Physics AQA Topic 2 (Energy transfers)

4.2.4

State two equations for the power of a circuit

P = IV (Power = current x voltage), P = I^2R (Power = (current)^2 x resistance)

What are the units for the equations for power

Power = watts (W), current = amperes (A), potential difference = volts (V), resistance = ohms (Ω)

State an equation linking energy transferred, power and time

E = Pt (Energy transferred = power x time)

What are the units of the equation linking energy transferred, power and time

Energy = joules (J), power = watts (W), time = seconds (secs)

State an equation linking energy transferred, charge flow and potential difference

E = QV (Energy = charge x potential difference)

What are the units of the equation linking energy transferred, charge flow and potential difference

Energy - joules (J), charge = coulombs (C), potential difference = volts (V)

What two main factors does the amount of energy transferred by an appliance depend on

How long the appliance is being used for and the power of the appliance

Describe the energy transfers in a battery powered torch

Battery - chemical energy to electrical energy, bulb - electrical energy into light energy and waste energy in the form of thermal energy

Describe the energy transfers in a battery powered motor

Battery - chemical energy to electrical energy, motor - electrical energy to kinetic energy and waste energy in the form of thermal energy due to friction

What three things determine the power of a circuit device

The potential difference across the circuit (P = IV), the current through the circuit (P = IV), and the amount of energy transferred in a given time (E = Pt - P = E/t)

When is work done

Work is done when charge flows in a circuit

What is the purpose of the national grid

To link power stations to consumers so that they have access to a source of electricity

What are the two types of transformers used in the national grid

Step-up and step-down transformers

Where are step-up transformers found in the national grid and what do they do

Step-up transformers are used when connecting power stations to transmission cables, and they increase the potential difference

Where are step-down transformers found in the national grid and what do they do

Step-down transformers are used in connecting transmission cables to domestic buildings (like houses), and they decrease the potential difference

Why do transmission lines transfer electricity at high potentials

A high potential difference results in a low current, and the lower the current, the less energy that is wasted as heat energy as the energy is transferred across the grid, so it is more efficient

Why does the potential need to be decreased between transmission lines and houses

Lower potential differences are safer for domestic use and reduces the likelihood of severe electrocution so many appliances are also designed for 230V