Year 4 Science: Will the lamp light? The complete loop
Part of the Year 4 Science electricity and circuits chapter. Chapter overview · whole chapter on one page
Will the lamp light? The complete loop
The electricity can only flow when there is a complete loop, from the cell, through the wires and the lamp, and back to the cell. If the loop has a gap, or a switch is open, the loop is broken, and the lamp does not light.
So a lamp lights only when it is part of a complete loop with the cell.
The table shows five set-ups, and whether the lamp lights in each one.
| Set-up | Does the lamp light? |
|---|---|
| a cell, wires and a lamp in one complete loop | lights |
| a gap in one of the wires, so the loop is broken | does not light |
| a switch that is open, so the loop is broken | does not light |
| a switch that is closed, so the loop is complete | lights |
| a lamp with a wire to only one end of the cell | does not light |
Key ideas
- A lamp lights only when it is part of a complete loop with the cell.
- A gap in a wire breaks the loop, so the lamp does not light.
- An open switch breaks the loop, so the lamp does not light.
- A closed switch completes the loop, so the lamp lights.
Method. Check whether the loop is complete.
- Check the loop. The loop goes all the way round, from the cell to the lamp and back.all the way round
- Decide. A complete loop lets the electricity flow, so the lamp lights.lights
- Write the answer. lights
Check. The first row of the table says the lamp lights. ✓
Method. Check the loop.
- Check the wire. The gap breaks the loop, so the electricity cannot flow.breaks the loop
- Decide. Without a complete loop, the lamp does not light.does not light
- Write the answer. does not light
Check. The second row of the table says the same. ✓
Method. Check the switch.
- Check the switch. A closed switch completes the loop.closed
- Decide. A complete loop lets the electricity flow, so the lamp lights.lights
- Write the answer. lights
Check. The table says a closed switch lights the lamp. ✓
Watch out
Watch out: a lamp touching one wire is not in a complete loop. The lamp must be joined to the cell by a loop that goes all the way round.
Try it at home or school (with a grown-up). With a grown-up, join a cell, a bulb in a holder and two wires to make a loop. Check that the bulb lights. Then take one wire off, and check again. Do not touch the bulb while it is hot.
Exam tip
Grown-up tip. Ask: “Is the loop complete? What happens when you take one wire off?” Let your child explain the loop in their own words.
Exercise 9C Will the lamp light? The complete loop
Method. Read the first row of the table in section 9.3.
- Read the row. The complete loop row says the lamp lights.lights
- Write the answer. yes
Check. A complete loop lights the lamp. ✓
Method. Read the gap row in the table.
- Read the row. A gap breaks the loop, and the row says the lamp does not light.does not light
- Write the answer. no
Check. The gap breaks the complete loop. ✓
Method. Read the open switch row.
- Read the row. An open switch breaks the loop, so the lamp does not light.does not light
- Write the answer. no
Check. Open means the loop is broken. ✓
Method. Read the closed switch row.
- Read the row. A closed switch completes the loop, so the lamp lights.lights
- Write the answer. yes
Check. Closed means the loop is complete. ✓
Method. Read the one-wire row.
- Read the row. The lamp has only one wire, so the loop is not complete.not complete
- Write the answer. no
Check. Only a complete loop lets the electricity flow. ✓
Method. Count the rows that say lights.
- Count the rows. The complete loop and the closed switch both light the lamp.two rows
- Write the answer. 2
Check. Two lit and three not lit make five set-ups. ✓
Method. Count the rows that say does not light.
- Count the rows. The gap, the open switch and the one-wire set-up do not light the lamp.three rows
- Write the answer. 3
Check. Three not lit plus two lit makes five. ✓
Method. Read the key idea in section 9.3.
- Read the key idea. A lamp lights only when it is part of a complete loop.complete
- Write the answer. complete
Check. A complete loop has no gaps. ✓
Method. Think about the loop.
- Think about the wire. Touching one wire does not complete the loop.one wire
- Decide. So Ruby is not right.no
- Write the answer. no
Check. The loop must go all the way round. ✓
Method. Think about what the closed switch does.
- Think about the switch. Closing the switch joins the loop together.joins
- Decide. So the loop became complete.complete
- Write the answer. complete
Check. A closed switch makes a complete loop. ✓
Method. Use what you know about open switches.
- Think about the loop. An open switch breaks the loop.breaks
- Make the prediction. So the lamp will not light.no
- Write the answer. no
Check. Test your prediction with a grown-up and a bulb holder. ✓
Method. Think about the electricity.
- Think about the flow. Electricity needs a complete loop to flow round the circuit.complete loop
- Write the reason. With a gap, the electricity cannot flow, so the lamp does not light.does not light
- Write the answer. Accept any sentence that says electricity cannot flow round a broken loop, so the lamp does not light.
Check. A gap is a break in the loop. ✓