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EMPA Information. Provisional Dates:- Weds 5 th May – Practical (Section A) – All Day (no time limit) consists of two tasks. Fri 7 th May – Written Paper (Section B) 1hr 1/4. Remember Unit 1 exam is on Thursday 27 th May. EMPA Practice. Next week in Friday P1 with Dr. Poulton.

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EMPA Information

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  1. EMPA Information Provisional Dates:- Weds 5th May – Practical (Section A) – All Day (no time limit) consists of two tasks. Fri 7th May – Written Paper (Section B) 1hr 1/4. Remember Unit 1 exam is on Thursday 27th May.

  2. EMPA Practice Next week in Friday P1 with Dr. Poulton. Following week on Thursday P1 (BK) +P2 (CJP). More will follow...Don’t Panic! I can do more in Phys. Workshop (need advance warning.)

  3. Resistor Rules & Resistance Heating Electricity Lesson 7

  4. Learning Objectives To know and derive the rules for resistors in series and parallel. To know how to calculate resistance heating.

  5. Recap What are the circuit rules for current and potential difference?

  6. Potential Difference Rule 1

  7. Resistors in Series The pd across all the components is equal to the sum of the pds across each component:- Using R=V/I, we can write:-

  8. Resistors in Series The resistance across all three resistors is:- Then cancelling the Is on the right hand side:-

  9. Quick Question (a) What is total resistance of the three resistors connected in series?

  10. Resistors in Parallel

  11. Resistors in Parallel The current splits at a junction but the potential difference is the same across both:- Using R=V/I we can say:-

  12. Resistors in Parallel So it follows that:- Dividing throughout by V gives:-

  13. Resistors in Parallel • What is the resistance across the two resistors if R1 = 10 Ω? • R2 = 5 Ω? • If it is a 1.5 V cell evaluate I, I1 and I2?

  14. Practice Calculation What is the resistance across the 4 ohm and 8 ohm resistors? What is the total resistance? What is the current in each part of the circuit?

  15. Resistance Heating The heating effecting of an electric current in a component is due to the resistance of the component. The charge carriers repeatedly collide with the positive ions of the conducting material. So there is a net transfer of energy from the charge carriers to the positive ions.

  16. Resistance Heating When a current I passes through a component, the pd across the component is given by V=IR. The power supplied to the component is given by:- This is energy transferred per second as thermal energy. and rate of heat transfer to the surroundings (if the component is at constant temperature).

  17. Resistance Heating If a component heats up, its temperature rise depends on the power the supplied to it and the rate of heat transfer to the surroundings. Energy transferred in time t = power × time = I2Rt The energy transferred does not depend on the direction of the current only its magnitude.

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