Y11 Physics - Electricity
Electric Charge
Opposite charges attract each other.
Same charges repel each other.
/ Like charges repel, unlike charges attract.
Conductors & Insulators
Static Electricity
Electric Fields
A conductor allows negatively charged electrons to flow through it because the electrons are delocalised.
Electrons can move from one atom to another.
Current electricity is concerned with the movement of electric charge in conductor.
An insulator does not allow electrons to flow. Electrons are still present in an insulator but are normally stationary or ‘static’. Static electricity is concerned with electric charges in insulators.
Definition: An accumulation of charge in an insulator
Can occur when two insulators
They cannot flow, but it is possible for electrons to be removed from the surface of an insulator, making it positive.
This is because the material now has more protons.
It is also possible to add electrons to the surface of an insulator, making it negative.
Electrons are the only charges that are transferred between materials.
Protons never move!'
This is because there are now more electrons than protons.
It depends on the materials as to which insulator will become positive or negative. But one will always end up positive and one will always end up negative.
Anything with a charge has an electric field around it. An electric field is a volume of space in which an electric charge experiences a force.
The direction of an electric field is the direction of the force on a positive charge in that field.
Current and Potential Difference
Potential difference: When an object becomes charged, we say that it stores electrical potential(volts). The more charge that object has, the higher its electrical potential.
Electric Current: A flow of electric charge is called current. It is measured in amps (A). The higher the flow of charge, the higher the current.
Current(amps, A) = Charge(coulombs, Q) / Time(seconds, s)
Current (I) = Charge (Q) / Time (t)
Potential Difference (volts, V)
Circuit Quantities
Charge
Electrons carry a charge it is measured in Coulombs (C), it takes billions of electrons make 1 C of charge. Current is the rate of flow of charge, it is measured in Amps (A). If lots of charge passes a point in a small time, this is a large current.
Potential Difference
The potential difference (or voltage) across a component is the difference in the amount of energy used by it per unit of charge. It’s the difference in potential energy of the electrons before they reach the component and after they leave it.
Electromotive Force (emf)
Emf is the amount of energy given to a coulomb of charge by the power source in a circuit.
Resistance
Resistance is what makes it difficult for electrons to flow. Every conductor has resistance, it is caused by electrons colliding with atoms in the conductor. A big resistance means it is hard for electrons to flow, therefore current is small. Resistance is measured in Ohms (Ω).
Resistance of a Wire
the electrons are freely flowing. There are few collisions between the delocalised electrons and the metal ions. This wire has a low resistance.
the electrons are not flowing as freely. There are more collisions between metal ions and the delocalised electrons. This wire has a high resistance.
The resistance of a piece of wire is directly proportional to length.
The resistance of a wire is inversely proportional to radius.
Ohm's Law and I-V Graph Analysis
Ohm's Law
When there is a current through a component, that component transfers away a certain amount of energy. The energy that it transfers is the potential difference across the component, per unit of charge.
The higher the potential difference, the more energy the electrons have, the faster they are moving and therefore the higher the current. In other words, potential difference is proportional to current.
If the component has a higher resistance, more energy is needed to make the same amount of current pass through. In other words, potential difference is proportional to resistance.
The potential difference across a component is directly proportional to the current flowing through it, provided that resistance is constant.
Circuit Components
Series and Parallel Circuits
Ammeters are always placed in series to measure current through a component.
Voltmeters are always placed in parallel to measure voltage across a component.
Potential Dividers and Relays
Potential divider
A potential divider does what it says: it splits potential difference across two components. This is useful. Imagine we have a e.m.f. supply of 12V but a bulb that only needs 8V. We can use the potential divider to split up the 12V into a 4V and an 8V, thereby powering the bulb without risking breaking it.
Electrical Power
Power is the rate at which energy is transferred, it is how quickly energy is used, it is the joules per second. Power is measured in Watts (W). When studying mechanical power, you were concerned with force, distance, energy, or time. With electrical power we are concerned with voltage, current, resistance, energy, charge, or time. However, it is essentially the same thing.
Electrical Safety
