6- Potential Difference, E.M.F And Power

Question 1

Potential difference

Answer 1

• electrical energy used in a circuit/charge passing = p.d (JQ^-1),(V)
• measured by voltmeters, which have very high resistances to minimise the current taken by the voltmeter
• voltmeters must be connected in parallel to the circuit

Question 2

E.M.F

Answer 2

• the initial force applied to electrons by a power supply
• has the same units as p.d (V)
• Internal resistance of the power supply is the resistance the current experiences before it even leaves the power supply and can be measured
• Therefore V= E.M.F - Internal resistance
• Therefore when there is no current, there is no internal resistance and V=E.M.F, which has to be extrapolated from a V/I graph

Question 3

Power

Answer 3

• P(W)=IV
• change in power (W)= IV×change in time
• P = I^2*R
• P = V^2/R

Question 4

Voltage formula

Answer 4

• potential difference (v) = work done (J) /charge (C)

Question 5

Voltage distribution

Answer 5

• in a series circuit the voltage is split between resistors, in a parallel circuit it is even across all branches
• this is due to conservation of energy

Question 6

Difference between emf and terminal pd

Answer 6

• EMF is the voltage across the cell when no current flows through it, terminal p.d is the voltage across the cell when there is a current, and it will be lower than EMF
• When there is a current the terminal p.d is equal to EMF as there is no internal resistance