A Level Physics Electrical Quantities Current and Electrical

A –Level Physics: Electrical Quantities: Current and Electrical Energy

Objectives: Additional skills gained: • Deriving equations • Accurate definition of quantities

Starter: Experimental Design an experiment that you would undertake to calculate the Young’s modulus of a wire. Include all the measurements etc. that you would need to take and graphs that you would draw You have 15 mins ONLY Extension: Draw a diagram of this experiment and fully label it

GCSE link: Charge Copy down the following questions and then answer them using what you remember from GCSE (20 mins) 1) An electrostatic paint sprayer is going to be used to paint a metal object. When the paint particles leave the sprayer, they are negatively charged. Explain the benefits of using this sprayer compared with one that does not charge the paint 2) Vicky combs her hair with a metal comb. Then she tries to pick up some small pieces of metal foil with the comb. The metal comb does not pick up any pieces of metal foil. Explain why the metal foil is not picked up by the comb. 3) The student rubs a cloth against a balloon and holds the balloon against a wall. The balloon sticks to the wall when he lets go. Explain how this happens.

Charge • The Greek philosopher Thales (c. 600 BC) discovered that when amber was rubbed with a cloth, it could attract small pieces of dried leaf. • The Greek word for amber is elektron which is the origin of our words electron and electricity • Under normal circumstances (due to equal number of protons and electrons), there is no overall charge on an object (conservation of charge) but we can induce charge via friction

Charge • Some particles are said to have an electric charge such as the electron which has a negative charge. The charge on a single electron is 1. 6 x 10 -19 C So…how many electrons make up 1 C of charge? 6. 25 x 1018 • This fundamental charge on a particle is unchangeable, remember we can’t destroy nor create charge (conservation of charge)

Definition Challenge There are so many misconceptions with defining electrical quantities. Copy and fill in the table below. The last column will be filled in after we have discussed them! Electrical Quantity Symbol Unit My original definition Official Definition Current The rate of flow of charge Potential Difference The energy transferred by a component per coulomb of charge Electromotive Force The energy supplied to each coulomb of charge (not a force at all!)

Current We defined current as the rate of flow of charge and this can be applied to anything that is carrying a charge (charge carriers) such as conventionally, the electron. So if 1 C worth of charge flows by in 1 s, then 1 A of current is flowing! The ampere is the base unit of current but as 1 A is actually a large current, often values are given in m. A, so don’t forget to convert to amperes. What current is produced when a lightning strike that lasts for 0. 1 s transfers 1150 coulombs of charge?

What’s the (potential) difference? There are four separate quantities that are often interchanged in GCSE but are subtly different! Electromotive Force Voltage Potential Difference Electronvolt Weirdly, less first consider how they are similar and why they get confused! Generally speaking, they have almost identical formula to calculate. It’s just the symbols that alter!

What’s the (potential) difference? Voltage is the generalised term, and is the one you are most familiar with from previous years. It is very rarely used at A-level but is simply the measure of the amount of energy a component uses per unit charge It’s not specific enough when discussing circuits and current, so let’s try to avoid it

What’s the (potential) difference? Potential Difference Potential difference is the more specific term for voltage, as it highlights the transformation of energy from electrical into another type It is the amount of electrical energy transferred by a component into a different form per unit charge In order to transfer the energy, the component is said to have done work and this is why the symbol has changed.

What’s the (potential) difference? Electromotive Force This is basically the supply voltage, i. e. if you were to put a voltmeter around the power-pack or battery etc, this would be the reading It is the energy transferred to each coulomb of charge from a supply. It is given symbol of epsilon.

What’s the (potential) difference? Electronvolt This quantity is likely to be new to you and is only really considered for sub-atomic particles (i. e. looking at an individual electron itself) If an electron is accelerated by a p. d of 1 V then it gains energy equal to its charge.

Practice Questions Explain how potential difference is different from electromotive force a) A cell provides 76 C of charge with 120 J of energy, what’s the emf? b) What is the p. d. across a lamp when the lamp carries a current of 2. 4 A for 5. 4 s transferring 120 J of energy? Convert a) 9. 6 x 10 -19 J into elecronvolts and b) 4. 8 Me. V into joules Write an evaluation of the strengths and weaknesses of a ski slope model of an electric circuit including discussion of: • Representation of the charge carriers • Analogies of electrical energy • Ski lift as a cell • Snowpark obstacles as components • Differing speeds by different skiers • How each quantity is represented

Objectives: Additional skills gained: • Deriving equations • Accurate definition of quantities