The table and Fig. 2 are from a workshop trial of an electromagnet used to lift steel bottle caps from a conveyor. A technician wound insulated copper wire ...
Question 1 Report
The table and Fig. 2 are from a workshop trial of an electromagnet used to lift steel bottle caps from a conveyor. A technician wound insulated copper wire around a removable steel core. A switch connects the coil to a 12 V d.c. supply. The results table shows how the number of caps lifted changed as the current was adjusted. The core must release the caps promptly when the current is switched off, so that they fall into a collection box.
| current in coil / A | number of steel caps lifted |
|---|---|
| 0.50 | 3 |
| 1.00 | 7 |
| 1.50 | 12 |
| 2.00 | 18 |
(a) What material should be used for the removable core? [1]
(b) Which row of Table 1 gives the greatest magnetic force from the electromagnet? [1]
(c) What conclusion about current and electromagnet strength is supported by the results? [2]
(d) Calculate the resistance of the coil when the current is 1.50 A and the potential difference is 12 V. Give your answer in ohms. [3]
(e) Sketch a circuit diagram for the test. Include the 12 V supply, switch, ammeter and coil connected in series. [2]
(f) When the switch is opened, what property of the chosen core allows the caps to be released quickly? [1]
Answer Details
An electromagnet needs a core that magnetises strongly but loses its magnetism promptly when the current is switched off. The results show strength by the number of caps lifted.
- (a) Use soft iron for the removable core. [1]
- (b) The row with 2.00 A and 18 caps gives the greatest magnetic force. [1]
- (c) As current increases, the number of caps lifted increases. Therefore, increasing the current makes the electromagnet stronger and increases its magnetic force. [2]
- (d) Use \(R=\frac{V}{I}\):
\[R=\frac{12\ \text{V}}{1.50\ \text{A}}=8.0\ \Omega\]Resistance = 8.0 \(\Omega\). [3] - (e) The supply, switch, ammeter and coil must all be in one complete series loop. [2]
- (f) Soft iron has low retentivity, so it is easily demagnetised when the current stops and releases the caps quickly. [1]
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