The exercise

1. Briefly describe how metals and semiconductors conduct electricity.
2. How does a rise in temperature affect each?

1. In a metal, free electrons (loosely held outer electrons) carry the current. In a semiconductor, bonds broken by heat free electrons and leave holes, and both electrons and holes carry the current.
2. Heating a metal increases the random motion of its electrons and lowers its conductivity. Heating a semiconductor breaks more bonds, making more carriers, and raises its conductivity.

1. An LED does not light from one dry cell but lights from two in series. Explain.
2. Give three everyday uses of LEDs.
3. Give three reasons white LEDs are fast replacing filament bulbs.

1. An LED lights only when forward biased beyond its minimum forward voltage, about 1.8–2 V. One cell gives 1.5 V, below it; two in series give 3 V, above it.
2. Any three of: indicator lights on appliances, television screens, house and street lighting, torches, traffic lights.
3. They use much less energy, last about 50 000 hours, and waste little energy as heat.

Activity 11.6 lights a bulb when it gets dark. A student wants to change it so that a car's headlights falling on it at night open the garage door, using a small 3 V DC motor. Design the circuit and show where the motor goes.

Swap the LDR and V_R: put the LDR between the + terminal and the base, and V_R between the base and the − terminal. In the dark the LDR's resistance is high, the base voltage stays below 0.7 V and the transistor is off. When headlights fall on it, its resistance drops, the base rises above 0.7 V and the transistor switches on. Put the motor in place of the bulb, between the + terminal and the collector, so it runs when the transistor is on. V_R sets how bright the light must be.

Every diode in the bridge is an LED with a forward voltage of 1.8 V (D₁, D₄, D₅, D₆ in the bridge; D₂ and D₃ back to back in the load). A 6 V battery is connected to X and Y as shown.

1. Which LEDs light?
2. Mark the path of the current.
3. What happens if the battery is reversed?
4. What happens with a 3 V battery instead? Why?

1. Three LEDs light: the bridge LED that carries current from the positive terminal to the load, the one of D₂ and D₃ that points the way the current goes through the load, and the bridge LED opposite the first that returns the current to the negative terminal.
2. Positive terminal → forward-biased bridge LED → load LED → the opposite bridge LED → negative terminal. The other two bridge LEDs are reverse biased.
3. The other two bridge LEDs light instead — and the same load LED lights, because a bridge always sends current through the load the same way. The other load LED never lights.
4. None light. Every path has three LEDs in series, needing at least 3 × 1.8 = 5.4 V, more than 3 V.

Which bridge LEDs are which depends on how they face in the textbook's figure; the answer follows the bridge rule of figure 11.12.