Describe as fully as you can the route for electrical energy from power station to your home.

Electromotive Force and Potential Difference 26/06/2018 LO: Describe the difference between e.m.f. & p.d. & reinforce with calculations Define the volt State Kirchhoff’s 2nd Law from measurements made of circuits

From Power Station to Home Back in the power station energy is transferred from other forms of energy into electrical energy. Once this electrical energy has been created it can be used to power components. Only in capacitors can electrical energy be stored, everyday devices cannot store electrical energy. Electromotive force (e.m.f.) is the energy transferred per unit charge from one type of energy to electrical. The charge is gaining energy.

e.m.f e.m.f is measured in volts. It is an expression of the maximum potential difference across the source. e.m.f is measured in volts where 1 volt is equal to one joule of energy per coulomb of charge. Mains electricity (230V) is therefore supplying each coulomb with 230 J. Based on this definition can you derive the formula?

E.m.f. tells us how much work is done on the charge Electromotive Force = Electrical energy transferred Charge Electromotive Force = Work Done E = W/Q So: E.m.f. tells us how much work is done on the charge

P.d. tells us how much work is being done __ each Coulomb of charge. P.d (Potential Difference) What is happening to the energy the charge has gained when it moves through a filament lamp? Potential difference is the amount of energy transferred per unit charge from electrical to another form. The charge is _______ energy. V = W/Q P.d. tells us how much work is being done __ each Coulomb of charge.

Joules per Coulomb (J/C) = Volt What are both e.m.f and p.d measured in? Joules per Coulomb (J/C) = Volt An e.m.f. of 1V means that 1J of energy is transferred to every Coulomb of charge from the supply (cell, generator). A p.d. of 1V means that 1J of energy is transferred away from every Coulomb of charge to a component (buzzer, lightbulb).

Another Way to Think About it… e.m.f is the property of the battery whereas p.d is the property of the circuit (including the battery). "emf' is the potential difference across a source of electricity when there is no current through the source. Close the circuit and current will flow but the cell may internally resist the flow. The potential difference seen across the terminals of the source will therefore be less than the potential difference when there is no current.

Example 1 A cooker requires 1.5MJ to heat a pie. The e.m.f. of mains is ____V. How many Joules per Coulomb does the charge gain? 230J/C In the heating filament, how much energy does the charge lose? 230J/C How much charge is supplied in total? 1.5x106/230 = 6520C (3.s.f)

If finished Q. 2 on P. 95 Example 2 A 12V battery drives a current of 2A around a circuit for one minute. How much charge flows around the circuit in this time? How much energy is transferred to the charge? How much energy does the charge transfer to the components in the circuit? 120C 1440J 1440J

Experiment: By building a simple test circuit compare the e.m.f and p.d. in both series and parallel circuits. If you finish early: Qu2 Page 95 AS Textbook

Kirchhoff’s Second Law: Measuring e.m.f. & p.d. in a circuit – Conclusions: Series circuit – e.m.f. is equal to the sum of …. Parallel circuit – The e.m.f is equal to the... Kirchhoff’s Second Law: In any closed loop in a circuit the sum of the e.m.f.’s is equal to the sum of the p.d.’s This is a result of conservation of _____

What does the variable resistor do? In pairs, set up the standard test circuit A V What does the variable resistor do?

Investigate how current varies with potential difference in a series circuit with a fixed resistor. -Draw a results table which includes currents in both directions -Plot a graph of p.d. (x) against current (y). -Calculate the gradient. How does this relate to the value of resistance of your resistor? -How are V, I & R related?