V = IR.

Series circuit

A circuit with one path, where the current is the same everywhere and the voltage is shared.

Parallel circuit

A circuit with two or more branches, where each branch has the full supply voltage and the branch currents add up.

Conventional current

The agreed direction of current, from the positive to the negative terminal outside the cell; electrons move the opposite way.

Quick quiz

Frequently asked questions