phy 102
Quiz yourself by thinking what should be in
each of the black spaces below before clicking
on it to display the answer.
Help!
|
|
||||
|---|---|---|---|---|---|
| Electric charge |
🗑
|
||||
| • Intrinsic property of the particles that make |
🗑
|
||||
| up matter |
🗑
|
||||
| Electric charge |
🗑
|
||||
| • Charge can be positive or negative |
🗑
|
||||
| Electric charge |
🗑
|
||||
| • Atoms are composed of negatively-charged |
🗑
|
||||
| electrons and positively-charged protons |
🗑
|
||||
| Electric charge |
🗑
|
||||
| • Charge is measured in Coulombs [unit: C] |
🗑
|
||||
| Electric charge |
🗑
|
||||
| • Charge is measured in Coulombs [unit: C] |
🗑
|
||||
| • Proton and electron have equal and |
🗑
|
||||
| opposite elementary charge = 1.6 x 10-19 C |
🗑
|
||||
| • Charge on proton = +1.6 x 10-19 C |
🗑
|
||||
| • Charge on electron = -1.6 x 10-19 C |
🗑
|
||||
| Electric charge |
🗑
|
||||
| • We now know that protons and neutrons |
🗑
|
||||
| are made up of quarks with 2/3 and -1/3 |
🗑
|
||||
| charges (electrons are still fundamental) |
🗑
|
||||
| Electric charge |
🗑
|
||||
| • Charge cannot be created or destroyed (it is |
🗑
|
||||
| conserved) but it can be moved around |
🗑
|
||||
| A balloon is rubbed against a nylon |
🗑
|
||||
| jumper, and it is then found to cause a |
🗑
|
||||
| force of attraction to human hair. |
🗑
|
||||
| From this experiment it can be |
🗑
|
||||
| determined that the electrostatic |
🗑
|
||||
| charge on the balloon is |
🗑
|
||||
| 1. positive |
🗑
|
||||
| 2. negative |
🗑
|
||||
| 3. Impossible to determine |
🗑
|
||||
| 0% |
🗑
|
||||
| 0% |
🗑
|
||||
| 0% |
🗑
|
||||
| 1. |
🗑
|
||||
| 2. |
🗑
|
||||
| 3. |
🗑
|
||||
| Electric charge |
🗑
|
||||
| • Charges feel electrostatic forces |
🗑
|
||||
| Electric charge |
🗑
|
||||
| • Rub a balloon on your hair and it will stick to things! Why?? |
🗑
|
||||
| • Friction moves electrons from your hair to the balloon |
🗑
|
||||
| • The balloon therefore becomes negatively charged, so your |
🗑
|
||||
| hair becomes positively charged (charge conservation) |
🗑
|
||||
| • Your hair will stand on end (like charges repel), and the |
🗑
|
||||
| balloon will stick to your hair (opposite charges attract) |
🗑
|
||||
| • Now move the balloon near a wall. The wall’s electrons are |
🗑
|
||||
| repelled, so the wall becomes positively charged. |
🗑
|
||||
| • The balloon will stick to the wall! (opposite charges attract) |
🗑
|
||||
| Electric charge |
🗑
|
||||
| • Rub a balloon on your hair and it will stick to things! Why?? |
🗑
|
||||
| Electrostatic force |
🗑
|
||||
| • The strength of the electrostatic force between |
🗑
|
||||
| two charges q1 and q2 is given by Coulomb’s law |
🗑
|
||||
| 𝐹𝑒 𝐹𝑒 |
🗑
|
||||
| 𝑘 =9×109𝑁𝑚2𝐶−2 |
🗑
|
||||
| • The direction of the force is along the joining line |
🗑
|
||||
| Electrostatic force |
🗑
|
||||
| • The electrostatic force is a vector, written Ԧ |
🗑
|
||||
| 𝐹 |
🗑
|
||||
| • Vectors have a magnitude and a direction. This |
🗑
|
||||
| may be indicated by components Ԧ 𝐹 = (𝐹𝑥,𝐹𝑦,𝐹𝑧) |
🗑
|
||||
| Ԧ |
🗑
|
||||
| • The magnitude is sometimes written as |
🗑
|
||||
| can be evaluated as | Ԧ 𝐹| = |
🗑
|
||||
| 𝐹 . It |
🗑
|
||||
| � |
🗑
|
||||
| �𝑥 |
🗑
|
||||
| 2 +𝐹𝑦 |
🗑
|
||||
| 2 +𝐹𝑧 |
🗑
|
||||
| 2 |
🗑
|
||||
| • The direction can be indicated by a unit vector |
🗑
|
||||
| Electrostatic force |
🗑
|
||||
| Example |
🗑
|
||||
| Two 0.5 kg spheres are placed 25 cm apart. Each sphere has a |
🗑
|
||||
| charge of 100 μC, one of them positive and the other negative. |
🗑
|
||||
| Calculate the electrostatic force between them and compare it to |
🗑
|
||||
| their weight. |
🗑
|
||||
| 𝐹=𝑘|𝑞1||𝑞2| |
🗑
|
||||
| 𝑟2 𝑘=9×109𝑁𝑚2𝐶−2 Coulomb’s Law: |
🗑
|
||||
| |𝑞1|=|𝑞2|=100𝜇𝐶=100×10−6𝐶=10−4𝐶 |
🗑
|
||||
| 𝑟=25𝑐𝑚=0.25𝑚 |
🗑
|
||||
| 𝐹𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑠𝑡𝑎𝑡𝑖𝑐=9×109×10−4×10−4 |
🗑
|
||||
| 0.252 =1440𝑁 |
🗑
|
||||
| 𝐹𝑤𝑒𝑖𝑔ℎ𝑡=𝑚𝑔=0.5×9.8=4.9𝑁 |
🗑
|
||||
| Electrostatic force |
🗑
|
||||
| • Where multiple charges are present, the forces |
🗑
|
||||
| sum as vectors (“principle of superposition”) |
🗑
|
||||
| +ve |
🗑
|
||||
| +ve |
🗑
|
||||
| +ve |
🗑
|
||||
| What is the combined force |
🗑
|
||||
| on the blue charge from the |
🗑
|
||||
| two red charges? |
🗑
|
||||
| Electrostatic force |
🗑
|
||||
| • Where multiple charges are present, the forces |
🗑
|
||||
| sum as vectors (“principle of superposition”) |
🗑
|
||||
| +ve |
🗑
|
||||
| +ve |
🗑
|
||||
| +ve |
🗑
|
||||
| 𝐹2 |
🗑
|
||||
| 𝐹1 |
🗑
|
||||
| 𝐹𝑡𝑜𝑡𝑎𝑙 = |
🗑
|
||||
| 𝐹1 + |
🗑
|
||||
| 𝐹2 |
🗑
|
||||
| Electrostatic force |
🗑
|
||||
| • Where multiple charges are present, the forces |
🗑
|
||||
| sum as vectors (“principle of superposition”) |
🗑
|
||||
| +ve |
🗑
|
||||
| +ve |
🗑
|
||||
| +ve |
🗑
|
||||
| 𝐹2 |
🗑
|
||||
| 𝐹1 |
🗑
|
||||
| Electric field |
🗑
|
||||
| • The electric field at a point is the force a unit |
🗑
|
||||
| charge (q = +1 C) would experience if placed there |
🗑
|
||||
| 𝐸 = |
🗑
|
||||
| Ԧ 𝐹 |
🗑
|
||||
| 𝑞 Ԧ 𝐹=𝑞 |
🗑
|
||||
| 𝐸 |
🗑
|
||||
| (Units of E are N/C) |
🗑
|
||||
| • It is a vector and its direction can be represented |
🗑
|
||||
| by electric field lines |
🗑
|
||||
| • Let’s look at some simple examples! |
🗑
|
||||
| Electric field |
🗑
|
||||
| • Electric field around a positive charge +Q |
🗑
|
||||
| +q |
🗑
|
||||
| Test charge +q at separation r |
🗑
|
||||
| feels an outward force |
🗑
|
||||
| |𝐹| = |
🗑
|
||||
| 𝑘 𝑄𝑞 |
🗑
|
||||
| 𝑟2 |
🗑
|
||||
| Electric field is also outward |
🗑
|
||||
| |𝐸| = |
🗑
|
||||
| |𝐹| |
🗑
|
||||
| 𝑞 |
🗑
|
||||
| = |
🗑
|
||||
| 𝑘 𝑄 |
🗑
|
||||
| 𝑟2 |
🗑
|
||||
| Now imagine placing the test charge at many different |
🗑
|
||||
| places to map out the whole electric field |
🗑
|
||||
| Electric field |
🗑
|
||||
| • Electric field around a positive charge +Q |
🗑
|
||||
| Magnitude of electric field at |
🗑
|
||||
| any point: |
🗑
|
||||
| |𝐸| = |
🗑
|
||||
| |𝐹| |
🗑
|
||||
| 𝑞 |
🗑
|
||||
| = |
🗑
|
||||
| 𝑘 𝑄 |
🗑
|
||||
| 𝑟2 |
🗑
|
||||
| Direction of electric field is |
🗑
|
||||
| radially outward |
🗑
|
||||
| Electric field |
🗑
|
||||
| • Electric field around a negative charge -Q |
🗑
|
||||
| Magnitude of electric field at |
🗑
|
||||
| any point: |
🗑
|
||||
| |𝐸| = |
🗑
|
||||
| |𝐹| |
🗑
|
||||
| 𝑞 |
🗑
|
||||
| = |
🗑
|
||||
| 𝑘 𝑄 |
🗑
|
||||
| 𝑟2 |
🗑
|
||||
| Direction of electric field is |
🗑
|
||||
| radially inward |
🗑
|
||||
| Electric field |
🗑
|
||||
| • Electric field lines start on positive charges and end |
🗑
|
||||
| on negative charges |
🗑
|
||||
| • The more closely spaced the field lines, the |
🗑
|
||||
| stronger the force |
🗑
|
||||
| Electric field |
🗑
|
||||
| • The direction of the field lines show how a positive |
🗑
|
||||
| charge would move if placed at that point. A |
🗑
|
||||
| negative charge would move the opposite way. |
🗑
|
||||
| 𝐸 |
🗑
|
||||
| +q |
🗑
|
||||
| Ԧ |
🗑
|
||||
| 𝐹 =− |
🗑
|
||||
| 𝐸/𝑞-q |
🗑
|
||||
| Ԧ |
🗑
|
||||
| 𝐹 = |
🗑
|
||||
| 𝐸/𝑞 |
🗑
|
||||
| Electric field |
🗑
|
||||
| • Electric field lines between two charges |
🗑
|
||||
| Unlike charges |
🗑
|
||||
| Like charges |
🗑
|
||||
| Electric field |
🗑
|
||||
| • Electric field lines between charged plates |
🗑
|
||||
| Electric field |
🗑
|
||||
| • Electric field lines between charged plates |
🗑
|
||||
| 𝐸 |
🗑
|
||||
| • A constant electric field is obtained (see later |
🗑
|
||||
| material on capacitors) |
🗑
|
||||
| Electric field |
🗑
|
||||
| ExampleA +5.0 mCcharge is located at the origin, |
🗑
|
||||
| and a -2.0 mCcharge is 0.74 m away on the x-axis. |
🗑
|
||||
| Calculate the electric field at point P, on the y-axis |
🗑
|
||||
| 0.6 m above the positive charge. If a +1.5 mCwas |
🗑
|
||||
| placed at P, what force would it experience? |
🗑
|
||||
| 0.74 0 |
🗑
|
||||
| 0.6 |
🗑
|
||||
| P |
🗑
|
||||
| Electric field at P due to green charge q = +5x10-6C |
🗑
|
||||
| Electric field is superposition of 2 charges |
🗑
|
||||
| 0 |
🗑
|
||||
| P |
🗑
|
||||
| 0.6 |
🗑
|
||||
| 𝐸=𝑘𝑞 |
🗑
|
||||
| 𝑟2=9×109×5×10−6 |
🗑
|
||||
| 0.62 =1.25×105𝑁/𝐶 |
🗑
|
||||
| Direction is along y-axis: 𝐸𝑥,𝐸𝑦 =(0,1.25×105) |
🗑
|
||||
| E= kq/r2along joining line, k=9x109 |
🗑
|
||||
| Electric field |
🗑
|
||||
| Example |
🗑
|
||||
| A +5.0 mC charge is located at the origin, |
🗑
|
||||
| and a -2.0 mC charge is 0.74 m away on the x-axis. |
🗑
|
||||
| Calculate the electric field at point P, on the y-axis |
🗑
|
||||
| 0.6 m above the positive charge. If a +1.5 mC was |
🗑
|
||||
| placed at P, what force would it experience? |
🗑
|
||||
| Electric field is superposition of 2 charges |
🗑
|
||||
| E= kq/r2 along joining line, k=9x109 |
🗑
|
||||
| 0.6 |
🗑
|
||||
| P |
🗑
|
||||
| 0.6 |
🗑
|
||||
| P |
🗑
|
||||
| 0 |
🗑
|
||||
| 0.74 |
🗑
|
||||
| Electric field at P due to purple charge q = -2x10-6 C |
🗑
|
||||
| � |
🗑
|
||||
| �= |
🗑
|
||||
| 𝑘 |𝑞| |
🗑
|
||||
| 𝑟2 Pythagoras: r2 = 0.62 + 0.742 = 0.91 m2 |
🗑
|
||||
| 0.74 |
🗑
|
||||
| r = 0.95 m |
🗑
|
||||
| Electric field |
🗑
|
||||
| ExampleA +5.0 mCcharge is located at the origin, |
🗑
|
||||
| and a -2.0 mCcharge is 0.74 m away on the x-axis. |
🗑
|
||||
| Calculate the electric field at point P, on the y-axis |
🗑
|
||||
| 0.6 m above the positive charge. If a +1.5 mCwas |
🗑
|
||||
| placed at P, what force would it experience? |
🗑
|
||||
| 0.74 0 |
🗑
|
||||
| 0.6 |
🗑
|
||||
| P |
🗑
|
||||
| 0.74 |
🗑
|
||||
| P |
🗑
|
||||
| Electric field is superposition of 2 charges |
🗑
|
||||
| E= kq/r2along joining line, k=9x109 |
🗑
|
||||
| Electric field at P due to purple charge q = -2x10-6C |
🗑
|
||||
| 0.6 |
🗑
|
||||
| 𝐸=𝑘|𝑞| |
🗑
|
||||
| 𝑟2 =9×109×2×10−6 |
🗑
|
||||
| 0.952 =0.20×105𝑁/𝐶 |
🗑
|
||||
| Electric field |
🗑
|
||||
| ExampleA +5.0 mC charge is located at the origin, |
🗑
|
||||
| and a -2.0 mC charge is 0.74 m away on the x-axis. |
🗑
|
||||
| Calculate the electric field at point P, on the y-axis |
🗑
|
||||
| 0.6 m above the positive charge. If a +1.5 mC was |
🗑
|
||||
| placed at P, what force would it experience? |
🗑
|
||||
| 0.74 0 |
🗑
|
||||
| 0.6 |
🗑
|
||||
| P |
🗑
|
||||
| Electric field is superposition of 2 charges |
🗑
|
||||
| E= kq/r2along joining line, k=9x109 |
🗑
|
||||
| Electric field at P due to purple charge q = -2x10-6C |
🗑
|
||||
| � |
🗑
|
||||
| �=𝑘|𝑞| |
🗑
|
||||
| 𝑟2 =9×109×2×10−6 |
🗑
|
||||
| 0.952 =0.20×105𝑁/𝐶 |
🗑
|
||||
| 0.74 |
🗑
|
||||
| 0.6 |
🗑
|
||||
| 𝐸𝑥,𝐸𝑦 =(0.16×105,−0.13×105) |
🗑
|
||||
| Electric field |
🗑
|
||||
| Example |
🗑
|
||||
| A +5.0 mC charge is located at the origin, |
🗑
|
||||
| and a -2.0 mC charge is 0.74 m away on the x-axis. |
🗑
|
||||
| Calculate the electric field at point P, on the y-axis |
🗑
|
||||
| 0.6 m above the positive charge. If a +1.5 mC was |
🗑
|
||||
| placed at P, what force would it experience? |
🗑
|
||||
| Electric field is superposition of 2 charges |
🗑
|
||||
| Green charge: |
🗑
|
||||
| Purple charge: |
🗑
|
||||
| � |
🗑
|
||||
| �𝑥,𝐸𝑦 = (0,1.25×105) |
🗑
|
||||
| 0.6 |
🗑
|
||||
| P |
🗑
|
||||
| 0 |
🗑
|
||||
| 𝐸𝑥,𝐸𝑦 = (0.16×105,−0.13×105) |
🗑
|
||||
| Total: |
🗑
|
||||
| 𝐸𝑥,𝐸𝑦 = (0.16×105,1.12×105) |
🗑
|
||||
| Electric field strength at P: |
🗑
|
||||
| 𝐸 = |
🗑
|
||||
| 0.74 |
🗑
|
||||
| 𝐸𝑥 |
🗑
|
||||
| 2 +𝐸𝑦 |
🗑
|
||||
| 2 = 1.13×105 𝑁/𝐶 |
🗑
|
||||
| 𝐹 =𝑞𝐸 =1.5×10−6×1.13×105 =0.51𝑁 |
🗑
|
||||
| Force: |
🗑
|
||||
| Electric dipole |
🗑
|
||||
| Dipole moment |
🗑
|
||||
| Electric dipole |
🗑
|
||||
| • A dipole in an electric field will feel a torque |
🗑
|
||||
| but no net force |
🗑
|
||||
| 𝐸 |
🗑
|
||||
| 𝜏 =𝐹𝑙sin𝜃 = 𝐸𝑄𝑙sin𝜃 Ԧ 𝜏= |
🗑
|
||||
| 𝐸 × Ԧ 𝑝 |
🗑
|
||||
| Electrostatic |
🗑
|
||||
| analyzer |
🗑
|
||||
| • Charged particles will experience a force in an |
🗑
|
||||
| electric field F=qE, hence acceleration a=F/m=qE/m |
🗑
|
||||
| Electrostatic analyzer |
🗑
|
||||
| •An electrostatic analyzerselects velocities |
🗑
|
||||
| Uniform electric field E applied |
🗑
|
||||
| between curved surfaces |
🗑
|
||||
| r |
🗑
|
||||
| 𝑎= 𝐹 |
🗑
|
||||
| 𝑚=𝑞𝐸 |
🗑
|
||||
| 𝑚 |
🗑
|
||||
| 𝑎=𝑣2 |
🗑
|
||||
| 𝑟 |
🗑
|
||||
| Acceleration a is given by: |
🗑
|
||||
| 𝑣2 |
🗑
|
||||
| 𝑟 =𝑞𝐸 |
🗑
|
||||
| 𝑚→𝑣= 𝑞𝐸𝑟 |
🗑
|
||||
| 𝑚 |
🗑
|
||||
| Conductors and Insulators |
🗑
|
||||
| • In metals (e.g. copper, iron) some electrons are weakly held |
🗑
|
||||
| and can move freely through the metal, creating an electric |
🗑
|
||||
| current. Metals are good conductors of electricity. |
🗑
|
||||
| Conductors and Insulators |
🗑
|
||||
| • In metals (e.g. copper, iron) some electrons are weakly held |
🗑
|
||||
| and can move freely through the metal, creating an electric |
🗑
|
||||
| current. Metals are good conductors of electricity. |
🗑
|
||||
| • In non-metals (e.g. glass, rubber, plastic) electrons are |
🗑
|
||||
| strongly held and are not free to move. Non-metals are |
🗑
|
||||
| poor conductors of electricity, or insulators. |
🗑
|
||||
| • Semi-conductors (e.g. germanium, silicon) are half-way |
🗑
|
||||
| between conductors and insulators. |
🗑
|
||||
| Freely moving electrons make metals good conductors of electricityand heat |
🗑
|
||||
| Summary |
🗑
|
||||
| • Matter is made up of positive and negative charges. |
🗑
|
||||
| Electrons/protons carry the elementary charge 1.6 x 10-19 C |
🗑
|
||||
| 𝐹= |
🗑
|
||||
| • Forces between charges are described by Coulomb’s Law |
🗑
|
||||
| 𝑘 𝑞1 𝑞2 |
🗑
|
||||
| 𝑟2 𝑘=9× 109𝑁𝑚2𝐶−2 |
🗑
|
||||
| • Forces from multiple charges sum as vectors |
🗑
|
||||
| • Electric field describes the force-field around charges |
🗑
|
||||
| 𝐸 = |
🗑
|
||||
| Ԧ 𝐹 |
🗑
|
||||
| 𝑞 Ԧ 𝐹=𝑞 |
🗑
|
||||
| 𝐸 |
🗑
|
||||
| 1. Two 0.5 kg spheres are placed few meters apart. Each sphere |
🗑
|
||||
| has a charge of 60 μC, one of them positive and the other |
🗑
|
||||
| negative. If the electrostatic force between them is 1400 N, |
🗑
|
||||
| calculate the distance between them. 5 marks |
🗑
|
||||
| 2. You measure an electric field of 1.25 X 106 N/C at a distance |
🗑
|
||||
| of 0.150 m from a point charge. There is no other source of |
🗑
|
||||
| electric field in the region other than this point charge. (a) |
🗑
|
||||
| What is the electric flux through the surface of a sphere that |
🗑
|
||||
| has this charge at its center and that has radius 0.150 m? (b) |
🗑
|
||||
| What is the magnitude of this charge? 6 marks |
🗑
|
||||
| 3. A parallel-plate air capacitor is to store charge of |
🗑
|
||||
| magnitude 240.0 pC on each plate when the potential |
🗑
|
||||
| difference between the plates is 42.0 V. (a) If the area of |
🗑
|
||||
| each plate is 6.80 cm2, what is the separation between the |
🗑
|
||||
| plates? (b) If the separation between the two plates is |
🗑
|
||||
| double the value calculated in part (a), what potential |
🗑
|
||||
| difference is required for the capacitor to store charge of |
🗑
|
||||
| magnitude 240.0 pC on each plate? 6 marks |
🗑
|
||||
| 4. What is the resistance of a carbon rod at 25.8°C if its |
🗑
|
||||
| resistance is 0.0160 Ω at 0.0°C? 3 marks (Take α for |
🗑
|
||||
| carbon as– 0.0005 [(°C)−1]) |
🗑
|
||||
| Lectures I and II: |
🗑
|
||||
| Electric charge, force, field |
🗑
|
||||
| • What is electric charge and how do we |
🗑
|
||||
| measure it? |
🗑
|
||||
| • Coulomb’s Force Law between charges |
🗑
|
||||
| • How an electric field can be used to describe |
🗑
|
||||
| electrostatic forces |
🗑
|
||||
| • Some simple applications of these principles |
🗑
|
Review the information in the table. When you are ready to quiz yourself you can hide individual columns or the entire table. Then you can click on the empty cells to reveal the answer. Try to recall what will be displayed before clicking the empty cell.
To hide a column, click on the column name.
To hide the entire table, click on the "Hide All" button.
You may also shuffle the rows of the table by clicking on the "Shuffle" button.
Or sort by any of the columns using the down arrow next to any column heading.
If you know all the data on any row, you can temporarily remove it by tapping the trash can to the right of the row.
To hide a column, click on the column name.
To hide the entire table, click on the "Hide All" button.
You may also shuffle the rows of the table by clicking on the "Shuffle" button.
Or sort by any of the columns using the down arrow next to any column heading.
If you know all the data on any row, you can temporarily remove it by tapping the trash can to the right of the row.
Embed Code - If you would like this activity on your web page, copy the script below and paste it into your web page.
Normal Size Small Size show me how
Normal Size Small Size show me how
Created by:
Ajaxx
Popular Physics sets