What is potential difference?
Potenial difference is defined as the work done (or energy transferred) per unit charge.
V Eq.
V = W/Q
What is emf?
The emf (V) of a source is defined as the electrical energy produced per unit charge passing through the source.
How is energy transferred to a torch.
When a torch bulb is connected to a battery, electrons deliver energy from the battery to the torch bulb. Each electron moves around the circuit and takes a fixed amount of energy from the battery as it passes through it. The electrons then deliver energy to the bulb as they pass through it. After delivering energy to the bulb, each electron re-enters the battery via the positive terminal to be resupplied with more energy to deliver to the bulb.
What does a battery have?
A battery has the potential to transfer energy from its chemical store if the battery is not part of a complete circuit. When a battery is in a circuit, each electron does work to pass through the electrical component and therefore transfers some or all of its energy. The work done per unit charge is defined as the potential difference or voltage across the component.
For a source of emf in a circuit:
The electrical energy produced when charge passes through the source = Qε. This energy is transferred to other parts of the circuit and some may be dissipated in the source due to the source’s internal resistance.
What are the effects of an electrical current?
An electric current has a heating effect when it passes though a component with resistance. It also has a magnetic effect, which is made use of in electric motors and loudspeakers.
Energy transfer in a heater.
An electric heater has resistance. The work done on the heater is transferred as thermal energy. This happens because the charge carriers repeatedly collide with the atoms in the device and transfer energy to them, so the atoms vibrate more and the resistor become hotter.
Energy transfer in an electric motor.
In an electric motor turning at a constant speed, the work done on the motor is equal to the energy transferred to the load and surroundings by the motor, so the kinetic energy of the motor remains constant. The charge carriers are electrons that need to be forced through the wires of the spinning motor coil against the opposing force on the electrons due to the motor’s magnetic field.
Energy transfer in a louspeaker
For a loudspeaker the work done on the loudspeaker is transferred as sound energy. Electrons need to be forced through the wires of the vibrating loudspeaker coil against the force on them due to the loudspeaker magnet.
Work done formula
W = IV ΔT = QV
Electrical power
Work done = IV ΔT
Power = energy/time = IV ΔT/ ΔT = IV
Potential difference and charge carriers.
- If the charge carriers lose energy, the potential difference is a potential drop.
- If the charge carriers gain energy, which happens when they pass through a battery or cell, the potential difference is a potential rise equal to the pd across the battery or cell’s terminal.
Voltage rules
- For 2 or more components in series, the total pd across all the components is equal to the sum of the potential difference across each component.
- The pd across components in parallel is the same.
- For any complete loop of a circuit, the sum of the emfs round the loop is equal to the sum of the potential drops around the loop.
