Do you guys still remember electric current in AS/A-level Physics?

• Electric current is a flow of electric charges. A lamp lights because charged particles are moving through it.
• Ions can only vibrate about their fixed point, but electrons are free to move randomly from one ion to another through the lattice.
• When a battery is attached to a wire. The free electrons are repelled by the negative terminal but are attracted to the positive one.
• They still have random movement but now move in the same direction. This is a flow of charge- an electric current!

• Electric charge flows along wires.
• It flows along the complete conducting path.
• Charge is the source of electric force.
• Electric charges can be either positive or negative.

• The unit of electric charge is the coulomb.
• 1 coulomb is the amount of charge that passes a point when a current of 1 ampere flows for 1 second.
• The charge on 1 electron is 1.6 x 10 ^-19 Coulombs.

• The coulombs entering a lamp have electrical potential energy; those leaving have very little potential energy.
• The p.d between two points is the electrical potential energy transferred to other forms, per coulomb of charge that passes between two points.

• P.d is measured in Volts and is often called Voltage.
• If the p.d is one volt, then 1 joule of electrical energy is transferred for each coulomb of charge.

• The greater the resistance of a component the more difficult it is for charge to flow through it.
• The electrons make many collisions with a tungsten fillament which gives it high resistance.
• The resistance of a conductor is the ratio of the p.d applied across it, to the current passing through it.

• Resistance is measured in Ohms.
• The current through a metal wire is directly proportional to the p.d across it ( providing that temperature remains constant)- this is Ohm’s law.

• The resistance of a wire at a constant temperature depends on it’s dimensions and the material from which it’s made.
• Every material has a property called it’s resistivity ‘rho’ it is measured in ohm metres.
• If we know the resistivity, the cross-sectional area and the length of a sample of material we can calculate it’s resistance.

• A thermistor is a semiconductor device made of materials whose resistance varies as a function of temperature; can be used to compensate for temperature variation in other components of a circuit.

This is the end of the topic

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