IB Physics Topics 5.2 and 5.3

Question	Answer
Current (I) Definition	The rate of flow of electric charge (no. of coulombs flowing past a point in the circuit every second)
Current (I) Formula	I = ∆Q/∆t
Direction fo electric current	Always flows from positive to negative, or from big dash to small dash
e.m.f. Location on diagram	Electromotive force Checks the difference in energy per coulomb between cell
p.d. Location on diagram	Potential Difference Checks the difference in energy per coulomb between lamp
e.m.f. Definition	The total energy difference per unit charge around the circuit. (Is potential difference when NO current flows in circuit)
p.d. Definition	The work done per unit charge to move a small positive charge between two points
p.d. and e.m.f. in a series circuit	∑p.d. = e.m.f. across all cells e.g., three lamps of 3V (p.d.), the cell has 9V (e.m.f.)
p.d. and e.m.f. in a parallel circuit	p.d. = e.m.f.
Resistance Definition	Measures how difficult it is for current to flow Measured in Ohms (Ω)
Resistance Formula	Resistance (R) = Voltage (V) / Current (I)
Resistance proportionalities	R α L R α 1/A R depends on type of material
Resistivity	R = (ρL)/A Where R = resistance (Ω) ρ = resistivity of the material (Ωm) L = length (m) A = cross sectional area of conductor (m2)
Diameter in mm to A in m2	d = 0.1mm --> r = 0.05mm = 5E-5 cross sectional area = πr2 = π(5E-5)2 = 7E-9 m2
Ohm's Law	For a metallic conductor at a constant temperature, the current passing α applied p.d.
Power Definition	Amount of energy used by a device per second (W)
Power Formula	P = VI
Power 2nd Formula	P = VI Since V = IR P = I (IR) P = (I2)R
Energy transformed Definition	Total energy transformed by a lamp is the power (J/s) x time the lamp is on in seconds
Energy transformed Formula	E = VIt
Adding resistances Series circuit	R1 + R2 ... Rn = Rtotal
Adding resistances Parallel circuit	1/R1 + 1/R2 ... 1/Rn = 1/Rtotal
Ideal meters Voltmeters	Infinite resistance
Ideal meters Ammeters	Zero resistance
Potential Divider Formula	Vout = Vin (R2/R1+R2)
Assumption of power source having no resistance - true or false?	FALSE Internal resistance
Internal resistance	p.d. across cell is less than e.m.f. due to energy lost in the internal resistance
p.d. Formula using internal resistance	V = ε - Ir e.g., find p.d. across cell terminals when e.m.f. = 12, A of internal resistance = 3, internal resistance = 1.5Ω V = 12 - 3x1.5 V = 7.5 V
Kirchoff's 1st Law	Conservation of charge ∑Amps = 0 by a junction I1 + I2 = I3 + I4 + I5
Kirchoff's 2nd Law	∑e.m.f. = ∑p.d. in a closed loop or ∑V = 0 by a loop E = V1 + V2 + V3 or E - V1 - V2 - V3 = 0
Procedure for Kirchoff's Laws	1) Assume all voltage sources are given. If not, label. 2) Label each branch with a branch current 3) Apply Kirchoff's 1st law at each node, ∑I = 0 4) Apply Kirchoff's 2nd law for each independent loop of the current 5) Solve equation
Primary cell	Used once, then thrown away Electrochemical reaction is not EASILY reversible
Primary cell Discharge characteristics	Terminal p.d. loses initial value quickly Stable and constant value for most of it's lifetime Then rapid decrease to zero as the cell discharges completely.
Secondary cell	Reaction CAN be reversed by running a current INTO the cell
Secondary cell Advantages	Can charge High uses
Secondary cell Disadvantages	- Not good for small uses, such as tv remote control or mouse - Lose charge on their own - high "self-discharge", chemical reactions reduce the energy available - this is why primary cells are used more often
Light Dependent Resistor (LDR)	A device that has a resistance that varies according to the amount of light falling on it's surface High resistance in the dark Low resistance in light
Thermistor	Resistance decreases with increasing temperature

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IB Physics Topics 5.2 and 5.3

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	Created by Christine Laurich almost 8 years ago