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# level: key concepts

### level questions: key concepts

what is kirchoffs first law?charge flowing into a circuit junction must equal the sum of charge leaving the junction
what is kirchoffs second law?within any closed loop the sum of the emfs is equal to the sum of the products of current and resistance
what happens when pd is applied across a conducter?the electrons accelerate and gain kinetic energy
what is ohms law?current flowing within a conductor is directly proportional to the pd across it provided that physical conditions such as temperature remain constant
what factors affect resistance ?cross sectional area , material, length and temperature
how does cross sectional area of a wire affect its resistance?a thicker wire means lower resistance as there is a bigger volume of space for the electrons to flow in
how does the material of a wire affect its resistance?metals have delocalised electrons making them good conductors so resistance is lower . plastics and insulators do not so they have a higher resistance
how does the length of a wire affect its resistance?as the length of the wire increases so does the electrical resistance as electrons experience a smaller pd gradient as wire gets longer. the size of voltage per metre value decreases causing a lower drift velocity and more ions for electrons to collide with
how does temperature affect its resistanceas tempt increases it causes ions to gain kinetic energy and move faster making it more difficult to pass without collisions resulting in more resistance
explain the current in series circuitscurrent is the same
explain the current in parallel circuitscurrent splits into each branch and add to make the total current
explain the potential difference in series circuitspd splits in ratios according to resistance
explain the potential difference in parallel circuitsvoltage in each branch is the same
what happens when you add resistors in parallell?the total resistance decreaces
how do you find total emf if there are two emf in seriesadd them
how do you find total emf if there are two emf in parallelsubtract them