Carter is planning a backpacking trip in the mountains. To prepare for the trip, he has purchased the gear he needs and is planning his meals. He knows it will be cold in the evenings, and he would like a warm meal. Although it will cost more than using a camp stove to heat purified water from the trail and prepare dehydrated food, Carter is considering the use of self-heating containers. He evaluates containers that use the heating systems given in the table.

1. Write your answers on the lines. Identify two potential constraints for the system Carter may choose, one based on the specific needs of the situation and one considering environmental impacts or costs.

2. Develop criteria based on scientific understanding of energy transfer that can be used to evaluate potential solutions for his choice of system.

3. Analyze the costs and benefits of using self-heating containers compared with using a camp stove.

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Answer:

Sam squinted against the sun at the distant dust trail raked up by the car on its way up to the Big House. The horses kicked and flicked their tails at flies, not caring about their owner's first visit in ten months. Sam waited. Mr Carter didn't come out here unless he had to, which was just fine by Sam. The more he kept out of his boss's way, the longer he'd have a job.

Carter came by later while Sam was chopping wood. Carter lifted his hat as if he were waiting for an appointment with the town priest, and then removed it completely as if he were talking to his mother. He pulled out a pile of paper from his back pocket and held it out.

'Don't pick up your mail often, do you?'

Sam took it without a glance and dropped the envelopes onto the bench.

'Never,' he replied and waited for Carter to say why he was here. The fact it was Carter's house was no explanation and they both knew it. Carter twisted his hat round and round, licking his lips and clearing his throat.

'Nice work fixing those fences,' he said finally.

'I'll be back to the beginning soon,' Sam said. It wasn't a complaint. A fence that took a year to repair meant another year's work to the man who did it well.

'Don't you ever want to take a holiday?'

'And go where?' A holiday meant being back out in the real world, a place even people like Carter travelled to escape from. Sam's escape was his reality and he wasn't going back.

Mr Carter wiped the sweat from the back of his neck. The damp patches on his shirt drew together like shapes in an atlas. His skin was already turning ruddy in the June sun. Otherwise he had the indoor tan of a man that made money while other people did the work.

'I've brought my son with me on this trip. He's had some trouble at school.' Mr Carter's eyes flicked up, blinked rapidly and then shifted back to the hat occupying his hands. 'Not much trouble out here for a young boy.' He attempted a laugh but it came out like a dog's bark.

The two men looked towards the northern end of the property. It stretched as far as the eye could see. Even the fences were barely visible from where they stood. However bored and rebellious a teenage boy might get, it wasn't possible to escape on foot. Sam looked at the biggest of the horses, kicking at the ground with its heavy hooves. Could the boy ride? he wondered. There was a whole load of trouble a good rider could get into out here, miles away from anyone. But maybe there was even more trouble for someone who knew nothing about horses and wanted to get away from his father.

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