# SAT physics help?

right now, im trying to read a chapter on thermal physics and electric forces. Im trying to do the practice questions, but I can't seem to understand them. Can someone please do these few questions and explain them as well? thanks!

1) During each cycle, a heat engine absorbs 400 J of heat from its high-temperature source and sicards 300 J of heat into its low-temperature sink. What is the efficiency of this engine?
a) 1/7
b) 1/4
c) 3/7
d) 4/7
e) 3/4

2) Through a series of thermodynamic processes, the internal energy of a sample of confined gas is increased by 560 J. if the net amount of work done on the sample by its surroundings is 320 J, how much heat was transferred between the gas and its environment?
a) 240 J absorbed
b) 240 J dissipated
c) 880 J absorbed
d) 880 J dissipated
e) none of the above

3) a sphere of charge +Q is fixed in position. A smaller sphere of charge +q is placed near the larger sphjere and released from rest. The small sphere will move away from the large sphere with

a) decreasing velocity and decreasing acceleration
b) decreasing velocity and increasing acceleration
c) decreasing velocity and constant acceleration
d) increasing velocity and decreasing acceleration
e) increasing velocity and increasing acceleration

4) If the electric field does negative work on a negative charge as the charge undergoes a displacement from position A to positioon B within an electric field, then the electrical potential energy
a) is negative
b) is positive
c) increases
d) decreases
e) cannot be determined from the information given

5) The potential at point A in an electric field is 10V higher than at point B. If a negative charge, q=-2C, is moved from point A to point B, then the potential energy of this charge will

a) decrease by 20 J
b) decrease by 5 J
c) increase by 5 J
d) increase by 20 J
e) increase by 100 J

## Answers and Replies

rl.bhat
Homework Helper
Hi. Welcome to PF.
Please read the rules of PF.
We are here to help you in your home work. For that you have to your attempt.
What are the relevant equation you use to solve the above problems?

(1) Ideally you would like a heat engine to take in the heat from a hot resevoir and convert all of that energy into mechanical work, but that's not possible. It can convert some of that energy into work, but it must also throw away some of that energy by releasing it to a cold reservoir. A formula for the efficiency of a heat engine will be a measure of that situation.