AS REVISION UNIT 1

1. AS REVISION UNIT 1 TOPIC : PARTICLES, QUANTUM PHENOMENA AND ELECTRICITY

2. Explain what is meant by an electric current. An electric current consists of a flow of electrons.

3. What is the difference between “conventional current” and the an “electron flow current”? Conventional current flow from positive to negative, electron flow current flows from negative to positive.

4. Define the Volt This is the p.d. when the work done of 1J when 1 coulomb of charge flows.

5. Define the Ohm The Ohm is the resistance when 1 volt is applied when a current of 1 ampere flows.

6. Explain the meaning of resistance. Resistance is the opposition to flow of an electric current.

7. Explain the origin of resistance in a conductor. The atoms in the conductor form the basis of opposition to the flow of current.

8. State the three formulas for power P = I.V , P = V2 /R , P = I2 R.

9. Define the term resistivity Resistivity is the resistance in a unit cross-sectional area per unit length.

10. State two factors that affect the resistance of a material Length, cross-sectional area, material and temperature of the material.

11. State Ohm’s Law The p.d. across a conductor is proportional to the current flowing through it, provided the temperature remains constant.

12. Describe what happens to the resistance of a thermistor when it is heated. As the temperature of the thermistor increases its resistance decreases.

13. What is a kilowatt-hour? A kilowatt-hour is the energy an appliance of 1kW used in one hour. IkWh = 3.6 x 106 J

14. What happens to the resistance of a material if the length doubles? The resistance doubles .

15. What happens to the resistance of a material if the cross-sectional area doubles? The resistance halves.

16. What happens to the resistance if the diameter of a wire doubles? The resistance decreases by a factor of 4. Since doubling the diameter increases the area by a factor of four.

17. Define Resistance Ratio of the p.d. across a conductor to the current through it.

18. Can resistivity change with temperature? Yes, the resistivity increases with temperature.

19. Does a filament bulb obey Ohm’s Law? No, the resistance changes due to a change in temperature.

20. Does a diode obey Ohm’s Law? No, the resistance changes due to a change in charge carriers.

21. State a formula for the work done in a circuit. W = VxIxt V = P.D., I = Current in amp t = time in seconds

22. How is power related to work? Power = Work Done / Time Taken

23. What is the meaning of electric charge? Electric charge is a quantity of electricity on charged particles.

24. Why does an electric current move slowly? An electric current consists of free electrons moving superimposed on random motion of electrons

25. Define the term potential difference. Potential difference is defined as the work done per unit charge in converting electrical energy into other forms

26. Define the term resistivity. Resistivity is defined as the resistance of a conductor in a unit cross-sectional area per unit length.

27. Define Electromotive Force Work done per unit charge in converting the source energy into electrical energy in a circuit.

28. How is emf different from p.d.? Emf is the energy per unit charge in converting energy in a circuit. P.d in energy/charge converted across a component.

29. How is charge conserved in a circuit? In any closed circuit the amount of charge flowing is constant, the rate of flow of charge is constant.

30. Explain how energy is conserved in a circuit. In any circuit the energy supplied by the source is dissipated in the components.

31. State the formulas for the sum of three resistors in parallel. 1/RT= 1/R1 + 1/R2 + 1/R3 For 2 resistors RT = Product/Sum

32. State the formulas for the sum of three resistors in series. . RT= R1 + R2 + R3

33. What is the purpose of a potential divider? A potential divider gives a proportion of the source potential to another circuit.

34. What is meant by an ‘open circuit’? It is a circuit where a current does not flow.

35. Sketch the Current-Voltage curve for a Resistor.

36. Sketch the Current-Voltage curve for a Lamp.

37. Sketch the Current-Voltage curve for a diode.

38. What is meant by a closed circuit? • It is a circuit where a current flow

39. What is meant by the terminal pd? State a formula in terms of the emf and the internal resistance. Terminal p.d. is the pd across the terminals of a cell. IR = E - Ir

40. Explain why the pd across a cell drops when a resistor is connected to a cell. P.d. is dropped across the internal resistance which reduces the terminal pd. from the formula, IR = E -Ir

41. How is a voltmeter connected in a circuit? Why is the resistance of a voltmeter high? In parallel. The resistance is high so no current flows through it.

42. How is an ammeter connected in a circuit? Why is the resistance of a ammeter low? In series. So that it take all the current and it does not change the current in a circuit.

43. What is the purpose of a potential divider? A potential divider gives a proportion of the source potential to another circuit.

44. State the purpose of a wave To carry and transfer energy.

45. What is the photoelectric effect? Emission of electrons off the surface of a metal using light.

46. How can the the photoelectric effect be demonstrated? Using a gold leaf electroscope which is negatively charged, a clean piece of zinc and a uv lamp. The leaf slowly collapses as uv light is shown onto the zinc plate.

47. What is meant by the threshold frequency? Smallest frequency of light required for electron emission.

48. What is the work function? Smallest energy of light photons required for electron emission.

49. What is Einstein’s Photoelectric equation. hf =  + ½ mv2max hf = incident photon energy  = work function ½ mv2max = Max ke of photoelectrons

50. Explain why the photoelectric current is proportional to the intensity of the electromagnetic radiation. The greater intensity of the incident radiation (i.e more photon hitting the metal surface) the number of electrons emitted per second.